Testis Structure in Three Species of Teleosts with Tubular Gonopodia

Among the livebearing atheriniform fishes, males in two families, the Anablepidae and Jenynsiidae, possess tubular intromittent organs (gonopodia) unlike the gonopodia in all other groups. In the Goodeidae and Poeciliidae, which lack tubelike gonopodia, sperm are transferred to the female reproductive tract in the form of unencapsulated bundles called spermatozeugmata. Sperm packaging is apparently requisite for efficient sperm transfer in these two families of fishes. The evolution of tubular gonopodia comprises an alternate mechanism of sperm transfer whose efficiency is not diminished because sperm are not packaged as spermatozeugmata or spermatophores. In both Anableps anableps andJenynsia lineata, only free spermatozoa are observed within the efferent ducts and the main testis duct. In the most primitive of the Anablepidae, Anableps dowi, spermatozeugmata are partially formed during spermiation and this process is described. It is hypothesized that A. dowi represents an intermediate condition between a hypothetical ancestral type that produced spermatozeugmata in a manner similar to the closely related poeciliids, and the more advanced A. anableps whose testis produces only free spermatozoa.