Opisthobranch of the Week Data
Metaruncina setoensis is considered to be common on Okinawa, but quite seasonal. The first time I saw these small cephalaspids I thought they were small black acoel flatworms, which they closely resemble. Forty-eight specimens have been collected, all during the months of March and July. Others have been seen but, unfortunately, the dates were not recorded. Normally I wouldn't collect such a large series (44 specimens on the 23rd and 31st of march, 2000) of a particular species, but due to a request from Paula Mikkelsen at the American Museum of Natural History, reference specimens were collected for a growing opisthobranch collection to support molecular and anatomical studies. In several recent dives to the same locality (mid-May, 2000), no animals were found, in spite of an in-depth search of the same general collection site.
This small cephalaspid was originally described by Baba (1954) [as Runcina setoensis] from Seto, Kii, Japan; Baba's description was based upon 12 specimens, collected in the early 1950's (Jan. & Mar., 1951; Jan. & Apr., 1953) from "beneath the pebbles in the tide pools of high water level...". All observed Okinawan animals have been found crawling on the surface of silty-sand during daytime SCUBA excursions and the majority were from a depth of three meters. These small black, sometimes dark brown, cephalaspids crawl in a continuous flowing serpentine motion and the only other opisthobranch found here which superficially resembles M. setoensis is Gymnodoris nigricolor. Under field conditions, e.g., SCUBA, these two black animals are similar in appearance but the high profiled rhinophores and gills of G. nigricolor and the highly sinuous crawling activity of M. setoensis immediately allow the two to be separated from each other.