Pascal Kobeh's Image

Each year, thousands of deep-sea Australian majid spider crabs set off to walk over the seabed to shallow waters off South Australia.

In their drive to migrate, they climb over each other, sometimes forming great piles. 'They walked like an army on the march,' says Pascal. 'If I lay on the bottom, they would just clamber over me as though I was a lump of rock or coral.' Once in shallow water, many of them moult out of their exoskeletons (shells), emerging with soft new ones. It takes a while for the new, expanded shell to harden - a very vulnerable time for a crab. And this may be one reason for the great congregation: there is safety in numbers from predators such as rays. Great gatherings are also great places to find mates, and receptive females will attract large numbers of males. But aggregations don't always include receptive females, and males can't mate when they have soft shells. So the reason for the great migration is still partly a mystery.