CMNFI Canadian Museum of Nature Fish Collection, Ottawa, Canada

TGU Tomsk State University, Tomsk, Russia;

ZIN (ZISP also used) Zoological Institute, Russian Academy of Sciences, St. Petersburg, Russia;

ZMB Museum für Naturkunde, Berlin, Germany.

• ZMB 6475, 1 adult, 418.3 mm TL, neotype of Petromyzon marinus camtschaticus Tilesius, 1811 and lectotype of Petromyzon japonicus von Martens, 1868, Japan: Tokyo (originally Jeddo) and Yokohama, Honshu Island, Pacific Ocean basin, 1860–1863.
• ZMB 6476, 1 adult, 397.3 mm TL, paralectotype of Petromyzon japonicus, Japan: Tokyo (originally Jeddo) and Yokohama, Honshu Island, Pacific Ocean basin, 1860–1863.
• TGU 3 [no. 3699 in Ioganzen (1935b, table 3)], 1 adult, 182 mm TL, syntype of Petromyzon kessleri Anikin, 1905, Russia: Kirgizka River near Tomsk, Tom’ River system, Ob’ River drainage, Kara Sea basin, Arctic Ocean basin, 24 Dec. 1899, A. Neiland.
• TGU 9 [no. 3696 in Ioganzen (1935b, table 3)], 6 of 10 adults, 128–165 mm TL, syntypes of Petromyzon kessleri, Russia: near mouth of Tom’ River at Kozyulino, taken from a Common gull’s (Larus canus) digestive tract, Ob’ River drainage, Kara Sea basin, Arctic Ocean basin, 28 June 1903, G.E. Ioganzen.
• ZIN 12159, 8 of 10 adults, 281–374 mm TL, syntypes of Lampetra japonica septentrionalis Berg, 1931, Russia: Onega River at Podporozh’e, White Sea basin, Arctic Ocean basin, December 1901, N.A. Varpakhovskiy.
• ZMB 20638, 2 adults, 138.3–140.1 mm TL, syntypes of Lampetra mitsukurii Hatta, 1901, Japan: small watercourses on Hondo (= Honshu), Shikoku, Kyushu, and Hokkaido islands, Pacific Ocean basin.
• CMNFI 1984–274, 2 adults, 147.5–163.5 mm TL, paratypes of Lethenteron matsubarai Vladykov & Kott, 1978, Japan: Shokotsu River, Hokkaido Island, 44°22'N, 143°20'E, Sea of Okhotsk basin, Pacific Ocean basin, 1950–1952, T. Hikita.
• CMNFI 2008–59, 1 adult, 149.5 mm TL, paratype of Lethenteron ninae Naseka, Tuniyev & Renaud, 2009, Russia: Chakhtsutsyr Stream at Gumariya, Sochi District, Psou River drainage, Black Sea basin, 17–24 Dec. 2006, S.B. Tuniyev.

The common name in Itelmen, formerly Kamchadal, a language spoken in Kamchatka, is Canaháisch. The written description and the drawings of the body (life size) and of the oral disc (enlarged) are based on a metamorphosed specimen collected 30 July 1804 (Julian calendar; 11 August 1804 Gregorian calendar) from marine waters in the harbor of Petropavlovsk-Kamchatsky, Russia. The drawing of the body in side view is thrown into three curves and by using a string along the curves we estimated the total length to be 308 mm. Disc length is ca. 16 mm (≈ 5.2% TL) and urogenital papilla length ca. 9.5 mm (≈ 3.1% TL). The two dorsal fins are separate; the interspace being ca. 22 mm. Caudal fin is lanceolate (= spade-like). Dentition: teeth are yellow; supraoral lamina with two bicuspid teeth separated by a wide bridge; two bicuspid endolaterals on each side; infraoral lamina with seven unicuspid teeth; transverse lingual lamina u-shaped, with seven blunt unicuspid teeth, the median one not enlarged in the figure, but noted as distinct in the text. The figure of the oral disc also shows a total of eleven anterials arranged in two rows; the first one consisting of five unicuspid teeth. Posterial and marginal teeth are not mentioned in the text, nor are they shown in the figure of the oral disc. Body pigmentation is not mottled, dorsal surface of head olive-brown, ventral body surface bluish-silvery, and tips of fins blackish; further on in the text he specifies that the posterior (= second) dorsal fin is blackish. The type specimen is lost, but Kottelat (1997) designated the lectotype of Petromyzon japonicus von Martens, 1868 as the neotype of Petromyzon marinus camtschaticus Tilesius, 1811, and it was studied (Tables 1–3).

The Japanese common name is Yats’-me-anango meaning eight-eyes-eel, in reference to the eye and seven branchial openings on the side of the body and eel-like body shape. The written description and the drawing of the oral disc (scale not provided) are based on an unspecified number of metamorphosed specimens collected between 1860 and 1863 from Tokyo (appeared as Jeddo) and Yokohama, Honshu Island, Japan. It is not clearly stated whether von Martens collected these himself or if they were obtained from the fish market. The habitat from which they came (i.e., freshwater, brackish or marine) is, therefore, not certain. Total length, 454 mm; snout length, 29 mm (= 6.4% TL). The two dorsal fins are separate, the interspace is 2.5 times the eye diameter (eye diameter not provided). Dentition (Fig. 2): row of similarly-sized, slender and pointed marginals; between the marginals and the infraoral lamina is a row of 16 small posterials (we counted 19 on the drawing); between the marginals and supraoral lamina are several larger teeth in a quincunx arrangement (the quincunx arrangement is not apparent on the drawing); supraoral lamina is crescent shaped, each side with a strong unicuspid tooth; three bicuspid endolaterals on each side (the drawing shows 1–2–2 on the left and 2–2–1–2 on the right side); infraoral lamina with six unicuspid teeth (we counted seven on the drawing), the two outermost larger (smaller on the drawing); transverse lingual lamina without a furrow, with a strong middle unicuspid tooth and four unicuspid teeth on each side (only three unicuspid teeth on each side were discernible on the drawing); longitudinal lingual laminae paired, narrow, with numerous cusps facing each other. Body pigmentation is slate gray on the dorsal surface and silvery on the lateral and ventral surfaces. One lectotype and one paralectotype was studied (Tables 1–3).

The written description is based on 16 metamorphosed specimens (Fig. 3) collected in the Tom’ River and at the mouth of its tributary the Kirgizka River, Ob’ River drainage; both localities near Tomsk, Russia. Total length, 160–210 mm. Two dorsal fins; the interspace varying from 0 (touching at their bases with swelling at the site of contact) to 10 mm. The second dorsal fin is usually angular and 1.5–2 times higher than the rounded first dorsal fin. The second dorsal fin is continuous with the caudal fin. The caudal fin shape is rhomboid (= spade-like). Dentition: row of minute marginals; 20–25 anterials becoming progressively smaller towards the anterior end of the oral disc and arranged in staggered order in five rows; supraoral lamina with two large, either sharp, blunt or intermediate shape, usually dark yellow teeth separated by a bridge; three bicuspid endolaterals on each side; infraoral lamina with six or seven, usually six, exceptionally nine (Mean = 6.6), large, either sharp or blunt, usually dark yellow teeth, the lateralmost being bicuspid and the internal ones unicuspid; a single row of 20–25 small posterials; semi-circular transverse lingual lamina with a large, sharp median tooth and eight or nine smaller teeth on each side; pair of longitudinal lingual laminae with an unspecified number of fine teeth facing each other. Body pigmentation is pale brown, ash gray or black on the dorsal surface, white or yellowish on the ventral surface and the end of the tail is dark brown. The line of demarcation between the dorsal and ventral pigmentation is not always distinct. The dorsal and caudal fins are white or yellowish except that the apex of the second dorsal fin is ash gray and the tip of the caudal fin is dark brown. Seven syntypes were studied (Tables 1–3).

The written description is based on ten metamorphosed specimens from the Onega River at Podporozh’e, White Sea basin, Russia, collected December 1901 by N.A. Varpakhovskiy (ZIN 12159). The common name given is Arctic lamprey. Total length, 284–377 mm (Mean = 327 mm). Dentition: infraoral lamina with six sharp teeth (Berg’s notation is 1+4+1, but he does not explain the reason for the distinction between the types of teeth). Berg (1931) provides a drawing of the oral disc of a non-type specimen collected from the type locality in December 1929 (Berg 1931: pl. V, fig. 4) and it becomes clear that 1+4+1 means four unicuspid teeth flanked on each side by one enlarged bicuspid tooth. This is the condition (1b4u1b) of all eight syntypes examined in this study (Table 2). The drawing also shows a row of marginals, two rows of anterials, the first row with three unicuspid teeth, one row of posterials with 19 unicuspid teeth, no exolaterals on either side, supraoral lamina with two unicuspid teeth separated by a wide bridge, three bicuspid endolaterals on each side, transverse lingual lamina with an enlarged median cusp flanked on the left side by six unicuspid teeth and on the right by at least five. Additional characteristics reported by Berg (1931) in non-type specimens from the Onega River are an infraoral lamina count of 1+6+1 and four adult individuals with their ventral body surface mottled brown and one with the body entirely black. In a specimen from the Tura River the infraoral lamina count is 1+5+1 and in a specimen from the Vyg River a third cusp is placed asymmetrically between the two larger cusps on the supraoral lamina. Based on material examined and literature, Berg (1931) determined that the distribution of La. j. septentrionalis occurs from the White Sea basin to the Ob’ River drainage. The specific rivers listed from west to east are: Umba (only apparently because based on an ammocoete), Vyg, Onega, Severnaya Dvina, Pesha, Shapkina (Pechora River drainage), Tura, Tobol, Irtysh, Tom’, and also the Gulf of Ob’. The shortest adult is 227 mm TL from the Vyg River at Soroka (= Belomorsk) and the longest is 430 mm TL from the Tura River at Tyumen. In the Onega River it is fished commercially. Berg (1931) diagnosed this subspecies from La. j. japonica based on the smaller length of its upstream migrants (227–430 mm TL versus 352–625 mm TL, respectively) and its lower fecundity: 24,086–25,144 eggs in two females, 335–339 mm TL, collected from the Onega River at the end of November 1929 compared to 80,825–107,015 eggs in six females, 403–492 mm TL, respectively (the TL of the female with the lowest fecundity was unknown), collected from Tneyvakh, below Nikolayevsk-on-Amur, lower Amur River, Russia between September and 20 November 1929. Berg (1931) diagnosed this subspecies from La. j. kessleri based on the latter having few eggs (no specific numbers given) and adults only reaching a maximum of 260 mm TL. Eight syntypes were studied (Tables 1–3).

The written description is based on a 310 mm TL metamorphosed specimen from the mouth of the Amur River, Russia. At the periphery of the suctorial disc is a series of small pointed teeth (= marginals); a second more centrally-located circular series of teeth consists in its lower zone of a row of small pointed teeth (= posterials), in its central zone, on each side, of three transversely-positioned tooth plates (= endolaterals), of which only the top two are bicuspid (the condition of the bottom endolateral is not given), and in its upper zone of two rows of more pointed conical teeth (= anterials) numbering eleven. Maxillary arch (= supraoral lamina) with two strong, sharp canine teeth. Mandibular arch (= infraoral lamina) with an average (therefore, the description involved more than one individual, but only one measuring 310 mm TL is mentioned) of four smaller-pointed teeth, and on either side a strong bicuspid tooth. Tongue crescent bar (= transverse lingual lamina) with 19 teeth; a narrow, weakly convex-concave (= parenthesis-shaped) bar (longitudinal lingual lamina) with 12 pointed teeth at both extremities of the crescent bar. First dorsal fin separated from the higher second dorsal fin by a wide gap. Body ash gray colored above and silvery below. The whereabouts of the type specimen is unknown and is presumed lost.

The written description is based on two metamorphosed specimens ca. 160 mm long collected in the summer of 1903 at the mouth of the Yenisei River near Gol’chikha, Russia. These specimens were presumed to have come from the digestive system of a bird or a fish because their mucous layer was absent and all the fins were destroyed, leaving only occasional shreds. The dorsal body surface was black and the ventral surface yellowish white, with a sharp boundary between the two areas. The posterior end of the body and the tip of the caudal fin were black. Dentition is identical to that of P. kessleri in terms of number and arrangement of teeth in the upper jaw (= supraoral lamina), lower jaw (= infraoral lamina), paired lateral teeth (= endolaterals) and all the fine teeth (= anterials and posterials), but resembles that of Petromyzon (= Lampetra) fluviatilis in the teeth being larger and sharp. It may be assumed that the Siberian lampreys are represented by two forms, parallel to the European Petromyzon (= Lampetra) planeri and P. fluviatilis; P. planeri corresponds to P. kessleri and P. fluviatilis corresponds to P. dentex. The teeth are brown. The number of teeth on the infraoral lamina is not the same in the two specimens; one has six teeth, and the other has eight teeth, with two of the inside teeth being incompletely separated. This indicates that each of these incompletely divided teeth originate from a single tooth, resulting in the total number of eight instead of six teeth. The outermost teeth of the infraoral lamina have two inconspicuous cusps (i.e., bicuspid) in both specimens. There are 17–20 anterials arranged in four oblique rows and these teeth decrease in size from the center to the periphery of the oral disc. The row of small posterial teeth extends from one side to the other, its ends reaching the lower lateral pairs of teeth (= endolaterals). The two syntypes are presumed lost because Ioganzen (1935b) could not find them at TGU where they had been deposited and nor could AMN during his visit in 2011.

