PREDATOR PREY INTERACTIONS IN SEAGRASS BEDS .2. DISTRIBUTION AND DIET OF THE BLUE MANNA CRAB PORTUNUS-PELAGICUS LINNAEUS AT CLIFF-HEAD, WESTERN-AUSTRALIA

The blue manna crab Portunus pelagicus grows extremely rapidly. At the Cliff Head study site, crabs increased from ≈ 35 mm carapace length ( = 60 mm carapace width) after recruitment to ≈65 mm carapace length (=116 mm carapace width) within 1 yr. Slow-moving invertebrate species, in particular molluscs (31.6% of the volume of foregut contents) and polychaetes (16.1% of volume), were the major dietary items of P. pelagicus; however, the crabs also consumed lesser quantities of seagrass (10.2% of volume). Small (<50mm carapace length) crabs predominantly foraged on shallow sand flats, whereas larger individuals occurred most abundantly amongst seagrass and unvegetated habitats further offshore. Although P. pelagicus generally foraged in the habitat in which they rested diurnally, they readily moved to other habitats where prey >2.8 mm sieve size were abundant. The diet of P. pelagicus was similar to the diet of the rock lobster Panulirus cygnus, the other major decapod species at Cliff Head, when the two species foraged in the same area. P. pelagicus was, however, a more selective and wider-ranging predator. The active foraging behaviour and the rapid growth rate of P. pelagicus are typical of other portunid crabs but are unusual amongst benthic invertebrates. These two life-history characteristics may be directly related. © 1990.