Giant crab horde gathers in Australia

Image copyrightSHEREE MARRIS

Image captionSheree Marris said she swam for hundreds of metres without reaching the end of the crab horde

A horde of giant spider crabs has amassed in waters near the Australian city of Melbourne.

Hundreds of thousands of the crabs migrate to Australia's southern shores each year as ocean waters cool.

Australian aquatic scientist Sheree Marris filmed an enormous gathering of the crustaceans in Port Phillip Bay.

Ms Marris said she hoped to raise awareness of the diversity of sea life in Australia's southern waters.

"Who would have thought something like this, that is so spectacular, could be happening in Australia on the southern shore," she said.

The exact reason for the behaviour is not known, but scientists speculate it is most likely to do with the process of moulting.

Image copyrightSHEREE MARRIS

Image captionIt is believed the crabs gather together for protection, although an alternative theory posits that mating is behind the behaviour

When crabs shed their hard outer shell in order to grow, they are vulnerable to predators such as cormorants and stingrays.

Bunching together in large numbers provides a level of protection against being eaten.

"People think Port Phillip Bay's a marine wasteland … but this is really unique and it's really spectacular," Ms Marris said.

"Not only is [Melbourne] the most liveable city above the water, it is also the most liveable below the water."

Image copyrightSHEREE MARRIS

Image captionCrabs that become isolated from groups are more vulnerable to predators

Image copyrightSHEREE MARRIS

Image captionMs Marris is a sea life enthusiast who once won a Young Australian of the Year award

Image copyrightSHEREE MARRIS

Image captionScientists are banding together to document and understand the reasons for the crabs' behaviour