Opisthobranch of the Week Data
Although I've collected 10 individuals during four separate dives, I consider Pleurobranchus mamillatus to be rare on Okinawa. I found the first individual crawling on the top of a patch of open sand during a daytime dive (the featured 145mm specimen collected on 18 February, 1990). The next individuals were seen on the 14th (eight animals) and the 15th (six animals) of March of the same year. There were scores of egg masses seen in the general depth area and some of these notaspids were observed to have wounds which appeared to have been predator(?) inflicted.
All of these animals were seen at one specific dive site, Seragaki; one specimen was found on open sand, all others, including their ova, were found amid mixed sand and strewn coral rubble. The animals were all found within an approximate spread of perhaps fifty meters and a 120 ~ 160ft (36.6 ~ 48.8m) depth range. There was relatively little consistency in color of these attractive animals. The observed colors were a very light gray, mottled gray, light tan, dark brown, and black; many of the individuals had prominent dorsal projecting papillae and all had a consistently brightly colored lunate pattern on the notum.
Apparently these animals are quite ephemeral in nature in as much as I dive this site on a regular basis without seeing many (only during February and March of 1990 and March and April of 1994).
There are to date the following six described species of Pleurobranchus known from Okinawan waters, as well as several undescribed species.