The written description is based on one metamorphosed specimen (holotype, USNM 21524) 15 inches (= 381 mm) TL collected in 1877 (Bean 1882) from the Yukon River at Anvik (63°N, 160°W), Alaska, U.S.A. Dentition: maxillary (= supraoral lamina) with two cusps and mandibular (= infraoral lamina) with seven, the lateralmost one on each side enlarged. Eye length (O), nearly twice the width of the largest branchial opening, but the latter not given. Head or prebranchial length (d-B₁), 9.7% of TL; tail length (a-C), 25% of TL (uncertain proportion because not clear if the measurement was taken from the anterior or posterior edge of the cloaca). Two dorsal fins with an interspace of ca. 24 mm. Coloration (in alcohol) of the back plumbeous, sides and belly golden yellow (hence, the specific name), and underside of the head and branchial region silvery. The collector, L.M. Turner, noted that the species was extremely abundant and used for food. The holotype was not studied.

The written description is based on an unspecified number of metamorphosed specimens measuring 80–156 mm TL and collected from small watercourses (streamlets between fields, springs, and small canals) on Hondo [= Honshu], Shikoku, Kyushu, and Hokkaido islands, Japan. Hatta (1901) states that the new species is distinct from La. japonica measuring 390–507 mm TL that occurs in waterbodies (rivers, lakes, and ponds) of Honshu and Hokkaido islands, Sea of Japan basin, Japan. Additionally, it is diagnosable from La. japonica by having a more protruded suctorial disc; less prominent and more obtuse teeth; an unspecified lesser number of teeth in the series outside the mandibular tooth-plate (row of posterial teeth); the cusps at the lateral extremities of the mandibular tooth-plate (infraoral lamina) not bifurcated; the first and second dorsal fins are not separated by an interspace, only a notch; the anal fin (= fin-like fold) is of considerable height in females during the spawning season; the labial tentacles (= oral fimbriae) are mostly palmate; the skin is dark brown with faint irregular spots. Hatta (1901) notes that the two most important diagnostic characters are the smaller body size (80–156 versus 390–507 mm TL) and the unicuspid (versus bicuspid in La. japonica) lateralmost teeth on either side of the infraoral lamina. He suggests that the American brook lamprey, La. wilderi is probably the closest relative to La. mitsukurii. Two syntypes were studied (Fig. 4, Tables 7–9).

The written description of Lampetra mitsukurii minor is based on an unspecified number of metamorphosed specimens measuring 80–165 mm TL from 14 localities across Japan as follows: Sapporo (Hokkaido Island), Tsuyama, Takayama, Akita, Aganogawa, Tamagawa, Kawagoye, Yamagata, Yamaguchi, Sakura, Hamamatsu, Gifu (eleven localities on Honshu Island), Matsuyama (Shikoku Island), Kumamoto (Kyushu Island). It is supplemented by drawings of the body of a spawning male and a spawning female from Sapporo in side-view, as well as close-ups of their cloacal regions (Hatta 1911: figs 3, 4 and 7, 8, respectively). The written description of La. m. major is based on an unspecified number of metamorphosed specimens measuring 350–410 mm TL from Sapporo (Hokkaido Island); the maximum TL in the range having been determined from Hatta (1911: fig. 1). It is supplemented by drawings of the body of a spawning male and a spawning female in side-view, as well as close-ups of their cloacal regions (Hatta 1911: figs 1, 2 and 5, 6, respectively). The males of both subspecies possess well-developed urogenital papillae and the females of both subspecies possess well-developed anal fin-like folds. Both subspecies are diagnosable from Japanese La. japonica because the latter is larger (450–507 mm TL), not externally sexually dimorphic, its supraoral and infraoral laminae cusps are sharp instead of blunt and its intestine relatively thick instead of thread-like. Additionally, while La. m. major arrives on its spawning grounds at the end of April, La. japonica arrives on its spawning grounds in late May or early June. La. m. major is sympatric with La. m. minor at Sapporo and is often found attached to the latter, but is allopatric with La. japonica. No type material was studied.

The written description is based on seven metamorphosed specimens (holotype + six paratypes) measuring 150–174 mm TL collected in 1950–1952 from Shokotsu River, Hokkaido Island, Japan (44°22'N, 143°20'E). It is supplemented by photographs of the holotype (whole body in side-view and oral disc) and paratypes (intestine and velar apparatus). Lethenteron matsubarai is diagnosable from five metamorphosed Le. japonicum (= Le. camtschaticum) collected sympatrically and measuring 172–372 mm TL by being nonparasitic (intestinal diameter < 1 mm versus 3.5 mm), having weaker dentition (poorly cornified, pale yellow blunt cusps versus strongly cornified, orange sharp cusps) and by the absence of pigmentation versus dark pigmentation on the second dorsal and caudal fins. Additionally, disc length and eye diameter, as a percentage of TL, was respectively, 4.4–5.3 and 1.7–2.3 in Le. matsubarai measuring 150–174 mm TL versus 5.3–6.0 and 2.6–3.2 in sympatric Le. camtschaticum measuring 172–193 mm TL. Dentition: supraoral lamina with one cusp at each end; three bicuspid endolaterals on each side; infraoral lamina with six cusps; single row of 18–23 unicuspid posterials; transverse lingual lamina with an enlarged median cusp and up of eleven cusps in total. Body pigmentation in 4–5% formalin is light brown on back and sides and very light brown on ventral aspect. Trunk myomeres are 66–70 and velar tentacles seven or eight. Vladykov and Kott (1979b) provided an amended caption to a figure in the original description (Vladykov and Kott 1978b: fig. 4). Two paratypes were studied (Tables 7–9).

Hubbs (1925) proposed that what he called Entosphenus mitsukurii was a degenerate, dwarf, brook lamprey derived from anadromous, parasitic Entosphenus japonicus (= Le. camtschaticum). Whether morphologically-similar (other than maximum adult size attained), but trophically distinct paired subspecies or species sensu Zanandrea (1959) or stem-satellite species sensu Vladykov and Kott (1979a) [reviewed and updated by Salewski (2003)] represent distinct taxa, or a single trophically-plastic taxon sensu McPhail and Lindsey (1970), Yamazaki et al. (1998), Sawatzky et al. (2007), Kucheryavyi et al. (2007a, 2007b), Kucheryavyy et al. (2010a, 2010b, 2016), April et al. (2011), Nazarov et al. (2011), Yamazaki et al. (2011), Makhrov et al. (2013), Yamazaki and Nagai (2013), Kucheryavyy (2014), Artamonova et al. (2015), Makhrov and Popov (2015), and Yamazaki and Goto (2016) needs to be treated on a case by case basis as there appears to be a continuum of differentiation within lampreys [see reviews by Docker (2009) and Docker and Potter (2019)]. Between 17–21 June 1972 Savvaitova and Maksimov (1978) observed communal spawning of two forms (large and small) of La. japonica (= Le. camtschaticum) in Levyy Kolkalvayam River, tributary to Utkholok River, Sea of Okhotsk basin, western Kamchatka, Russia. The communal redds of large spawners (230–320 mm total length) and small spawners (100–140 mm total length) were located 60–80 km from the sea while redds consisting of small spawners only were located both closer to the coast and further upriver. Savvaitova and Maksimov (1978) proposed that the large spawners are the anadromous form and the small spawners the precociously mature resident freshwater form of Le. camtschaticum. Gritsenko (2002) also reported communal spawning of Le. japonicum (= Le. camtschaticum) (340–570 mm TL) and what he identified as Le. kessleri (< 230 mm TL), but could belong to another species (see below ‘Sympatric parasitic and nonparasitic taxa’, page 40) in the Tym’ River, Sakhalin Island, Sea of Okhotsk basin, Russia. However, he concluded that the two species were distinct because, in addition to their difference in total lengths, their trunk myomeres were non-overlapping by over 90%. In their review of lampreys in the Eurasian Arctic, encompassing over 70 rivers from the basins of the Barents, White, Kara, Laptev, East Siberian, Chukchi, and Bering seas, Makhrov et al. (2013) determined that Le. camtschaticum comprised both anadromous parasitic and resident nonparasitic forms and considered the latter form, represented by Le. kessleri, to be a junior synonym. Kucheryavyi et al. (2007a) had previously treated sympatric lampreys in the Utkholok River drainage possessing three distinct modes of life [i.e., normally-maturing anadromous lampreys feeding as adults, early-maturing (i.e., praecox form) anadromous lampreys that spend up to a year at sea, presumably for feeding, although this is not explicitly stated, but in Savvaitova et al. (2007) and Kucheryavyy et al. (2010b) it is, and freshwater-resident lampreys non-feeding as adults] as belonging to a single species, Le. camtschaticum. Kucheryavyi et al. (2007a, 2010b) proposed a mechanism to explain the co-existence of these morphologically-similar, other than mature adult total length attained (i.e., respectively, 174–350 mm, 145–220 mm, and 100–165 mm), and synchronously spawning lampreys. They suggested that what determined the adult mode of life was the larval diet, with the larvae that switch from their usual diet of organic detritus and algae to an energy-rich diet of decomposing carcasses of Pacific salmons (Oncorhynchus spp.) forego feeding as an adult and become residents, while those that fed only on organic detritus and algae begin feeding post-metamorphosis in fresh water and later at sea before returning to fresh water for spawning. Unfortunately, this does not explain why there are normally maturing and praecox anadromous forms (Renaud 2011). Additionally, while decaying salmonids may be available in this particular river drainage, this is not the case in other parts of Russia and in Kazakhstan where nonparasitic lamprey occur thousands of kilometers from the sea, namely the upper Ob’ and Irtysh rivers, respectively (Romanov et al. 2017). Kucheryavyy et al. (2010a) also suggested that size differences in spawning individuals of Le. camtschaticum was not a barrier to successful spawning in that species. In the Kolkavayam River, Utkholok River drainage, communal and paired matings of different size individuals of all three forms (i.e., means of 270–280 mm, 170 mm, ≈130 mm TL for anadromous, praecox-anadromous, and resident, respectively) in various combinations during spawning ensure what Kucheryavyy et al. (2010a) refer to as evolutionary stasis. Kucheryavyi et al. (2007b) had previously reported on six so-called spawning balls of Le. camtschaticum in the Utkholok River drainage that varied in size from six to 44 individuals, three of which (six, seven, 43 individuals) consisted of the resident form only and the other three (eight, 27, 44 individuals) consisted of a mixture of anadromous and resident forms in which the resident form was always predominant, constituting respectively, 62, 85, and 98% of individuals. On the other hand, Mateus et al. (2013) found 166 fixed allelic differences, 12 of which were assigned to genes, some of them controlling migratory behavior (i.e., anadromy vs. freshwater residency), between the species pair La. fluviatilis (anadromous parasitic) and La. planeri (resident nonparasitic) collected sympatrically at a spawning site in Portugal; thus indicating reproductive isolation. While such a study has not been conducted on Le. camtschaticum and its nonparasitic derivatives, we treat lampreys with different trophic modes of life as distinct species until this hypothesis has been falsified. Renaud et al. (2009) proposed common garden experiments as a way to elucidate this conundrum. Interestingly, preliminary studies by Yamazaki et al. (2011) and Yamazaki and Nagai (2013) indicate significant differences in allelic frequencies of polymorphic nuclear microsatellite loci between anadromous and what they characterize as fluvial nonparasitic landlocked (10 dams separating them from the sea) populations of Le. camtschaticum in Japan, and this may represent the first step in lamprey speciation. However, the results of these studies must be taken with caution because the two nonparasitic populations of Le. camtschaticum identified were represented only by ammocoetes (Ina and Tateiwa rivers, Agano River system, Honshu Island). Yamazaki and Goto (2000a) had previously proposed that in Japan speciation of lampreys of the genus Lethenteron had occurred from ancestral populations of anadromous parasitic Le. japonicum (= Le. camtschaticum) via precocious dwarf individuals leading independently to distinct nonparasitic Le. kessleri and what they called a northern form of Le. reissneri (see below ‘Taxonomic identity of nonparasitic lampreys in Japan and Sakhalin Island, Russia with low trunk myomere counts’, page 42). Salewski (2003) argued that the worldwide trend of satellite species in lampreys perhaps represented cases of sympatric speciation. Although lamprey species are usually either parasitic or nonparasitic, Le. appendix is the only nonparasitic species within the genus that exhibits very rarely what has been termed facultative parasitism (Docker 2009; Renaud 2011; Renaud and Cochran 2019). Eight “giant” Le. appendix adults (260–354 mm in total length) have been reported from Lake Huron and Lake Michigan basins (Manion and Purvis 1971; Vladykov and Kott 1980; Cochran 2008). Because these adults exceed the maximum total length of 240 mm reported for the larvae of the species (Mundahl et al. 2005) they must have fed post-metamorphosis. The retained capacity of some individuals to feed post-metamorphosis argues for a recent divergence of Le. appendix from Le. camtschaticum (Renaud 2011; Renaud and Cochran 2019).

A number of individuals of various Lethenteron species possess exolaterals [see Kott (1974) in reference to Le. appendix (appeared as Lethenteron lamottei), in which 21.1% of individuals possessed one or two exolaterals in one or both exolateral fields; Vladykov and Kott (1978a) in reference to Le. alaskense, in which the holotype possesses one exolateral in each one of the exolateral fields and one paratype possesses on both exolateral fields a complete exolateral row, which they refer to as a supplementary marginal row; Renaud and Naseka (2015) in reference to one individual each of Le. reissneri and Le. camtschaticum, in which they respectively possessed two and one exolateral teeth on one of the fields]. In this study, we recorded one or two exolaterals per exolateral field in two individuals of Le. camtschaticum (one from the Kamchatka River and one from the Tobol River), one individual of Le. kessleri from the Ob’ River and in one syntype of Le. mitsukurii (Tables 5, 8).

