Opisthobranch of the Week Data
Risbecia tryoni is considered to be common on Okinawa, but only at a single collection site. The great majority of observed individuals were seen beneath Tengan Pier amid silty-sand and strewn oyster shell rubble. Three of the collected individuals were from Horseshoe Cliffs (including the specimen from 170ft), the other two individuals were collected from Tengan Pier. Many additional individuals have been seen at the Tengan collecting site, but not collected. The animals are frequently found crawling in tandem, usually as pairs, but occasionally found as triads.
There are two species of Risbecia currently found in the waters of Okinawa's main island, R. godeffroyana and the above featured R. tryoni. Quite remarkably, the dorid most similar to R. tryoni is Chromodoris leopardus which is very similar in both morphology and pigmentation; the best field identification device for discriminating between these two similar appearing species is that in Chromodoris leopardus the dark notum spots are relatively incomplete and often quite irregular. Risbecia tryoni, on the other hand, has the notum spots appearing as relatively consistent solids. The body profile of Risbecia spp. is high while that of the similar Chromodoris leopardus is relatively low. It should be noted that both species are commonly found simultaneous at the same collecting site.