Dimorphism and the functional basis of claw strength in six brachyuran crabs

Abstract

By examining the morphological basis of force generation in the chelae (claws) of both molluscivorous and non-molluscivorous crabs, it is possible to understand better the difference between general crab claw design and the morphology associated with durophagy. This comparative study investigates the morphology underlying claw force production and intraspecific claw dimorphism in six brachyuran crabs: Callinectes sapidus (Portunidae), Libinia emarginata (Majidae), Ocypode quadrata (Ocypodidae), Menippe mercenaria (Xanthidae), Panopeus herbstii (Xanthidae), and P. obesus (Xanthidae). The crushers of the three molluscivorous xanthids consistently proved to be morphologically ‘strong,’ having largest mechanical advantages (MAs), mean angles of pinnation (MAPs), and physiological cross-sectional areas (PCSAs). However, some patterns of variation (e.g. low MA in C. sapidus, indistinguishable force generation potential in the xanthids) suggested that a quantitative assessment of occlusion and dentition is needed to understand fully the relationship between force generation and diet. Interspecific differences in force generation potential seemed mainly to be a function of differences in chela closer muscle cross-sectional area, due to a sixfold variation in apodeme area. Intraspecific dimorphism was generally defined by tall crushers with long in-levers, though O. quadrata exhibited an extreme dimorphism suggesting that factors unrelated to a speed–strength dichotomy (e.g. sexual selection) have shaped dimorphism of that species. It is concluded here that: (1) the majority of interspecific claw strength variation is a function of closer muscle cross-sectional area; (2) variation in claw morphology related to force production and transmission does have some relation to hardness of diet; (3) claw dimorphism in many species does seem to be related to strength and speed trade-offs; (4) factors besides molluscivory must be considered to understand claw evolution fully; (5) a quantitative assessment of force distribution, lacking in the literature, is essential for a more complete understanding of the relationship between claw design and ecological function.