Long and thin in shape. Prostomium rounded anteriorly, lateral parts of the peristomium not especially well demarcated. Two pairs of red eyes arranged somewhat in trapezoidal shape, lateral pair kidney‐shaped and situated anteriorly. Body extremely transparent (Fig. 5D). Green pigmentation on pharynx and pygidium (Fig. 5C, D). Caruncle develop in late larvae, extends posteriorly from posterior part of prostomium (Fig. 5C). In late larvae, branchial anlages occur from chaetiger II, pygidium acquires anal cirri.

Adult individuals of this species were collected from shallow subtidal muddy bottom sediments in Sasuhama in August 2011 by using a hand‐scoop. The species was referred to P. krusadensis as the adult morphology agrees with the descriptions of this species by Imajima (1990a). The larvae and adults were confirmed to match (18S: 1749/1749, 16S: 506/507 bp) using molecular data (Fig. 2).

Planktonic larvae of this species were found in Sasuhama and Onagawa Bay in July and August, but they were rare in the plankton samples. Green pigmentation on the pharynx and pygidium is characteristic of the larvae of this species.

Long and thin in shape. Prostomium rounded anteriorly, lateral parts of peristomium not especially well demarcated. Two pairs of red or dark red eyes arranged somewhat in trapezoidal shape, lateral pair kidney‐shaped and situated anteriorly. Small caruncle develop in late larvae, extends posteriorly from posterior part of prostomium (Fig. 5G). In late larvae branchial anlages occur from chaetiger II, pygidium acquires anal cirri (Fig. 5G). Palps not yet developed in 15- and 20‐chaetiger larva (Fig. 5E, F) but developed in 24‐chaetiger larva (Fig. 5G). Pigmentation absent except eyes.

Adult individuals of this species were collected from muddy bottom sediments at 22 m depth in Onagawa Bay in December 2011 by using a Smith‐McIntyre grab sampler. The species was referred to P. membranacea as the adult morphology agrees with the descriptions of this species by Imajima (1990b). The larvae and adults were confirmed to match (18S: 1752/1752 and 1747/1750 except for gaps, 16S: 502/505 bp) using molecular data (Fig. 2). Planktonic larvae of this species were found in Onagawa Bay during August to October.

Long and thin in shape. Prostomium rounded anteriorly, lateral parts of the peristomium not especially well demarcated. Two pairs of red or dark red eyes arranged somewhat in trapezoidal shape, lateral pair kidney‐shaped and situated anteriorly. Small caruncle develop in late larvae, extends posteriorly from posterior part of the prostomium. In Prionospio sp. 2, branchial anlages occur from chaetiger II, pygidium acquires anal cirri (Fig. 5I). Pigmentation absent except eyes in Prionospio sp. 1. Gut pigmented in orange in Prionospio sp. 2. Palps developed in 19‐chaetiger larva of Prionospio sp. 2.

Two unidentified species of planktonic larvae of the genus Prionospio other than P. krusadensis and P. membranacea were collected from Onagawa Bay. The adult individuals of these species were not collected in the present study. Even though the 18S and 16S rRNA gene sequences obtained from these larvae in the present study did not match any Prionospio sequences obtained in the present study nor with those registered in DDBJ/EMBL/GenBank, these species were referred to Prionospio sp. 1 and 2 as the larvae were similar to the other Prionospio species in their morphology and gene sequences (Figs 2, 3). The larvae of Prionospio sp. 1 and 2 are similar to each other and to that of P. membranacea, and it is difficult to distinguish among them based only on their morphology.

Overall body shape short and thick. Prostomium broad and straight or slightly notched anteriorly. Lateral parts of peristomium clearly demarcated from prostomium. Palps attached to outer end of lateral parts of peristomium. Two pairs of red or black eyes present. Faint yellow pigment may be present in anterior part of prostomium and posterior part of pygidium. In late larvae, pair of prominent antero‐lateral processes on prostomium developed. Melanophore absent, black or yellowish pigmentation occur in some species. Nototrochs weakly developed, occur in all chaetigers except first chaetiger. Gastrotrochs occur regularly in every other chaetiger from chaetiger III onwards (Carrasco 1976; Radashevsky 2007).

Overall body shape short and thick in relation to length. Prostomium broad, stumpy, somewhat notched anteriorly. Peristomium well developed, forming wide collar on sides of prostomium. Two pairs of red or dark red eyes arranged in straight line, lateral pair in kidney‐shape. Parapodia strongly differentiated in late larvae. Larval chaetae occur only in notopodia. Pygidium large and round, acquires dorsal cirri in late larvae. Late larvae have two antero‐lateral processes on prostomium (Fig. 4H). Pigmentation usually absent, some individuals have brownish pigmentation on peristomium, dorsum, and/or pygidium, and/or two medial black pigmentation ventrally on approximately chaetiger VI and anterior margin of the pygidium.

Adult individuals of this species were collected from intertidal and shallow subtidal sandy or muddy bottom sediments in Gamo Lagoon in January 2011 and Sasuhama in September 2011 by using a hand‐scoop. To date, three Rhynchospio species, R. foliosa Imajima, 1991, R. gutaea (Ehlers, 1987), and R. tuberculata Imajima, 1991, have been recorded from Japan (Imajima 1991a). However, extensive morphological and molecular studies revealed the absence of records of R. glutaea from the northern Pacific Ocean (Radashevsky 2007; Radashevsky et al. 2014, 2016a). Radashevsky et al. (2014) also referred to R. arenicola Hartman, 1936, R. asiatica Chlebovisch, 1959, R. aff. asiatica, and R. glutaea as members of the R. glutaea complex because they resembled each other so closely. Adult morphology and 18S and 16S rRNA gene sequences of Rhynchospio specimens obtained in the present study agree (18S: 1716/1716, 16S: 486/492 bp) with those of R. aff. asiatica (Fig. 3) from South Korea (KJ546296) reported by Radashevsky et al. (2014); therefore, this species was referred to R. aff. asiatica sensu Radashevsky et al. (2014). The larvae and adults were confirmed to match (18S: 1783/1783, 16S: 471/477 bp) using molecular data (Fig. 2).

