Geographical and taxonomic variation in the pattern of natural sex reversal in the teleost fish order Synbranchiformes

Two types of males have been found in all members of the Synbranchidae. The primary males are born as males and possess single, but bilobular, testes with two gonadal sinuses. Secondary males evolve by sex reversal of functional females and possess a lamellar organization of the testes with three gonadal sinuses and often ovarian residua. Undifferentiated races lack primary males and are recorded only from certain geographical areas. Rudimentary hermaphroditism is classified as a derived evolutionary stage and gonochorism, which is represented in the Amphipnoidae, a primitive evolutionary condition. The alternative of primary versus secondary male is decided by a genetic shift mechanism instead of by environmental conditions. The precarious poise of sexual identity in the Synbranchidae is defined as a switch mechanism to compensate for the vicissitudes of the natural habitat, allowing reproduction in spite of severe population depletions. Based on an examination of its gonadal structure, Alabes is considered to have no phylogenetic affinities with the Synbranchiformes. The occurrence of natural hermaphroditism in phylogenetically unrelated groups of fishes in radically different environments is regarded as the results of chance evolution, rather than convergence.