Footage Shows Giant Spider Grabs Congregating For Annual Winter Moult
Scientists still aren't totally sure why so many of the crustaceans gather at the same time each year during the Australian winter, which runs from June to August.
But when they do, they shed their shells for new ones - clambering on top of each other in the process and creating something of a natural spectacle.
The video footage, which was shared by Museums Victoria a few months back, shows the crabs coming together en masse in Port Phillip Bay, which sits on the central coastlike of southern Victoria.
An explanation on the Museums Victoria website explains that it takes up to an hour for the crabs to lose their shells - which they do 'almost simultaneously'.
"Like many crustaceans, Giant Spider Crabs are protected by their hard body shell, rather like a suit of armour. The trouble is that a hard shell doesn't allow room for growth. Crabs must shed their old skin to get bigger; they can expand their size in the brief window before the new skin hardens.
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"The process of moulting takes up to an hour and all the crabs in an aggregation moult almost simultaneously."
While the whole thing is 'still a bit of a mystery', Dr Julian Finn - senior curator for Museums Victoria - has come up with some theories based on several years of observation.
One idea is based on the fact that crabs become particularly tempting to predators when they've shed their shell, making them very vulnerable - perhaps hence the power in numbers.
The website continues: "A soft, freshly-moulted crab is irresistible to predators such as rays, seals and birds. By aggregating in the thousands an individual crab reduces its chance of being eaten, much the same way as mammals in herds find protection in numbers.
"Movement into shallow waters may help the crabs, usually dispersed throughout Port Phillip Bay, aggregate in a single mass and gain refuge from the strong tidal currents that scour the deep channels.
"An earlier explanation that the annual aggregations were related to mating has thus far proved unlikely, as following the moulting of tens of thousands of crabs, only the odd couple has been observed to mate.
"We still don't know however what happens when they disperse back into deep water.
"Julian believes this sudden influx of tender crab meat is an important part of the Port Phillip Bay food chain."
Featured Image Credit: Museums Victoria/YouTube