Diving for an explanation to the mysterious migration of giant crustaceans

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Every winter, thousands of hoary-backed giant spider crabs move from the open ocean into Port Phillip Bay and congregate en masse, before their annual moult.

This mysterious species has presumably been making this annual migration for millennia – yet very little is known about them.

Where, exactly, do they come from? We don’t know. How do they know it’s time to march? Not sure. And why do they come here? Well, we can guess, but it’s not definitive.

Now, for the first time, researchers from Deakin University have studied giant spider crabs –Leptomithrax gaimardii and also called great spider crabs – to try to understand more.

“I’m amazed how little we know, not just about spider crabs but the biodiversity of the bay in general,” says Deakin post-doctoral researcher Dr Elodie Camprasse. “There’s many species that we love and interact with that we know very little about, like basic questions of biology and ecology.”

Researchers also hope their work can help ease tensions between environmentalists and fishers. These last erupted in 2020, when the crabs congregated around Rye pier and the water became crowded with people hauling hundreds of crabs out of the water and dropping litter in the water.

An aggregation of giant spider crabs in Port Phillip Bay.

Giant spider crabs are a native species and can be found in marine water across the Great Southern Reef: the stretch of coast, abundant in forests of golden kelp, that runs for more 8000 kilometres from Kalbarri in Western Australia to the NSW-Queensland border.

They move into Port Phillip Bay in May and June. They have been seen in aggregations in Tasmania, the Gippsland Lakes and South Australia, but the bay is where they reliably gather, year after year.

It’s not clear exactly where they come from. “Either they live in the bay, and when it’s time they come together and moult, or they could come in from Bass Strait, perhaps even marching together and entering the bay through the heads,” Camprasse said.

The Deakin research included a citizen science component, where more than 60 snorkelers, divers and other bay users logged hundreds of sightings of spider crabs on iNaturalist, a global social network that maps nature across the globe.

Giant spider crabs are a charismatic, species living in Port Phillip Bay. New research sheds some light on this mysterious species.

Underwater time-lapse cameras were deployed to monitor the crabs at Rye and Blairgowrie piers on the Mornington Peninsula and at St Leonards near Geelong. More than 65,000 images were uploaded, which allowed 2500 citizen scientists to help identify spider crabs and other marine life.

Divers also put acoustic tags on 50 spider crabs to increase understanding of their movement, with most crabs found to remain after the moulting season in the southern part of Port Phillip Bay.

Observers used to think the crabs came together to moult and then mate, but Deakin researchers did not observe many examples of mating. This is fascinating because, based on similar species, the female crab needs to be soft-shelled to mate. Her genital opening is covered when she has a hard shell and becomes inaccessible.

“Our main theory is they come together when they’re moulting as a ‘safety in numbers’ approach because that’s when they’re most vulnerable to predators,” Camprasse said.

Giant spider crabs seem to prefer shallow water.

This year, for the first time, the crabs aggregated in piles near Blairgowrie but did not moult. And then they disappeared altogether. It was a nervous wait until a member of the public spotted a gathering of crabs off the coast at St Leonards, on the other side of the bay.

The project greatly increased the quantity and quality of spider crab sightings, despite a challenging season where spider crab activity was low at the main sites where people look for them. Researchers also found the community was reluctant to log spider crab sightings because of fear of revealing locations of aggregations.

Camprasse said more funding would allow her to complete a second year of observations. She is motivated to share the incredible biodiversity of Australia’s southern waters with a new audience.

“I’m amazed at how little people know what’s in their blue backyard. But here on the Great Southern Reef, we have a very high number of species found nowhere else in the world,” she said.

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