Reproductive biology and systematics of phallostethid fishes as revealed by gonad structure

Testis and ovary structure was examined histologically in seven of the 19 species in the three tribes of the teleost fish family Phallostethidae, series Atherinomorpha. These diminutive species have testes in which spermatogonia are restricted to the distal ends of lobules, a diagnostic character of atherinomorphs. Sperm in the ovarian lumen and chorionic attachment filaments on eggs confirms observations that phallostethids are internally fertilizing and lay fertilized eggs. The immense number of sperm in ovarian cavities means that all, or nearly all, ovulated oocytes will be fertilized. As revealed in histological sections, testicular ducts in most phallostethids examined contain 'granular' secretions that have not been reported in any other atherinomorphs. Species in the tribes Neostethini and Gulaphallini form unique spermatozeugmata that differ from those of other internally fertilizing atherinomorphs examined in that they have sperm nuclei that are oriented towards one side of the sperm bundle. Spermatozeugmata are not formed in species in the tribe Phallostethini. A unique spermatozeugmatum is interpreted as being a diagnostic character of phallostethids that has been lost or modified in phallostethins. Gonads of phallostethids and hypothesized close relatives are posterior and posteroventral to the gut rather than dorsal to the gut, as they are in most other fishes. Museum specimens preserved over sixty-five years ago are as useful for demonstrating gonad histology as are those preserved in the past few years.