Pang Quong has been scouring the depths of Port Phillip Bay for more than 40 years.

But despite decades of experience, the diving enthusiast has been unable to solve the many mysteries that surround the giant spider crab migration.

"You never, ever say I've seen it all," said Mr Quong, with a smile.

"There's always something new."

It's a Wednesday morning as Mr Quong meets up with his regular diving partner and marine biologist Sheree Marris at Blairgowrie, just near the mouth of Port Phillip Bay.

The wind is calm and the clouds are sparse. It's perfect diving weather.

Sheree Marris (left) and Pang Quong (right) are trying to understand the migration pattern of giant spider crabs. ( ABC News: James Oaten )

We board a boat, wriggle into our wetsuits and check the diving equipment before setting off in search of a mass of giant spider crabs.

Mr Quong stands up the front of the boat, eyes scanning the water, looking for large dark shadows.

"The way we're going at the moment there should be thousands of them," he explains with a calm confidence.

"I don't believe we'll have much trouble finding them."

His prediction is correct.

Just a short trip from Blairgowrie Yacht Squadron, a large mob of spider crabs is seen below the surface. It's time to dive.

Clear skies are a good sign when you're diving in search of spider crabs. ( ABC News: James Oaten )

Bay a safe haven as crabs shed shells

Every year, giant spider crabs migrate to the shallows of Port Phillip Bay, reaching their biggest mass by June.

It's commonly accepted among divers and biologists that the crabs stick together to provide safety in numbers, as they squeeze out of their old, hardened shells and expose their new, soft shells that allow them to grow in size.

But what happens afterwards is a mystery.

The spider crabs disappear into the depths for almost a year and no-one really knows where they go or why.

Researchers still do not know where the crabs go after leaving the bay. ( Supplied: Pang Quong )

Mr Quong suspects the mobs stick together and move to the deeper parts of the bay where fewer divers explore.

Other divers think they disperse throughout the bay and maybe even leave the heads to explore deeper ocean waters.

"In some ways they're the gardeners of the sea," Mr Quong said.

"They move zig-zag fashion, back and forth, feeding on all the algae and sponges.

"I believe they're actually doing that stuff all the time, but in areas that the divers aren't seeing."

But where the swarms of crabs go and why is just one of many questions surrounding the crustacean.

"The thing I really love about the crabs is the mystery that kind of surrounds it," said Ms Marris.

"Plus, it's one of the most spectacular aggregations that you'll see anywhere in the world.

"There's so much that we still don't know about the crabs."

Researchers say the spider crabs are under threat from an invading starfish species. ( ABC News: James Oaten )

Crabs turn to cannibalism during moulting season

As we sink below the water on our dive near Blairgowrie pier, the great mound of thousands of crabs crawling over each other becomes clearer.

But just a few metres away from main group we see something that Mr Quong has only recently been able to film himself: crab cannibalism.

A spider crab is ripping into the flesh of a fellow spider crab.

The victim is turned on its back, missing a couple of legs and helplessly twitching with those that remain.

Once the crabs fully moult they're vulnerable to predators. ( Supplied: Pang Quong )

As we approach, the predator spider crab drags away its meal, fearful of losing its lunch.

"Once the crabs fully moult they're actually open to be predated on, even by their own kind, which is really quite fascinating," Mr Quong said.

"It's a terrible thing to do, really, to eat one of your own kind but when a crab moults, but they do that sort of thing."

Ms Marris said the behaviour is typical for crabs, who are a scavenging species.

The giant spider crabs sometimes resort to cannibalism during the moulting period. ( ABC News: James Oaten )

"When they're moulting, they're growing their new shell, they need as much nutrition and food as what they can get," she said.

It's also common to see large stingrays preying on the spider crabs at this time of year.

Giant spider crabs can be a tasty meal for Port Phillip Bay's stingrays. ( Supplied: Pang Quong )

Crabs under threat from life-sucking starfish

But another lesser known predator is the invasive Northern Pacific Star Fish, a brightly coloured yellow and purple starfish that sticks to newly minted crabs to suck out their juices.

"They fall over the animal and digest the animal, suck in the juice," Mr Quong said.

In footage that Pang Quong has only been able to shoot during this year's migration, large armies of the invasive star fish are seen clamouring over giant spider crabs.

It's a slaughter. The sand is littered with empty crab shells.

"I think we definitely should be concerned," Ms Marris said.

"For starters, it's an introduced species. And it's not only having an impact on spider crabs."

The invasive starfish species poses a serious threat to the giant spider crab. ( Supplied: Pang Quong )

Apart from the dangers surrounding the migration, Pang Quong has this year been able to capture strange mating rituals.

One piece of footage showed a male spider crab lifting a female crab above its head as if to claim her as a mating partner.

"The male will have a female and he'll actually grab it by the claw, hold her up in the air and run around and say, 'mine, mine,'" said Mr Quong, laughing.

"But the funny thing is, among 10,000 crabs you might only see two or three pairs doing it.

"I think what's happening is when the male crab sees the female, the male knows that she's going to release eggs. He can't mate with her until she releases those eggs."

Marine life abundance on Melbourne's 'front door'

The giant spider crab migration has gained a lot of attention in recent years since featuring on the BBC's Blue Planet documentary series.

But despite the abundance of enthusiasm among local divers to monitor and document the giant spider crabs, Sheree Marris and Pang Quong want to see more money flow into researching the migration.

"It would be great to tag them and do some genetic testing," said Ms Marris.

"So we can see where they're moving and who's related to who."

But even without extra money flowing into research, the two hope the giant spider crabs will encourage others to take an interest in Port Phillip Bay.

"Most people think to see diversity or any action under the water you need to go to the tropics or the Great Barrier Reef," Ms Marris said.

"You don't have to. We have kelp forests, sponge gardens and so many weird and wonderful animals.

"If the crabs can be an introduction to showcasing what else we have on our front door step it can only be a great thing."