Ophryotrocha japonica Paxton & Åkesson, 2010, sp. nov.

Ophryotrocha japonica sp. nov.

Figure 1 B; Table 1

Ophryotrocha japonica nom. nud. Pleijel & Eide, 1996; Dahlgren et al. 2001; Simonini 2002; Åkesson & Paxton 2005; Heggøy et al. 2007; Simonini et al. 2009; Wiklund et al. 2009.

Material examined. Type material: Holotype (AM W36869), complete female specimen, 2.2 mm long, 0.30 mm wide without parapodia (preserved) for 16 chaetigers; allotype (AM W36870), complete male specimen, 2.4 mm long, 0.30 mm wide without parapodia (preserved) for 16 chaetigers; 10 paratypes (AM W36871); 10 paratypes (SMNH T- 8029); cultured from specimens collected in 1989 near Amakusa Marine Biological Laboratory in southern Japan. Other material: Live cultures from same collection.

Description. Length of most live adults 3–4 mm (15–18 chaetigers), maximum length 6 mm (28 chaetigers). Live animals (Fig. 1 B) translucent, preserved opaque white. Pigmentation consisting only of very small lateral red spots on some chaetigers. Prostomium anteriorly rounded, with pair of short, ovate antennae; palps absent; two distinct eyes, not medially connected. Two peristomial achaetous segment-like rings.

Parapodia uniramous, lacking dorsal and ventral cirri, with dorsal protrusion; with retractile ventral lobe; 2–3 supra-acicular simple chaetae, 3–4 subacicular heterogomph falcigers and inferiormost simple chaeta; distal part of simple chaetae and blades of falcigers finely serrated. Pair of pygidial cirri present, pygidial median stylus absent in adults. Rosette glands, one per segment, present mid-dorsally on posteriormost segments of mature animals, up to five in males and females.

Mandibles with elongate shafts and bifid cutting plates with 20–24 tiny pointed teeth at anterior edge. Maxillary apparatus of P- and K-type in both sexes, with falcate P1-forceps, bidentate P2-forceps, K-forceps right bidentate, left falcate.

Reproduction and development. Gonochoristic; chromosomes 2n = 6; diameter of eggs varies from 145–160 µm in different populations, released larvae with 2–3 chaetigers, with short pygidial median stylus.

Etymology. The new species was first discovered in Japan, hence the name.

Remarks. The new species was originally identified through crossbreeding experiments in 1989 and has been confirmed by gene sequence studies (Dahlgren et al. 2001; Wiklund et al. 2009). Only four species of the O. labronica group have eyes not medially connected. Two of these (O. robusta sp. nov. and O. rubra sp. nov.) differ from O. japonica in having 10 diploid chromosomes rather than 6, in addition to different jaw and reproductive characteristics (Table 1). Ophryotrocha olympica, nom. nud. has the same number of chromosomes as O. japonica, similar egg size and the released larvae have three chaetigers, but it differs in that the eggs are white in colour rather than yellow as in O. japonica.

Distribution. North Pacific: Japan and Southern California, USA; Mediterranean.