The original description of P. marinus camtschaticus based on a single adult (now lost) is very different from that of P. japonicus represented by two extant adult syntypes, one of which was selected as the neotype of the former taxon by Kottelat (1997) thereby making it a lectotype of the latter taxon. The original description of P. marinus camtschaticus refers to a supraoral lamina bearing two bicuspid teeth separated by a wide bridge, two pairs of bicuspid endolaterals and no row of posterial teeth is mentioned or depicted, while in that of P. japonicus (Fig. 2) the supraoral lamina has two unicuspid teeth separated by a wide bridge, three pairs of bicuspid endolaterals (although the accompanying drawing shows 1–2–2 on the left and 2–2–1–2 on the right side) and a single row of posterials. However, both taxa exceed 300 mm TL indicating that they represent parasitic species and possess a u-shaped transverse lingual lamina with an enlarged median cusp. The number of teeth on the transverse lingual lamina in the original description of P. m. camtschaticus is seven and nine in that of P. japonicus although in the type material from Japan we counted 2u–I–6u and 2u–I–3u (Table 2). The disc length of 5.2% TL estimated from the original description of P. m. camtschaticus is similar to the disc length of 4.1–4.8% TL for the type material from Japan (Table 1). The original description of P. m. camtschaticus reports seven unicuspid teeth on the infraoral lamina and that of P. japonicus reports six unicuspid teeth (although we counted seven in the accompanying drawing). However, the type material from Japan exhibits counts of 1b4u1b and 1b5u (Table 2). The second dorsal fin is stated to be blackish in the original description of P. m. camtschaticus and although no mention is made of its condition in the original description of P. japonicus both types from Japan exhibited a blotch on the second dorsal fin (Table 3). We agree with the action by Kottelat (1997) of making P. japonicus a synonym of P. marinus camtschaticus and suggest that the specimen described by Tilesius (1811) is atypical in the condition of its supraoral lamina and number of bicuspid endolaterals, while the absence of a row of posterials is an oversight on his part as these teeth are relatively small in comparison to the others that he did describe.

Because Tilesius (1811) had one lamprey specimen only, he added the unpublished description (Indice Piscium Camtschaticorum, manuscript F) by Georg Wilhelm Steller (1709–1746) of a metamorphosed lamprey collected near the mouth of Bolschaja River brought to the latter on 18 June 1738 (Julian calendar; 29 June 1738 Gregorian calendar). However, Steller left St. Petersburg in January 1738 to join the Second Kamchatka Expedition (1733–1743) of Vitus Jonassen Bering (1681–1741), and by January 1739, had only reached Yeniseysk (on the Yenisei River), where he met Johann Georg Gmelin (1709–1755); still over 3,900 km from the Kamchatka Peninsula. Although there is a Bolshaya River on the western coast of Kamchatka, it is, therefore, improbable that this lamprey was obtained from there. It is more probable that the lamprey came from the Yenisei River, the name being derived from Ionessi (= Yonessi) meaning Bolschaja (= Bolshaya) voda or Big water in Evenki, the language spoken by the local Evenks people. This 333 mm TL lamprey adult is described as having variegated (mottled) body coloration with a brown dorsal surface and brown lines extending down in a wavy pattern into the shiny green-bronze sides of the body while the type of P. m. camtschaticus is not mottled. Otherwise, the characters described are similar to those of P. m. camtschaticus: body anguilliform; two dorsal fins, interspace between them ca. seven mm; posterior (second) dorsal fin blackish at the top; teeth yellowish; supraoral lamina with two teeth; infraoral lamina with six teeth. Additional characters described are: snout length is ca. 25 mm (≈ 7.5% TL); eye length is ca. 2.5 mm (≈ 0.8% TL); iris bronze colored. The common name Kanaháisch of this variegated lamprey is almost identical (Canaháisch) to the one reported for P. m. camtchaticus by Tilesius (1811). Although both Le. camtschaticum and Le. kessleri occur in the Yenisei River (see below ‘Sympatric parasitic and nonparasitic taxa’, page 40), the lamprey in Steller’s manuscript is Le. camtschaticum based on its size (333 mm TL). The only other study that reports a mottled body coloration in metamorphosed lampreys from Russia is that of Berg (1931) based on four non-type adults of La. j. septentrionalis [= Le. camtschaticum; see below ‘Lampetra japonica septentrionalis a synonym of Petromyzon japonicus’, page 26] measuring 279–345 mm TL from the Onega River.

We reject the hypothesis proposed by Ioganzen (1935a) that the similar dentition of La. j. septentrionalis and La. j. japonica is not indicative of their common origin and that La. j. septentrionalis is instead derived from La. fluviatilis and should therefore be called La. fluviatilis septentrionalis. Ioganzen (1935a) argued that the differences in dentition in La. j. septentrionalis versus La. fluviatilis in the second endolateral tooth (bicuspid versus tricuspid, exceptionally quadricuspid, respectively) and posterial row (present versus absent, respectively) were not stable taxonomic characters to distinguish the two. However, Ioganzen (1935a) provided no evidence for the presence of a tricuspid or quadricuspid second endolateral tooth in La. j. septentrionalis and merely suggested that this was due to insufficient samples. Berg (1948) hypothesized that the absence of lower labials (i.e., posterials) in three of 19 adults of La. japonica from the Vyg River, White Sea basin, could be explained through hybridization of this species with La. fluviatilis in the post-Tertiary period during which the White and Baltic sea basins were in communication. Unfortunately, we did not examine this particular material, although the single adult we did examine from the Vyg River (Table 5, ZIN 8545) possessed a row of posterials. Given that all lamprey individuals that we examined and were stated by Berg (1931) to be without posterials in fact possessed them (this study; Renaud and Naseka 2015), we believe that the hybridization explanation does not need to be invoked. Additionally, the cladistic analysis of Gill et al. (2003) based on morphological characters that included dentition showed that La. fluviatilis was not sister to Le. camtschaticum.

Berg (1931) distinguished La. j. septentrionalis from La. j. japonica by the smaller length of its upstream migrants (227–430 mm TL versus 352–625 mm TL, respectively) and its lower fecundity (24,086–25,144 eggs versus 80,825–107,015 eggs, respectively). Although Berg (1948) did not explain why he synonymized La. j. septentrionalis with La. j. japonica, he stated that the upstream migrating lampreys from the Mezen’ River, White Sea basin, were not smaller than those of the Pacific Ocean basin [540 mm TL (Manteyfel’ 1945) versus 625 mm TL (Berg 1931), respectively], and hence, one of his two diagnostic characters no longer held. Furthermore, Berg (1948) stated that the average number of eggs within lamprey from the Taui River, Okhotsk Sea basin, was ca. 40,000. This is intermediate between the 24,086–25,144 eggs for La. j. septentrionalis and 80,825–107,015 eggs for La. j. japonica reported in Berg (1931), thereby weakening the strength of his second diagnostic character. Additionally, Morozova (1956) recorded fecundities ranging from 50,000 to 124,000 eggs in La. japonica measuring 335–481 mm TL from the Amur River near Elabuga and Malmyzh, Russia, Yamazaki et al. (2001a) recorded fecundities ranging from 62,936–119,180 eggs in Le. japonicum measuring 363–442 mm TL from Hokkaido and Honshu islands, Japan, and Nursall and Buchwald (1972) recorded fecundities ranging from 9,790 to 29,780 eggs in 18 mature Le. japonicum females (TL not stated) from Great Slave Lake, Slave River, and Hay River, Northwest Territories, Canada. Our comparison of morphometrics, trunk myomeres, dentition, oral papillae and fimbriae, caudal fin shape and pigmentary characters of the type material of Petromyzon marinus camtschaticus, P. japonicus and La. j. septentrionalis (Tables 1–3) did not reveal any diagnostic differences between these three nominal taxa and we therefore consider them to be synonyms. Makhrov and Lajus (2018) suggest that Le. camtschaticum originated in the Pacific Ocean basin and colonized the Eurasian Arctic Ocean basin postglacially because in the median joining network analysis of mtDNA haplotypes, the Northern European haplotypes occur at the end of branches indicating their recent origin. Furthermore, on the eastern portion of its distribution, Walters (1955) proposes that Le. camtschaticum (appeared as La. japonica) colonized Arctic Canada through two possible routes: Arctic Alaska and the Yukon Valley; and that this could have occurred during the presence of the Bering Strait land bridge linking Eurasia and North America. Walters (1955) further states that Le. camtschaticum can also disperse across marine waters, and therefore, this is another possible route for its colonization of Arctic Canada. Yamazaki et al. (2014) provide evidence based on their study of seven polymorphic nuclear microsatellite loci in 12 anadromous Le. camtschaticum populations distributed between Velikaya River, Chukotka, Russia in the north and Jinzu River, Honshu Island, Japan in the south, that the species has considerable marine dispersal ability and low homing ability. There is a caveat, however, associated with that study. Only three localities on Honshu and Hokkaido islands, Japan out of the 12 contained adults; the remaining samples were ammocoetes which are notoriously difficult to identify to species and were not dealt with in the present study.

Petromyzon ernstii is considered a junior synonym of Le. camtschaticum on the basis of its adult total length of 310 mm (Dybowski 1872a). The adult size attained by P. ernstii indicates that it feeds post metamorphosis and Le. camtschaticum is the only species to do so in the genus. The dentition of P. ernstii is not fully described by Dybowski (1872a), but what is described generally agrees with that of the neotype of P. m. camtschaticus (Table 2): supraoral lamina with two strong cusps; three endolaterals on each side with the top two endolaterals bicuspid (the type of the bottom endolateral is not given); infraoral lamina with two bicuspid teeth laterally and an average of four unicuspid teeth internally; two rows of anterials; one row of posterials. While the counts of 19 teeth on the transverse lingual lamina and 12 on each of the longitudinal lingual laminae reported by Dybowski (1872a) are respectively lower (9) and undetermined in the neotype of P. m. camtschaticus (Table 2), in the types of the latter’s junior synonyms, P. japonicus and La. j. septentrionalis, the ranges observed are much closer to reaching those values (respectively, 6–16 and seven or eight; Table 2). If we include topotypes of La. j. septentrionalis the counts are even closer or encompassing (respectively, 12–17 and 9–13; Table 2). Notwithstanding the fact that we did not observe a count of 19 teeth on the transverse lingual lamina, we found a count of 18 in a specimen from the Tobol River identified as La. j. septentrionalis by Berg (1931) (Table 5), and therefore, we believe that P. ernstii is a junior synonym of Le. camtschaticum.

Berg (1948) reported that La. j. japonica + La. j. septentrionalis (= Le. camtschaticum) usually had six mandibular (infraoral) teeth, occasionally seven (exceptionally nine in the Kamchatka River). We found six in the types (Table 2) and six to eight in the non-types (Table 5). According to Kucheryavyi et al. (2007a), the infraoral lamina in typically anadromous Le. camtschaticum from the Utkholok River drainage, Kamchatka possessed one bicuspid tooth on either side, and three to seven unicuspid teeth internally, giving a total of five to nine teeth. Kucheryavyi et al. (2007a) and Nazarov et al. (2011) for the Kol’ River drainage, Kamchatka, also reported that rarely one of the lateralmost teeth was unicuspid. We also found a lateralmost unicuspid tooth in the paralectotype of Petromyzon japonicus (ZMB 6476; Table 2) and one topotype of La. j. septentrionalis (ZIN uncat.; Table 2).

In regard to the wide discontinuity in the distributions of the populations of parasitic Le. camtschaticum reported by Berg (1931) as La. j. septentrionalis and La. j. japonica and later (Berg 1948) combined as La. j. japonica, this study has re-identified as Le. camtschaticum some of the samples in the intervening Siberian waters and considered by the aforementioned author as well as Ioganzen (1935b) as nonparasitic La. j. kessleri, thereby narrowing the gap between the distributions. Firstly, Berg (1931, 1948) tentatively considered and Ioganzen (1935b) undoubtedly considered P. dentex Anikin, 1905 to be a junior synonym of La. j. kessleri. However, we suggest instead that it is a junior synonym of Le. camtschaticum because Anikin (1905) stated that the teeth of the two approximately 160 mm TL specimens collected at the mouth of the Yenisei River are brown (indicating strong keratinization) instead of dark yellow as they usually are in P. kessleri and that although they have the same number and arrangement of teeth as in P. kessleri, these are larger and sharp, resembling those of Petromyzon (= Lampetra) fluviatilis rather than P. (= L.) planeri. Furthermore, Egorov (1985) reports the presence of La. j. kessleri adults up to 322 mm TL from the Yenisei Gulf, which on the basis of size we identify as Le. camtschaticum. Therefore, in our opinion, the range of Le. camtschaticum extends further east than the Ob’ River drainage to the Yenisei River. Secondly, six specimens from the Anadyr Liman (= Estuary) are also re-identified as Le. camtschaticum rather than Le. kessleri. Anikin (1905) stated that La. j. kessleri (appeared as P. kessleri) reached an adult TL of 210 mm, but Berg (1931) stated that it reached, albeit rarely, 220–260 mm. Although we acknowledge that a TL of 221 mm can be attained (Table 4, ZIN 6174; only 212 mm when re-measured by us likely caused by shrinkage over time), we suggest that the 258 mm TL spent female [ZIN 23154 in Berg (1931)] is a Le. camtschaticum rather than La. j. kessleri because it was collected in brackish water of the Anadyr Estuary and was attached to a dog salmon, also called chum salmon (Onchorhynchus keta). Furthermore, it exceeds by 38 mm the longest ammocoete reported by Berg (1931) for La. j. kessleri (ZIN 6306, 220 mm TL from the Irtysh River at Omsk) even without taking into account the shrinkage in body length that occurs in a nonparasitic species between the beginning of metamorphosis and spawning. The two adults 135–138 mm TL (ZIN 23158) with very sharp teeth identified as La. j. kessleri by Berg (1931) are also re-identified as Le. camtschaticum because they were also collected in brackish water and attached to a chum salmon. The adult 144 mm TL (ZIN 14371) identified as La. j. kessleri by Berg (1931) is also re-identified as Le. camtschaticum (Tables 4–6) because it possesses the silvery body coloration of an anadromous downstream migrant. Later, Berg (1948) suggested that what he called La. j. japonica is possibly present in the Anadyr River based on a 260 mm specimen with sharp teeth collected in its estuary on 7 August 1938. Two other specimens with sharp teeth and thick intestines from the Anadyr Estuary identified by Berg (1948) as La. j. kessleri are re-identified as Le. camtschaticum; one (155 mm TL) was attached to a sculpin (Cottidae) and the other (144 mm TL) to a chum salmon. Chereshnev (2008) also reports the presence of anadromous Le. camtschaticum in the Anadyr River from the estuary to ca. 600 km upstream. Therefore, we believe that Le. camtschaticum occurs in the Anadyr River and this extends its range northward from Kamchatka. Between the Yenisei and Anadyr River drainages are only two major rivers: the Lena and Kolyma; both within the region of Yakutia, also known as Republic of Sakha. According to Kirillov and Chereshnev (2006) and Kirillov et al. (2014), only nonparasitic Lethenteron lampreys occur in this entire region from the Anabar (west of the Lena) to the Kolyma rivers. However, these authors provide very little information about the lampreys and two things suggest that, at least in the case of the Kolyma River, anadromous, parasitic lamprey may be present. According to Kirillov et al. (2014) the mature adults at the beginning of the spawning season attain 240 mm TL, which slightly exceeds the maximum total length of 230 mm attained by Le. kessleri (see below), and lamprey are known down to the estuary, which would not be expected in a brook lamprey. Further investigations are required to confirm this.

