Stegastes diencaeus, the longfin damselfish, is a damselfish in the family Pomacentridae from the Western Atlantic. Anal fin long and pointed, reaching well beyond base of tail. Adults dark gray-brown, the edges of the scales blackish; a wash of yellowish often present dorsally on head, nape, and on back below spinous portion of dorsal fin; a small black spot at upper base of pectoral fins. Juveniles bright yellow with two bright blue lines dorsally on head, extending to beneath middle of dorsal fin where they break into spots; a large blue-edged black spot basally on dorsal fin centered on last spine. It grows to a size of 12.5 cm in length. Longfin damselfish have been reported to have a mutualistic relationship with mysid shrimp of the species Mysidium integrum. The interaction is described as a form of domestication with the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators.
