Opisthobranch of the Week Data
The above featured attractive little costasiellid, Costasiella sp. 2, was photographed and collected by Atsushi Ono from the waters of Zamami Island in the Kerama Islands, one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa Capitol, Naha. This featured animal was discovered by Atsushi on the 20th of July, 2000, in 4m of water while hand-fanning the surface of brown algae (Phaeophyceae) at the edge of a reef. I've not personally seen Costasiella sp. 2 in the waters of Okinawa's main island and Atsushi reports (pers. comm.) collecting two of three observed individuals and considers the animal to be very rare in the Kerama Islands.
There are a series of images concerning this animal posted on the Sea Slug Forum and there is a fair amount of confusion regarding the animal's taxonomic placement. Rudman (2000) mentions in the Sea Slug Forum the following:
Kathe Jensen (2000) additionally mentions in the Forum the following:This animal has similarities to an aeolid, a Janolus and a sacoglossan. .... The consensus seems to agree that it is a sacoglossan, and the close set eyes suggest a species of Costasiella. Points of importance are grooved rhinophores, the eyes close together, the 'melanin gland' (black spot behind the eyes and in front of the pericardial sac).
I am mostly inclined to agreeing with the Costasiella placement. I have one undescribed species collected in Guam and also in Phuket, Thailand, in which the eyes are also behind the rhinophores. That species is very darkly pigmented, and it looks like the eyes are in little white "goggles", which must mean that they are some distance apart (namely 2 white rings). That species also has orange glands at the bases of the cerata. .... It looks to me like your Costasiella sp. 2 has long stalks to the cerata with unbranched tubule of digestive gland, and that the dark lobules of the digestive gland (characteristic of other species of Costasiella as well) occur only in the outer part of the cerata. It certainly is an interesting little "feller", and I look forwards to hearing of anybody finding it on an identifiable alga (apparently nobody has found it on Avrainvillea, which is the most common food for this genus), and for somebody taking a look at its radula (if it is a Costasiella, the teeth should be small, narrow blade-shaped - and be of almost uniform size along the radular ribbon).
Previously I've featured several species of Costasiella on these pages and as of the current date there are several described and undescribed species of Costasiella, reported from Okinawan waters. The featured animals are as follows:
C. kuroshimae C. paweli C. usagi C. vegae C. sp. 1 C. sp. 2 (the above featured animal) C. sp. 3