Determinants of reproductive success in freshwater amphipod species that experience different mortality regimes

Abstract

Selection on traits conferring enhanced mating success, and thus favoured through sexual selection, may be opposed by selection imposed by predators. This study compares determinants of reproductive success in two species of freshwater amphipods,Hyalellaspp., that differ substantially in adult body size and other life-history traits, and occupy ecologically dissimilar habitats. The small-bodied (‘lake’) species occurs in a lake with size-selective fish predators, and experiences increasing mortality with increasing body size. The large-bodied (‘marsh’) species inhabits a fishless marsh, and experiences decreasing mortality with increasing size. Although male reproductive success increased with body size in both species, the form and degree of the relative mating advantage for large males appeared to differ between species. In the marsh species, male pairing success increased throughout the range of variation in male body size. In the lake species, small males had lower pairing success than males of intermediate and large sizes, but large males did not have greater pairing success than males of an intermediate size. Female reproductive success (through fecundity) increased with body size similarly in both species. Positive assortative mating by size was weak in the marsh species, but stronger in the lake species. Species also differed in the intensity and form of sexual selection acting on the size of male gnathopods, a pair of sexually dimorphic appendages. In the marsh species, sexual selection favoured larger gnathopod width (relative to body size) throughout its range of variation, but no sexual selection for gnathopod width was detected in the lake species. Species differences in the relative importance of large body and gnathopod size in males may reflect an evolutionary divergence in mating behaviour stemming from species differences in countervailing mortality selection.