This species is recorded from Japan for the first time in the present study. The brooding of larvae beneath dorsal branchiae in this species was observed in September 2011 (Fig. 6). The larvae adhere to their parents and are enclosed by branchiae present on the posterior chaetigers (26^th–39^th chaetigers). The larvae are retained on the parents’ dorsum even when the parent individuals leave their tube, unless the parent is disturbed. Larvae seemed to be released at around the 3‐chaetiger stage; the fact that planktonic larvae with more than three chaetigers were commonly collected from plankton supports this observation. Similar dorsal larval brooding was reported in other Rhynchospio species (Levin 1982; Radashevsky 2007; Radashevsky et al. 2014) and in Streblospio benedicti Webster, 1879 (Levin 1982, 1984).

Figure 6. 

Light micrographs showing dorsal brooding of Rhynchospio aff. asiatica. Hermaphroditic individual broods their larvae between dorsal branchiae on posterior chaetigers. Scale bars: 1 mm.

Planktonic larvae were found in Onagawa Bay, Gamo Lagoon, and Sasuhama in almost every season of the study period, but few were found in winter season (November to March). Larval morphology of this species resembles that of R. glutaea and R. nhatrangi Radashevsky, 2007 described by Carrasco (1976) and Radashevsky (2007), respectively. The overall larval morphology of Rhynchospio species is quite similar to that of the genus Malacoceros described in Hannerz (1956, as Scolelepis), but it differs in the latter having three pairs of black eyes, the most lateral pairs with double-eyes.

Overall shape thick and fusiform. Prostomium pointed anteriorly, terminates in retractile, muscular tip. Lateral parts of peristomium clearly demarcated from prostomium, forming large peristomial umbrella. Short palps on lateral-most parts of peristomium. Two pairs of red eyes present. Melanophore absent, black, brown, orange, red, or green pigmentation patches often present in body surface, pharynx, gut, and/or proctodaeum. Nototrochs present or absent. Gastrotrochs occur in all chaetigers from chaetiger II or III onwards. Pygidium large, inflated, and surrounded by thick telotroch (Okuda 1946, as Spio filicornis; Hartman 1941, as Nerinides; Thorson 1946, as Nerine in part see Hannerz 1956; Hannerz 1956, as Nerine and Nerinides; Imajima 1959, as Nerinides; Dean and Hatfield 1963, as Nerinides; Carrasco 1976, as Nerine and Nerinides; Plate and Husemann 1994; Scheltema et al. 1997; Blake and Arnofsky 1999; Blake 2006).

Thick and fusiform in shape. Prostomium pointed anteriorly, terminates in retractile, muscular tip. Lateral parts of peristomium clearly demarcated from prostomium, forming peristomial umbrella. Peristomial umbrella carrying well‐developed prototroch. Two pairs of red eyes arranged in somewhat trapezoidal shape, medial pair situated anteriorly. Greenish pigment in pharynx and proctodaeum. Pygidium very broad and horseshoe‐shaped.

Only three individuals of early larvae of this species were collected from Sasuhama in January 2012. The 18S rRNA gene sequences obtained in the present study for these specimens match (464/464 bp) that of S. kudenovi from Lizard Island, Australia (KP636517: Meißner and Götting 2015). Since the species identification is unreliable because of the short reference sequence, this species was referred to S. cf. kudenovi.

The sequence of an adult individual, which collected from the surf zone of the sandy beach in Rishiri Island and previously identifies as Scolelepis kudenovi (Abe et al. 2019c) as the morphology agrees with the descriptions of S. kudenovi by Imajima (1992) and Meißner and Götting (2015), 100% matched with that of the larvae of Scolelepis cf. kudenovi in 18S rRNA gene (1819/1819 bp) but largely differed in 16S rRNA gene (462/505 bp). Because the 16S rRNA gene of the adult individual was rather closer to S. daphoinos (430/455 bp) from China (GU362676, Zhou et al. 2010), it is referred to S. aff. daphoinos in the present study (Table 1, Figs 2, 3).

Thick and fusiform in overall shape. Prostomium pointed anteriorly as a small process, tip of prostomium terminates in retractile, muscular tip. Lateral parts of peristomium clearly demarcated from prostomium, forming large peristomial umbrella. Peristomial umbrella carrying well‐developed prototroch. Short palps developed in late larvae, attached on lateral-most parts of peristomium. Two pairs of red eyes arranged in an approximately straight line. Pharynx pigmented orange and the gut pigmented brown. Pygidium broad and spherical.

Adult individuals of this species were collected from muddy sediments at 22 m depth in Onagawa Bay in December 2011 by using a Smith‐McIntyre grab sampler. These adults were morphologically identified as Scolelepis, but they were not identified to species level as these specimens were all incomplete and in poor condition. As the 18S and 16S rRNA gene sequences obtained in the present study did not match any available Scolelepis sequences (Figs 2, 3), this species was referred to Scolelepis sp. 1. Planktonic larvae of this species were collected in Onagawa Bay in October during the study period. The larvae and adults were confirmed to match using molecular data (Fig. 3).

Thick and fusiform in overall shape. Prostomium bluntly pointed anteriorly, terminates in retractile, muscular tip. Lateral parts of peristomium clearly demarcated from prostomium, forming large peristomial umbrella. Peristomial umbrella carrying well‐developed prototroch. Short palps developed in late larvae, attached on lateral-most parts of peristomium. Two pairs of red eyes arranged in an approximately straight line. Pharynx widely pigmented orange and the gut pigmented brown. Pygidium broad and spherical shaped.

