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Comment from expert, Richard Willan: This is a prosobranch snail related to the Bean cowries (Triviidae). It has an internal shell (which lies under the skin beneath the large ‘hump’ in the centre of the animal, and it feeds on compound ascidians. So it is not an nudibranch and you will have to move this photo to the non-nudibranch pages of Nudi Pixel.