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This specimen was identified by Clay Carlson and Patty Jo Hoff as Atys cylindrica at the Sea Slug Forum, thanks!
Very similar is Aliculastrum debile (Pease, 1860) [Atys] which differs by a narrower shell posteriorly and was described and known for a long time only from Hawaii.
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.
Aliculastrum cylindricum is in IPN as Atys cylindricus, many websites of shell-collectors use the name Aliculastrum cylindricum, many websites and UW-guides use the name Atys cylindrica, A. cylindricus is because of the gender ending wrong.
in a recent (26 September 2011) email wrote Clay Carlson: "Aliculastrum is fine for the genus."
According to WoRMS is Aliculastrum cylindricum as of 20 August 2015 the accepted name.
This species was posted on 18 April 2003 at these websites as Atys cylindrica (Helbling, 1779) [Bulla], changed on 27 September 2011 into
Aliculastrum cylindricum (Helbling, 1779) [Bulla].
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 Aliculastrum cylindricum as Atys cylindrica are on Bill Rudman's Sea Slug Forum!
How to cite:
Köhler, E. (2021), published 27 February 2021, Aliculastrum cylindricum (Helbling, 1779)
available from https://www.Philippine-Sea-Slugs.com/Cephalaspidea/Aliculastrum_cylindricum.htm