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Pretty similar are:
Ceratosoma trilobatum which also has 3 lobes, but a mantle edge between the head and the lobes and
Ceratosoma tenue which has 5 mantle lobes.
More information on Ceratosoma gracillimum are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2017), published 15 February 2017, Ceratosoma gracillimum Semper in Bergh, 1876
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Euctenidiacea/Ceratosoma_gracillimum.htm