Size assortative mating by the hermit crab <i>Pagurus filholi</i> (Decapoda: Anomura: Paguridae)

Abstract

The hermit crab Pagurus filholi shows precopulatory guarding behavior in which males drag the shells occupied by ripe females. It bas been suggested by our previous study on male mate choice in this species that males would adopt a mating strategy of pairing with the first ripe female they encounter. If it is true, we would expect random pairing by size in a field population. However, we observed positive size assortative pairing in the actual field population; larger males tended to mate with larger females and smaller males with smaller females. In this study, field observations and laboratory experiments were conducted to examine the following possible causes of the size assortative mating: 1) mate choice, 2) mate availability, and 3) constraints of size and/or weight of possible partners. The results were: 1) no strong preference for larger females was found in males; 2) males often took over females guarded by smaller males; 3) smaller males could not drag larger females and thus did not pair; and 4) larger females often rejected smaller males. These results suggest that the loading constraint of size and/or weight of the partners and female's rejection of pairing with smaller males explain size assortative mating by P. filholi.