Sperm depletion as a consequence of increased sperm competition risk in the European bitterling, Rhodeus amarus

Sperm depletion occurs when males are unable to maintain ejaculate size over successive ejaculations and appears to be widespread across multiple taxa. We used the European bitterling, a fish that lays its eggs in the gills of freshwater mussels, in experiments to investigate sperm depletion over different temporal scales at different levels of sperm competition risk. The spermatozoa density of territorial male ejaculates declined over five consecutive ejaculations, but mean ejaculate size was significantly larger if the male was exposed to a rival, suggesting males were sensitive to sperm competition risk and modulated ejaculate size in accordance with theoretical models. However, following successive matings over the course of a day, males exposed to rivals produced significantly smaller ejaculates than males with a low risk of sperm competition. Results are discussed with reference to a possible intersexual conflict over the size and distribution of ejaculates in the context of male sperm depletion and female sperm limitation.