Competitive mating, mate choice and mating associations of Brachyuran crabs

The mating associations of brachyuran crabs are reviewed and classified according to apparent modes of competition among males for mates. Males obtain mates in three general ways: they may compete directly for females and then defend them for relatively long periods before mating (female centered competition), they may compete for resources females use during breeding or refuges they occupy (resource centered competition), or they may search for or intercept receptive females but defend neither females nor resources (encounter rate competition). Male modes of competition for females are influenced by patterns of predation, competitor density, female density, distribution, mobility, habitat requirements for breeding and mate choice. Some form of mate guarding, multiple copulations and sperm plugs occur in nearly every type of mating association. Sperm competition may be common in brachyurans and may have a pervasive influence on male competitive tactics. A major goal of those interested in the evolution of mating behavior is to understand how sexual selection leads to differences in mating associations (the spatial and temporal relationships between the sexes that result from social interactions and lead to mating) in different species. Classification of the mating associations exhibited within a group is useful for summarizing the diversity of mating behavior and for spotting trends in how sexual selection operates. Choice of a classificatory scheme is important as the criteria used to sort species' mating associations into types reflect judgments about which differences are superficial and which arise from fundamental contrasts in the way males compete for females and how females choose mates. In this paper I will review and classify mating associations in brachyuran crabs. The scheme employs criteria which focus attention on ecological and social determinants of male mating behavior (Bradbury and Vehrencamp, 1977; Emlen and Oring, 1977). It is appropriate to stress male mating behavior because how males compete for mates largely determines the spatial and temporal relations between the sexes. Female interests, as reflected in female mate choice and control of receptivity, can have profound effects on how males compete (Borgia, 1979; Knowlton and Greenwell, 1984; Parker, 1984; Thornhill and Alcock, 1983). Therefore, I will also discuss how female crabs may choose among competitors in each type of mating association. Reviews of courtship and mating behavior in crabs by Hartnoll (1969), Hazlett (1975a), Ridley (1983) and Salmon (1983), each with a slightly different focus, stimulated many of the ideas I develop here and provided numerous references. Field studies of sufficient depth and scope to permit clear judgments about modes of mating are few or lacking for most families. I will focus on these (mostly recent) studies when possible. MATING ASSOCIAnONS IN BRACHYURAN CRABS I distinguish eight kinds of mating associations in brachyuran crabs that fall into three general categories (Table 1). Males may (1) search for or attract individual receptive females that they defend directly from other males, (2) defend resources that females require for breeding or survival and mate with the females