Sperm traits dependent on body size in masu salmon Oncorhynchus masou

Abstract

Most comparative studies on the sperm traits (sperm concentration, swimming speed, and longevity) of salmonid fish involve males with alternative life histories (anadromous versus resident forms) that exhibit different reproductive strategies. However, males exhibit a dominance hierarchy within the same life history, in which smaller males have a tendency to be lower rank. This phenomenon prompted us to compare the sperm traits of large anadromous and small resident males in masu salmon Oncorhynchus masou, and to identify the size-dependent sperm traits within these two life history groups. We detected a significant difference in sperm concentration between the two groups, with resident males having higher sperm concentrations than anadromous males. For resident males, we detected a significantly negative correlation between body size and sperm swimming speed, i.e., faster sperm speed was associated with smaller body size. These findings support earlier data and suggest that smaller or younger males utilize more energy to produce high concentration and high speed sperms.