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

Abstract

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.

An initial description of alternative male reproductive phenotypes in the bluebanded goby, Lythrypnus dalli (Teleostei, Gobiidae)

“…In general, females have short, blunt genital papillae ('female-typical') and typically deposit ripe eggs from April to September (although they begin developing the eggs as early as February; Wiley 1976, Behrents 1983. During this time, males -which are characterized by a long, pointed genital papilla ('male typical') and longer finscourt gravid females, maintain nesting sites and presumably lay trails of sperm before and after the courted female releases her eggs (as in other gobies, see Scaggiante et al 1999;Reavis & Grober, unpublished data). The males then proceed to defend the nests of demersal eggs (Behrents 1983).…”

“…One essential male characteristic in gobies is the accessory gonadal structure (AGS) (Cole & Robertson 1988), a pair of multi-chambered lobes containing sperm as well as mucins and/or steroid derivatives (Miller 1984, Fishelson 1991, Scaggiante et al 1999). This structure is similar to seminal vesicles and sperm duct glands found in other fish species (reviewed in Lahnsteiner et al 1992).…”

“…Wiley 1976;Behrents 1983;St. Mary 1993, 1994 Zosterisessor ophiocephalus; Scaggiante et al 1999;Mazzoldi et al 2000. 3 Salaria pavo; Ruchon et al 1995;Oliveira et al 2001.…”

“…While this does not conform to classic sperm competition theory, these two male phenotypes may use their structures differently, thus providing a different form of reproductive differentiation. The higher mucus content in nesting male AGS is probably important for laying sperm trails in their nests (Fishelson 1991, Scaggiante et al 1999, however, mini males (which most likely have to sneak fertilizations) will not have the opportunity to lay sperm trails and thus do not need a mucus-filled AGS. Therefore, mini males may not need a relatively large testis (contrary to sperm competition theory) because they have an AGS that is packed with sperm, releasing the testes from this constraint.…”

“…Therefore, ejaculate quality and quantity in L. dalli males may be the defining characteristic of their reproductive strategy and requires further investigation. This supports the suggestion by Scaggiante et al (1999) that the actual function of accessory structures must be considered when studying sperm competition in diandric (two male phenotypes) systems.…”