Although Berg (1931) considered La. j. kessleri to be a small non-migratory (i.e., resident) form in which the larvae at metamorphosis were larger than the adults, the material at his disposal did not support this contention. Berg (1931) stated that the longest ammocoete was 220 mm and adults reached 220–260 mm. We have explained above that Le. kessleri adults reach 221 mm TL, but have no direct evidence that they exceed this value. However, Poltorykhina (1971, 1974) conducted the most extensive studies of Le. kessleri examining 50 ammocoetes, 104 metamorphosing ammocoetes and 100 adults in the first study and 300 adults in the second study collected from the upper Irtysh River system (Ob’ River drainage) and reports that 218 mm is the mean TL attained by 50 ammocoetes, 233 mm is the mean TL attained by 12 metamorphosing ammocoetes at the penultimate stage of metamorphosis, and 230 mm is the maximum TL attained by adults. According to Poltorykhina (1971) the apparent increase in length between ammocoetes and adults is not the result of parasitic feeding post metamorphosis, but due to continued feeding of metamorphosing ammocoetes, at first on detritus and unicellular algae and later exclusively on unicellular algae, even though the oral cirri are lost and replaced by the rudiments of teeth over the course of metamorphosis. The latter is perhaps achieved through grazing of unicellular algae on rocks via the action of the lingual laminae and piston cartilage as suggested by Renaud (2011). Although Poltorykhina (1971) did not comment on whether or not the digestive tract remained open throughout metamorphosis, one must assume that it is patent at least for most of the period because growth occurred and algae were found in the intestine up until and including the last stage of metamorphosis. According to Poltorykhina (1971, 1974) the adults possess 67–72 trunk myomeres, which overlaps the range based on our examination of Le. kessleri type material (70–74; Table 2).

Ioganzen (1935b) reported that La. j. kessleri from the Ob’ River drainage usually has two teeth on the supraoral lamina (n = 15) and exceptionally four (n = 1). However, his drawing (1935b: fig. 7) of the supraoral lamina in the latter specimen from the Ob’ River near Bogorodskoye is interpreted as two bicuspid teeth instead of four unicuspid teeth and Egorov (1985) is also in agreement with this interpretation. However, Zhuravlev and Lomakin (2017) reported the case and provided a photograph of four unicuspid teeth on the supraoral lamina, one at each end and two on the bridge, in one of 54 individuals of Le. kessleri from the Belokurikha River, upper Ob’ drainage (53 individuals had only one tooth at each end). In her extensive study of Le. kessleri from the upper Irtysh River system (Ob’ River drainage), Poltorykhina (1974) reported usually two teeth on the supraoral lamina (n = 297) and three cases in which an additional tooth was found on the bridge. We also found a case of three teeth on the supraoral lamina (ZIN 6310, Table 5). Poltorykhina (1974) stated that the infraoral lamina possesses five to ten teeth, usually six or seven, with the lateralmost being bicuspid and in 16 of 300 individuals some of the internal ones are also bicuspid. In five syntypes of P. kessleri (Table 2) we found six or seven teeth, with the lateralmost bicuspid and in one individual from the Yenisei River (ZIN 14441, Table 5) we recorded five teeth with the lateralmost bicuspid. Ioganzen (1935b) reported four bicuspid endolaterals on either side of the oral disc in a syntype of P. kessleri from the Tom’ River (Ioganzen 1935b: table 3, TGU 3700). We did not find this specimen during AMN’s visit to TGU in 2011. All of the syntypes that we examined (Table 2) had three endolaterals only on either side, most of which were bicuspid (n = 9) and one with the formula 2–2–3. Additionally, Anikin (1905) only reported three bicuspid endolaterals in the original description of P. kessleri. Poltorykhina (1974) also only recorded three endolaterals on either side of the oral disc (n = 300), and while the majority were bicuspid, in 36 individuals the formula was 2–2–1 and in 16 it was 2–1–2. Nazarov (2012) stated that usually there were three, but sometimes only two endolaterals on one side in material from the middle Yenisei River drainage. Nazarov (2012) reported three to five teeth in the first anterial row in individuals from the middle Yenisei River drainage, while we recorded invariably five in five syntypes of P. kessleri (Table 2) and two to six in non-types of Le. kessleri (Table 5). Poltorykhina (1974) reported that the lower circumoral teeth (= first posterial row) consist of a single row with 16–25 teeth and an unspecified number of individuals do not possess any. We recorded 24–29 teeth in three syntypes of P. kessleri (Table 2) and 18–31 in non-types of Le. kessleri (Table 5).

The presence of well-developed lower labial teeth (= posterial row) in Le. kessleri versus weakly developed or completely absent lower labial teeth in Le. reissneri was the diagnostic character used by Berg (1931, 1948) to distinguish the two taxa. However, Renaud and Naseka (2015) showed that a complete and well-developed posterial row was usually present in Le. reissneri. Berg (1931) reported “a very aberrant specimen” of Le. kessleri from the upper Yenisei River (ZIN 14441) without teeth on the lower lip (i.e., no posterials) and Berg (1948) suggested that it was possibly an aberrant Le. kessleri resembling Le. reissneri. However, we re-examined this specimen and found one row of posterials with 18 unicuspid teeth (Table 5). This situation is reminiscent of the case of three specimens identified by Berg (1931) as Le. reissneri without posterials, but found by Renaud and Naseka (2015) to possess them. While one of the three specimens was determined by Renaud and Naseka (2015) to be Le. reissneri (Shangshi River, People’s Republic of China), they re-identified the other two as Le. camtschaticum (Samarga and Sedanka rivers, Russia).

We suggest that the ten syntypes of P. kessleri retrieved in relatively good condition from the digestive tract of a Common gull [TGU 9 = TGU 3696 in Ioganzen 1935b: table 3] on 28 June 1903 were spawning because lampreys congregate in shallow waters during that time making them susceptible to bird predation. This was also the contention of Holčík (1986b). Additionally, Ioganzen (1935b) stated that the two dorsal fins touched in all ten individuals indicating that they were mature adults.

Kucheryavyi et al. (2007a) reported the presence of three forms of Lethenteron camtschaticum in the Utkholok River drainage, Kamchatka, Russia; typically anadromous, anadromous forma praecox, and resident. All three forms spawned together in June 2005 and possessed a dark blotch at the apex of their second dorsal fin and well pigmented spade-like caudal fins. The total lengths of the three forms were: 174–350 mm (typically anadromous, mature individuals of both sexes combined), 145–220 mm (anadromous forma praecox, mature individuals of both sexes combined), and 100–165 mm (resident, mature individuals of both sexes combined). Kucheryavyy (2014) reported anadromous adults of Le. camtschaticum on the Kamchatka Peninsula up to 452 mm TL. The supraoral lamina in the three forms possessed one unicuspid tooth on either end and in the case of the typically anadromous form rarely one bicuspid tooth on either end, as was also the case in the specimen of P. m. camtschaticus in Tilesius (1811: pl. IX, fig. II). The endolaterals in the three forms generally consisted of three bicuspid teeth on either side. Rarely, in the case of the typically anadromous form, the lower left or right endolateral was unicuspid or there were four bicuspid endolaterals or there were three bicuspid with one unicuspid tooth at the lower position, and rarely in the case of the resident form, the lower endolateral was unicuspid or there were four bicuspid endolaterals. The infraoral lamina in the three forms generally possessed one bicuspid tooth on either side, and respectively, three to seven, four or five, and four to six unicuspid teeth internally. Rarely, in the case of the typically anadromous form, one of the lateralmost teeth was unicuspid, and in the case of the resident form, one or both lateralmost teeth were unicuspid. The number of posterials for the three forms was respectively, 13–28, 12–22, and 12–25. These values are lower than the combined values of 18–29 posterials, based on our examination of type or topotype material of P. m. camtschaticus, P. japonicus, P. kessleri, and La. j. septentrionalis (Table 2), but may reflect the larger sample sizes examined by Kucheryavyi et al. (2007a). Remarkably, the range in trunk myomeres for the three forms was extremely broad for a small river drainage being respectively 55–79, 57–71, and 57–78. The lower end counts may at least partly be explained by the method of counting which was based on a vertical line drawn in front of the cloaca rather than using the lower angle of the posterior myoseptum lying at or anterior to the anterior edge of the cloaca as in Renaud (2011). We interpret the typically anadromous form as being Le. camtschaticum and both the anadromous forma praecox and the resident form as being Le. kessleri. The resident form is stated by Kucheryavyi et al. (2007a) as not feeding after metamorphosis. The reasons we identify the anadromous forma praecox as Le. kessleri is because there is no clear evidence that it is anadromous or feeding as an adult as the intestinal contents of 16 individuals were examined by Kucheryavyi et al. (2007a), and while five did not contain any food, six contained small algae and five contained brown material similar in color to the food of ammocoetes, and furthermore, their adult size range (145–220 mm TL) falls under the maximum 230 mm TL reported for adults of the species by Poltorykhina (1974). If the anadromous forma praecox had spent several months to a year in the sea, as Kucheryavyi et al. (2007a) contend, one would not expect any remnants of larval food in their intestine. While we identify as Le. camtschaticum four small adults collected in the Kamchatka River and measuring 154.5–198 mm TL (Table 4: ZIN 23440, 23441, 23590), these individuals possessed intestinal diameters 2.5–4.5 mm (Table 4) and were therefore at the beginning of their adult life unlike the spawning individuals in the Utkholok River drainage presumed by Kucheryavyi et al (2007a) to be anadromous forma praecox Le. camtschaticum. In fact, Berg (1931) stated that three of the four individuals collected June 1908 (ZIN 23441) and 7 July 1909 (ZIN 23590) measuring a combined 154.5–198 mm TL were descending towards the ocean. The fecundity of the typically anadromous form was 12,272–34,586 eggs (Kucheryavyi et al. 2007a), which overlaps the lower end of the fecundity (24,086–107,015 eggs) recorded by Berg (1931, 1948) for Le. camtschaticum. Kucheryavyi et al. (2007a) did not record the fecundity for the anadromous forma praecox, but stated that the fecundity of the resident form was 468–3,441 eggs. This we take to be the fecundity for Le. kessleri. It broadly overlaps with the fecundity reported by Poltorykhina (1973) [cited in Holčík (1986b) and Makhrov et al. (2013)] for Le. kessleri (i.e., 1,820–5,800 eggs) from the upper Irtysh River system, Ob’ River drainage, as well as the fecundity reported by Zhuravlev and Lomakin (2017) for Le. kessleri (i.e., 465–1,350 eggs) from the Belokurikha River, Ob’ River drainage, and slightly overlaps with the fecundity reported by Karasev (2008) for Le. kessleri in the lower Tobol River (i.e., 3,161–7,208 eggs), also in the Ob’ River drainage, and is very close to the fecundity reported by Kuderskiy and Mel’nikova (1970) for Le. kessleri (i.e., 651–3,096 eggs) in the Yemtsa River (Severnaya Dvina River drainage), and to that of Le. alaskense provisionally suggested here as a synonym of Le. kessleri (see below ‘Taxonomic key to adults of Lethenteron’, page 53). The fecundity of Le. alaskense from Brooks River, Alaska, U.S.A. and Martin River, Northwest Territories, Canada was reported by Vladykov and Kott (1978a) as 2,188–3,477 eggs. It is also very similar to the fecundity recorded by Karasev (1987) for Le. reissneri from the Ingoda River, Russia (1,720–3,360 eggs). The extremely low fecundity of 117 eggs recorded by Novikov (1966), in a 134 mm TL Le. kessleri retrieved from the stomach of a Northern pike, Esox lucius, collected in July 1963 in the Kolyma River, Russia, may have been a partially spent female.

