Polydora cornuta Bosc, 1802

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

Polydora cornuta Bosc, 1802 Fig. 8C, D

Larval morphology.

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.

Remarks.

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).

Planktonic larvae of this species were collected from Gamo Lagoon in August 2012. The larval morphology of this species generally agrees with the descriptions of P. cornuta by Hannerz (1956, as P. ligni ), Blake (1969, as P. ligni ), Plate and Husemann (1994, as P. ligni ), and Radashevsky (2005). Peristomial melanophores, which were reported by Hannerz (1956) and Blake (1969) but not by Radashevsky (2005), and middorsal vesiculate melanophores, which were reported by Radashevsky (2005) but not described by Hannerz (1956) and Blake (1969), were both present in specimens of the present study. Ventral pigmentation pattern was consistent with the description by Blake (1969) and Radashevsky (2005) instead of Hannerz’s (1956) description. The larval dorsal pigmentation pattern, similar to that of P. cornuta , is typically found in many other Polydora species. This species can, however, be distinguished by the characteristic ventral yellow pigmentation pattern as the yellow pigment on the ventral side of the other Polydora species is diffusely scattered and does not appear regularly arranged when present ( Radashevsky 2005).

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 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.