Variation pattern of sperm quality traits in two gobies with alternative mating tactics

Theoretical models predict that ejaculate expenditure will positively covary with the level of sperm competition but trade-offs between either different measures of sperm quality or sperm quality traits and sperm number are expected. However, empirical results have generally failed to find trade-offs between sperm number, size, velocity and longevity or viability. We analyzed the sperm traits of the grass goby, Zosterisessor ophiocephalus, and the black goby, Gobius niger, two fish characterized by alternative male reproductive tactics, long lasting egg deposition and intra-specific variability in ejaculate characteristics. Indeed, in both species sneaker males release ejaculates with greater sperm number and lower seminal fluid content than territorial ones. We showed that sperm of grass goby males have a similar quality, whereas in the black goby sneakers have faster swimming speeds, greater viability and higher ATP content than those of territorial males. The effectiveness of territorial male mate guarding differs in the two goby species and might account for the observed difference in the intra-specific variability of sperm quality. The pattern of variation that we found in the investment of sperm numbers and sperm quality in goby territorial and sneaker males, supports the results found in other species of fish with alternative mating tactics, suggesting that ejaculate quality traits are usually not traded off one against the other when ejaculate effort is increased in response to increased levels of sperm competition.