Dietary composition of the blue swimmer crab Portunus pelagicus L. Does it vary with body size and shell state and between estuaries?

The aim of this study was to determine the diets of Portunus pelagicus in the large Peel-Harvey and Leschenault estuaries in south-western Australia in order to ascertain whether the dietary composition of this crab changes with body size during two different moult stages and differs between the two estuaries. Portunus pelagicus, ranging in age and carapace width from ca. 2 months and 12 mm to ca. 112 years and 159 mm, were collected from the shallow basins of the Peel-Harvey and Leschenault estuaries. Examination of the cardiac stomachs of these crabs showed that P. pelagicus does not feed just before or immediately after moulting and that the stomachs of recently-moulted crabs contained significantly more food than those of intermoult crabs. Although the volumetric contribution made by calcareous material to the stomach contents was similarly high in all size classes of recently-moulted crabs, i.e. 47 to 55%, the volumetric contributions made by small bivalves decreased with body size, whereas the reverse occurred with shell fragments of large decapods and, to a lesser extent, polychaetes. The dietary compositions of intermoult crabs were shown by classification and multi-dimensional scaling ordination to differ markedly from those of recently-moulted crabs and to undergo similar progressive ontogenetic changes in both the Peel-Harvey and Leschenault estuaries. Thus, the contribution made by small benthic and epibenthic crustaceans, such as amphipods and tanaids, declined with increasing body size, whereas the reverse occurred with larger prey, such as nereid polychaetes, small decapods and teleosts. The dietary composition of P. pelagicus was influenced more by moult stage, i.e. recently moulted vs. intermoult, than by body size. Although the dietary compositions of P. pelagicus in the two estuaries were not significantly different, they did differ from those recorded from coastal marine waters in the same region, thereby reflecting differences in the potential prey in those two environments.