They look like a sinister, marauding mass as they emerge from the underwater gloom. Their reddish-white bodies appear covered in dusty debris – as if they were escaping an apocalypse. George Lucas could hardly have done a better job.

Marching en masse with legs so long they look like they're striding on stilts, the giant spider crabs clamber over anything in their path – including their slower or weaker peers.

This creepy crustacean corps numbers in the thousands. But it's an underwater army without a general. The crabs follow no one.

The usually solitary individuals, seemingly programmed, make their march from out of the inky depths, heading for the shallow waters off Blairgowrie in Port Phillip Bay with unrelenting determination.