Thuridilla bayeri & T. gracilis

September 28, 2000
From: C. Carlson & P.J. Hoff

UPPER RIGHT: th04 - Thuridilla bayeri colour form, 16mm; Bile Bay, Guam, 5m; 11 October 1975. th15 - Thuridilla gracilis colour form, 18mm; Cetti Bay, Guam, reef flat; 30 November 1985.
LOWER LEFT: th04 - open parapodia of Thuridilla bayeri colour form, 14mm; Guam, Apra Harbor, Sponge Mound, 21m, 5 July 1988.
LOWER RIGHT: th15 - Thuridilla gracilis colour form, 20mm; Mactan, Cebu, Philippines. depth?; 29 April 1997.

Dear Bill,
Concerning the question of whether Thuridilla bayeri and Thuridilla gracilis are distinct species. We have tended to keep them separate, based upon the blue spotting along the sides of 'T. bayeri' as well as the coloring differences inside the the parapodia. The 'T. bayeri' form shown here is one of the few that we have seen that has torquiose spots on the side of the parapodia. Generally we find dark blue. This would be a good pair for some

Clay & Patty Jo

Carlson, C. & Hoff, P.J., 2000 (Sep 28) Thuridilla bayeri & T. gracilis. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Clay & Patty Jo,
Thanks for the photos. There are two questions which need resolving.
• Are the two distinct species?
• Is Risbec's description of T. gracilis sufficient for us to identify it?

Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Sep 28). Comment on Thuridilla bayeri & T. gracilis by C. Carlson & P.J. Hoff. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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