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It is similar in shape to the smaller Flabellina bicolor, including the long oral tentacles with flattened tips. It differs by the colour pattern.
One more species with that kind of oral tentacles is Flabellina bilas.
In IPN is a picture of a specimen which lacks the left oral tentacle, and the right oral tentacle does not show the distinctive purple bands, which are also not mentioned in the text.
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp. at page 352
This picture shows 4 specimens ranging in size from 5 to 7 cm length.
More information on Flabellina riwo are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2016), published 3 November 2016, Flabellina riwo Gosliner & Willan, 1991
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Dexiarchia/Flabellina_riwo.htm