By Dr Daryl McPhee
Crabs are always a welcome addition to the festive season menu and are locally abundant in Moreton Bay around the Christmas period.
The blue swimmer crab is abundant in Bramble and Deception Bays and off the Redcliffe peninsula. They can be caught all year in Moreton Bay but are more active in the warmer months. The scientific name of the blue swimmer crab is Portunus armatus and it is found around most of Australia and New Caledonia.
It has been a long-standing management arrangement in Queensland that recreational and commercial fishers must only retain male crabs. The males are bright blue in colour with white spots, longer claws and a narrow abdomen. Females on the other hand are duller with shorter claws and a much wider abdomen.
Female crabs are more abundant in shallow areas (particularly the tops of sandbanks) while male crabs prefer deeper water and the lower slopes of sandbanks. Tagging results showed the movements of blue swimmer crabs within Moreton Bay were generally limited and random.
Blue swimmer crab mating activity has two seasonal peaks – a major peak being in May/June and a minor peak in November. Female egg bearing crabs occur throughout the year, with the proportion of females bearing eggs being greatest between August and October. The granular egg mass is easy to see under the female crab’s abdomen. Prior to hatching, the female crab moves into shallow marine habitats to release her eggs.
Juveniles recruit from the plankton into seagrass or tidal pools. Juvenile blue swimmer crabs are cannibalistic, and this behaviour is more frequent in the absence of appropriate shelter. Without healthy seagrass beds, the stock of blue swimmer crabs in Moreton Bay would most likely decline significantly.
Blue swimmer crabs grow rapidly and are relatively short-lived (three years). Like all crabs, blue swimmer crabs have a hard exoskeleton (the shell) and they need to shed it in order to grow. This is a process called moulting and ensures that growth is not linear but occurs in a stepwise fashion.
Blue swimmer crabs in Moreton Bay are prone to infection from the parasitic barnacle called Sacculina granifera and this may have impacts on their population. This is not the standard barnacle on the surface of the shell. The visual external symptom of an infected crab is a large mass of soft tissue under the abdomen that looks superficially like an egg mass of the crab, but on closer inspection no distinct eggs are present. Infection rates tend to be higher in more nearshore waters. The infection can prevent blue swimmer crabs from successfully reproducing. The prevalence of the infection has changed through time.
Enjoying your crabbing over the warm months of the year and keep an eye out for evidence of parasitic barnacles!
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