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Swim Like A… Crab

By Chris Doyle on September 20, 2012 in Other

Photo: George Evatt

Most people would be familiar with the sight of a crab scuttling across the sand or scurrying over the rocks, deftly moving in that odd but effective sideways motion that crabs are renowned for. But have you ever seen a crab that swims rather than walks?

The thought of a crab that swims might seem absurd, particularly with those flimsy-looking legs they have, but the aptly named blue swimmer crab does exactly that. With powerful, paddle-shaped back legs, blue swimmer crabs can lift off the bottom of the ocean and propel themselves through the water with surprising agility. And just as other crabs scurry sideways, blue swimmers swim sideways, as it is the fastest and most efficient way for them to move.

If you happen to come across a blue swimmer crab in spring or summer, don’t be fooled into thinking you are seeing double. Blue swimmers usually mate when the water begins to warm up after winter and they have an unusual ritual where the male carries the female under his belly for over a week before mating with her. By doing so, the male protects the female while she moults – a process where she will shed the hard shell covering her body and lay down a new, bigger shell.

Crabs, like all crustaceans, must moult every now and again in order to grow. However, the new shell is paper-thin and remains soft for a few days. This is when crabs are at their most vulnerable to predators and also at their yummiest to humans (ever heard of ‘soft-shell crab’? Now you know why it’s soft!). With a male to protect her and carry her around, the female blue swimmer can moult in relative peace.

As for the male, he busily fends off predators and other males interested in his new beau. But while the female is still frail from moulting, he takes the opportunity to flip her over and mate with her. He then continues to carry her for another few days until her new shell has completely hardened and she can protect herself once again.

While they are agile swimmers and can cover considerable distances by swimming, don’t expect a blue swimmer to pull alongside you on your Sunday afternoon splash at Bondi. Blue swimmers are usually shy and bury themselves in the sand for most of the day, coming out only to feed. Your best chance of seeing them is at dusk when they are most active, trying to catch a dinner of mussels, oysters, fish and seagrass.

Blue swimmer crabs are actually quite tasty to eat and are highly sought after by both recreational and commercial fishermen. Catching a blue swimmer crab or two can be a lot of fun, particularly during the hot summer months when they are most active. Remember though, if you do go catching crabs, be sure to abide by all fisheries regulations for this species and do not take any females that have eggs. That way we can ensure that future generations get to enjoy the same benefits of our unique wildlife as we do.