Pictures of Aldisa williamsi
Author: Dr. Bill Rudman
Quoted from Sea Slug Forum: The example I illustrate here are the phyllidiid dorid nudibranchs such as Phyllidiella pustulosa. The phyllidiids exude very noxious and toxic chemicals from their skin and appear to be avoided by most fish. They are often the only sea slugs which blatantly crawl out in the open during the day. Other nudibranchs, flatworms, and even a holothurian are known to mimic phyllidiid nudibranchs.