Similar to the case reported above in the Utkholok River drainage, Nazarov et al. (2011) stated the presence of three forms of Le. camtschaticum in the Kol’ River drainage, Kamchatka, Russia; typically anadromous (243–297 mm TL, mature individuals sexes combined), anadromous forma praecox (190.5–237 mm TL, two spent females only), and resident (110–141 mm TL, mature individuals sexes combined). However, whereas all three forms are reported to possess a blotch on the second dorsal fin (their table 2), only the typically anadromous and anadromous forma praecox possess one (their fig. 5a, b, respectively), whereas the resident form appears to possess none (their fig. 5c depicting a male and a female). Additionally, the typically anadromous and anadromous forma praecox possess a strongly pigmented caudal fin (their fig. 5a and 5b, respectively), whereas the resident form appears to possess no caudal fin pigmentation (their fig. 5c). The two anadromous forms are said to be parasitic and the resident form nonparasitic. The supraoral lamina in the three forms possessed one unicuspid tooth on either end. The endolaterals in the three forms generally consisted of three bicuspid teeth on either side. Rarely, in the case of the typically anadromous form, and sometimes, in the case of the resident form, the lower left or right endolateral was unicuspid. The infraoral lamina in the three forms generally possessed one bicuspid tooth on either side, and respectively, four to seven, five or six, and five or six unicuspid teeth internally. Rarely, in the case of the typically anadromous form, one of the lateralmost teeth was unicuspid. The number of posterials for the three forms was respectively, 13–20, 17–22, and 15–21. The range in trunk myomeres for the three forms was respectively, 71–76, 73–75, and 67–74. The fecundity of the typically anadromous form was 24,038–31,050 eggs, which virtually completely overlaps with the lower end of the fecundity (24,086–107,015 eggs) recorded by Berg (1931, 1948) for Le. camtschaticum. Nazarov et al. (2011) did not record the fecundity for the anadromous forma praecox, as the only two females were spent, but stated that the fecundity of the resident form was ca. 3,200 eggs. Nazarov et al. (2011) reported that resident lamprey either spawned communally with typically anadromous and anadromous forma praecox or independently. This is reminiscent of the situation reported by Savvaitova and Maksimov (1978) of communal spawning between large (230–320 mm TL) and small (100–140 mm TL) lampreys and independent spawning of small lamprey in the Levyy Kolkalvayam River, Utkholok River drainage. We interpret the typically anadromous and anadromous forma praecox as being Le. camtschaticum and the resident form as very similar to Le. mitsukurii in terms of the absence of a blotch on the second dorsal fin and the unpigmented caudal fin, but different in its possession of bicuspid rather than unicuspid lateralmost teeth on the infraoral lamina (see below ‘Lethenteron mitsukurii distinct from Le. kessleri and Le. reissneri’, page 37). We, therefore, defer judgment on the identity of the resident form until more information becomes available. However, it is significant that Vladykov and Kott (1979c) reported the presence of Le. matsubarai on the Kamchatka Peninsula (see below ‘Lethenteron matsubarai, a synonym of Le. mitsukurii’, page 38). Khusainova and Karpenko (2017) confirmed the presence of Le. camtschaticum in the Kol’ River based on prespawning individuals measuring 312–351 mm TL.

Karpenko et al. (2013) described 37 spawning individuals of Lethenteron measuring 147.1–172.0 mm TL and collected on 12 July 2012 from Dyakonovskiy Creek, Lake Azabach’e basin, but they could not identify them to species. It would appear that the individuals were partially spent because the four females examined possessed only 16–126 eggs and in the two in which the eggs were measured they were ca. 1.0 mm in length. According to Karpenko et al. (2013) these adults possess two or three teeth on the supraoral lamina (usually one unicuspid tooth at each end, but in one case the tooth at one end is bicuspid, and in another case an additional unicuspid tooth is present on the bridge between the unicuspid lateral teeth), three bicuspid endolaterals on each side, the infraoral lamina usually with two lateralmost teeth bicuspid and four or five internal teeth unicuspid (sometimes the two lateralmost as well as the four, five or six internal teeth are unicuspid), one row of small posterials, and 66–74 trunk myomeres. While the characters of dentition and trunk myomeres all agree with Lethenteron, the small total length of the spawning individuals would indicate a nonparasitic species; either Le. reissneri or Le. kessleri. However, the diagnostic characters distinguishing these two species (i.e., second dorsal fin pigmentation and transverse lingual lamina dentition; see below ‘Lethenteron kessleri distinct from Le. reissneri’, page 35) were not recorded, and therefore, we cannot determine the species’ identity. Bugayev et al. (2007) had reported the presence of Le. reissneri in Lake Azabach’e, but we have re-identified the 185 mm TL adult as Le. camtschaticum because its intestine contained half-digested fish flesh. Lake Azabach’e belongs to the Kamchatka River drainage, and the presence of Le. camtschaticum is well documented in this river drainage (Berg 1931, 1948; Bugayev et al. 2007). Khusainova and Karpenko (2017) collected additional spawning individuals from Dyakonovskiy Creek in 2014 (n = 145) and compared them with prespawning material from the Kol’ River (western Kamchatka) collected in 2013 (n = 19). These authors concluded based on total length that the lamprey from the Dyakonovskiy Creek measuring 132–190 mm TL is Le. reissneri and the one from the Kol’ River measuring 312–351 mm TL is Le. camtschaticum. While we agree with the identification of Le. camtschaticum for the Kol’ River material, we believe that the identification of the material from Dyakonovskiy Creek cannot be established because, as in Karpenko et al. (2013), neither the second dorsal fin pigmentation nor the transverse lingual lamina dentition was examined by Khusainova and Karpenko (2017).

Despite having been placed in the genus Ammocoetes, A. aureus is clearly based on an adult individual. The holotype measures 381 mm TL, and it possesses supraoral and infraoral laminae and eyes. Even though the original description is fragmentary, the size of the adult individual, the condition of its supraoral (two cusps) and infraoral (seven cusps, the lateralmost one on either end enlarged) laminae indicate that A. aureus is a junior synonym of Le. camtschaticum. Nelson (1887) reports that A. aureus (= Le. camtschaticum) ascends the Yukon River, Alaska, at least up to Nulato and that the native Alaskan name for the Arctic lamprey is Nû-mug-û-shûk. He further reports that the upstream-migrating lamprey passed through Anvik in the evening of 26 Nov. 1879 and that the native Alaskans catch them through holes in the ice with sticks having two short cross bars at the lower end or dipnets and extract their oil for eating and as a substitute for seal oil in lamps. On the other hand, Turner (1886) reports that A. aureus (= Le. camtschaticum) ascends the Yukon River in the latter part of December, reaches the Russian Mission – Anvik river section by the middle of February and by the latter part of April, Fort Yukon, Alaska, over 1,600 km upriver of the mouth. Turner (1886) also states that the spawning run passing through a given locality lasts approximately three weeks.

Because no character was found to distinguish the adults of Le. kessleri from those of Le. alaskense, we provisionally consider the latter to be a junior synonym of the former despite the fact that they occur on separate, but adjoining continents; Le. kessleri in Eurasia and Le. alaskense in North America. Further study is required to test this hypothesis. Interestingly, Berg (1948) had suggested that what he called La. j. kessleri was probably also present in Alaska and Poltorykhina (1974) treated the Alaskan La. japonica of Heard (1966) as La. kessleri. Additionally, in the original description of Le. alaskense, Vladykov and Kott (1978a) stated that they did not compare it to Le. kessleri because they lacked sufficient material for comparison. Holčík (1986c) treated Le. alaskense as a landlocked form of Le. japonicum (= Le. camtschaticum), but we reject this on the basis that the former is nonparasitic and the latter parasitic.

A comparison of the original descriptions and extant type material of these two nominal taxa (this study; Renaud and Naseka 2015) did not reveal any significant differences other than total length (160–210 mm in P. kessleri versus 120–140 mm in P. reissneri), and this we believe is due to the incomplete descriptions and the poor condition of the specimens, in particular Le. reissneri, preventing a full comparison. However, Renaud and Naseka (2015) reported that the adult specimen identified by Berg (1931) and accepted by them as Le. reissneri (ZIN 14457) from the Shangshi River, Amur River drainage, does not have a blotch at the apex of its second dorsal fin and has a transverse lingual lamina with 2u–I–2u while the original description of Petromyzon kessleri Anikin, 1905 stated that the apex of the second dorsal fin is ash gray (i.e., blotched) and the transverse lingual lamina has a large median tooth flanked by eight or nine smaller teeth (i.e., 8u–I–8u or 9u–I–9u and combinations thereof). In two of the seven syntypes of P. kessleri for which we could determine the character state of the transverse lingual lamina (Table 2) these were 5u–I–7u and 7u–I–7u. Therefore, the pigmentation of the second dorsal fin and the transverse lingual lamina dentition are diagnostic characters for these two taxa. The suggestion by Yamazaki et al. (2006) that Le. kessleri is a synonym of Le. reissneri on the basis of similarities in their larval trunk myomere counts and identical electrophoretic profiles is thus rejected. Sato (1951) was the first to suggest that Le. kessleri (appeared as La. j. kessleri) was present on Hokkaido Island on the basis of 26 spawning adults 144–193 mm TL collected in the Shibechari River (now known as Shizunai River). However, we have re-identified this material as Le. reissneri because according to the description by Sato (1951) the specimens do not have a dark blotch on their second dorsal fin, but do have a pigmented caudal fin (++, +++; according to his fig. 1), an infraoral lamina bearing five to seven teeth, the lateralmost being bicuspid, usually three bicuspid endolaterals on each side, the third one rarely unicuspid on one or both sides, 17–23 teeth in the first posterial row, and 66–74 trunk myomeres. Additionally, material from the Shizunai River have an identical electrophoretic profile to Le. reissneri from the type locality of Onon River, Siberia (Yamazaki et al. 2006).

In their redescription of the species, Renaud and Naseka (2015) restricted the distribution of Le. reissneri to the Shilka and Songhua river systems within the Amur River drainage, until a more geographically comprehensive study is undertaken. Loshakova and Knizhin (2015) reported the presence of a nonparasitic lamprey in the geographically proximate Angara River drainage, Yenisei River system, Russia; specifically, in the Chuksha River. Their reported adult TL of 135–182 mm, caudal fin strongly pigmented [i.e., +++, based on figs 3, 4 in Loshakova and Knizhin (2015)], trunk myomeres 66–77, supraoral lamina with two unicuspid teeth, three bicuspid endolaterals on either side, infraoral lamina with usually four, less frequently five unicuspid teeth and one bicuspid lateralmost tooth on either side, 18–26 posterials, spawning at the end of May middle of June, absolute fecundity of 1,042–3,166 eggs with diameter 0.79–0.87 mm, all point to the identity being Le. reissneri and the reported absence of a dark blotch on the second dorsal fin distinguishes it from Le. kessleri (see above ‘Lethenteron kessleri distinct from Le. reissneri’, page 35). Indeed, Karasev (1987) reported an almost identical adult TL (i.e., 137–182 mm) for lamprey, identified as Le. reissneri by Renaud and Naseka (2015), from the upper Amur basin (Ingoda [type locality of Le. reissneri] and Shilka rivers), as well as a fecundity in females from the Ingoda River of 1,720–3,360 eggs with diameter 0.68–0.84 mm. Both syntypes of Le. reissneri examined by Renaud and Naseka (2015) also had a supraoral lamina with two unicuspid teeth and the only one in which endolaterals and trunk myomeres could be determined had three bicuspid endolaterals and 70 trunk myomeres. Although the only syntype in which the infraoral lamina could be studied had only one lateralmost bicuspid and five unicuspid teeth, another individual from the Shangshi River (Songhua River drainage, Amur River system), and identified by Berg (1931) as Le. reissneri, had two lateralmost bicuspid and four internal unicuspid teeth, as well as 24 posterials, a strongly pigmented (i.e., +++) caudal fin and no blotch on its second dorsal fin (see Renaud and Naseka 2015). Finally, Dybowski (1869) stated that Le. reissneri spawns in June. Unfortunately, the number of cusps on the transverse lingual lamina, another diagnostic character distinguishing Le. reissneri from Le. kessleri, was not recorded by Loshakova and Knizhin (2015). According to Enikeev (2018) a Transbaikalian paleolake existed in the Late Pleistocene connecting the Angara River to the upper Amur including the Onon and Ingoda rivers, the type locality of Le. reissneri, and we believe this could explain the present-day occurrence of this lamprey species in the Angara and Amur River drainages.

Lampetra mitsukurii is placed by us in the genus Lethenteron because it possesses a row of posterials according to the original description (Hatta 1901) and the examination of two syntypes (Table 8). Lethenteron mitsukurii is distinct from Le. kessleri based on the absence of bicuspid lateralmost teeth on the infraoral lamina (Hatta 1901; Table 8) and the absence of pigmentation on its second dorsal and caudal fins (Table 9). The original description of Le. kessleri states that the lateralmost teeth on the infraoral lamina are bicuspid (Anikin 1905) and this was confirmed in five syntypes for which the character could be determined (Table 2). While examination of the pigmentary characters was uninformative in the syntypes of Le. kessleri (Table 3), the original description states that the apex of the second dorsal fin is ash gray (Anikin 1905) and an adult identified by Berg (1931) as La. j. kessleri from the Ob’ River (ZIN 6310, Table 6) has a heavily pigmented caudal fin (+++). Lethenteron mitsukurii is distinct from Le. reissneri based on syntypes of the latter possessing a bicuspid tooth on one or both sides of the infraoral lamina (Dybowski 1869; Renaud and Naseka 2015) and a heavily pigmented caudal fin (+++) in the adult from the Shangshi River, Amur River system (ZIN 14457) identified by Berg (1931) and accepted by Renaud and Naseka (2015) as Le. reissneri.

Hatta (1901) identified as La. mitsukurii a recently metamorphosed 150 mm TL female from Pervaya Rechka Brook near Vladivostok, Russia. Contrary to what is stated in Hatta (1901) this brook is not a tributary to the Amur River, but empties directly into the Sea of Japan. He notes that the first and second dorsal fins are separated by a deep notch; the supraoral lamina has two well separated blunt cusps; the infraoral lamina has six blunt cusps; three bicuspid endolaterals occur on each side; other labial teeth are simple and form a circle immediately inside the fringe of labial tentacles (= oral fimbriae). The latter character refers to the row of marginals. There is no mention of a row of posterials. Renaud and Naseka (2015) identified as Le. camtschaticum a 161.5 mm TL adult, previously identified by Berg (1931) as La. reissneri, from the Sedanka River emptying directly into the Sea of Japan near Vladivostok. Although the six unicuspid teeth on the infraoral lamina point to Le. mitsukurii, the absence of information on other key diagnostic characters (i.e., posterial row, second dorsal fin and caudal fin pigmentation) prevent us from confidently identifying as such the specimen from Siberia in Hatta (1901).