No adult individuals of this species were collected in the present study. Even though the 18S and 16S rRNA gene sequences obtained from larvae in the present study did not match any available Scolelepis sequences, this species was referred to Scolelepis sp. 2 as the larvae constitute a monophyletic clade with the other Scolelepis species with robust statistical support (Figs 2, 3).

Planktonic larvae of this species were found in Onagawa Bay in September. The larvae of this species are quite similar to those of Scolelepis sp. 1; however, the prostomium is more broadly pointed anteriorly and the gut diverticula are more strongly segmented in the former species.

Overall shape slender. Prostomium small or broad, rounded or slightly notched anteriorly. Lateral parts of the peristomium slightly or moderately demarcated from the prostomium, palps on lateral-most parts of peristomium. Two pairs of red eyes present. In late larvae, a pair of prominent or small antero‐lateral processes on prostomium are often developed. Melanophore absent, some pigmentation patches of various colors are present on pharynx, proctodaeum, prostomium, peristomium, pygidium, and/or various locations of the body in late larvae. Nototrochs occur in all chaetigers from chaetigers II–IV onwards. Gastrotrochs occur in all chaetigers from chaetiger II onwards (Thorson 1946; Hannerz 1956; Carrasco 1976; Plate and Husemann 1994; Blake 2006).

Overall shape slender. Prostomium small and rounded anteriorly. Two pairs of red or dark red eyes present, lateral pair situated anteriorly. Late larvae bear very small antero‐lateral processes on prostomium. Lateral parts of peristomium slightly demarcated from prostomium, palps attached on lateral-most parts of peristomium. Nototrochs occur from chaetiger IV onwards (Fig. 4N). Yellow pigments on the prostomium and pygidium, intense yellow-brown pigment on peristomium, inside of pharynx, and pygidium. Small red pigment spots present on lateral part of body (Fig. 4N). Black pigment in pharynx and proctodaeum absent.

No adult individuals of this species were collected in the present study. However, gene sequences obtained from larvae of this species were almost identical (18S: 1732/1732, 16S: 341/342 bp) to that of S. uschakowi (KM998760 and MG878915) from Russia (Radashevsky et al. 2020a); therefore, this species was referred to S. uschakowi. Imajima (1991b) recorded four Spiophanes species from Japan: S. kroyeri Grube, 1860 (as S. kroeyeri); S. japonicum Imajima, 1991; S. bombyx (Claparède, 1870); and S. urceolata Imajima, 1991. Then, Meißner and Hutchings (2003) synonymized S. urceolata with S. wigleyi Pettibone, 1962. Additionally, the specimens from Japan formerly identified as S. bombyx were morphologically reexamined and identified as S. cf. uschakowi by Meißner and Blank (2009). In the present study, the presence of S. uschakowi in Japan was further supported by molecular analysis.

Only a few larvae of this species were collected in Onagawa Bay in November 2011. The overall larval morphology of this species somewhat resembles that of S. kroyeri described by Hannerz (1956) in the following aspects: prostomium is relatively small and anteriorly rounded, the peristomium is not quite sharply demarcated from the prostomium, nototrochs occur from chaetiger IV onwards, and the brown pigmentation is present on the pygidium and inside the pharynx. However, S. kroyeri lacks small red pigment spots on the lateral part of the body and lateral processes on the prostomium even in 22‐chaetiger larvae.

Overall shape slender. Prostomium broad and slightly notched anteriorly. In late larvae, a pair of prominent antero‐lateral processes on prostomium developed. Two pairs of red or dark red eyes present, lateral ones situated somewhat anteriorly. Lateral parts of peristomium moderately demarcated from prostomium, palps on lateral-most parts of peristomium. Nototrochs occur from chaetiger II onwards. Pharynx and proctodaeum black in color internally. Pygidium acquires dorsal cirri in late larvae. Some brownish, yellowish, or greenish pigmentation occurred on various locations of body in late larvae.

Adult individuals of this species were collected from muddy bottom sediments at 22 m depth in Onagawa Bay in April and May 2012 by using a Smith‐McIntyre grab sampler and from bottom sediments of the shallow subtidal zone in Sasuhama in February 2012. Adult morphology agrees with the description of S. cf. uschakowi by Meißner and Blank (2009) as well as with that of S. bombyx by Imajima (1991b). However, the 18S and 16S rRNA gene sequences of this species did not match those of S. uschakowi obtained from DDBJ/EMBL/GenBank (KM998760): there was a 0.29% (5/1750 bp) and 0.88% (3/342) difference, respectively between these two species. Therefore, this species was referred to S. aff. uschakowi. The larvae and adults were confirmed to 100% match using molecular data (Fig. 2).

Planktonic larvae of this species were collected in Onagawa Bay in November 2011 and in Sasuhama in February 2012. The larval morphology of S. aff. uschakowi was different from that of S. uschakowi in the following aspects: prostomium of the former is broad and slightly notched anteriorly, whereas that of the latter is relatively small and anteriorly rounded; the peristomium of the former is well demarcated from the prostomium, but that of the latter is relatively less demarcated; nototrochs of the former occur from chaetiger II onwards, whereas those of the latter occur from chaetiger IV onwards; pigmentation inside the pharynx is black in the former but brown in the latter; and black pigmentation in the proctodaeum is present in the former but absent in the latter. Black pigmentation in the pharynx and proctodaeum were also reported in the larvae of S. bombyx (Hannerz 1956), S. cf. bombyx (Blake 2006), and S. duplex (Chamberlin, 1919) (Blake 2006). However, the illustrations of S. cf. bombyx provided by Blake (2006: fig. 13.10C, D) are seemingly more similar to larvae of Rhynchospio than to those of Spiophanes.

