Figure 1 D; 3 A, B; 5 B; Table 1
Ophryotrocha robusta nom. nud. Åkesson, 1975: 378; Pleijel & Eide 1996; Dahlgren et al. 2001; Simonini 2002; Heggøy et al. 2007; Simonini et al. 2009; Wiklund et al. 2009.
Material examined. Type material: Holotype (AM W 36878), complete female specimen, 4.4 mm long, 0.60 mm wide without parapodia (preserved) for 22 chaetigers; allotype (AM W 36879) complete male specimen, 2.3 mm long, 0.35 mm wide without parapodia (preserved) for 15 chaetigers; 10 paratypes (AM W 36880); 10 paratypes (SMNH T- 8032); cultured from specimens collected at Malaga, Spain in 1978. Other material: Live cultures from same collection.
Description. Length of most live adults 3–4 mm (14–16 chaetigers), maximum length 6 mm (22 chaetigers). Live animals (Fig. 1 D) 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 (Fig. 3 A, B).
Parapodia uniramous, lacking dorsal and ventral cirri, with dorsal protrusion, with retractile ventral lobe; 2–3 supra-acicular simple chaetae, 2–4 subacicular heterogomph falcigers and inferiormost simple chaeta; distal part of simple chaetae and blades of falcigers coarsely 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 seven in males and females.
Mandibles with elongate shafts and bifid cutting plates with 22–25 tiny pointed teeth at anterior edge. Maxillary apparatus of P- and K-type in both sexes, with falcate P 1 -forceps, bidentate P 2 -forceps (Fig. 5 B), K-forceps right bidentate, left falcate.
Reproduction and development. Gonochoristic; chromosomes 2 n = 10. Diameter of eggs 120-130 µm; tubular egg masses; released larvae without parapodia, with long pygidial median stylus.
Etymology. The name of the new species is derived from the fact that sexually mature females of O. robusta are bigger than females of other species, e.g. O. macrovifera, with the same segment number.
Remarks. The new species was originally identified through crossbreeding experiments (Åkesson 1975) and has been confirmed by gene sequence studies (Dahlgren et al. 2007; Heggøy et al. 2007; Wiklund et al. 2009). Of the four O. labronica group species with separate eyes, O. robusta is the only one with small eggs producing larvae without parapodia when released from the egg mass. It is further characterised by having a diploid chromosome complement of 10, a characteristic it shares with only O. rubra in the group.
Distribution. Mediterranean and Strait of Gibraltar: Malaga, Ceuta, Tarifa, Spain; Genoa, Sicily, Italy.