Sperm competition and the mode of fertilization in the grass goby Zosterisessor ophiocephalus (Teleostei: Gobiidae)

Accessory structures to the male reproductive system are known in several families of teleost, but their role in sperm production patterns and fertilization dynamics is still unclear. The intraspecific variability in seminal vesicles, shown by the grass goby Zosterisessor ophiocephalus, presents an opportunity to examine both its possible correlation to alternative male mating tactics and the function of these accessory structures. In this species, males are known to release sperm in the form of sperm trails, bands of mucosubstances in which sperm are embedded. Surgical removal of seminal vesicles and histochemistry demonstrate that mucins, involved in the production of sperm trails, are secreted by the seminal vesicles. Gametes show a high longevity; sperm motility lasts on average 80 minutes. Eggs can be fertilized for several hours and do not need to be laid over trails because sperm are able to reach them via the surrounding water. Gonosomatic and seminal vesicle somatic indices, histology and histochemistry of gonads and seminal vesicles, sperm counts, and sperm trail longevity, suggest the presence of alternative male mating tactics in this species. Larger males have smaller testes and larger seminal vesicles compared to those of smaller males. The major role of seminal vesicles is mucin secretion in larger males, sperm storage in smaller ones. Trails of larger males last longer and release sperm more constantly over time than those of smaller males. Overall differences between males support the presence of differences in the intensity of sperm competition, with larger males performing nesting behaviour while smaller ones sneak spawnings