Overall shape thick or slender and fusiform. Prostomium small or broad and rounded anteriorly. Three pairs of black eyes present, most lateral often double-eyes. Dorsal pigment pattern consists of single row of branching melanophores in most species, some species lack distinct dorsal melanophore. Lateral pigments present or absent. Ventral pigments absent. Nototrochs occur in all chaetigers except first two chaetigers. Gastrotrochs occur in irregular pattern. Modified chaetae develop in chaetiger V in late larvae (Söderström 1920, as Polydora natrix; Hartman 1941; Carrasco 1976; Woodwick 1977; Blake and Kudenov 1981; Duchêne 1984, 1989; Guérin 1991; Gibson 1997; Blake and Arnofsky 1999; Gibson and Smith 2004; Blake 2006; Kamel et al. 2010; Oyarzun and Brante 2015; Blake 2017).

Slender and fusiform in overall shape, widest in middle of body. Prostomium rounded and slightly notched anteriorly. Three pairs of eyes present, most median pair rounded, lateral pairs double‐eyes. Body entirely faint green in color. A prominent row of dorsal melanophores occurs medially from chaetiger III, lateral black pigment spots present on chaetigers VII and VIII in late larvae (Fig. 7B). Pygidium has dorsal gap, pigmented with weak dark color. Internally, vestibule light brown, gut either yellow or brown. Gastrotrochs on chaetigers V and VII.

Figure 7. 

Light micrographs showing the morphologies of living spionid larvae of genera Boccardia, Boccardiella, and Dipolydora A, B Boccardia proboscidea, dorsal view of accidentally hatched 9‐chaetiger (A) and 15‐chaetiger larvae (B) C Boccardia pseudonatrix, dorsal (left) and ventral (right) view of accidentally hatched 10‐chaetiger larvae D–F Boccardia sp. 1, dorsal view of 9‐chaetiger (D) and dorsal (E) and ventral view (F) of 17‐chaetiger larvae G Boccardia sp. 2, dorsal view of 15‐chaetiger larva H, I Boccardiella hamata, dorsal view of 16‐chaetiger (H) and 18‐chaetiger larvae (I) J, K Dipolydora bidentata, dorsal view of 13‐chaetiger (J) and 18‐chaetiger larvae (K) L Dipolydora cf. commensalis, dorsal view of 21‐chaetiger larva M, N Dipolydora giardi, dorsal view of 19‐chaetiger (M) and lateral view of 21‐chaetiger larvae (N) O Dipolydora cf. socialis, dorsal view of 18‐chaetiger larva P Dipolydora sp., dorsal view of 7‐chaetiger larva. Scale bars: 300 μm.

Adults of this species were non‐boring and collected from mud deposits in crevices of shells of living Crassostrea gigas (Thunberg, 1793) (recently assigned to Magallana: see Backeljau 2018) oysters in Sasuhama in May 2011 and February 2016. Adult morphology agrees with the description of B. proboscidea by Sato‐Okoshi (2000). The 18S and 16S rRNA gene sequences obtained in the present study match (18S: 1748/1748, 16S: 435/435 bp) that of B. proboscidea from USA (KJ546254) reported by Radashevsky et al. (2014) (Fig. 3). Therefore, this species was referred to B. proboscidea. The larvae and adults were confirmed to match (18S: 1768/1768, 16S: 472/472 bp) using molecular data (Fig. 2).

Planktonic larvae of this species were rare, and only one 15‐chaetiger larva (Fig. 7B) was collected in Sasuhama in May 2011. Another 9‐chaetiger larvae, which accidentally hatched from its egg capsule in an adult tube during the process of extraction of the adult specimens (Fig. 7A), was also collected on the same date. Boccardia proboscidea has been reported to have poecilogonous development (Gibson 1997; Oyarzun et al. 2011). However, Sato-Okoshi (2000) reported that Japanese populations only show lecithotrophic development, with no (or a very short) planktonic stage after hatching. The larval morphology of this species agrees with the description of that of B. proboscidea documented in Hartman (1941), Woodwick (1977), Blake and Kudenov (1981), Gibson (1997), Gibson and Smith (2004), Kamel et al. (2010), and Oyarzun and Brante (2015). The dorsal pigment pattern of these larvae resembles that of the larvae of B. tricuspa (Hartman, 1939) described by Carrasco (1976, as B. proboscidea; fide Blake and Kudenov 1978), B. natrix (Söderström, 1920) described by Söderström (1920, as Polydora natrix), and B. columbiana Berkeley, 1927 described by Blake and Arnofsky (1999) and Blake (2006) in having a single row of mid‐dorsal melanophores. However, the dorsal pigment pattern of the larvae of B. tricuspa differs from that of B. proboscidea in having branching dorsal melanophores on chaetiger I and in lacking small black lateral pigment spots on chaetigers VII and VIII. The larvae of B. natrix also lack small black lateral pigment spots on chaetigers VII and VIII. Boccardia columbiana has extensively branching mid‐dorsal melanophores from chaetiger II onward, whereas mid‐dorsal melanophores are less branching and start from chaetiger III in B. proboscidea.

Slender and fusiform in overall body shape. Prostomium rounded with a slight anterior notch. Three pairs of eyes present, most median pair rounded and lateral pairs double‐eyes. Body entirely faint yellow. Dorsal melanophore absent, slight black pigmentation present (Fig. 7C, left). Pygidium with dorsal gap, pigmented with yellow. Internally, the vestibule and pharynx brown or black, gut green in color. Gastrotrochs on chaetigers V and VII.

