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It rapidly drops off many of its cerata when disturbed (autotomise). Like other members of the genus Phyllodesmium it is what Bill Rudman calls "Solar-powered Sea Slugs".
Pretty similar is:
Phyllodesmium parangatum which is a smaller species, up to 20 mm, and has more cylindrical cerata with white apices. It differs by its milky-white body colour, by the position of the anal papilla, and by the short rhinophores, and the long translucent white oral tentacles. Let me quote IPN: "There are also major internal differences in the form of the radula and jaws."
More information on Phyllodesmium magnum are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2016), published 25 December 2016, Phyllodesmium magnum Rudman, 1991
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Cladobranchia/Phyllodesmium_magnum.htm