ABSTRACT: Crabs grown experimentally on fully shelled prey developed larger and stronger claws than those raised on nutritionally equivalent unshelled prey. When one claw was immobilized, claws also became asymmetrical. These use-induced changes differ from skeletal remodelling in vertebrates and many invertebrates because changes in the rigid exoskeleton can occur only after molting, and claw muscle mass must be reduced substantially before the molt. Such short-term adaptive responses to environmental stimuli, if heritable, could yield long-term evolutionary changes in claw size, and if combined with behavioral biases towards one side (handedness), could also promote the evolution of claw dimorphism.
Figure 1. Effect of diet and claw treatment on the relative manus volume of Cancer productus grown in the laboratory. (A) Data expressed as unstandardized deviations from regression. (B) Data expressed as deviations from regression after standardizing for statistically significant among-individual differences in initial claw size in the 'Pre-treatment' molt (P< 0.001, ANOVA). All points are approximate manus volume expressed as a % deviation from that expected at a given carapace width (±SE). H2- hard diet, both claws free. S2- soft diet, both claws free. H1- hard diet, one claw glued closed and the other free. S1- soft diet, one claw glued closed and the other free. Free- unconstrained claw. Glued- claw glued permanently in the closed position. Field- exoskeleton at time of collection, Pre-treatment- first exoskeleton produced in the laboratory after variable amounts of time on identical diets, Expt. #1, Expt. #2- first and second exoskeletons produced after complete intermolt periods under experimental conditions.
Figure 2. Mean maximum crushing force (Newtons ± SE) generated by wild and laboratory-reared Cancer productus. Laboratory-reared crabs experienced at least two entire intermolt periods under different experimental conditions (see Fig. 1 legend for treatment abbreviations). Letters identify groups of means that differed significantly. Numbers inside each bar indicate sample sizes. The dashed line highlights the mean force generated by wild crabs.
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