Rare phenomenon on quiet beach amazes Aussies: 'Vanish as quickly as they appear'
Hundreds, and possibly thousands, of crab shells have been spotted strewn across beaches in the country's south, prompting confusion among locals and fears of a mass die-off.
Tasmanians shared photos online this week showing the shorelines of some beaches in the Hobart area "littered with all sorts of pieces of spider crabs". Particularly dramatic images show shells on one beach in the Southern Channel dotting the shore for metres on end, with some bodies almost fully intact.
"They actually form mini mountains like a huge pile on. Amazing to see, but it's only temporary because they vanish as quickly as they appear," one local said.
While the scenes may look alarming at first, Dr Simon Grove said there's "no cause for concern" with the phenomenon likely "an entirely normal occurrence". Curator of zoology at the Tamanian Museum, Grove spoke to Yahoo News and revealed the fascinating truth behind what's occurred.
Grove said in parts of southern Australia, spider crabs periodically head into shallow water to moult. As the water temperature drops in late autumn and early winter, the crabs come together in their thousands, forming towering clusters that can cover the seafloor. The migration is driven by the need to shed their hard outer shells in order to grow, a vulnerable process known as moulting.
"This is the only way the crabs can grow — they shed their old exoskeleton and a new, bigger one then swells out and hardens to replace it," he said. "The theory is that, if they were to moult on their own in deep water, they would be vulnerable to being eaten mid-process, before the new exoskeleton hardened, for instance by a ray, skate or shark.
If they "instinctively" head towards shallow water, Grove explained, they "all end up getting funnelled into shallow bays" where there's safety in numbers. "Even if some of them are predated, the individual risk of being eaten is lower. Then, because of wave action, their shed exoskeletons are often cast ashore, sometimes intact but more usually already fragmented," he said.
This process has been documented before, particularly in Port Phillip Bay in Victoria.
Deakin University's spider crab expert Dr Elodie Camprasse, who has extensively studied the spectacle, says "we still don't know what exactly triggers it". "It could be moon phase, water temperature, or other environmental parameters," she told Yahoo. "We know spider crab moulting happens every year in Port Phillip Bay, but we have very little information about where this phenomenon occurs outside of Victoria.
"As such, getting reports from Tasmania is very exciting."
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Moulting leaves the crabs soft and defenceless for several days, so it's believed they congregate in large groups to reduce the risk of predation. By shedding their shells en masse, they create safety in numbers — a strategy that confuses predators like rays and octopuses, which can't easily pick off individuals in the crowd. The occurrence often sees vast piles of empty exoskeletons washed ashore, startling locals unfamiliar with the phenomenon.
"Typically, this takes place in the cooler months, but in southern Tasmania it can also occur in spring and summer," Grove said. So while it might look like a crustacean catastrophe, it's actually a natural part of a spider crab's life cycle.
Deakin University runs a citizen science program, Spider Crab Watch, to encourage the community to log sightings on iNaturalist when they see spider crabs aggregating in the water or remains on the beach.
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