Dichotomous male asymmetry in metapopulations of a marine amphipod

Spatially structured populations result in spatially segregated individuals experiencing heterogeneous habitats and a limited number of conspecifics. Spatial segregation can affect mating systems to the extent where mate selection is a function of mate availability, giving rise to frequency-dependent selection. In species with secondary sexual characters, dichotomous asymmetry in traits may reflect tradeoffs between alternative morphs. The amphipod Dulichiella appendiculata (Say, 1818) shows strong sexual dimorphism where males have one large asymmetric gnathopod that can either be left or right of the body. In Florida seagrass beds, D. appendiculata occupies discrete reef habitats created by pen shells. In this metapopulation, "right-clawed" males have a larger gnathopod than "left-clawed" males. While each male morph occurs in isolation from the other morph 28% of the time, left-clawed morphs occupy 84.4% of pen shells and right-clawed morphs 55.4%. More males are present when "right-clawed" males are present, suggesting "right-clawed" males are more gregarious. More females are present when "right clawed" males occur in a pen shell but sex ratio varies independently of morph frequency. Results suggest a tradeoff between the larger, more gregarious right-clawed morph that attracts more females and the smaller left-clawed morph that seems to rapidly disperse and occupy more habitats. Such tradeoff may allow both morphs to persist in nature driven by frequency-dependent mating strategies.