Abstract

Field studies on the mating system of the amphipod Eogammarus oclairi Bousfield in the intertidal shell habitat of Grays Harbor estuary (WA, USA) showed weight assortative mating. There was no spatial difference in the size distribution of individuals, and assortativeness was not affected by habitat complexity. Laboratory experiments showed that males tended to select larger females, and that larger males may either outcompete or take over the females of smaller males. Interaction between a lone, searching male and an amplexing couple was influenced by proximity of the female's moll, the size of the female and size of the intruder male compared to the amplexing male. Males increased the tenacity of their hold on a female during interactions with intruder males as pending female molt grew closer. Among environmental variables, flow velocity up to 30 cm.s(-1) in an artificial flume did not affect males' ability to pair and carry a female. Males were able to remain in amplexus for 24 h with females larger than themselves at this velocity. All evidence suggests that assortative mating is mostly driven by male-male competition and that sexual selection favors large males.