Thousands of Giant Spider Crabs (Leptomithrax gaimardii) congregate in Port Phillip Bay every year before their winter moult.
These crabs are hard to spot when they’re on their own, as algae, sponges and sea squirts set up house on their shells. But it’s hard to miss thousands of orange crabs marching across the sea floor, especially if you’re a hungry predator.
The Spider Crab is usually protected by its hard shell, but this body armour doesn’t expand as they grow. As a result, they have to shed it every year and expand in size before the new one hardens. It’s a necessary process, but one that leaves them soft and vulnerable.
This vulnerability may explain the huge congregation of crabs in the shallow waters of the bay. By moulting in a huge group, an individual crab can reduce its chances of being eaten by a stingray or a seal. They even appear to moult simultaneously in an effort to increase their chances.
Source: Museum Victoria