242 S
. de Lestang et al. J. Exp. Mar. Biol. Ecol. 246 2000 241 –257 Keywords
: Diets; Estuary; Moult stage; Ontogenetic changes; Portunus pelagicus; Stomach fullness
1. Introduction
The blue swimmer crab Portunus pelagicus Linnaeus is found in coastal marine and estuarine waters throughout the Indo-West Pacific Stephenson, 1962. This species
forms the basis of substantial recreational and or commercial fisheries in certain embayments and estuaries, such as the Peel–Harvey and Leschenault estuaries in
south-western Australia Potter et al., 1983; Kailola et al., 1993; Potter and de Lestang, 2000. In south-western Australia, P
. pelagicus spawns in mid spring and early summer and small crabs are recruited into the estuaries of that region in late summer and more
particularly the following spring Potter et al., 1983; Potter and de Lestang, 2000. Portunus pelagicus reaches maturity in the Peel–Harvey and Leschenault estuaries at the
end of its first year of life, with the ovigerous females migrating into coastal marine waters where their eggs are then released. Some of these crabs subsequently return to the
estuary. Although 1
1 crabs are found in these estuaries in summer and autumn, they tend to leave the estuary during winter when freshwater discharge increases markedly
and, as a consequence, salinity declines precipitously Potter et al., 1983; Potter and de Lestang, 2000.
Previous studies have shown that, in marine waters, P . pelagicus consumes a wide
range of benthic invertebrates and, to a lesser extent, teleosts and plant material Patel et al., 1979; Williams, 1982; Wassenberg and Hill, 1987; Edgar, 1990a; Sukumaran and
Neelakantan, 1997; Wu and Shin, 1998. Edgar 1990a found that the compositions of the diets of small and large P
. pelagicus in coastal marine environments differed, a feature he attributed to variations in the potential prey in the nearshore, shallow and
offshore, deeper waters where small and large crabs, respectively, typically occur. On the basis of work carried out on P
. pelagicus in a large coastal embayment, Williams 1982 concluded that the dietary compositions of this species did not change with
increasing body size in either intertidal or subtidal waters. However, her analyses of the dietary compositions of different-sized crabs were based on pooled data for different
shell states and thus did not take into account any differences in the type of material ingested by crabs at different stages in the moult cycle.
We have determined the dietary composition of the full size range of P . pelagicus in
the confines of the shallow Peel–Harvey Estuary. The resultant data have been used first to examine whether, irrespective of their size, recently-moulted crabs ingest a large
amount of calcium-rich material, which would thereby facilitate a rapid subsequent calcification of their new shell. The data were next used firstly to determine whether
sequential size groups of intermoult hard-shelled P
. pelagicus ingest far smaller amounts of calcium-rich material than recently-moulted crabs, and secondly to ascertain
whether, with increasing body size, the diet of this portunid shifts progressively from small invertebrate prey to larger invertebrates and teleosts. Although the dominant
species of benthic macroinvertebrate and fish in the shallow basins of the Peel–Harvey and Leschenault estuaries are essentially the same, several of the less abundant species
S . de Lestang et al. J. Exp. Mar. Biol. Ecol. 246 2000 241 –257
243
are restricted to one or the other of those two systems cf. Loneragan et al., 1986; Rose, ¨
1994; Potter et al., 1997; Durr and Semeniuk, 2000; Semeniuk, 2000; Semeniuk and Wurm, 2000. We have thus compared the dietary compositions of P
. pelagicus in these two estuaries to ascertain whether any of the latter differences in potential prey are
reflected in a significant difference between the compositions of the food ingested by crabs in those two systems. Comparisons between the dietary compositions of crabs of
different size and shell state and in the two estuaries were facilitated by the use of non-metric ordination, which is particularly well suited for analysing dietary data Platell
et al., 1998.
2. Materials and methods
