GROWTH IN THE AMPHIPOD GAMMARUS LAWRENCJANUS

A trade-off is found between growth and the length of time male amphipods (Gammarus lawrencianus) spend in amplexus. Males spending the majority of time in amplexus showed 45% less growth than unamplexed males. The inability of males to use their gnathopods to feed while in amplexus appears to cause this reduced growth. Growth rates of females appear unaffected by amplexus. Since male size is correlated with male reproductive success in G. lawrencianus, a 45% decrease in size increment at the next molt would represent a similar loss in the incremental male reproductive success. Male mating decisions are therefore based not only on immediate past male investment in amplexus, male size, and population characteristics, but also on the trade-off between present reproduction and future size.