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Facelinidae Family

Family Facelinidae

Facelinids range in size from small to large and body shapes are very diverse. Facelinids have elongate, slender and deep bodies, often with a wide foot and a long tapering tail. The anterior corners of the foot can be developed into propodial tentacles – either short or extremely long filiform structures. The oral tentacles are usually long and tapering. Across the Facelinidae, the rhinophores have a diverse range of form, from simple, to annulate, to lamellate, or papillate. There is a diversity of form with the cerata as well. They can be long, or tubular, or tapering, or curved, or only curved apically, or coiled, or club-tipped. Their surface can be smooth, or papillate, or nodulose, or tuberculate. The arrangement of cerata also varies with rows, multiple rows or arches all being represented in different genera. Most facelinids feed on hydroids. All genera except Phyllodesmium and Favorinus have functional cnidosacs in the cerata tips for the storage of nematocysts. Members of the genus Phyllodesmium feed on soft corals and gorgonians, and have developed modified cerata shape to “farm” zooxanthellae obtained from their prey. Members of the genus Favorinus feed upon the eggs of other sea slugs. Species of the genus Pteraeolidia have extremely long and slender bodies with very numerous ceratal clusters. The Facelinidae are characterised by the position of the anus posterior to the inter-hepatic space amongst the cerata. It has been observed feeding upon hydroids, but is also capable of storing symbiotic zooxanthellae. There are no specific gill structures. They are usually fast moving and can be of an “aggressive” nature directing their cerata at a perceived threat.