Biogeography of Crab Claw Size: Assumptions and a Null Hypothesis

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Abstract
It was recently proposed that relative strength of the master claw of shell-breaking crabs, indirectly measured by the ratio of chela height or thickness to carapace breadth, within the tropics decreases in the order Indo-West Pacific > Eastern Pacific > Western Atlantic > Eastern Atlantic; is greater in the tropics than in the temperate regions; and increases as the number of co-occurring congeners or total number of brachyurans increases. These patterns were hypothesized to result from diffuse and intrageneric competition in the tropics and from metabolic limitations in the temperate zones. The data show that, because the ratio varies with body size, any comparisons depend more upon the size of the animals being compared than on their geographical origin. Crabs with similar ratios may differ significantly in claw dentition, musculature and leverage properties; the ratios are therefore not good measures of claw strength. The crabs in the analysis do not represent a random sample of crabs from the regions being compared. Using the same data as Vermeij (1977), the null hypotheses that there are no relationships between relative claw size and geographical region, number of co-occurring congeners and total number of brachyuran species cannot be rejected. Tests of the hypotheses require actual measurements of crab claw strength.