The crabs mass together to increase their chances of survival during the half hour it takes to moult and shed their old shells. During the full moon cycle, the ocean currents are stronger which helps the crabs shake free of their shells but once they are released they are still at high risk as their soft shells make them the choice prey of the rays.
When the masses head to the piers for safety the rays follow. The crabs eat as they march - seastars, seaweed and sponges on the pylons nourish the crustaceans. When the moulted crabs emerge the back of the shell cracks, body fluids ooze out which the rays detect and swiftly move in to crush their prey with plate-like teeth. The rays have sensitive rostrums which they use to smell out the recently moulted and easily consumed crabs. With up to twenty rays feeding at the same time the crabs' only hope is to moult quickly and hide on the pylons until their shells harden enough to no longer be of interest to the rays. Once their shells harden these survivors will head to deeper water until the next annual migration.
