Male ejaculate volume is adjusted according to sperm stock and not influenced by female reproductive potential or mating status in Japanese pygmy squid

Abstract Due to the cost of sperm production, males are expected to adjust ejaculate volume to maximize reproductive success. This study investigates how male ejaculate volume in Idiosepius paradoxus is influenced by (1) female quality (reproductive potential and mating status) and (2) the sperm stock of the male. Males are hypothesized to increase ejaculate volume when mating with females of high reproductive potential or non-virgin females with sperm-competition risk. However, no evidence supported these assumptions, suggesting the presence of other factors. We found that males consistently allocated a moderate volume of ejaculate with approximately 12% and 14 % of their sperm stock in the early and late reproductive season, respectively. This may be the most efficient way to use male reproductive resources in conditions where the male-driven mating frequency is high. It was noted that, early in the reproductive season, the ejaculate volume transferred by smaller males was similar to that transferred by larger males. This was achieved by smaller males allocating a higher proportion of their total sperm stock, despite possessing relatively fewer sperm. However, this trend was not observed in the late reproductive season. Possible explanations for the evolution of these unique patterns of ejaculate volume allocation are discussed.