| Sumario: | The aim of this study is to evaluate the variations in trophic relationships among species of blue crabs, which may lead to a better understanding of the mechanisms that enable closely related species to exploit the same trophic niche. Callinectes sapidus, C. rathbunae, and C. similis from Tamiahua Lagoon, Mexico, were used in the present study to investigate this question. A total of 4,038 crab-gut contents were analyzed. Of these 1,350 belonged to C sapidus, 1,128 to C rathbunae, and 1,560 to C similis. Six groups of food items were identified: plant fragments (Halodule spp.), micromolluscs (Mulinia lateralis and Macoma atenta, crustaceans (Penaeus spp. and Callinectes spp.), animal residues, and sand and mud. A seasonal variation in prey composition was observed, as well as differences among ages of the three species. In general, as the size class increased, plant matter, sediment, and animal residues decreased, and micromolluscs and crustaceans increased. A clear overlap was detected for C sapidus-C. rathbunae of their trophic niches, with a maximum overlap in winter when both species ate practically the same type of food. The same pattern was observed also for C sapidus-C. similis. Callinectes similis-C rathbunae show overlap in spring and winter, when ovigerous females are on their breeding grounds. Ontogenetic changes in the diet and niche overlap in relation to seasonal changes in reproductive behavior allow the three species to utilize the same resources at Tamiahua Lagoon, Mexico.
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