Mating biology and selection on body weight in Eogammarus oclairi Bousfield (Crustacea: Amphipoda)

In this dissertation I investigate the role of: (a) females (besides mating) in the amphipod's mating system; (b) different processes leading to assortative mating; (c) whether operational sex ratio, weight structure of the population, local population density influence precopulatory guarding; and (d) the relationship between population density and the opportunity for selection on body weight.; There was no evidence of any active selection of mates by females. Females molted during the night and were fertilized immediately. When males were not present females delayed molting and did not ovulate if molted under this circumstance. Fecundity rate decreased as delay in encounter of males after her molt increased. Molting delay may increase the female's chances of successful reproduction.; There was weight-assortative mating, but no spatial variation in the weight distribution of individuals. Males selected larger females, and larger males take over the females paired with smaller males. Interaction between a lone male and an amplexing couple was influenced by proximity of the female's molt, the size of the female and the relative size of the intruder male compared to the amplexing male. Since flow velocity (up to 30 cm s{dollar}sp{lcub}-1{rcub}{dollar}) did not affect males' abilities to carry females, assortative mating may be driven mostly by male-male competition.; Population structure affect males' mating behavior, mainly the operational sex ratio. These results suggest that the male's impetus to guard has a variable threshold dependent on the intensity of male-male competition.; Density was lower when oyster shells were whole than when fragmented. Predation rate on single amphipods by juvenile Dungeness crabs (Cancer magister) was not different between whole shell vs. fragmented shell, but mating pairs were consumed at higher rates in the whole shell treatment. There were no significant differences neither in size of individual amphipods between habitat types, nor in size of males in pairs consumed. The opportunity for selection on a male body weight increases with density, which complicates evaluation of the strength of a selective pressure in shaping traits such as body weight, because population abundance fluctuates widely in space and time.