In our opinion Le. matsubarai is a junior synonym of Le. mitsukurii because both possess only unicuspid teeth on the infraoral lamina (Table 8) and unpigmented second dorsal and caudal fins (Table 9), three diagnostic characters that distinguish them from Le. camtschaticum, in which the infraoral lamina possesses at least one bicuspid tooth, the second dorsal fin has a blotch at its apex, and the caudal fin is strongly pigmented (Tables 2, 3). Likewise, the infraoral lamina of both Le. reissneri and Le. kessleri possess at least one bicuspid tooth (Renaud and Naseka 2015; Table 2). Unfortunately, the poor condition of the type material of Le. reissneri (see Renaud and Naseka 2015) and Le. kessleri (Table 3) did not permit an evaluation of the pigmentation of their second dorsal and caudal fins. However, Renaud and Naseka (2015) determined the second dorsal fin to be unpigmented (i.e., no blotch) and the caudal fin to be strongly pigmented in an adult of Le. reissneri from the Shangshi River (Amur River system), People’s Republic of China and Anikin (1905) described Le. kessleri as having the apex of the second dorsal and caudal fins pigmented, thus further distinguishing both species from Le. mitsukurii. Therefore, we reject the synonymy of Le. matsubarai with Le. kessleri proposed by Iwata et al. (1985), which was based on material presumed as belonging to the latter species that came from Hokkaido Island following Sato (1951) instead of examining type material or using the original description. The material identified as Le. kessleri by Sato (1951) has been re-identified by us as Le. reissneri (see above ‘Lethenteron kessleri distinct from Le. reissneri’, page 35). The material identified as Le. kessleri by Iwata et al. (1985) we believe to be a mixture of Le. reissneri and Le. mitsukurii because the authors report it to possess 65–73 trunk myomeres, an unpigmented second dorsal fin and a strongly pigmented caudal fin; a few individuals however having an unpigmented caudal fin. Those individuals with a strongly pigmented caudal fin correspond to Le. reissneri, while those with an unpigmented caudal correspond to Le. mitsukurii.

Hatta (1911) described two forms of Lampetra mitsukurii; one small (80–165 mm TL) with a widespread Japanese distribution he called minor and one large (350–410 mm TL) restricted to Sapporo, Hokkaido Island, he called major. The two forms occurred sympatrically on spawning grounds at Sapporo. Both were distinct from La. japonica (= Le. camtschaticum) based on characteristics related to sexual maturity (i.e., well-developed urogenital papilla in males and anal fin-like fold in females in the former versus undeveloped structures in both sexes in the latter, blunt cusps on the supraoral and infraoral laminae and thread-like intestines in the former versus sharp cusps on these laminae and relatively thick intestines in the latter). The subspecies minor is a junior synonym of Le. mitsukurii (Hatta, 1901) based on their very similar total lengths (80–165 versus 80–156 mm, respectively) and their identical geographic distributions (i.e., Hokkaido, Honshu, Shikoku, and Kyushu islands). The subspecies major is distinct from La. mitsukurii because figs 5 and 6 in Hatta (1911) showing close-ups of the cloacal regions of male and female La. m. major, respectively, possess a dark blotch at the apex of the second dorsal fin similarly to fig. 9 showing a close-up of the cloacal region of La. japonica (= Le. camtschaticum), whereas our study of a La. mitsukurii syntype revealed that its second dorsal fin is unpigmented (Table 9). Thus, even though Hatta (1911) states that the forms minor and major were often found attached to each other while spawning, they belong to distinct species. We therefore agree with Creaser and Hubbs (1922) and Berg (1931) that La. m. major is a synonym of what they respectively called Entosphenus (Le.) japonicus and La. j. japonica (= Le. camtschaticum). We disagree with Berg (1931) that La. m. minor is a synonym of Le. reissneri (see above ‘Lethenteron mitsukurii distinct from Le. kessleri and Le. reissneri’, page 37). Ioganzen (1935c) reported on adults of nine La. m. minor (130.7–162.3 mm TL) and two La. m. major (334.0–366.0 mm TL) identified by Prof. S. Hatta and collected sympatrically in Sapporo in April 1911. These were sent to Tomsk State University where they were studied in 1914 by the author’s father, Prof. G. E. Ioganzen. Unfortunately, in 1933 Ioganzen (1935c) could only find the La. m. minor and AMN could find neither when he visited TGU in 2011. According to Ioganzen (1935c) the supraoral lamina in both forms consisted invariably of two teeth while the infraoral lamina in La. m. minor possessed either 6(2), 7(4) or 9(1) unicuspid teeth or the formulae 5u1b(1) or 2b3u1b(1). Additionally, one count of six unicuspid teeth and three counts of seven unicuspid teeth are followed by unexplained exclamation marks and one count of seven unicuspid teeth is followed by an unexplained question mark. Both La. m. major possessed six teeth. However, since these two individuals were not personally examined by Ioganzen (1935c), we do not know whether the counts given represent only unicuspid or a mixture of unicuspid and lateralmost bicuspid teeth. While Hatta (1911) did not give tooth counts in his descriptions of La. m. minor and La. m. major, the presence of one or two bicuspid teeth on the infraoral lamina in two of the nine La. m. minor specimens is perplexing because it is different from the original description of La. mitsukurii by Hatta (1901) and our study of two syntypes (Table 8). Because this material of La. m. minor and La. m. major is now lost and relied in part on second-hand information, we treat it with suspicion and stand by our interpretations of the taxonomic identities of La. m. minor and La. m. major.

Sato (1951) hypothesized that Le. mitsukurii was Le. camtschaticum (appeared as La. j. japonica) in its spawning phase. However, type material of Le. mitsukurii has unpigmented second dorsal and caudal fins (Table 9) and that of Le. camtschaticum is pigmented for both those characters (Table 3), and thus, we reject this hypothesis.

According to Berg (1931) and Ioganzen (1935a, 1935b) the parasitic La. j. septentrionalis (= Le. camtschaticum) is distributed throughout the Ob’ River drainage, where it occurs sympatrically with the nonparasitic La. j. kessleri (= Le. kessleri). Specifically, Berg (1931) stated that La. j. septentrionalis (ZIN 6308: 355 mm TL) was sympatric with La. j. kessleri (ZIN 6307, ZIN 6311: 180–224 mm TL [ZIN 6311 as measured by us is very shriveled and only 144 mm instead of 180 mm as measured by Berg]) in the Irtysh River at Omsk and based on literature in the Tom’ River near Tomsk (the type locality of P. kessleri). In regard to the Tom’ River, Berg (1931) placed the 389 mm TL adult reported by Ruzskiy (1920) under the synonymy of La. j. septentrionalis contrasting with the 160–210 mm TL type series of P. kessleri reported by Anikin (1905). Ioganzen (1935a, 1935b) also stated that these two taxa were sympatric in the Tom’ River near Tomsk based on two adults of La. j. septentrionalis 215–408 mm TL and 12 adult syntypes of P. kessleri 132–207 mm TL. Six of the latter syntypes (TGU 9 = TGU 3696 in Ioganzen 1935b, table 3) were 128–165 mm TL as measured by us. In their study of the fishes of the Ob’-Irtysh system, Pavlov and Mochek (2006) reported Le. camtschaticum (appeared as Le. japonicum) to be sympatric with Le. kessleri in the Upper Ob’ and Middle Irtysh regions, as well as in the Southern and Northern Ob’-Taz Guba estuaries. The reported presence, albeit rare, of the nonparasitic Le. kessleri in estuarine waters is unexpected and may perhaps be explained by extensive freshwater plumes. It appears that the lower Tobol River, Ob’ River drainage, also contains both Le. camtschaticum and Le. kessleri. In the lower Tobol River Karasev (2008) reported (as Le. kessleri) five lamprey adults measuring 200–407 mm TL and seven mature females, the latter collected in late April and May, with absolute fecundity 3,161–7,208 eggs. We believe that the adults measuring over 230 mm TL (see above ‘Adult total length attained by Le. kessleri’, page 29) refer to Le. camtschaticum while the adults with the reported fecundity < 10,000 eggs refer to Le. kessleri. Unfortunately, Karasev (2008) did not report the total lengths of the females with the recorded fecundities. Contrary to the contention by Zhuravlev and Lomakin (2017) that anadromous Le. camtschaticum is presumed to have become extirpated from the upper Ob’ River drainage since the mid-20^th century, it would appear that it is still present (Pavlov and Mochek 2006; Karasev 2008). Further west, Le. camtschaticum and Le. kessleri are sympatric in the Severnaya Dvina River drainage (Berg 1931; Altukhov et al. 1958; Kuderskiy and Mel’nikova 1970; Sotnikov and Solov’ev 2002). According to Berg (1948) Le. camtschaticum and Le. kessleri are sympatric in the Pechora River drainage based on a 349 mm TL adult of the former from Shapochnaya (= Shapkina) River (ZIN 20802: 334 mm TL as measured by us, Table 4) and a 118 mm TL spent female of the latter from the Pechora River (ZIN 23909: 110 mm TL as measured by us). Lethenteron camtschaticum and Le. kessleri are also sympatric in the Yenisei River based on our re-interpretation of P. dentex as a junior synonym of Le. camtschaticum (see above ‘Real or perceived distributional discontinuity between the populations of parasitic Le. camtschaticum in Russia?’, page 28) and one individual (ZIN 14441) of Le. kessleri (Tables 4–6). Nazarov (2012) also reports Siberian lamprey (= Le. kessleri) in the middle Yenisei (i.e., Chernaya and Beryozovka rivers) based on 152 adults 122–211 mm TL. Lethenteron camtschaticum and Le. kessleri are sympatric in the Utkholok River drainage, Kamchatka (see above ‘Identity of lampreys in the Utkholok River drainage, Kamchatka’, page 31) based on our re-interpretation of the data presented in Kucheryavyi et al. (2007a). Lethenteron camtschaticum and an unidentified nonparasitic species very similar to Le. mitsukurii are sympatric in the Kol’ River drainage, Kamchatka (see above ‘Identity of lampreys in the Kol’ River drainage, Kamchatka’, page 33). Lethenteron camtschaticum and an unidentified nonparasitic species are also sympatric in Lake Azabach’e basin, Kamchatka (see above ‘Identity of lampreys in the Lake Azabach’e basin, Kamchatka’, page 34). On Sakhalin Island, Le. camtschaticum is sympatric with nonparasitic Le. reissneri in the Tym’ River (Gritsenko 1968, 2002; Dyldin and Orlov 2016; this study – see below ‘Taxonomic identity of nonparasitic lampreys in Japan and Sakhalin Island, Russia with low trunk myomere counts’, page 42). While Gritsenko (2002) recorded the presence of another nonparasitic species in the Tym’ River, Le. kessleri, the identity of the species was not clearly established because he used the number of trunk myomeres to distinguish it from Le. reissneri and the number recorded (i.e., 64–74) could apply to either species as the type material of Le. kessleri possessed 70–74 (this study) and that of Le. reissneri possessed 70 (Renaud and Naseka 2015). Interestingly, out of several hundred specimens of brook lamprey from the Tym’ River examined by Gritsenko (2002), three possessed two rows of posterials, like the specimen of Le. reissneri mentioned above that we identified from that river. Unfortunately, the key diagnostic characters separating the two species (i.e., the pigmentation of the second dorsal fin and the transverse lingual lamina dentition; see above ‘Lethenteron kessleri distinct from Le. reissneri’, page 35) were not recorded. In the People’s Republic of China, Le. camtschaticum (as La. japonica) is sympatric with nonparasitic Le. reissneri in the Mudan River (not Mutantiang River), tributary to Sungari (Songhua) River (Hensel 1963). Li and Liu (2011) confirm the presence of Le. camtschaticum (as La. japonica or Le. japonicum used interchangeably) in the Songhua River system based on prespawning adults 385–470 mm TL. In Japan, Le. camtschaticum is sympatric with nonparasitic Le. mitsukurii in Sapporo, Hokkaido Island (see above ‘Taxonomic identity of Lampetra mitsukurii minor and La. m. major’, page 38) and Shokotsu River, Hokkaido Island (Vladykov and Kott 1978b in which Le. matsubarai is a synonym of Le. mitsukurii). Additionally, Yamazaki et al. (1998) reported the presence of mature dwarf male and female Le. camtschaticum (as Le. japonicum), which they suggested were nonparasitic, together with anadromous Le. camtschaticum (also as Le. japonicum) in the Ohno River, Hokkaido Island, Japan. However, neither photograph of a male and a female dwarf individual in their figure 1 exhibit a dark blotch on the second dorsal fin, and therefore, these would not be Le. camtschaticum, but rather belong to an unidentified nonparasitic species. Later, Yamazaki and Goto (2016) provided another photograph of a mature nonparasitic lamprey, measuring 140–160 mm TL, from the Ohno River (their photo 2–1B) showing a dark blotch on the second dorsal fin and a heavily pigmented caudal fin, which in our opinion could be Le. kessleri, but this needs confirmation. Yamazaki et al. (2011) and Yamazaki and Goto (2016) referred to the nonparasitic lamprey from the Ohno River as a population of fluvial Le. camtschaticum. In Lake Sopochnoe basin, Iturup Island, Kuril Archipelago, Russia, Le. camtschaticum is sympatric with Le. kessleri (Sidorov and Pichugin 2005). However, these authors used the identification criteria of Iwata et al. (1985) and we have argued above (see ‘Lethenteron matsubarai, a synonym of Le. mitsukurii’, page 38) that the specimens identified as Le. kessleri by the latter represented a mixture of Le. reissneri and Le. mitsukurii. Unfortunately, Sidorov and Pichugin (2005) did not comment on the pigmentation of the caudal fin, and therefore, we cannot make a definitive identification between these two species. In Alaska, Le. camtschaticum (as Le. japonicum in Vladykov and Kott 1978a) is sympatric with nonparasitic Le. alaskense (= Le. kessleri?; see key below, page 53) in the Naknek River system (Vladykov and Kott 1978a), as well as the Chatanika and Chena rivers, Yukon River drainage (Vladykov and Kott 1978a; Sutton 2017).

