Consider Hyas araneus, also known as the great spider crab. Now consider thousands of great spider crabs—hundreds of thousands. Consider how such a crusty group of crustaceans might scuttle across the ocean floor, piling into a hellish horde that for some is the stuff of nightmares but for others is just plain cool. As Darren Gray reports for The Age, that scenario is playing out right now in Port Phillip Bay in southern Australia—and it has to be seen to be believed.
The video above documents an annual migration that Gray calls “an underwater secret” for Melbourne residents used to the spectacle. Every year between May and June, a gigantic group of great spider crabs sweeps across the ocean floor en masse. Together they walk, eat, and pile up in an epic, swarming traffic jam.
The crabs aren't all struck with the inspiration to take a bay-bound vacation each year. Rather, they head to the bay to molt. As NOAA explains, crabs and other crustaceans continually outgrow their own bodies. Once a year, they molt, reabsorbing some of the old shell and slowly shedding their outdated one. Unsurprisingly, crabs without fully-formed shells are vulnerable to predators. So instead of facing that danger alone, they tend to molt together in a sheltered area.
In the case of the crabs of Port Phillip Bay, the molting animals don’t just huddle up—they pile up. Sheree Marris, an aquatic scientist from the area, tells ABC Australia’s Lucia Stein that each crab must fend for itself. “There’s no hierarchy,” she says. “It’s just this orange chaos of legs and claws. It’s a moving blanket of legs and claws really, it’s pretty awesome.”
Awesome is one word for it. But if you have ostraconophobia (fear of shellfish), you might want to stay away from Port Phillip Bay until June is through and the newly crusty crabs have scuttled back home.