The Blue Swimmer is also known as Blue Manner Crab, Sand Crab, Blue Crab and Sandy. It has a broad and flattened carapace with nine teeth on each side, the last tooth being very pronounced as horns (see Image}. The clawed legs are long, elongated and ridged. These crabs can grow up to 200mm in carapace width. The colour is mottled blue on males and mottled brown on females, but the intensity and pattern of the colouration are very variable.
The Blue Swimmer has the following classification:
The following figures show the morphology of the Blue Swimmer: A; Dorsal view (From Kailola et al. 1993). B; Ventral view (From Ruppert and Barnes 1994). * Note the paddle shaped swimming legs.
The class Malacostraca contains the majority of well-known crustaceans, especially the edible species. Malacostraca have paired compound eyes, usually two branched (biramous) antennujles and antennae, 8 pairs of thoracic walking legs (including up to three pairs of maxillipeds and five pairs of swimmerets (pleopods) on the abdomen. The female sexual openings, or gonopores, are located on the sixth thoracic segment or its appendages, while male gonopores are on the 8th segment or its appendages. The sub-class Eumalacostraca contains most of the large and commercially important crustaceans such as crabs.
The order Decapoda includes prawns, shrimps, rock lobsters and crabs, all having three pairs of maxillipeds and five pairs of thoracic walking legs which give the name, meaning 'ten footed'. Decapods are the largest crustaceans and support several major and minor fisheries in Australia.
The family Portunidae includes most of the crab species fished both commercially and by amateurs in Australian waters. P. pelaagicus can attain large sizes and are renowned for their culinary properties. These crabs have broadly flattened rear legs, an adaptation to swimming.
The distribution of P.pelagicus is Australia wide except for extensive areas of the southern coastline. Exceptions to this are the Spencer Gulf amd Gulf St. Vincent in South Australia.
Much of the information on the animal’s life history has in the past been sourced from other states (Kailola et al. 1993, Potter et al. 1983 and Smith 1982). To further our knowledge of the distribution of P. pelagicus in this estuary a combination of trapping and mark-recapture experiments were conducted in Cowan Creek which is a major tributary of the Hawkesbury River estuary.
Cowan Creek is a typical flooded river valley with a series of minor catchments entering from either side along its length. This results in a deeper main creek (> 20m) with more shallow bays along its length. Each of these small bays has a sandy alluvial delta entering at its headwaters from a freshwater creek. The catchment is protected as National Park and there is no commercial fishing in the area.
Population assessment was proposed to follow two methods. The first phase was a range finding experiment to determine the crab’s distribution in the Cowan Creek estuary. This was followed by a series of mark-recapture experiments based on the modified “T” bar anchor tag.
The distribution of P.pelagicuswas found to equate with depth. The distribution results in a series of geographically distinct populations in the small bays along Cowan Creek during summer. Mark-recapture assessments in one of these bays, using the Weighted Mean Method over four consecutive capture release days, on two assessment periods, gave population estimates of 26.0 (+/-14.9) and 15.2 (+/-7.6). On two other sampling occasions during winter, no crabs were captured suggesting that the animals either move to deeper waters during colder months or lower their metabolism and remain in the sediment for these periods.