Ophryotrocha costlowi

Description. Length of most live adults 2 – 3 mm (12 – 14 chaetigers), maximum length 4 mm (18 chaetigers). Live animals (Fig. 1 A) 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 eyes 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 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 five in males and females. Mandibles with elongate shafts and bifid cutting plates with 23 – 26 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, K-forceps right bidentate, left falcate. Reproduction and development. Gonochoristic; chromosomes 2 n = 6; diameter of eggs 125 µm; tubular egg masses; released larvae without parapodia, with long pygidial median stylus.

Remarks. Åkesson (1978) reported the discovery of Ophryotrocha costlowi, sp. nov. from Beaufort and Morehead City, North Carolina, and the aquaria at the Bermuda Biological Station. However, the name was invalid as he did not designate type specimens, which is herewith rectified. The new species was originally identified through crossbreeding experiments with other members of the O. labronica group (Åkesson 1978; Åkesson & Paxton 2005) and has been confirmed by gene sequence studies (Dahlgren et al., 2001; Wiklund et al. 2009). According to the crossbreeding experiments it is most closely related to O. labronica, from which it can be distiguished only by the following small differences: rosette glands in O. costlowi number up to five in both sexes, while in O. labronica they are less numerous in females, reaching a maximum of three. The small teeth at the margin of the mandibular plate range from 23 – 26 in the former and 25 – 28 in the latter. See Table 1 for comparisons with other species.

Distribution. North Atlantic: Morehead City and Beaufort, North Carolina, USA; Bermuda.

Etymology. This species is named in honour of Dr. John D. Costlow Jr., former Director of Duke University Marine Laboratory, who facilitated the work of B. Å. in all possible ways during his stay at the laboratory.

Material examined. Type material: Holotype (AM W 36856), complete female specimen, 2.9 mm long, 0.35 mm wide without parapodia (preserved) for 15 chaetigers; allotype (AM W 36867), complete male specimen, 1.7 mm long, 0.25 mm wide without parapodia (preserved) for 12 chaetigers; 10 paratypes (AM W 36868); 10 paratypes (SMNH T- 8028); cultured from specimens collected at Pivers Island, near Duke University Marine Laboratory, Beaufort, North Carolina, USA, in 1974. Other material: Live cultures from same collection.