Ophryotrocha dimorphica

Diagnosis. Prostomium with short, ovate antennae, palps absent; two eyes; parapodia uniramous, lacking dorsal and ventral cirri, with dorsal protrusion; supra-acicular simple chaetae and subaciular falcigers; pygidium with two cirri, pygidial median stylus absent in adults; dorsal median rosette glands on posterior segments; anterior edge of mandibles with 17 - 22 teeth, maxillae with falcate P 1 - forceps, bidentate P 2 - forceps, K-forceps right bidentate, left falcate; diameter of eggs 140 µm; tubular egg masses; released larvae without parapodia, with long pygidial median stylus.

Remarks. This species was described as a protandrous hermaphrodite with strongly pronounced sexual dimorphism and asexual reproduction. The authors described the males as smaller than females, but with a wider prostomium and larger K-forceps. During the sex change from male to female the K-maxillae were stated to moult once more resulting in K-maxillae with the smaller K-forceps typical of females. From our experience, Ophryotrocha maxillae do not moult any more once the K-type is in place. As to the asexual reproduction, Zavarzina & Tzetlin (1986) described a restriction after chaetigers 5 – 7 and deterioration of the following segments resulting in fission. While the anterior part regenerated a new posterior end, the isolated posterior part remained alive for some weeks, but no regenerated heads were observed. The authors added that the frequency of fission increased with increased environmental stress. In our cultures we have recognised similar fissions in many species. It may occur if the animals are infected by intestinal or coelomic parasites or if the culture bowls are allowed to remain without adequate renewal of fresh water. The described fission is asexual, but it does not represent any reproduction as the number of individuals is the same before and after the process. On the basis of its morphology, O. dimorphica is a typical member of the gonochoristic O. labronica group. Most members of the group display some features of sexual dimorphism, especially difference in the size of the K-forceps. However, the difference in prostomial width of male and female O. dimorphica appears to be greater than in other species.

Distribution. Near Popov Island, Japan Sea.