Homology of Pelvic Fin Structures in Female Phallostethid Fishes (Atherinomorpha, Phallostethidae)

Abstract

Male phallostethid fishes have an elaborate, subcephalic copulatory organ, the priapium. They are bilaterally asymmetric, such that the aproctal side of the body is the left in sinistral males and the right in dextral males. Bones of the priapium generally are hypothesized to be homologous with those of the pelvic and parts of the pectoral fins. Female phallostethids usually have vestigial or no pelvic fins and exhibit no bilateral asymmetry. However, report is made herein of a bony structure ventral to the pectoral fins which is offset to the left or right of the anus and urogenital opening in, hence, bilaterally asymmetric females of a Philippine phallostethid, Manacopusfalcifer (Manacop). Position, size and shape of this unsegmented element in adults suggest it is homologous with either a postcleithral or pelvic fin structure. Ontogenetic series indicate that: 1) the bilaterally asymmetric element in adult females is a modified pelvic fin ray; 2) the pelvic girdle and other fin rays are lost during ontogeny; and 3) pelvic rays are unsegmented in early stages of development. These conclusions support the idea that several elements within the bilaterally asymmetric priapium are homologous with pelvic-fin rays.