Moulting and mating

Spider crabs spend most of their lives in deep water, but once a year off the coast of southern Australia, quarter of a million crabs set off on a long journey to the shallows to moult and mate. They all share in the same problem. As the crabs grow their armoured shells get uncomfortanbly tight. Each crabs has to grow a new shell and get rid of the old one, but this is no simple process. First the crab grows an entirely new skin underneath the old one. It then flexes its body to force its shell to split along the back before gingerly backing itself out. The crabs are also here to mate and grab the opportunity with considerable enthusiasm, but the commotion of so much activity attracts predators. Alarm spreads amongst the crabs as a stingray hovers over them. Stingrays are picky eaters and like to go for the newly moulted, soft-shelled crabs. However, once a target is singled out, there is little chance of escape. The ray positions itself over the crab and sucks it up.

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