Opisthobranch of the Week Data
Okenia japonica is considered to be rare on Okinawa; thirteen specimens have been collected between 22 April, 1989 and 11 April, 1997. All of these specimens have been collected from the same general site in approximately the same environment, ranging between 52 and 61m. In all cases the animals were found in similar-appearing colonies of the arborescent ctenostome bryozoan (Amathia sp.). These bryozoan colonies were found in shallow depressions of mixed sand, and coral/shell rubble, in areas frequently experiencing high bottom currents. In several instances there were two or three of these attractive little goniodorids found in the same bryozoan colony. It should be noted that a second Okenia species, O. purpureolineata, is also occasionally found in the same bryozoan colonies along with that of O. japonica.
Baba (1949) described the species from five specimens collected in 1939 from Sagami Bay (one from 17m and four from "shallow water"). The Okinawan material closely fits Baba's original species description, including the presence of a single mid-dorsal papilla and the anterior median plume of the gills being sometimes bifurcated. I've included a second image of a 10mm specimen simultaneously collected from the same bryozoan as the featured animal above. This second animal has ova of a parasitic copepod attached just posterior to the gills.