Mate searching by males of the intertidal amphipod Corophium volutator (Pallas)

In this paper we describe the mate-searching behaviour of male Corophium volutator (Pallas), an intertidal amphipod. We expected male choice of mates because males invest in mating by mate guarding, and also because adult females greatly outnumber adult males at our study site. We predicted that males would choose large females carrying many eggs and females close to moulting or close to being receptive. We found that individual males ''assessed'' an average of ca. 5 females in their respective burrows before entering the burrow of a prospective mate. Males were more likely to cohabit with females that were close to being receptive than with females that were far from being receptive. Over the season, the size of mate-searching males declined, whereas the number of females encountered by males increased, which may relate to the seasonal distribution of receptive females. However, there were no differences in size between paired and unpaired females regardless of time of season. Our results support mate discrimination by males based on one criterion (stage of receptivity). The proximate and ultimate causes of the biased sex ratios of C. volutator need to be determined before other predictions based on mate-choice theory can be made.