Inter- and intra-male variation in sperm length is associated with fertilization mode in marine fishes

Several evolutionary factors, including sperm competition and fertilization modes (external and internal), influence sperm phenotypes. Comparative studies in some animal taxa have demonstrated that sperm competition reduces inter- and intra-male variation in sperm morphology. However, the influence of fertilization mode on variation in sperm traits remains poorly understood. In this study, we compared the residual variance in sperm traits among and within males of 11 marine fish species with different fertilization modes and sperm competition risks. Both inter- and intra-male variation in sperm length was smaller in internally fertilizing species than in externally fertilizing species. We found no clear evidence that sperm competition risk was associated with inter-male variation in sperm length, whereas intra-male variation decreased with increasing sperm competition risk in externally fertilizing species. Sperm length showed lower variation than other sperm traits when comparing inter- and intra-male variation across sperm traits. Lower inter-male variation is consistent with stabilizing selection on sperm length, whereas lower intra-male variation may reflect tighter quality control during spermatogenesis. Our study provides insights into how fertilization mode and sperm competition risk may be associated with sperm trait variation across hierarchical levels, particularly among males and within ejaculates.