Adults of this species were non‐boring and collected from mud deposits in crevices of shells of living C. gigas oysters in Tomiura. Adult morphology (see Abe et al. 2019b) agrees with the descriptions of B. pseudonatrix from South Africa (Day 1967; Simon et al. 2010) and Australia (Sato‐Okoshi et al. 2008, as B. knoxi; see Walker 2013). The 18S rRNA gene sequences obtained in the present study completely match (1714/1714 bp) that of B. pseudonatrix from South Africa (KY677895) reported by Williams et al. (2017) (Fig. 3). Therefore, this species was referred to B. pseudonatrix.

Boccardia pseudonatrix has been reported to have adelphophagic larvae with a short or absent planktonic phase (Sato‐Okoshi et al. 2008; Simon 2015). The larvae herein reported accidentally hatched from egg capsules in an adult tube during the process of extraction of the adult specimens.

Slender and slightly fusiform in overall shape, widest in anterior part of body. Prostomium rounded anteriorly. Three pairs of black eyes present, most median pair rounded and lateral pairs double‐eyes. Body entirely yellowish in color. A prominent row of dorsal melanophores occurs medially from chaetiger IV, lateral black pigment spots present from chaetiger V onwards (Fig. 7E). Pygidium with dorsal gap, pigmented with black color. Vestibule black, gut orange in color internally. Larval chaetae on first chaetiger long especially in early larvae (Fig. 7D). Gastrotrochs on chaetigers III, V, VII, X, and XIII.

No adult individuals of this species were collected in the present study. The 18S and 16S rRNA gene sequences obtained from the larvae did not match any available Boccardia sequences, but this species is very similar to the other Boccardia species in larval morphology and gene sequences (Figs 2, 3). Therefore, this species was referred to Boccardia sp. 1.

Planktonic larvae of this species were collected from Onagawa Bay in April, May, November, and December 2011, and January, February, March, and May 2012. The overall body shape of these larvae is slender and slightly fusiform, similar to those of B. proboscidea. However, other larval morphological characteristics differ between these two species: overall body color is faint yellow in the former species and faint green in the latter; the larval chaetae are longer in the former species than in the latter, especially in early larvae (Fig. 7D); lateral black spots are present on chaetiger V onwards in the former species but only on chaetigers VII and VIII in the latter species.

Thick and fusiform in overall shape, widest at middle part of body. Prostomium extensively broad and anteriorly rounded. Three pairs of black eyes present, most median pair rounded and lateral pairs double‐eyes. Body entirely faint green in color in late larvae. A prominent row of dorsal ramified melanophores occurs medially from chaetiger IV onwards, lateral black pigment spots absent. Pygidium with dorsal gap, pigmented with weak dark color. Internally, vestibule black, gut orange in color. Gastrotrochs on chaetigers III, V, VII, X, and XIII.

No adult individuals of this species were collected in the present study. The 18S and 16S rRNA gene sequences herein obtained from the larvae did not match any of the available Boccardia sequences, but this species is similar to the other Boccardia species in larval morphology and gene sequences (Figs 2, 3); therefore, this species was referred to Boccardia sp. 2.

Planktonic larvae of this species were collected from Onagawa Bay in December 2010 and November and December 2011, from Sasuhama in January 2013, and from Sendai Port in December 2010. The larval morphology of this species differs from that of other Boccardia larvae in having a thick and fusiform body shape.

Overall shape thick and fusiform. Prostomium extensively broad and rounded anteriorly. Three pairs of black eyes present, most lateral pairs usually double‐eyes. More than two pairs of dorsal melanophores from chaetiger III onwards. Lateral and ventral pigments present. Nototrochs occur in all chaetigers except first two. Gastrotrochs occur in irregular pattern. Modified chaetae develop in chaetiger V in late larvae (Rullier 1960, as Polydora redeki; Dean and Blake 1966, as Boccardia).

Thick and fusiform in overall shape, widest at middle part of body. Prostomium broad and anteriorly rounded, usually dusky brown anteriorly. Three pairs of black eyes present, most median pair rounded, lateral pairs usually double‐eyes, occasionally divided into respective eyes. Black pigmentation usually presents ventrally on each lateral lip, occasionally absent. Dorsal pigmentation basically consists of a pair of medial bands, lateral branching melanophores, and small pigment patch at the base of notopodia in each chaetiger from chaetiger III onwards (Fig. 7H). These melanophores undergo expansion and contraction, sometimes coalescing to cover almost the whole of the dorsal surface as ramified pigmentation (Fig. 7I). Four transverse lines of black pigmentation sometimes fused as a single transverse band in chaetiger I. One or two pairs of lateral black pigmentation on chaetiger II. Two rows of band‐shaped ventral pigmentation usually located on posterior edges of some chaetigers posterior to second chaetiger. A pair of black pigment patches on pygidium. Gastrotrochs on chaetigers III, V, VII, X, and XIII.

Adults of this species were non‐boring and collected from mud deposits in crevices of shells of living C. gigas oysters in Sasuhama in May 2011 and February 2016. Adult morphology agrees with the description of B. hamata by Sato‐Okoshi (2000). Therefore, this species was referred to B. hamata. The larvae and adults were confirmed to match (18S: 1772/1772, 16S: 480/481 bp) using molecular data (Fig. 2).

Planktonic larvae of this species were frequently collected from Onagawa Bay, Gobu‐ura, and Sasuhama in July and August. The larval morphology of this species agrees with that of B. hamata described by Dean and Blake (1966, as Boccardia).

