Morphology-based taxonomic re-assessment of the Arctic lamprey, Lethenteron camtschaticum (Tilesius, 1811) and taxonomic position of other members of the genus

Abstract The lamprey genus Lethenteron Creaser & Hubbs, 1922 is widespread across Eurasia and North America, but the number and distribution of its constituent species is not firmly established. After a morphological examination of extant type material of the currently recognized species and their synonyms, Lethenteron mitsukurii (Hatta, 1901) is resurrected with Le. matsubarai Vladykov & Kott, 1978 as its junior synonym. Amongst nonparasitic species Le. reissneri (Dybowski, 1869) and Le. mitsukurii are confirmed as present in Japan and the former is also present on Sakhalin. 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. The material from the Anadyr Estuary identified by Berg (1931, 1948) as Lampetra japonica kessleri has been re-identified as Le. camtschaticum and there is no evidence that Le. kessleri occurs there. Lethenteron reissneri is reported from the Angara River system, Yenisei River drainage, Russia. Lethenteron alaskense Vladykov & Kott, 1978 is provisionally considered to be a junior synonym of Le. kessleri (Anikin, 1905). Petromyzon ernstii Dybowski, 1872, Ammocoetes aureus Bean, 1881, Petromyzon dentex Anikin, 1905, Lampetra mitsukurii major Hatta, 1911, and Lampetra japonica septentrionalis Berg, 1931 are junior synonyms of Petromyzon marinus camtschaticus Tilesius, 1811. A key is provided to adults of the six species recognized as belonging in the genus Lethenteron.

described from Shokotsu River, Hokkaido Island, Japan. The first was synonymized with La. reissneri by  and later tentatively resurrected by Hubbs and Potter (1971), while the second was synonymized with Le. kessleri by Iwata et al. (1985).
• re-evaluate synonymization of Le. kessleri with Le. reissneri by Yamazaki et al. (2006) and the two names with Le. camtschaticum by Kucheryavyy (2014) as not based on a re-examination of all of the relevant type material.
• establish relationships of Lampetra mitsukurii minor and La. m. major Hatta, 1911with Le. camtschaticum. Berg (1931 considered La. m. minor a synonym of La. reissneri and La. m. major a synonym of La. japonica japonica. Molecular data of Lang et al. (2009), Pu et al. (2016), and Ren et al. (2016) have suggested that Lampetra (Eudontomyzon) morii  should be assigned to Lethenteron and we will provide a justification for its placement in Eudontomyzon.
The remaining three species recognized by Potter et al. (2015) as belonging to the genus Lethenteron, Le. alaskense, Le. appendix, and Le. ninae Naseka et al. 2009, will be included in a taxonomic key to the adults of the species of the genus that will also include the other species as established in this study. Artamonova et al. (2011) synonymized Le. ninae with Le. camtschaticum, but Tuniyev et al. (2016) confirmed the specific distinctiveness of the former. Although Tuniyev et al. (2016) showed that Le. ninae usually possesses a row of posterial teeth (complete or incomplete) characteristic of the genus Lethenteron, other morphological evidence (tricuspid middle endolateral, low number of trunk myomeres, straight longitudinal laminae, no velar wings) suggested that it should be assigned to Lampetra. However, they recommended the status quo until a total evidence cladistic analysis had been completed that incorporated parasitic and nonparasitic species and morphological and molecular characters to prevent re-assignment of a species to a different genus based on incomplete information.

