Dive into the world of these ten-legged crustaceans that can be found scurrying across Queensland’s rocky shores, sandy mud flats, and coral reefs.
Crabs are one of the largest groups of crustaceans, and the most diverse in both shape and size. The group’s scientific name, Brachyura, means 'short tail', and refers to the major evolutionary breakthrough that has made crabs so successful. Instead of using the tail for swimming, crabs have it greatly reduced and tucked completely under their body (carcinisation), and no longer have a tail-fan. This gives them a compact body shape and increased mobility, allowing them to spend their adult lives in a variety of habitats on the sea floor. Though some shore crabs can run sideways at great speed, most crabs can move in any direction they choose, belying the popular myth.
Considered to have first appeared in the Jurassic period (144-213 million years ago), the world fauna is now known to consist of more than 7,000 species, of which nearly 1,200 are known from Australia. The majority of crabs live in shallow marine environments, but others occupy a huge variety of habitats, ranging from deep-sea hydrothermal vents to tropical rainforests, freshwater streams, and even inland deserts. While important commercially, the crab fishery is dominated by a few members of a single family, the Portunidae, which includes Mud Crabs (Scylla species), and the Blue Swimmer Crab (Portunus armatus). More than half a million tonnes of these crab are caught and eaten each year in the Indo-West Pacific region.
Examples of Queensland crabs include:
The abdomen of a male crab is much narrower and more triangular than a female of the same species. The female possesses a broader, more rounded abdominal flap. This is because the female crab can use her abdomen to hold eggs.
Male fiddler crabs have one grossly oversized and gaudily coloured claw. They use this large claw to court females, and in battles with other rival males. When courting, the males wave the claw back and forth like a violin player, which gives rise to the name ‘fiddler crab’.
Actually, not all species of soldier crab march, but the commonly encountered species, Mictyris longicarpus, will emerge by the thousands on the falling tide and march across the sand flats to feed. It is thought by some that this behaviour may provide protection from predators by ‘safety in numbers’.
The Smooth-handed Ghost Crab is often seen scurrying about beach campsites. It occurs in northern Australia from Kimberley, Western Australia, to northern New South Wales.
The Shiny Bait Crab is found in crevices and pools on exposed rocky shores in eastern Queensland. It is common at Point Lookout, on North Stradbroke Island.
The Red and White-spotted Reef Crab hides in cavities in dead coral or mussel clumps, intertidal zone down to a depth of about 30 metres. Known only from northern Australia, south to Moreton Bay.
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