Overall shape slender or slightly fusiform. Prostomium small rounded anteriorly. Three pairs of black eyes present, most lateral pairs often double‐eyes. Ramified melanophore between central and lateral pairs of eyes usually absent, but present in some species (e.g., D. cf. commensalis). Dorsal pigment pattern consists of two rows of band or spot shaped melanophores or a transverse row of small melanophores at each chaetiger in most species, while some species have single row of branching mid‐dorsal melanophores (e.g., D. cf. commensalis) or completely lack melanophores (e.g., D. armata). Lateral and ventral pigments are present or absent. Nototrochs occur in all chaetigers except the first two chaetigers. Gastrotrochs occur in irregular pattern. Modified chaetae develop in chaetiger V in late larvae (Andrews 1891, as Polydora; Hannerz 1956, as Polydora; Hatfield 1965, as Polydora; Blake 1969, as Polydora; Carrasco 1976, as Polydora; Day and Blake 1979, as Polydora; Radashevsky 1989, as Polydora; Plate and Husemann 1994, as Polydora; Lewis 1998; Blake 2006; Blake 2017).

Overall shape elongated. Prostomium and pygidium small. Three pairs of black eyes present, most lateral pairs double‐eyes. Black pigmentation patches on lateral peristomium absent. Some patchy black pigment occurs between head and first chaetiger. Two dorsal black bands begin on chaetiger II and continue to posterior end. Dorso‐lateral pigment extend posteriorly along lateral side found on most chaetigers. Some black or brown pigment may occur on pygidium. Ventral pigment absent. Gastrotrochs on chaetigers V, VII, X, XIII, and XV.

Adults of this species are shell-borers and were collected from shells of wild C. gigas oysters in Sasuhama in July 2012. Adult morphology agrees with the description of D. bidentata by Sato‐Okoshi (1999). The 18S and 16S rRNA gene sequences obtained in the present study match (18S: 900/900, 16S: 473/475 bp) that of D. bidentata from Russia (JX228065) reported by Radashevsky and Pankova (2013) (Fig. 3). Planktonic larvae of this species were collected from Onagawa Bay in November 2011 and from Sasuhama in February 2012. The larvae and adults were confirmed to 100% match using molecular data (Fig. 2).

Overall shape elongated and slender. Prostomium small but wider than body and rounded anteriorly. Three pairs of eyes present, most lateral pairs double‐eyes of kidney‐shaped appearance. Ramified melanophores present around eyes. Black pigmentation on lateral peristomium absent. Median row of ramified melanophores from chaetiger I onwards. Lateral and ventral pigments absent. A central black pigment spot and a pair of dark brown pigments on pygidium. Pygidium has a dorsal notch and lacks appendages. Gastrotrochs on chaetigers III, V, VII, X, XIII, XV, XVII, XIX, XXI, and XXIII. Modified chaetae develop in chaetiger V in late larvae.

No adults of this species were collected in the present study. The 18S and 16S rRNA gene sequences obtained from the larvae of this species neither match nor constitute a monophyletic clade with any of the other available spionid sequences (Figs 2, 3). However, this species was tentatively identified as D. cf. commensalis based on its larval morphology, as it includes the characteristic dorsal pigment pattern of larvae of D. commensalis as described by Andrews (1891), Hatfield (1965), Blake (1969), and Radashevsky (1989) (described as Polydora commensalis by all of these authors). The combination of a slender body and a single dorsal median row of distinct melanophores from chaetiger I to the end of the body is distinctive among spionid larvae and has not been reported for any other spionid species. Currently, there are no records of D. commensalis from Japan; however, the presence of this species in Japan is expected as it has been reported from the Asian continental coast of the Sea of Japan and the Kurile Islands (Radashevsky 1993). This species is an obligate symbiont of hermit crabs (Blake 1996; Williams and McDermott 1997), but little effort was devoted to collecting hermit crab shells in the present study.

Notably, the results of the phylogenetic analysis in the present study showed that D. cf. commensalis deviates from the monophyletic clade constituted by many other Dipolydora species. This result supports the suggestion by Blake (1971) that D. commensalis may represent a distinct genus as its morphology deviates widely from other species of the genus Polydora and Dipolydora.

Only three individuals of planktonic larvae of this species were collected in Sasuhama in January 2013. A small patch of lateral black pigments on the anterior margin of each chaetiger in late larvae was described in Hatfield (1965) and Blake (1969). However, these pigments were not observed in the present study, as in Andrews (1891) and Radashevsky (1989). Although Hatfield (1965) and Blake (1966) noted the high similarity between the larval morphologies of D. commensalis and Polydora hermaphroditica Hannerz, 1956, the adult morphologies of these two species were reported to be completely different (Bhaud 1966). The dorsal pigment pattern of P. hermaphroditica larvae reported by Hannerz (1956) rather resembles those of Polydora glycymerica and Polydora cf. glymymerica larvae reported by Radashevsky (1989) and in the present study, respectively.

Overall body shape elongated and slender. Prostomium small and rounded anteriorly. Three pairs of black eyes arranged in transverse row, most lateral pairs double‐eyes. Black pigmentation on lateral peristomium absent. Two pairs of dorsal black spots begin on chaetiger III onwards and continue to posterior end, sometime medial pair in first 2–4 chaetigers band‐shaped. Small medial spot of black pigment on posterior margin of each chaetiger usually from chaetigers III, rarely from V or VI, in late larvae. Two small spots of black pigmentation occur lateral to the medial black pigmentation from approximately chaetiger VI or VII. A small black pigment spot, not visible dorsally, present on antero‐lateral edges from chaetiger II onwards. Black pigment occurs on pygidium. Rust‐colored pigment occurs in pharynx. Ventral pigment absent. Some metamorphosing larvae reduce pigmentation over the entire body and present whitish appearance with eyes fused and appears as one pair (Fig. 7N). Gastrotrochs on chaetigers III, V, VII, X, XIII, and XV.

