Each female produces between 180,000 and two million eggs in a single spawning – and may spawn more than once in a season. Estuarine crabs tend to spawn later in summer, having moved to the sea or estuary mouth during winter rains. In the ocean, big females mostly spawn in late spring and early summer. In southern WA waters, the females retain the males’ sperm over winter until their ovaries develop – helped, it’s thought, by rising water temperature in spring. After mating, he continues to carry and protect her for another three-to-four days while her new shell hardens. He helps her to moult then turns her over to mate while she is still soft-shelled. A courting male then catches a female and carries her beneath him for four to 10 days while fending off other males. Males moult first, so that their new shells have hardened beforehand. During the final moult to reach maturity, females mate for the first time. They continue growing rapidly.īy winter, the carapace is about nine centimetres wide. They turn into a more crab-like state called ‘megalopae’. They settle to the sea or estuary bottom and moult (shed their shell) frequently while rapidly growing. The survivors reach shallow nursery areas by late summer. They are prey for fish and the death rate is high. They drift in bays or along the coast up to 80 kilometres out to sea. Lifecycleīlue swimmer crabs begin life as tiny larvae, called ‘zoea’, which grow and change shape over a four-to-six-week period during early summer. At night they become mobile and search for food. Crabs in marine bays spend their entire lives in the bay.īy day, they usually hide beneath the sand with only their eyes protruding, then launch themselves at prey. They live in estuaries, sheltered bays and offshore waters up to 50 m deep.Įstuarine crabs tend to move from estuaries into nearby marine waters during winter. In WA they’re found mainly between Port Hedland and Dunsborough. In turn, they are prey for fish and birds. They eat small fish and crustaceans, molluscs, worms, and occasionally, algae and seagrass. In Western Australia, blue swimmer crabs can grow to up to 25 cm wide across the carapace with a claw span of up to 80 cm. They also have nine spikes, called horns, along their carapace, on either side of their eyes. The family also includes other large, edible crabs found in Australia, such as mud crabs.Ĭrabs from this family can usually be recognised by their flat, disc-shaped hind legs, used as paddles for swimming. pelagicus) are a tropical species and belong to the Portunidae family. They are targeted by commercial and recreational fishers.īlue swimmer crabs ( Portunus armatus formerly known as P. They are powerful swimmers and voracious hunters and scavengers. Blue swimmer crabs are sometimes called ‘blue manna’ or ‘blueys’.
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