Reproductive biology in males of the annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae)

Abstract

This study explored the male reproductive biology of the annual killifish Millerichthys robustus; the only annual fish distributed in North America. The structure of the testes was described, as well as the spermatogenesis, spermatozoon and reproductive cycle, using histological sections and gonadosomatic index. The testes of M. robustus correspond to the restricted spermatogonial type, with the spermatogenesis process of cystic and longitudinal type. Spermiation and mature spermatozoa were observed in the network of efferent ducts and in the central duct, uninterruptedly from the third week post hatching (WPH) until pool desiccation, during week 27 WPH. No testes were observed in regression or at rest, indicating the continuous active reproductive phase throughout its adult life. The gonadosomatic index showed that the reproductive potential of males increases towards the end of their life cycle prior to drying up of their pool, suggesting that fish prioritize their energy into reproduction in order to maximize the reproductive success up to the end of their life.