Adults of this species are shell-borers and were collected from shells of cultured Mizuhopecten yessoensis (Jay, 1857) (formerly as Patinopecten yessoensis) scallops suspended in Onagawa Bay in December 2010. Adult morphology agrees with the description of D. giardi by Sato‐Okoshi (1999). Therefore, this species was referred to D. giardi. The larvae and adults were confirmed to match using molecular data (Fig. 3).

The 18S rRNA gene sequences of this species are very similar to that of D. capensis 1PE from South Africa (KY677896) reported by Williams et al. (2017), but there is a slight difference between their sequences (0.12% difference: 2/1714 bp). It is unclear whether this difference indicates that these two are the same or different species because two different 18S rRNA gene sequences have been reported from South Africa and are currently under the same species name (D. capensis) (Table 2, Fig. 3). No gene sequences of D. cf. giardi previously recorded from South Africa (Simon 2011) are available.

Planktonic larvae of this species were collected from Onagawa Bay in December 2010, June, July, October, November, and December 2011, and December 2012, and from Sasuhama in January 2013. The larval morphology of this species was previously described from California by Day and Blake (1979, as Polydora giardi). The morphology and dorsal pigment pattern of late larvae described by these authors resembles that reported here, but there are slight differences: two golden pigment spots present on either side of chaetiger I in the former description but absent in the latter; two small lateral melanophores present on chaetigers I and II in the former description but absent in the latter; a medial black pigmentation beginning from chaetiger II onwards in the former description but from chaetiger III onwards in the latter description; and two small spots of black pigmentation lateral to the medial black pigmentation starting from chaetiger III in the former description but from more posteriorly in the latter description. These differences between specimens from Japan and California may indicate that they are different species, or that intraspecific variation occurs in larval dorsal pigmentation. Day and Blake (1979) pointed out differences in reproductive traits between the Californian and French populations and suggested the existence of two different species. Therefore, more than one species may be included under the name of D. giardi, which currently is reported with a worldwide distribution (Radashevsky and Petersen 2005).

Late larvae usually thick and slightly fusiform in shape, although not as much as the larvae of Boccardia sp. 2 (Fig. 7G), Boccardiella hamata (Fig. 7H, I), and Pseudopolydora species (Fig. 9A–I). Anterior margin of prostomium has yellow‐brown pigment. Three pairs of black eyes arranged in transverse row, most lateral pairs double‐eyes. Band of black pigment on each lateral part of the peristomium. First dorsal black melanophores occur as paired bands on chaetiger III and continue through to chaetiger V. From chaetiger VI, two pairs of dorsal black spots or bands occur and continue to posterior end of body. From chaetiger IV or V and continuing posteriorly, clusters of small black pigmented cells present in transverse row on dorsal posterior half of chaetigers. Lateral pigment found on late larvae on chaetiger II. Each notopodial lobe tipped with orange pigment, small patch of black pigment at the base of notopodial lobes. Ventral pigment consists of paired bars on posterior border of chaetigers, commencing with chaetiger II. Some black or brown pigment may occur on pygidium. Gastrotrochs occur on chaetigers III, V, VII, X, XIII, XV and XVII.

Adults of this species were non‐boring and collected from muddy bottom sediment at 22 m depth in Onagawa Bay in December 2010 by using a Smith‐McIntyre grab sampler and from bottom sediments of shallow subtidal zone in Sasuhama in April 2013. Adult morphology agrees with the description of D. socialis by Sato‐Okoshi (2000). The 18S rRNA gene sequence obtained in the present study showed a 0.35% (6/1715 bp) difference with that of D. cf. socialis from South Africa (KY677899) reported by Williams et al. (2017), which may indicate that these two are different species. The 18S rRNA gene sequence obtained in the present study rather closer to that of D. carunculata (940/942 bp match) reported by Radashevsky and Pankova (2013), but the 16S gene sequence showed a 2.3% (11/475 bp) difference with that of D. carunculata. As described above, since the taxonomic status of the species reported here is uncertain, we tentatively referred to it as D. cf. socialis.

Planktonic larvae of this species were collected from Onagawa Bay in November 2010 and 2011, and in October 2012. The larvae and adults were confirmed to match (18S: 1770/1770, 16S: 473/475 bp) using molecular data (Fig. 2). The larval morphology of this species agrees with that of D. socialis described as Polydora socialis by Blake (1969) and Carrasco (1976).

Overall body shape slender. Prostomium small and rounded anteriorly. Anterior margin of prostomium has weak yellow‐brown pigment. Three pairs of black eyes present in transverse row, most lateral pairs double‐eyes. Black pigmentation on lateral peristomium absent. A pair of dorsal black spots present on chaetiger III onwards. A small medial spot of black pigment on posterior margin of chaetiger III. Some black pigment occurs on pygidium. Ventral pigment absent. Gastrotrochs occur on chaetigers III and V.

No adult individuals of this species were collected in the present study. The 18S and 16S rRNA gene sequences obtained from larvae in the present study did not match any available Dipolydora sequences. As the larvae specimens formed a monophyletic clade with the other Dipolydora species (excluding D. armata, D. capensis 1GG, D. cf. commensalis, and D. quadrilobata) with robust statistical supports (Figs 2, 3), this species was referred to Dipolydora sp.

Overall shape slender or slightly fusiform. Prostomium broad or small and rounded anteriorly. Three pairs of black eyes present, most lateral pairs often double‐eyes. Some species have ramified melanophore between central and lateral pairs of eyes. Dorsal pigmentation usually consists of two rows of bands, spots, or branching melanophores in most species, while some species have a single row of mid‐dorsal melanophores (e.g., Polydora cf. glycymerica). Lateral and ventral pigments present or absent. Nototrochs occur in all chaetigers except first two. Gastrotrochs occur in irregular pattern. Modified chaetae develop on chaetiger V in late larvae (Wilson 1928; Thorson 1946; Hannerz 1956; Hopkins 1958; Woodwick 1960; Blake 1969; Carrasco 1976; Radashevsky 1986, 1988, 1989; 1994, 2005; Plate and Husemann 1994; Sato‐Okoshi 1994; Williams 2001; Radashevsky and Cárdenas 2004; Blake 2006; Radashevsky et al. 2006; Zhang et al. 2009; Gao et al. 2011; David et al. 2014; Barros et al. 2017; Blake 2017; Radashevsky and Migotto 2017; Ye et al. 2017).

