Rhynchospio aff. asiatica sensu Radashevsky et al., 2014

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

Rhynchospio aff. asiatica sensu Radashevsky et al., 2014 Figs 4G, H , 6

Larval morphology.

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.

Remarks.

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 (26th-39th 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).

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.

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 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.

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.