The nonparasitic Le. reissneri has long been reported from Japan (Berg 1931, 1948; Mori 1936; Okada 1960; Hubbs and Potter 1971; Vladykov and Kott 1978a, 1978b; Sato 1984). However, in the redescription of this species Renaud and Naseka (2015) restricted its distribution to mainland Asia (Russia, Mongolia, People’s Republic of China) pending a more comprehensive study across a wider geographic range, because of a marked difference in the number of trunk myomeres between Le. reissneri from mainland Asia versus nonparasitic lamprey in Japan. Indeed, the number of trunk myomeres in adult Le. reissneri (including a syntype) from mainland Asia is 70–72 (Renaud and Naseka 2015) versus 57–63 in adults referred to this species from Japan and Sakhalin Island (Vladykov and Kott 1978a). There are two available names for nonparasitic species described from Japan, Le. mitsukurii and its junior synonym Le. matsubarai. Their adults possess 66–70 trunk myomeres (Table 8; Vladykov and Kott 1978b), higher counts than the 57–63 recorded for Japanese “Le. reissneri” by Vladykov and Kott (1978a). Interestingly, the basis for the first report of Le. reissneri in Japan was the synonymy by Berg (1931) of Le. mitsukurii with Le. reissneri due to his re-identification of Japanese adult lamprey 86–147 mm TL (erroneously reported as 92–145 mm TL) with 56–67 trunk myomeres identified as Le. mitsukurii (appeared as Entosphenus mitsukurii) by Jordan and Hubbs (1925). Hubbs and Potter (1971) had remarked that unidentified nonparasitic lampreys from Hokkaido (Japan) and Siberia (Russia) possessed ca. 70 or more trunk myomeres, contrasting with the lower counts of Jordan and Hubbs (1925). It is important to note that Berg (1931) did not examine type material of Le. mitsukurii or Le. reissneri in reaching his conclusion. Yamazaki and Goto (1996) discovered fixed alternate allelic differences at eleven loci (AAT–1, G6PDH, GPI–2, IDHP–1, IDHP–2, IDHP–3, IDHP–4, MDH–3, MDH–4, MEP–1, PGM) among what they assumed were Japanese populations of Le. reissneri collected from 27 rivers across Hokkaido and Honshu islands, which they identified as northern (N) and southern (S) groups. Furthermore, they detected no evidence of hybridization between three sympatric river populations on Honshu Island (i.e., Uono, Shou-gawa, and Shoushu rivers) implying that the northern and southern groups were reproductively isolated. Yamazaki et al. (1999) extended the range of the S-group to include Shikoku and Kyushu islands, Japan, and the Korean Peninsula. They also added five other rivers on Honshu where the two forms (N and S) were sympatric (Ushiwatari, Takifuchi, Shouzenji, Makino, and Shourai) and Yamazaki et al. (2001a) added a further one (Gakko). Despite an overlap in the spawning season and size at spawning of the two forms in the Ushiwatari River, tributary to the Gakko River, Yamazaki and Goto (2000b) found only males and females of a given form on their own nests and no evidence of gene exchange between the two forms, which they suggested may be due to an unidentified premating isolating mechanism ensuring that they remain distinct. Additionally, Yamazaki et al. (2003) found differences in the mtDNA COI sequences (1095 bp) between the N and S forms. Interestingly, the mean percentage sequence differences between them (9.1%) was greater than between either of them and La. fluviatilis (7.0–8.8%). However, Yamazaki and Goto (1997) could find no diagnostic morphometric, countable (trunk myomere and dentition) or pigmentation (caudal fin) characters to distinguish adults of the two. Yamazaki and Goto (1997) state that the caudal fin of the northern and southern groups is translucent or slightly pigmented. This is highly significant because the caudal fin pigmentation of Le. reissneri is heavily pigmented (+++) according to Renaud and Naseka (2015), while it is unpigmented (-) in Le. mitsukurii (Table 9). Additionally, the range in adult total length reported by Yamazaki and Goto (1997) for the northern and southern groups combined (87.9–169.1 mm) is very similar to that (80–156 mm) reported in the original description of Le. mitsukurii. According to Yamazaki et al. (2006) the two undescribed Lethenteron spp. from Japan are distinguished from Le. reissneri of the upper Amur River system (from the type locality) on the basis of the number of larval trunk myomeres: 65–73 (upper Amur Le. reissneri), 51–66 (Lethenteron sp. N), and 49–62 (Lethenteron sp. S). Yamazaki et al. (2006) further added that Le. reissneri from the type locality (Onon and Ingoda rivers) and Le. sp. S from the Naktong (= Nakdong) River, South Korea are fixed for alternate alleles at eight loci (AAT–1, IDHP–1, IDHP–2, IDHP–3, IDHP–4, MDH–3, MDH–4, PGM). Remarkably, based on a strict consensus tree generated from maximum parsimony of a partial sequence of the mtDNA COI gene (1009 bp), Le. sp. N is sister to a Lethenteron clade (100% bootstrap support) that includes Le. japonicum (= Le. camtschaticum), Le. reissneri, and Le. kessleri, while Le. sp. S is sister to an Entosphenus–Lampetra–Lethenteron clade (99% bootstrap support) that also includes Le. sp. N (Yamazaki et al. 2006). Yamazaki and Goto (1998) conducted an electrophoretic analysis of Le. japonicum (= Le. camtschaticum) and Le. kessleri collected from Hokkaido and Honshu islands, Japan, and the Far Eastern region of Russia. These were identified based on morphological features reported by Iwata et al. (1985). Although Yamazaki and Goto (1998) found that the two species were fixed for alternate alleles at one locus (MDH–3) at the three Hokkaido Island localities (Sarufutsu, Hororo, and Mena rivers) where they were sympatric, the identity of Le. kessleri is in question because it did not possess a pigmented second dorsal fin as reported in the original description and none of its samples came from the type locality. Yamazaki et al. (2006) found that lamprey material from the type locality of Le. reissneri (Onon and Ingoda rivers, Russia) and from the Ob’ River drainage, the same river drainage as for the type locality of Le. kessleri (Tom’ and Kirgizka rivers, Russia), although very distantly removed from the latter (Irtysh and Uba rivers, Kazakhstan), shared the same MDH–3 allele. However, we have shown above that two morphological characters (transverse lingual lamina counts and pigmentation of the second dorsal fin) distinguish those two species. At this point, only two nonparasitic species are confirmed to occur in Japan: Le. reissneri (Shizunai River, Hokkaido Island, this study) and Le. mitsukurii. As adults of both Le. mitsukurii and Le. reissneri have high trunk myomere counts (66–72: this study; Renaud and Naseka 2015), an in-depth study of large samples of nonparasitic lamprey adults from Japan and Sakhalin Island is needed to determine whether the lower trunk myomere (< 66) individuals from these areas represent one or more undescribed species, or Le. mitsukurii, or Le. reissneri, or a mixture of these three alternatives. Yamazaki and Goto (2000a) made the suggestion that perhaps Lethenteron form N or Le. form S could in fact be Le. mitsukurii. Recently, using data extracted from two theses, Dyldin et al. (2019b) could not determine the specific identity of adult Lethenteron lampreys from four Sakhalin rivers (Bol’shoy Garomai, Nitui, Novikovka, Pugachovka) measuring 116–220 mm TL and with 65–75 trunk myomeres. In this regard we have identified as Le. reissneri a 146 mm TL (158 mm originally) adult with 68 trunk myomeres from Tym’ River, Sakhalin Island, Russia (ZIN 25204) that was treated as Lethenteron sp. in Tuniyev et al. (2016). Although placed under the La. reissneri species account in Berg (1948), he added (p. 43, fig. 30) that it was possibly a new species because it possessed multiple posterial rows. Two posterial rows have been reported by Renaud and Naseka (2015) in a specimen of La. reissneri from the Shangshi River, People’s Republic of China (ZIN 14457). We diagnosed the specimen from Sakhalin (Fig. 5) as Le. reissneri because it possesses two rows of posterials, the first one with 20 unicuspid teeth, the endolateral formula is 2–2–2, the infraoral lamina is 1b5u1b, the second dorsal fin is unpigmented and the caudal fin heavily pigmented (+++). According to Nikoforov et al. (1994), the paleo-Amur was hypothesized to be connected to the eastern littoral of Sakhalin Island in the geological past, and we believe this may explain the presence of Le. reissneri in the Tym’ River in present times. One of the syntypes of La. mitsukurii (Table 8) also possessed two rows of posterials indicating that this condition is not restricted to Le. reissneri.

Synonyms, new combinations, and misidentifications are included.

Petromyzon marinus Camtschaticus Tilesius, 1811: 240–247, pl. IX, figs I, II [original description, marine waters of St. Peter and Paul Camtschatici (= Petropavlovsk-Kamchatsky) harbor, Russia].

Petromyzon Fluvialis (non Linnaeus, 1758) – Richardson 1823: 705 [misspelling of P. fluviatilis Linnaeus, 1758; common name: Lesser lamprey, Great Slave Lake, Northwest Territories, Canada, attached to an Inconnu, salmo Mackenzii (= Stenodus leucichthys)]; Richardson 1836: 294 [common name: River lamprey, refers to the individual in the previous reference].

Petromyzon borealis Girard, 1858: 377 [available by indication to Petromyzon fluviatilis Richardson, 1836].

Petromyzon Japonicus von Martens, 1868: 3–5, pl. I, fig. 2 [original description, Tokyo (appeared as Jeddo) and Yokohama, Honshu Island, Japan].

Petromyzon Kameraticus – Dybowski, 1869: 948 [misspelling of P. camtschaticus Tilesius, 1811; treated as a distinct species, not a subspecies of P. marinus as Tilesius (1811) intended, ascends to Stretensk on the Shilka River, a tributary of the Amur River, Russia].

Petromyzon fluviatilis (non Linnaeus, 1758) – Günther 1870: 504 [presence of a transverse series of small teeth behind the mandibular tooth (= row of posterials) in Petromyzon japonicus von Martens, 1868 from Japan judged insufficient by Günther (1870) to distinguish it from P. fluviatilis.]

Petromyzon Ernstii Dybowski, 1872: 220 [original description, mouth of the Amur River, Russia].

Petromyzon Kameralicus – Dybowski 1872b: 221 [misspelling of P. camtschaticus Tilesius, 1811].

Petromyzon kamtschaticus – Dybowski 1872b: 221 [misspelling of P. camtschaticus Tilesius, 1811].

Ammocoetes aureus Bean, 1881: 159 [original description, Yukon River at Anvik (63°N, 160°W), Alaska, U.S.A.].

Petromyzon (Ammocoetes) fluviatilis var. (non Linnaeus, 1758) – Smitt 1895: 1190–1191, fig. 353 [Archangel, Russia, 315 mm TL male with middle pair of the lateral (= endolateral) teeth bicuspid and a curved but irregular row of teeth in the posterior part of the disc (= row of posterials)].

Lampetra aurea – Jordan and Evermann, 1896: 13 [comb. nov., Yukon River]; Jordan and Gilbert, 1899: 434 [Yukon River]; Schmidt 1904: 336 [northern Bering Sea]; Evermann and Goldsborough, 1907: 222, 227 [common name: Lamprey eel, Bering Sea basin].

Entosphenus camtschaticus – Jordan and Gilbert, 1899: 434 [comb. nov., allocation to Entosphenus is proposed as probable, but not certain, Kamchatka].

Lampetra japonica – Hatta 1901: 22–24 [comb. nov., Honshu and Hokkaido islands, Sea of Japan basin, Japan]; Jordan and Snyder 1901b: 733 [Japanese common name: Yatsumeunagi, which translates to eight-eyed eel, 470 mm TL, Shinano River, Honshu Island, Japan]; Soldatov and Lindberg 1930: 2–3 [common name: Japanese lamprey, adults up to 350 mm TL, but fig. 1 indicates 452 mm, Tumen River, Maikhe River mouth, Ussuri Bay, Amur estuary, Vorovskaya River (Kamchatka), Russia]; Mori 1936: 3, 6, 10, 15 [Maritime, Karafuto (= Sakhalin), Hokkaido, Amur zoogeographical districts, Japan, Chosen (= Korea)]; Berg 1948: 25, 29–34 [Arctic and Pacific Ocean basins]; Ivanova-Berg and Manteyfel’ 1949: 18 [prespawning adults feed on blood and muscle of Coregonus nasus in Gulf of Ob’ and Tazovskaya Bay and Clupea pallasii in Severnaya Dvina mouth and Pechorskaya Bay]; Birman 1950: 158–159 [common name: Pacific lamprey, lamprey marks on Oncorhynchus gorbuscha in the Amur River estuary and mouth of the Tumnin River, Russia, and less frequently on O. keta, in the Amur River estuary]; Walters 1955: 267, 272 [common name: Arctic lamprey, 355 mm TL adult, Point Barrow, Alaska, U.S.A.]; Morozova 1956: 149 [common name: Pacific lamprey, spawning run on Amur River near Elabuga and Malmyzh, Russia, 3 Dec. 1948 – 8 Jan. 1949, 6–30 Dec. 1949]; Nikol’sky 1956: 588–590 [common name: Pacific lamprey, 22 adults 147–293 mm TL, with a blue-gray dorsal aspect and silvery-white ventral aspect, except for the largest one in which the dorsal aspect is greenish and the ventral aspect yellowish, feeding on Osmerus dentex on 11 Aug. 1955 in marine waters two to three km off Ribnovsk, Sakhalin Island, Russia; their intestines contained O. dentex scales, muscle, intestine, gonad, and bones]; Heard 1966: 332, 334, 336, 338 [in part, common name: Arctic lamprey, presumed anadromous, parasitic on Gasterosteus aculeatus, Prosopium coulterii, Oncorhynchus mykiss, O. nerka, mature or spent adults 218–311 mm TL, Naknek River and Brooks River, Naknek River system, Alaska, U.S.A.]; Gritsenko 1968: 157 [lamprey marks on anadromous Oncorhynchus gorbuscha, O. keta, O. kisutch, O. masou, Salvelinus alpinus entering Tym’ River, Sakhalin Island, Russia]; McPhail and Lindsey 1970: 50–55 [in part, common name: Arctic lamprey, adults from Alaska ca. 90–411 mm TL some feeding on Oncorhynchus tshawytscha, Platichthys stellatus; according to map, Alaskan waters of the Bering Sea basin from the Alaska Peninsula northwards to St. Lawrence Island, U.S.A., Beaufort Sea basin from near Barrow, Alaska, U.S.A. to Anderson River, Northwest Territories, Canada, Yukon River and off Herschel Island (feeding on Osmerus dentex, Stenodus leucichthys), Yukon, Canada, Great Slave Lake basin northward to Mackenzie River estuary, eastward to Artillery Lake and southward to Slave River at Fort Smith, Northwest Territories, Canada]; Savvaitova and Maksimov 1978: 556 [230–320 mm TL spawning individuals 17–21 June 1972 in the Levyy Kolkalvayam River, tributary to the Utkholok River, western Kamchatka, Russia]; Sato 1984: 2 [Japanese common name: Kawa-yatsume, parts of second dorsal and caudal fins blackish in adults, Japan]; Novomodnyy and Belyaev 2002: 81 [young lamprey adults ≤ 210 mm in length feed on Oncorhynchus gorbuscha and O. keta smolts ≤ 85 mm in length and O. masou ≥ 120 mm in length in the Amur River estuary and Sakhalin Bay, Russia]; Li et al. 2019: 1501–1502, 1505, 1507, 1509–1512 [Tumen and Amur rivers, People’s Republic of China, early development].

