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Pretty similar are:
Phyllodesmium cf. opalescens which lacks the series of white marks down the midline.
Phyllodesmium undulatum which differs by a white median line, and by highly undulate digestive gland ducts,
Phyllodesmium acanthorhinum which lacks the white median marks, but has a network of white lines on the head and white specks on the notum, distinctive are the papillate rhinophores.
Many other species of the genus Phyllodesmium are what Bill Rudman calls "Solar-powered Sea Slugs", but Phyllodesmium opalescens is none of them.
More information on Phyllodesmium opalescens are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2016), published 25 December 2016, Phyllodesmium opalescens Rudman, 1991
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Cladobranchia/Phyllodesmium_opalescens.htm