Presence of primary and secondary males in a population of the protogynous Synbranchus marmoratus

Synbranchus marmoratus, the ‘ swamp eel ’, is a protogynous diandric fish. The primary and secondary males can be distinguished from each other easily by diVerences in gonadal morphology. Primary males have lobular, unrestricted testes with central eVerent ducts. The secondary male has a ‘ lamellar ’ testis, eVerent ducts are present in the ventral region (new formation), lateral supports and it is covered by the former ovarian capsule. The length of primary male varies from 13 to 88 cm while secondary males range from 56 to 91 cm. Transitional individuals vary between 45 and 60 cm in length. The swamp eel population studied is composed of 80% primary males and 20% secondary males. Although gonadosomatic indices are always higher in primary males, they increase in secondary ones as the newly acquired stage progresses. The lamellar organization of the gonad of secondary males, the absence of this kind of testis in individuals smaller than 56 cm, and the presence of hermaphroditic fish, is evidence for protogyny in this species. ? 1996 The Fisheries Society of the British Isles