Watch 'gobsmackingly amazing' giant spider crab horde scramble on sea floor

Hundreds of thousands of giant spider crabs have been caught on camera stacked on top of each other, in some cases 10 deep, in the shallow waters off the coast of Melbourne, Australia.

"It's a moving blanket of legs and claws really, it's pretty awesome," Australian marine scientist Sheree Marris, who shot video of the phenomenon while scuba diving in Port Phillip Bay, told the Australian Broadcasting Corporation.

These large crustaceans that resemble spiders with their eight nearly 2.5-foot legs and 6-inch-wide bodies, make their way every year to southern Australia's shores between May and July as the ocean waters cool. There, they molt and sometimes mate, finding safety among their masses.

"I've been diving for 15 to 20 years and I've seen a lot of cool things, but this is the largest aggregation I've ever seen – a never-ending mass of crabs," Ms. Marris told Australia's 9News.

During their annual molting process, the crabs shed their shells in order to make room for larger ones that can accommodate their growing bodies. While they wait for their hard, protective shell to form, they are left vulnerable to predators such as stingrays and sharks, says Marris.

Coming together in the thousands allows the crabs to molt safely, as they stretch across the ocean floor in a moving blanket that can stretch for hundreds of yards, according to Marris's video.

"It's gobsmackingly amazing," Marris told Australia's 9News.

Another diver last year filmed the giant crabs that had gathered at the same location. This time, they were stacked in a giant, crawling pyramid. The scuba diver, PT Hirschfield, said she had never seen the crabs stacked in such an arrangement.

"That's really unusual," she told 9News at the time.

"When they usually come in for their pre-migration, they usually stack about five or seven high," she said.

One type of the crab, the giant Japanese spider crab, is the largest known species of crab and can live up to 100 years. It's body can be 15 inches in diameter, with their legs stretching to 15 feet, according to the Tennessee Aquarium.