Materials and methods
Material examined follows the method of Renaud (2011) and for the gular pigmentation of Vladykov and Kott (1978a). TL refers to total length. All collection dates are according to the Gregorian calendar. We restricted our study to adults (i.e., metamorphosed individuals) because the diagnostic characters in the original descriptions and the type material of the three nominal subspecies and their synonyms were based on this life stage only. Abbreviations:

Results
The morphometric, countable, shape, and pigmentary character states for the extant adult type material of Petromyzon marinus camtschaticus, P. japonicus, P. kessleri, and La. j. septentrionalis are given in Tables 1-3. The original descriptions of these nominal taxa are also an integral part of this section because they supplement our observations on the type material and because they were published in various languages (Latin, German, Russian, and English, respectively), they are presented below in English for ease of comparison. Additionally, as they span a period  when lamprey dentition nomenclature in particular was not yet standardized (see Follett 1967, Hubbs andPotter 1971), we used the current tooth nomenclature or added the equivalent name in parentheses.
The morphometric, countable, shape, and pigmentary character states in all non-type adults identified by  as La. j. japonica, La. j. kessleri, and La. j. septentrionalis available to us are given in Tables 4-6. Furthermore, the original descriptions of two species, Petromyzon ernstii and P. dentex, for which extant type material of neither could be found, are presented below in their English translation of the original German and Russian, respectively.  considered the former to be junior synonym of La. j. japonica and the latter, tentatively, as a junior synonym of La. j. kessleri.  considered Ammocoetes aureus to be a junior synonym of La. j. japonica and although its extant holotype was not examined, its original English description is presented below.
The morphometric, countable, shape, and pigmentary character states for extant adult type material of La. mitsukurii and Le. matsubarai are given in Tables 7-9 and their original English descriptions are also presented. Finally, the English translation of the original German descriptions of La. m. minor and La. m. major are presented.
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 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).  (7) pigmented, undetermined (7) unpigmented (2), pigmented, undetermined (8) Gular undetermined undetermined undetermined (7) - (7), undetermined - (7), + (2), +++, undetermined Original description of Lampetra japonica septentrionalis Berg, 1931: 93, 100-102, pl. V, fig. 4.
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).  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  in nontype 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,  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  TGU 3 [no. 3699 in Ioganzen (1935b:    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). Table 4. Morphometrics in adults identified by  as Lampetra japonica japonica (von Martens, 1868), La. j. kessleri , and La. j. septentrionalis . Numbers in parentheses are sample sizes. 6.4-7.1 (4) 4.0-6.7 (4) 5.6-5.9 (2) 4.1 4.5 5.8 5.6 4.6 Prenostril length (d-n) 5.6-6.8 7.2-8.1 (4) 5.2-7.2 (4) 6.2-6.4 (2) 6.1 6.6 6.0 5.6 5.5 Snout length (d-O) 5.6-7.0 (5) 8.0-9.1 (4) 5.4-7.7 (4) 6.7-6.9 (2) 6.1 7.6 7.1 6.6 6.4 Eye length ( 0.5-0.8 (4) 0.0-1.0 (4) 0.7-1.1 (2) 1.2 0.0 2 0.6 0.7 0.6 Table 5. Trunk myomeres, dentition, oral papillae and oral fimbriae in adults identified by  as Lampetra japonica japonica (von Martens, 1868), La. j. kessleri , and La. j. septentrionalis . Numbers in parentheses are frequencies of character states. Abbreviations: b, bicuspid; u, unicuspid.  ?-I-?

Lampetra japonica japonica
Longitudinal lingual laminae undetermined (10) undetermined (8) 12u ( One exolateral tooth on the right side and one on the left side in the middle of the lateral field between the first and second endolaterals in one specimen. 3 One exolateral tooth on the right side next to the marginals between the first and second endolaterals in one specimen. 4 One exolateral tooth on the left side between the third endolateral and the marginals. 5 Two unicuspid teeth in the second row of posterials. Table 6. Shape and pigmentary characters in adults identified by  as Lampetra japonica japonica (von Martens, 1868), La. j. kessleri , and La. j. septentrionalis . Numbers in parentheses are frequencies of character states. Pigmentation coverage as follows: -, absent to < 1%; +, 1% to < 25%; ++, 25% to < 75%; +++, ≥ 75%.  Original description of Petromyzon ernstii Dybowski, 1872: 220.
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-B 1 ), 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.  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.  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 : 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   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 : 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 & Kott, 1978b: fig. 4). Two paratypes were studied (Tables 7-9).

