Poecilochaetus sp.

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

Poecilochaetus sp. Fig. 4F

Larval morphology.

Overall shape long and slender. Two pairs of dark red eyes present. Metatrochophore larvae with 17 chaetigers have broadened trapezoidal prostomium with tactile cilia in anterior part, broad and low caruncle, provisional larval chaetae, and well developed prototrochs (Fig. 4F ). Body of metatrochophore larvae transparent, characterized by small pigment spot between parapodia from chaetiger II onwards. Pygidium without anal cirri. Incomplete anterior fragment of nectosoma larvae characterized by extremely long body, rounded prostomium, broad and low caruncle, reduced prototrochs, parapodia with digitiform dorsal postchaetal lobes, and rapid serpentine swimming behavior. The body of nectosoma larvae transparent and small pigment spot laterally on each side of chaetigers. Occipital antenna and pair of palps not observed.

Remarks.

Adult individuals of this species were not 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 of the Poecilochaetus sequences from DDBJ/EMBL/GenBank, this species was referred to Poecilochaetus sp. because specimens formed a monophyletic clade with the other Poecilochaetus species with robust statistical support (Fig. 3 ).

Planktonic larvae of this species were collected in Onagawa Bay in August 2010 and January 2013. The larval morphology of this species is similar to that of Poecilochaetus serpens Allen, 1904 described by Hannerz (1956) and Plate and Husemann (1994). However, the former species differs from the latter by not having yellow chromatophores on the “head” and pygidium as described by Plate and Husemann (1994). The pigmentation pattern of Poecilochaetus sp. larvae also differs from that of Poecilochaetus anterospinus , which has a pair of ventral green melanophores on the lateral side of each segment, beginning from chaetiger IV throughout ( Magalhães et al. 2015).

Figures

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 4. Light micrographs showing morphologies of living spionid larvae of Aonides, Laonice, Rhynchospio, Scolelepis, and Spiophanes A-C Aonides cf. oxycephala, dorsal view of early planktonic (A), 8 - chaetiger (B), and 18 - chaetiger larvae (C) D Laonice sp. 1, dorsal view of early planktonic larva E Laonice sp. 2, dorsal view of 12 - chaetiger larva F Poecilochaetus sp., dorsal view of 17 - chaetiger larva G, H Rhynchospio aff. asiatica, dorsal view of 6 - chaetiger (G) and 12 - chaetiger larvae (H) I Scolelepis cf. kudenovi, dorsal view of 7 - chaetiger larva J, K Scolelepis sp. 1, dorsal view of 17 - chaetiger (J) and 19 - chaetiger larvae (K) L Scolelepis sp. 2, dorsal view of 20 - chaetiger larva M, N Spiophanes uschakowi, dorsal (M) and lateral view (N) of 18 - chaetiger larvae O, P Spiophanes aff. uschakowi, dorsal view of 16 - chaetiger larva (O) and 27 - chaetiger juvenile (P). Scale bars: 300 μm.