Dipolydora cf. commensalis (Andrews, 1891)

Abe, Hirokazu & Sato-Okoshi, Waka, 2021, Molecular identification and larval morphology of spionid polychaetes (Annelida, Spionidae) from northeastern Japan, ZooKeys 1015, pp. 1-86 : 1

Dipolydora cf. commensalis (Andrews, 1891) Fig. 7L

Larval morphology.

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.

Remarks.

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.

Figures

Figure 2. Maximum Likelihood tree inferred from nuclear 18 S and mitochondrial 16 S rRNA gene sequences of spionids obtained from Japan in the present and previous studies (provided in Table 1). The gene sequences of adult and larval spionid polychaetes are indicated by solid squares and circles in front of each species name, respectively. SH-aLRT / approximate Bayes support / ultrafast bootstrap support values of ≥ 80 %, ≥ 0.95, ≥ 95 %, respectively are given beside the respective nodes. Nodes with red circles indicate triple high support values of SH-aLRT ≥ 80, approximate Bayes support ≥ 0.95, and ultrafast bootstrap support ≥ 95. The scale bar represents the number of substitutions per site. Sequences of Laonome sp. and Sabella pavonina Savigny, 1822 obtained from DDBJ / EMBL / GenBank were used for outgroup rooting.

Figure 3. Maximum Likelihood tree inferred from nuclear 18 S and mitochondrial 16 S rRNA gene sequences of spionid polychaetes obtained from Japan in the present and previous studies (shown in Table 1) and from DDBJ / EMBL / GenBank (shown in Table 2). The tree is divided into two parts A, B. The gene sequences obtained in the present study are highlighted by bold and red color and the adult and larval sequences are indicated by solid squares and circles in front of each species name, respectively. SH-aLRT / approximate Bayes support / ultrafast bootstrap support values of ≥ 80 % / ≥ 0.95 / ≥ 95 %, respectively are given beside the respective nodes. Nodes with red circles indicate triple high support values of SH-aLRT ≥ 80, approximate Bayes support ≥ 0.95, and ultrafast bootstrap support ≥ 95. The scale bar represents the number of substitutions per site. Sequences of Laonome sp. and Sabella pavonina Savigny, 1822 obtained from GenBank were used for outgroup rooting.

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.