Discussion
One or multiple species  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 Table 7. Morphometrics in adult types of Lampetra mitsukurii  and Lethenteron matsubarai . Numbers in parentheses are sample sizes.

Lampetra mitsukurii Lethenteron matsubarai ZMB 20638 (2 syntypes)
CMNFI 1984-274 (2 paratypes) Type locality Japan Shokotsu River, Hokkaido Island, Japan Trunk myomeres 66-67 (2) Rows of anterials 1, 2 2 (2) Rows of exolaterals 0 (4) 1 0 (4) Rows of posterials  (2) 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., normallymaturing 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. (2007aKucheryavyi et al. ( , 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 (2) unpigmented (2)  Gular undetermined (2) - (2) mous 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 postmetamorphosis. 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).

Lethenteron species with exolaterals
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).
Petromyzon japonicus, a synonym of P. marinus camtschaticus 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.

Identity of the lamprey in Steller's unpublished manuscript
Because Tilesius (1811)

Lampetra japonica septentrionalis not derived from La. fluviatilis
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  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.
Lampetra japonica septentrionalis, a synonym of Petromyzon japonicus    (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, a synonym of Lethenteron camtschaticum
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   (Table 5), and therefore, we believe that P. ernstii is a junior synonym of Le. camtschaticum.
Variability in the infraoral lamina dentition 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).
Real or perceived distributional discontinuity between the populations of parasitic Le. camtschaticum in Russia?
In regard to the wide discontinuity in the distributions of the populations of parasitic Le. camtschaticum reported by  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 ( , 1948 tentatively considered and Ioganzen (1935b) undoubtedly considered P. dentex  to be a junior synonym of La. j. kessleri. However, we suggest instead that it is a junior synonym of Le. camtschaticum because  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. (= La.) 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.  stated that La. j. kessleri (appeared as P. kessleri) reached an adult TL of 210 mm, but  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 ] 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  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  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  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.

Adult total length attained by Le. kessleri
Although  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.  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 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) the adults possess 67-72 trunk myomeres, which overlaps the range based on our examination of Le. kessleri type material (70-74; Table 2).
Variability in the dentition of Le. kessleri 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),  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).  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,  only reported three bicuspid endolaterals in the original description of P. kessleri.  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 nontypes of Le. kessleri (Table 5).  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).

Lethenteron kessleri without posterials?
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 ( , 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.  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  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 . 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 . 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,  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 ( , 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  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  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.

Identity of lampreys in the Kol' River drainage, Kamchatka
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 ( , 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.
Identity of lampreys in the Lake Azabach'e basin, Kamchatka 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 reidentified 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 , 1948Bugayev 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).

Ammocoetes aureus, a synonym of Le. camtschaticum
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.

Lethenteron alaskense, a synonym of Le. kessleri?
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  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.

Lethenteron kessleri distinct from Le. reissneri
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  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  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).

Lethenteron reissneri present in the Angara River drainage, Russia
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  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.

Lethenteron mitsukurii distinct from Le. kessleri and Le. reissneri
Lampetra mitsukurii is placed by us in the genus Lethenteron because it possesses a row of posterials according to the original description ) 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 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 ) 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  and an adult identified by  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  and accepted by Renaud and Naseka (2015) as Le. reissneri.
Identity of the lamprey from Siberia described in  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  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  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 .

Lethenteron matsubarai, a synonym of Le. mitsukurii
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  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.
Taxonomic identity of Lampetra mitsukurii minor and La. m. major  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., welldeveloped 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  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  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  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  that La. m. major is a synonym of what they respectively called Entosphenus (Le.) japonicus and La. j. japonica (= Le. camtschaticum). We disagree with  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 1933Ioganzen (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  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  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.
Lethenteron mitsukurii not the spawning phase of Le. camtschaticum 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.