Lampetra mitsukurii (non Hatta, 1901) – Jordan and Snyder 1901b: 734 [in part, 305–356 mm TL, Ishikari River, Sapporo, Hokkaido Island, Japan].

Entosphenes camtschaticus – Schmidt 1904: 336 [misspelling of Entosphenus, brackish-water form, Bering Sea off Kamchatka].

Petromyzon dentex Anikin, 1905: 15–17 [original description, mouth of the Yenisei River, near Gol’chikha, Russia].

Lampetra fluviatilis (non Linnaeus, 1758) – Berg 1906: 177, 179 [in part, ZIN 7814].

Lampetra mitsukurii major Hatta, 1911: 266–268, pl. IX, figs 1, 2, 5, 6 [original description, Sapporo, Hokkaido Island, Japan, spawning male and female].

Entosphenus japonicus – Regan 1911: 201–202 [comb. nov., Echigo Province (= Niigata Prefecture minus Sado Island), Tokyo, and Hokkaido Island, Japan; Archangelsk, Russia]; Jordan et al., 1913: 6 [Japanese common name: Kawayatsume, northern Japan]; Jordan Hubbs 1925: 98 [supraoral lamina with a cusp at each end and at most a minute cusp on the bridge; infraoral lamina with six to eight teeth, the lateralmost bicuspid and the internal ones unicuspid; reported locality is Karafuto, near Otaru, Hokkaido Island, Japan, but this makes no sense as Karafuto is the former name of the southern part of Sakhalin Island].

Lampetra fluviatilis japonica – Berg 1911: 33–34 [comb. nov., ZIN 6308, 7814, 8545, 12159, 14371]; Ruzskiy 1920: 30 [common name: Siberian-Japanese lamprey, 389 mm TL adult, Tom’ River near Tomsk, Russia]; Soldatov 1924: 16 [common name: River lamprey, 330 mm adult in a drift net at the freshwater/brackish water interface of the Pechora River mouth, Russia]; Knipovich 1926: 51 [common name: Siberian river lamprey].

Entosphenus (Lethenteron) japonicus – Creaser and Hubbs 1922: 3, 6–7, 11 [new subgenus based on Lampetra wilderi Gage in Jordan & Evermann, 1896; coasts and streams from Bering Sea west to the White Sea and south to the Sea of Japan].

Lampetra borealis – Jordan et al. 1930: 10 [comb. nov., common name: Arctic lamprey, streams of northern Alaska and Kamchatka].

Lampetra (Lampetra) japonica septentrionalis Berg 1931: 93, 100–102, pl. V, fig. 4 [original description and key, Onega River at Podporozh’e, White Sea basin, Russia].

Lampetra (Lampetra) japonica japonica – Berg 1931: 93, 98 [nominative subspecies based on Petromyzon Japonicus von Martens, 1868].

Lampetra (Lampetra) japonica kessleri (non Anikin, 1905) – Berg 1931: 102 [Anadyr Liman (= Estuary), Russia; ZIN 23154, 258 mm TL spent female collected in brackish water attached to Onchorhynchus keta; ZIN 23158, two adults 135–138 mm TL with very sharp teeth collected in brackish water and attached to O. keta]; Berg 1948: 35 [Anadyr Estuary, Russia: two uncatalogued specimens with sharp teeth and thick intestines; one (155 mm TL) attached to a sculpin (Cottidae) and the other (144 mm TL) to O. keta].

Lampetra (Lampetra) reissneri (non Dybowski, 1869) – Berg 1931: 104 [Samarga (ZIN 15078) and Sedanka (ZIN 15747) rivers, Russia].

Lampetra fluviatilis septentrionalis (non Berg, 1931) – Ioganzen 1935a: 369 [comb. nov.].

Lampetra japonica japonica – Taranetz 1937: 47 [common names: Pacific lamprey, Japanese lamprey]; Shmidt 1950: 16, 232, 236 [Okhotsk Sea basin: mouth of Amur River, Gulf of Sakhalin, northern part of the Sea of Okhotsk, western coast of Kamchatka, eastern and northern coasts of Sakhalin Island, Russia].

Entosphenus japonicus septentrionalis – Rawson 1951: 208, 221 [comb. nov., common name: Northern lamprey, Great Slave Lake in Fort Resolution area, near town of Hay River and Gros Cap, Northwest Territories, Canada; the latter two in Lota lota and Stenodus leucichthys stomachs, respectively].

Entosphenus lamottei japonicus – Wilimovsky 1954: 281 [comb. nov., Arctic

Alaska, Bering Sea, north and east Asia].

Lampetra (Lethenteron) japonica – Hubbs and Potter 1971: 51 [comb. nov., Varanger Fjord throughout Siberia, along the eastern Pacific coast, Japan, Alaska, and northern Canada].

Lethenteron japonicum – Nursall and Buchwald 1972: iv, 14, 18, 24 [comb. nov., common name: Arctic lamprey, adults 168 to > 300 mm TL; Great Slave Lake, Slave River, Hay River, Northwest Territories, Canada; host of Catostomus catostomus, Coregonus artedi, C. clupeaformis, Lota lota, Salvelinus namaycush, Stenodus leucichthys in fresh water]; Vladykov and Kott 1978a: 3, tables 9–10, 22 [Mackenzie River drainage, Northwest Territories, Canada; Beaufort Sea; Naknek River system, Nushagak and Yukon rivers, Alaska, U.S.A; Honshu and Hokkaido islands, Japan; Amur River, Russia]; Iwata et al. 1985: 185–186 [Saru, Mu, Ryukei rivers and Tofutsu Lake, Hokkaido Island, Japan]; Iwata and Hamada 1986: 17–20 [common name: Arctic lamprey, 197 mm TL adult male with fully pigmented caudal fin and blotch on second dorsal fin, Assabu River, Hokkaido Island, Japan; 153–228 mm TL young adults, Tofutsu Lake and Saru River, Hokkaido Island, Japan; 365–533 mm TL adult males, Assabu, Hime, Toshibetsu rivers, Hokkaido Island and Mabechi River, Honshu Island, Japan]; Yamazaki et al. 2001b: 1135 [Japan: Ishikari, Ohno, Saru, and Uzura rivers, Hokkaido Island, 215.2–478.0 mm TL adults; Mogami River, Honshu Island, 397.0–431.0 mm TL adults]; Martynov 2002: 145 [common name: Pacific lamprey, upstream migrants in the Vashka River, Mezen’ River drainage, Russia, mean TL 417 mm for males and 424 mm for females, one out of 109 adults had a third cusp on the bridge of its supraoral lamina and two out of 111 adults had unicuspid instead of bicuspid lateralmost teeth on the infraoral lamina]; Gritsenko 2002: 13–21 [340–570 mm TL adults, Tym’ River, and lamprey marks on anadromous Oncorhynchus gorbuscha, O. keta, O. kisutch, O. masou, Salvelinus alpinus, S. leucomaenis entering Tym’ River and Nyyskiy Bay, Sakhalin Island, Russia]; Reshetnikov 2002: 28 [intestinal diameter in adult 4–20 mm].

Lethenteron japonica – Nikoforov et al. (1994): 26 [common name: Arctic lamprey, southern Sakhalin Island, Russia].

Lampetra (Lethenteron) camtschatica – Mecklenburg et al. 2002: 62 [comb. nov., common name: Arctic lamprey, Alaska and elsewhere in the northern hemisphere, but not Caspian Sea as erroneously stated].

Lethenteron camtschaticum – Sidorov and Pichugin 2005: 402–405 [common name: Japanese lamprey, silvery downstream migrants to the sea, 151–201 mm TL, feeding on Oncorhynchus gorbuscha, O. nerka, Salvelinus malma, Lake Sopochnoe basin, Iturup Island, Kuril Archipelago, Russia, 3–10 Aug. 2001]; Gritsenko et al. 2006: 16–17 [common name: Pacific lamprey]; Kucheryavyi et al. 2007a: 41–44 [in part, Kolkavayam and Utkholok rivers, western Kamchatka, Russia, typically anadromous form, 174–350 mm mature individuals of both sexes combined]; Bugayev et al. 2007: 32–33 [common name: Pacific lamprey, 310–320 mm TL adults, Kamchatka River estuary, Russia]; Sutton 2017: 1198 [anadromous, adults, 304–427 mm TL, Chatanika and Chena rivers, Yukon River drainage, Alaska, U.S.A.]; Chereshnev 2008: 25–31 [common name: Pacific lamprey, 310 mm TL individual from the stomach of a burbot, Lota lota, in the middle reaches of the Anadyr River, Russia]; Shevlyakov and Parensky 2010: 396, 399 [common name: Kamchatka lamprey, lamprey wounds and scars 6–22 mm in diameter above the lateral line between the dorsal and adipose fins of Oncorhynchus keta in the lower Kamchatka River and Kamchatka Bay, Russia]; Novikov and Kharlamova 2018: 296, 298–301 [common name: Arctic lamprey, adults 230–480 mm TL, Barents Sea up to 76°N and White Sea at depths 10–131 m]; Siwicke and Seitz 2018: 111 [common name: Arctic lamprey, 28 feeding individuals collected using a epipelagic trawl in the eastern Bering Sea at depths < 100 m between mid-August and early October 2012]; Shink et al. 2019: 1993–1998 [common name: Arctic lamprey, feeding adults, 187–465 mm TL, on Mallotus villosus, Clupea pallasii, Ammodytes hexapterus, Eleginus gracilis, Gadus chalcogrammus, Leptoclinus maculatus, Limanda aspera, Oncorhynchus gorbuscha, O. tshawytscha, Osmerus dentex, Cottidae, Gasterosteidae, eastern Bering Sea, Alaska, U.S.A.].

Lampetra camtschatica – Sawatzky et al. 2007: 10, 12–13 [in part, common name: Arctic lamprey, Northwest Territories, Canada, including Horton River].

Lethenteron reissneri (non Dybowki, 1869) – Bugayev et al. 2007: 33–34 [two males, 185 mm TL, one from Lake Azabach’e and one from Lake Kurzin, both in the Kamchatka River drainage, Russia with half-digested fish flesh in their intestines].

Synonyms, new combinations, and misidentifications are included.

Lampetra mitsukurii Hatta, 1901: 22–24 [original description, Hondo [= Honshu], Shikoku, Kyushu, and Hokkaido islands, Japan]

Lampetra mitsukurii – Jordan and Snyder 1901a: 126 [Japanese common name: Sunayatsume].

Lampetra mitsukurii minor Hatta, 1911: 263–266, 268, pl. IX, figs 3, 4, 7, 8 [trinomial based on Lampetra mitsukurii Hatta, 1901].

Lampetra planeri (non Bloch, 1784) – Regan 1911: 203 [misidentification based on synonymy with Lampetra mitsukurii Hatta, 1901]; Jordan et al. 1913: 6 [Japan]; Creaser and Hubbs 1922: 13 [Japan].

Entosphenus appendix (non DeKay, 1842) – Creaser and Hubbs 1922: 7, 12 [in part, eastern Asia, including Japan].

Entosphenus mitsukurii – Hubbs 1925: fig. 16, 589 [comb. nov., a degenerate, dwarf, brook lamprey derived from anadromous, parasitic Entosphenus japonicus]; Jordan and Hubbs 1925: 98–99 [in part, mature male, 147 mm TL, 67 trunk myomeres, mottled coloration and mature female, 142 mm TL, 63 trunk myomeres, plain coloration, Sapporo, Hokkaido Island, Japan].

Lampetra (Lethenteron) mitsukurii – Hubbs and Potter 1971: 52–53 [comb. nov., provisionally recognized, Japan].

Lethenteron matsubarai Vladykov & Kott, 1978: 1792–1800 [original description, Shokotsu River, Hokkaido Island, Japan (44°22'N, 143°20'E)]; Vladykov and Kott 1979c: 10 [common name: Japanese brook lamprey; we recommend this common name now be used for Le. mitsukurii].

Lampetra reissneri (non Dybowski, 1869) – Sato 1984: 2 [dorsal and caudal fins without blackish parts in adults, Japan].

Lethenteron kessleri (non Anikin, 1905) – Iwata et al. 1985: 186–188 [in part, Hokkaido Island, Japan, individuals with unpigmented second dorsal and caudal fins].

Petromyzon mitsukurii – Paepke and Schmidt 1988: 160 [comb. nov., Japan].