Overall shape slender and slightly fusiform. Prostomium broad and rounded anteriorly. Three pairs of black eyes present, innermost pair rounded, lateral pairs double‐eyes, ramified melanophore between innermost and lateral two pairs of eyes usually present. Black pigment on lateral peristomium absent. Dorsal pigmentation consists of two rows of melanophores from chaetiger III. Dorsal melanophores undergo expansion and contraction, may expand to branching melanophores or ramified appearance or covered almost whole of dorsal surface by very finely ramified black pigments (Fig. 8B), or they contract to dot‐like pigmentation patches (Fig. 8A). Lateral and ventral pigments absent. Vestibule and pharynx pigmented with black, gut pigmented with orange color. Modified chaetae develop on chaetiger V in late larvae. Gastrotrochs occur on chaetigers III, V, VII, X, XIII, XV, and XVII.

Figure 8. 

Light micrographs showing the morphologies of living spionid larvae of the genus Polydora A, B Polydora brevipalpa, dorsal view of 15‐chaetiger (A) and 17‐chaetiger larvae (B) C, D Polydora cornuta, dorsolateral view of 11‐chaetiger larva (C) and ventral view of 17‐chaetiger larva (D) E Polydora cf. glycymerica, dorsal view of 25‐chaetiger larva F Polydora hoplura, dorsal view of 15‐chaetiger larva G–I Polydora onagawaensis, dorsal view of 10‐chaetiger (G) and 18‐chaetiger larvae (H), and lateral view of 16‐chaetiger larva (I) J Polydora cf. spongicola, dorsal view of 17‐chaetiger larva K, L Polydora sp. 1, dorsal view of 7‐chaetiger (K) and 16‐chaetiger larvae (L) M Polydora sp. 2, dorsal view of 23‐chaetiger larva N–P Polydora sp. 3, dorsal view of 13‐chaetiger (N) and dorsal (O) and lateral view (P) of 18‐chaetiger larvae. Scale bars: 300 μm.

Adults of this species are boring and were collected from shells of cultured M. yessoensis scallops suspended in Onagawa Bay in February 2011 and Mutsu Bay in October 2011. This species was identified as P. brevipalpa as adult morphology agrees with the descriptions by Sato‐Okoshi (1999) and Sato‐Okoshi and Abe (2012). The larvae and adults were confirmed to 100% match using molecular data (Fig. 2).

Planktonic larvae of this species were collected from Onagawa Bay in April, May, and July 2011 and from Sasuhama in April 2011. The pair of large ramified or dot‐like melanophores from chaetiger III distinguishes larvae of this species from those of other Polydora species. Blake (2017) reported similar dorsal pigment patterns in the larvae of Polydora spongicola Berkeley & Berkeley, 1950. However, the larvae of P. spongicola in Blake (2017) differ from those of P. brevipalpa in having dorsal melanophores from chaetiger II instead of chaetiger III, dark green colored intestine instead of orange, and non‐pigmented pharynx instead of pigmented with black. Reproduction and life history of this species was reported in Sato‐Okoshi et al. (1990) and Sato‐Okoshi (1994) (both as P. variegata).

Overall shape slender. Prostomium broad and rounded anteriorly. Three pairs of black eyes present, median pair rounded, most lateral pairs double‐eyes, ramified melanophores between first median and the second lateral pair of eyes usually present. In late larval stage, anterior part of prostomium and lateral lips of peristomium pigmented yellow or brown. Small spots of black pigments occur on lateral parts of peristomium. Dorsal pigmentation consists of two rows of melanophores from chaetiger III with those of anterior four chaetigers band‐shaped and then replaced by rounded or ramified melanophores from chaetiger VII onwards. Three rows of small faint dorsal spots of brown pigment present on posterior edge from chaetigers III or IV onwards in late larvae. Lateral pigment on chaetigers II, III, and often VI–XI extensive compared to that on other chaetigers. Large yellow or brown chromatophores occur ventrally from chaetigers V or VI onwards, usually three chromatophores arranged in transverse line except on gastrotroch‐bearing chaetigers where single midventral chromatophores present. Black pigment spots occur on ventral side of body (Fig. 8D) and mid‐dorsal part on pygidium (Fig. 8C). Gastrotrochs occur on chaetigers III, V, VII, X, XIII, XV and XVII.

Adults of this species were non‐boring and collected from mud deposits in crevices of shells of living C. gigas oysters in Sasuhama in June 2011 and from intertidal bottom sediment in Gamo Lagoon in August 2012. This species was identified as P. cornuta as adult morphology agrees with the description by Sato‐Okoshi (2000) and Radashevsky (2005). The larvae and adults were confirmed to match (18S: 1770/1770, 16S: 468/470 bp) using molecular data (Fig. 2).

Rice et al. (2008) suggested that at least three sibling species may be involved in North America under the name of P. cornuta by differences of mitochondrial COI sequences between California, Florida, and Maine populations. Takata et al. (2011) reported that the P. cornuta from Fukuyama in the Seto Inland Sea, western Japan is genetically close with the California/New Zealand lineage. It is unclear to which lineage the eastern Japan populations belong. The 18S rRNA gene sequence obtained in the present study showed a 1.9% (5/421 bp) difference with that of P. cornuta from Netherlands (KC686637).