Reticulidia fungia from Bali

Dear Rainer,
This is Reticulidia fungia. At the time Neville's book was published it did not have a scientific name. 'Common names' in photo books like this are a bit of a misnomer. They are 'common' in the sense they use 'common' everyday English. They are certainly not 'common' in the sense they are commonly used for the species. In fact the common names were made up for that book. 'Common names' are not very helpful because they differ from book to book. I would encourage you to use scientific names - where they exist - because they are understood worldwide and so are the real 'common name' for each species.

Your animal is a particularly pale form of the species, the yellow being quite pale and the black region reduced to bands. Your lower photo, which I have included a close-up of alongside, is also very interesting because it shows a hole in the reddish sponge which looks like it could have been eaten out by the nudibranch. The family Phyllidiidae, to which Reticulidia belongs, have no teeth or hard parts to assist in feeding, the animals feeding by dissolving the sponge tissue externally and sucking the half-digested soup into their mouth. I suspect the hole you can see is a burrow or some other natural phenomenon, but the bare whitish tissue you can see appears to be damaged sponge tissue. An interesting observation, because we know so little about feeding in these animals, and nothing about feeding in this species.
Best wishes,
Bill Rudman