West Bali, about 10 meters deep. Body is roughly 10 cm across. Does not move.
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theforestecologist
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Tom Demeyer
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4Is that "does not move " in the timeframe you allowed? Or based on a timeframe of months of continuous observation. – Solar Mike Dec 16 '18 at 06:55
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2Lack of movement suggests it's not a sea slug. Possibly a strange tunicate? – theforestecologist Jun 30 '19 at 18:49
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I am convinced this is a nudibranch. The features that indicate this are the rhinophores (antennae-like sensory structures at the "peaks"), the cerata or brachial plume (hard to tell which based on this photo, but these are translucent extensions just visible in the "valley" between the two "peaks", if you zoom in). A look here and here shows there is a huge diversity in morphology, but many nudibranchs have some form of these structures. Also, the shape of margins around the edge of the animal matches that of many nudibranch species. These are mobile animals but it is possible it just was not moving during your observation.
Ian Jones
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I would say that it is a sea hare of some type based upon its appearance and location above course sand. While it was not moving, that could just be a factor of when it was observed. I wouldn't expect a tunicate to have this appearance or be affixed to sandy sediment.
TJ Rowell
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1Welcome to Biology.SE. You are more likely to get a positive response to this answer if you explain why your identification is correct. Specifically, please [edit] your answer to add discussion of key features that led you to this conclusion and supporting references or at least validated images. Without this your answer is indistinguishable from opinion and is thus inappropriate for this site. ——— You may also wish to take the [tour] and then consult the [help] pages for additional advice on [Answer] and [Ask] effectively on this site. – tyersome May 04 '22 at 22:12