In short:

Large numbers of giant spider crabs have gathered off Coffin Bay to moult, according to an expert.

An oyster farmer says spider crabs have been sighted before in the region, but not en masse.

What’s next?

Little is known about the arthropods, with a researcher asking the community to log sightings online to help scientists.

A “big invasion” of giant spider crabs in waters off South Australia’s Eyre Peninsula has surprised locals and experts alike.

Hundreds of the bright orange arthropods have been spotted at Kellidie Bay, near Coffin Bay, in recent days.

Oyster farmer Lester Marshall, who has worked in the area for more than 25 years, said he had never seen this many giant spider crabs gather in the region.

“We’ve had a big invasion of spider crabs turn up, they’ve marched up through the channels of Coffin Bay and come right across here into Kellidie Bay in half a metre, a metre of water in the thousands,” he said.

Thousands of spider crabs have aggregated in the waters of Coffin Bay.(Supplied: Mark Thomas)

“Particularly when the sun’s out and they’re bright orange and there are masses of them and groups of 200, 300, 500 — it is a sight to behold.

“They’re so slow-moving but they’ve obviously moved hundreds of kilometres, they come from outside the continental shelf into the bay here.”

Mr Marshall said the mass appearance was unusual but suspected it could be because of an upwelling from the continental shelf when deep, cold water rises to the surface.

Deakin University researcher Elodie Camprasse, who leads a citizen science project on spider crabs, said the creatures usually congregate during the moulting process, where they shed their shells and harden a new shell.

Dr Elodie Camprasse says very little is known about the moulting process.(Supplied)

“They actually have to shed their shell, extract themselves from the shell, pump themselves full of water then they harden a new shell on top of their bigger bodies,” she said.

“At that time they’re often vulnerable to predators.

“The main reason we think spider crabs come together in such big numbers is to seek protection from predators.”

Dr Camprasse said spider crabs have previously been spotted near Kangaroo Island and the Spencer Gulf — but that most reported sightings have been in Tasmania and Port Phillip Bay in Victoria.

She said she was “very excited” to hear about the spider crabs gathering near Coffin Bay.

“We do know very little about spider crabs, about the moulting and their gatherings,” Dr Camprasse said.

Experts believe the crabs gather to moult for safety in numbers.(Supplied: Mark Thomas)

“It could be that spider crabs do gather in this area but that’s just not been reported in a way that us as scientists can access.

“There’s also sightings of spider crabs in Tasmania but it doesn’t seem to be necessarily in reliable locations year after year.”

Dr Camprasse said moulting usually happens during winter but very little is known about the process.

“There are some theories around water temperature having to drop to a certain temperature for the environment to be suitable for spider crabs to moult,” she said.

“Other people think this is related to moon cycles, perhaps particularly around full moon.”

It is hoped citizen scientists will help researchers learn more about the spider crabs.(Supplied: Mark Thomas)

Dr Camprasse said the citizen science program hopes to give more clarity about where, when and how long spider crabs congregate.

“One of the big questions that we have is the role of spider crabs for the broader ecosystem — so are they important in keeping predator population healthy?” she said.

She said the community could report sightings to Spider Crab Watch on iNaturalist to help scientists understand more about the mysterious animal.

Posted , updated