Costasiella usagi

Photo courtesy of Atsushi Ono
Kerama Islands, Okinawa, Japan

Costasiella usagi Ichikawa, 1993

I canít tell you how much I love seeing all the fabulous photos of sacoglossids found In Rie Nakanoís new book Opisthobranchs of the Islands of Japan and Atsushi Onoís companion book Opisthobranchs of Ryukyu Islands . Here is Costasiella usagi described by Ichikawa in 1993. It can be found on page 68 of Ono-sonís book. This strikingly colored species is one of over three dozen sacoglossid nudibranchs presented in the two books.

Looking very much like an aeolid nudibranch, members of the genus Costasiella have rolled rhinophores. Although quite variable in color, as indicated on Bill Rudmanís Sea Slug Forum , Ichikawa's description the color is described as: "The head and tail are not pigmented, and the rhinophores are black. The median cerata are grey, while the outer ones are black. All the cerata possess a white end and a black tip. Moreover, all the cerata bear longitudinal striations reaching from the base of each ceras to the white apical portion. ... The ventral side of the foot is white. .... Smaller specimens have the median cerata colored green, instead of grey."

The geographic range of this species seems to be throughout the tropical western pacific, including northern Australia, Guam and Japan. Specimens measure about 3-7 mm in length and are reported to feed on the algal species, Avrainvilla.

Costasiella usagi was also presented a couple of weeks ago on Bob Bolland's Okinawa Slug Site as the slug of the week!

Between this web site, Bob's Site, and Bill Rudmanís Sea Slug Forum and the many new high quality books we are seeing come out today, we are all very lucky.


Ichikawa, M. (1993) Saccoglossa Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory, 36: 119-139.

Dave Behrens
Danville, Calif
Oct., 2004

Webmaster, Atsushi Ono, and Jerry Allen
May 2002, Photo courtesy of Akiko Miyamoto

Atsushi Ono is the owner of a dive service on Zamami Island in the Keramas off Okinawa that caters mainly to Japanese Divers. Atsushi evidently relocated from a small town north of Tokyo some fifteen years ago to Zamami Island. He subseqeuntly started a diving service. In fact, the economy of Zamami Island seems to based entirely on diving! For those of you who don't speak Japanese that is not a problem communicating with Atusushi, his written english is very good! Atsushi has a passion that so many of us also share and that is the observation and study of sea slugs. Atsushi published a field guide "Opisthobranchs of Kerama Islands " a couple of years ago that excels in the macro photography of small Japanese slugs. The most recent book is even better. His books are a must for any of those considering travel to that area of the world. We spent many an evening with Atsushi back in 2002 sharing pictures and experiences with branches. His enthusiasm is very intense, one can feel it in both talking and diving with him! Atsushi has been a regular contributor to the Okinawa Slug Site maintained by Bob Bolland.

I feel so strongly about the Ono-san's dedication to the study of Opisthobranchs that he is going to receive my unqualified recommendation for anyone planning a branching trip to Okinawa. His operation is first class and can be totally focused on branching if you bring enough people.

You can drop Atsushi a note at

Taxonomic information courtesy of:

David W. Behrens

Author: Pacific Coast Nudibranchs
Co-Author Coral Reef Animals of the Indo Pacific
Proprietor of Sea Challengers Natural History Books

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