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Pretty similar is Phyllodesmium poindimiei which differs by white spots on the body and on the cerata and lack the white median line.
The above specimen was identified by Dave Behrens, it lacks the white median line, I think it is Phyllodesmium poindimiei.
More information on Phyllodesmium serratum are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2016), published 29 December 2016, Phyllodesmium serratum Rudman, 1981
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Cladobranchia/Phyllodesmium_serratum.htm