Sympatric parasitic and nonparasitic taxa
According to  and Ioganzen (1935aIoganzen ( , 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,  stated that La. j. septentrionalis (ZIN 6308: 355 mm TL) was sympatric with La. j. kessleri (ZIN 6307, 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,  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 . Ioganzen (1935aIoganzen ( , 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 Altukhov et al. 1958;. 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 reinterpretation 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(Gritsenko , 2002Dyldin 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,  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).
Taxonomic identity of nonparasitic lampreys in Japan and Sakhalin Island, Russia with low trunk myomere counts The nonparasitic Le. reissneri has long been reported from Japan (Berg , 1948Mori 1936;Okada 1960;Hubbs and Potter 1971;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  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  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).  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  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.  Lampetra mitsukurii major Hatta, 1911: 266-268, pl -Nursall and Buchwald 1972: iv, 14, 18, 24 [comb. nov., common  Partial synonymy of Lethenteron reissneri (Dybowski, 1869), the Far Eastern brook lamprey based on this study A more extensive synonymy is given in Renaud and Naseka (2015).

Generic assignment of Lampetra (Eudontomyzon) morii
When  described this new non-migratory species from the upper Yalu River, in what was then known as Manchuria, but now more or less follows the border of the People's Republic of China and North Korea, the only other members of the subgenus, Lampetra danfordi (Regan, 1911) and La. mariae , were European in distribution. To explain the discontinuous distribution of the subgenus, he suggested two hypotheses: 1) the European and Asian species were relics of a former continuous distribution or 2) they arose independently, the former from La. fluviatilis and the latter from La. japonica (= Le. camtschaticum). However, in their cladistic study based on morphological characters Gill et al. (2003) showed that Eudontomyzon danfordi and E. morii constituted a clade supported by a single synapomorphy, the presence of alate rows of teeth in the laterals fields, and that this clade was sister to La. fluviatilis-La. ayresii rather than Le. camtschaticum. The four specimens of E. morii used in Gill (2003) included a syntype and three non-type adults from the upper Yalu River, People's Republic of China.
Later studies based on the mitochondrial DNA (Lang et al. 2009;White 2014;Pu et al. 2016;Ren et al. 2016) have suggested that E. morii should be assigned to the genus Lethenteron. However, the material used in these studies is of unconfirmed identity. In Lang et al. (2009) and White (2014) the specimen was a metamorphosing individual with exolaterals, but from the Liaohe River, west of the Yalu River. Although  suggested that La. morii probably occurred in the Liao-ho (= Liaohe), he had only seen an ammocoete from that river. Additionally, exolaterals have occasionally been reported in Lethenteron species (see above 'Lethenteron species with exolaterals', page 23). The specimen was the same in Pu et al. (2016) and Ren et al. (2016) and came from the city of Dandong, at the Yalu River estuary, Yellow Sea basin, and may in fact have been Le. camtschaticum. Unfortunately, no morphological description of the specimen is provided. Given the uncertainty surrounding the identity of the specimens used in these molecular studies, we prefer to continue treating E. morii in the genus Eudontomyzon. According to Renaud (1982), who examined four adults from the type locality, including a syntype, E. morii possesses one or two exolateral rows on either side of the oral disc (Fig. 6) and this distinguishes it from members of the genus Lethenteron that usually have no exolateral teeth, rarely one or two teeth and exceptionally one row.
Taxonomic key to adults of Lethenteron Lethenteron adults are characterized by the presence of two dorsal fins; spade-like caudal fin; supraoral lamina with a wide bridge bearing a unicuspid (rarely bicuspid) tooth at each end and rarely one or two unicuspid teeth on the bridge; three (rarely four) endolateral bicuspid teeth on either side of the oral disc (rarely the third tooth may be unicuspid or tricuspid and the fourth tooth unicuspid); infraoral lamina usually with lateralmost bicuspid teeth and unicuspid teeth internally, but much variability exists within the genus (see Table 10); one or two (usually one) rows of posterial teeth; no exolateral teeth (rarely one or two teeth on one or both sides, and exceptionally, a complete row on both sides); transverse lingual lamina with an enlarged median cusp. The key was constructed based on the character matrix compiled in Table 10.