This sea slug(Chromodoris annulata) is a very colorful dorid nudibranch (a shell-less marinegastropodmollusk in the family Chromodorididae) Chromodoris annulata is a large smooth pale-bodied nudibranch with many vivid yellow spots, though these may be absent in some individuals. It has one purple ring around the gills and another ring around the rhinophores. The mantle usually has a purple border.This nudibranch occurs in the Indian Ocean, from East Africa to Thailand. It may grow to 100mm in total length. See this previous post for another beautiful sea slug.