Opisthobranch of the Week Data
Fiona pinnata is considered to be very rare on Okinawa as I have seen and collected a total of only two individuals (Mar '87 and Jan '94) over a period of many years on Okinawa. In both instances the animals were found associated with the common floating hydroid, Velella velella ("By-the-wind Sailor"), found on the wrack line of a sandy beach during low tide conditions. These attractive aeolids are infrequently seen by divers due to the animal's habit of association with their prey, Velella (and perhaps other floating hydroids such as Physalia and Porpita), and miscellaneous additional floating materials, which include attached goose-neck barnacles (Lepas spp.) upon which F. pinnata is also known to feed.
These pelagic animals have a relatively unique reproductive strategy in which Dave Behrens comments on as follows [from Mike Miller's Slug Site]:
"Because of its pelagic existence the species is cosmopolitan, found in all northern seas. Unlike other nudibranchs this species is found only with its food resource. Its life cycle is quite interesting and far more problematic than other benthic species which can crawl around looking for more food. At the risk of violating the chicken before the egg dilemma, it literally feeds until the food is gone, lays eggs, eggs hatch, planktonic larvae ride the currents until the right food is located, and in the middle of the ocean who knows how long that can take, then metamorphose into a juvenile, feed, grow and repeat the cycle again. When found it is often is very large numbers."
There are two species of Okinawan aeolids which are superficially similar in appearance to F. pinnata, Aeolidiella indica, and the melanistic form of Phestilla melanobrachia. However, close examination quickly separates the three. Also, F. pinnata is apparently unique in as much as the trailing edge of the cerata have an undulating sail-like membrane (Gosliner, 1987: 120).