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Species of the family Haminoeidae are herbivorous cephalaspideans.
The above specimen was identified by the photographer Gordon Tillen, the ID was confirmed by David W. Behrens and by Clay Carlson, thanks!
In a recent (26 September 2011) email wrote Clay Carlson: "I already have records of A. debilis from many areas in the Pacific as well as the Indian Ocean."
Very similar is Aliculastrum cylindricum (Helbling, 1779) [Bulla] which is a common Indo-West Pacific species, it differs by a wider shell posteriorly.
In a recent (26 September 2011) email wrote Clay Carlson: "The major criteria appears to be the slight posterior hook/extension of the last whorl of the shell."
Wikipedia lists more than 50 species of Atys, and there are 13 species of Atys in IPN, pages 22 - 25,
I really don't know how to distinguish them.
All species of the genera Aliculastrum and Atys are herbivorous, related to Haminoea and Phanerophthalmus. At present most species are tropical and have heavy well-calcified shells. In most cases the true foot is short and the shell extends a considerable distance beyond its posterior end. It is possible when more of the many tropical species are studied that they will be placed in other genera.
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426 pp.,
More information on Atys debilis are on Sea Slugs of Hawaii!
How to cite:
Köhler, E. (2016), published 21 June 2016, Atys debilis Pease, 1860
available from https://www.Philippine-Sea-Slugs.com/Cephalaspidea/Atys_debilis.htm