A new species of coral-feeding nudibranch (Mollusca: Gastropoda) from the Gulf of Thailand

The validity and position of the Indo Pacific genus Phestilla Bergh, 1874 (Fionoidea: Trinchesiidae) have been subject of some controversy as a result of several recent studies, which have sequentially synonymized or re-established it. At present, the genus includes eight valid species, the most of them described in the nineteenth and twentieth centuries, and all but one feeding on scleractinian corals of four genera: Porites Link, 1807 (Poritidae), Tubastraea Lesson, 1830, Dendrophyllia Blainville, 1830 (Dendrophylliidae) and Montipora Blainville, 1830 (Acroporidae). The discovery of an unknown Phestilla species feeding on Pavona explanulata (Lamarck, 1816) (Agariciidae) in Koh Tao, Thailand, motivates this work, in which this new species is described providing morphoanatomical and molecular characters. In addition, its systematic position and ecological significance as coral parasite are discussed.