Morphometric and biomechanical characteristics [size, mechanical advantages (MAs), muscle mass ratio (MR)] of the chelipeds of Necora puber, Liocarcinus depurator and Liocarcinus arcuatus (Crustacea, Decapoda, Portunidae) were analyzed to investigate their relation to diet selection patterns observed from gut content analysis of crabs from the Ria de Arousa (Galicia, NW Spain). The size of the chelipeds relative to body size is similar in the 3 species, but there are important differences in biomechanical parameters. Both males and females demonstrate an interspecific morphometric gradient where there is an inverse relationship between MA and MR of the chelipeds. The prey consumed by portunids in the Ria de Arousa was classified according to its mobility and presence of a hard exoskeleton. The diet of L. depurator had a high morphological diversity as the functional structure of the chelipeds is more versatile than in the other species. Chelipeds of L. depurator are highly mobile because the MAs are relatively low, but the decrease in force produced is compensated by a relative increase in muscle mass. L. arcuatus has a lower MR but the highest MA, hence the force produced depends more on the design of the chelipeds. These characteristics give rise to appendages with Little mobility, which is reflected in a diet made up mainly of sedentary prey without an exoskeleton. N. puber has intermediate trophic and biomechanical characteristics; its prey are primarily mobile and have hard exoskeletons. Ontogenetic changes are the main factor reponsible for life history variation in the diet, and they are related to biomechanical factors due ro the absolute growth of the chelipeds. Growth gives access to prey with hard exoskeletons, especially in the case of L. depurator, whereas the changes in relative growth are associated with the onset of sexual maturity and reproductive behaviour. Within a species, the morphometric patterns observed may be attributed to non-trophic selection processes, and they have no major influence on the diet, which is conditioned mainly by the absolute growth of the chelipeds and muscle mass. On a longer evolutionary scale, the interspecific diversity in feeding habits and morphometry are correlated, although the causal mechanism could not be determined.
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