Use of the Sterile Male Technique to Investigate Sperm Competition, Storage, and Use in a Pill Box Crab, Halicarcinus cookii (Brachyura: Hymenosomatidae)

Halicarcinus cookii is a small intertidal New Zealand crab that has high levels of ovigerous females in all months. Continuous breeding requires continuous mating. Mating is not linked to moulting because the pubertal moult is terminal. Spermathecal fullness is modelled in terms of the difference between the rates of copulation and rates of brood production. It is estimated that females can fill their spermathecae with sperm after 3-4 copulations (depending on male size) and that approximately 15% of an ejaculate is used to fertilize each brood. Given the fact that females have multiple partners we asked the question: is it worthwhile for males to spend any time mate guarding? Using sterile males we showed that the majority of each brood is sired by the last male to mate. In females prevented from mating again, more of each successive brood is likely to be sired by earlier males who had mated with the female. This suggests that over periods exceeding one brood cycle (i.e., months) sperm slowly becomes mixed. Therefore, it is worthwhile for male H. cookii to invest time in guarding female partners.