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.

It's a once-a-year event. Only this year it began in February – almost two months ahead of peak season.

Julian Finn, Museums Victoria senior curator of marine invertebrates, said February was early but not unheard of for the crabs to gather.

"It is a rarer occurrence," he said. "And usually the ones I've seen coming in then have been smaller."

While little is known about the giant spider crab lifecycle, the spectacular annual gathering coincides with their moulting season, when each crab sheds its exoskeleton and regrows a new one that better fits its growing body.

This moulting moment, which usually takes place between May and June, is when they are vulnerable to predators. Stingrays, it turns out, are fans of soft-shell crab and slurp them up. As are sharks, dolphins and seabirds.

Though no one knows what makes the crabs form a crawling carpet and march into the shallows, it is thought that their vulnerability when shedding their shell makes them move away from hungry predators and seek safety in numbers.

"By coming into shallower waters, they lose some of their bigger predators," said Parks Victoria chief conservation scientist Mark Norman.

The crabs, which can number as many as 100,000 when they gather to moult, have attracted the attention of world renowned naturalist David Attenborough and a Japanese film crew.

"It's more famous worldwide than it is to Victorians," Dr Norman said.

Moulting starts when the crabs start picking at their own eyes. Researchers think this stimulates the moulting process. It takes a few hours to moult and then a few days for them to re-grow the exoskeleton.

Native to Port Phillip Bay, giant spider crabs have also been reported at Wilsons Promontory, St Leonards and Portarlington.

Dr Finn said when not in shallow waters, it is believed the crabs live in the bay's deeper channels that lead out towards the heads.

Footage, shot on February 25 showed crabs that had yet to start moulting. 

Dr Finn said just because they had arrived earlier, didn't mean the shedding would start before May.

Disability educator and diving instructor Chiharu Shimowada, filmed the start of the spider crab migration on February 25.

​"It's almost something from an alien movie," she said.

"But when you are in the water watching them, it gives you such a thrill."