The Great Spider Crab migration is an annual event in Port Phillip Bay.
This weekend hundreds of thousands of Spider Crabs were gatherd around Rye pier, waiting to molt their exoskeletons and breed.
They can move as the weather changes, this can also affect how long they hang around. It can go on for a number of weeks moving around the bay. However when it gets rough and blows from the north this can set off there molting, and once they get their new outfit they tend to disperse and vanish back in to the deep.