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This is most likely a very dark colour form of Nembrotha chamberlaini Gosliner & Behrens, 1997.
More informations on Nembrotha cf. chamberlaini as Nembrotha sp. 8 are at Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2017), published 3 October 2017, Nembrotha cf. chamberlaini Gosliner & Behrens, 1997
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Euctenidiacea/Nembrotha_chamberlaini_cf.htm