A horde of hundreds of thousands of giant spider crabs, Hyas araneus, is gathering off the coast in Australia and is the stuff of nightmares, or dreams, and has to be seen to be believed.
Scientists do not know why the thousands of crabs, some measuring up to three feet, gather each year between May and June off the coast of Port Phillips Bay, near Melbourne, but swimming through the horde of giant crabs is not for the faint-hearted. But swimming though it is just what one Australian aquatic scientist, Sheree Marris, did and she filmed the great horde of giant crabs up close.
She described the phenomenon gathering of giant spider crabs as several hundred meters long and said that in some places the crustaceans were stacked up to ten crabs high, according to the BBC. The horde of craps do everything together and walk, eat and tumble over each other as they march across the ocean floor.
“There’s no hierarchy,” Morris said. “It’s just this orange chaos of legs and claws. It’s a moving blanket of legs and claws really, it’s pretty awesome.”
Marris said it was impossible to tell how many giant grabs were in the horde, but she hopes her footage brings awareness to 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.
“People think Port Phillip Bay’s a marine wasteland … but this is really unique and it’s really spectacular”
Marris has been swimming in Australian waters for over 20 years and said she has never seen such a large horde of giant spider grabs, or any crabs, in all her years of diving.
“I swam in a straight line for 4.5 minutes and the crabs were thick on the sandy shallows. It was gobsmackingly amazing. [In previous years] I’ve swam maybe a minute-and-a-half to two minutes and [this year] I wasn’t going slow… It’s pretty awesome.”
Some people say that the horde of giant crabs, which can live up to 100 years, gather for mating each year but others think it is to molt and they gather in such large groups purely for protection when shedding their exterior shell.
Crabs often shed their outer shell so they can grow, but ridding themselves of their hard shell means they are more susceptible to attacks from stingrays, sharks and other predators. Staying in a large horde in the shallows reduces their risk of being attacked from above and eaten during molting.
The Nature of Science YouTube page where Marris’ footage is shared explains the molting process in more detail.
“They secrete a special enzyme that separates the old shell from the underlying skin, while a new soft paper-like shell is secreted beneath the old one. The crabs then start absorbing seawater and swell, causing the old shell to come apart. The… shell then simply opens up like a lid and the crab extracts itself.”
What is even more fascinating is that when one giant spider crab sheds its shell it sets off a chain reaction and all the crabs in the horde shed theirs too, so they are all equally vulnerable. The crabs then race to get to the bottom of the horde because the crab at the bottom is the most protected. All the crabs leave behind is a graveyard of removed shells, which is almost as eerie as the horde of live crabs.
The horde of giant spider crabs is expected to stay in the waters just off Port Phillip Bay until the end of June so shellfish enthusiasts still have the chance to swim over the top of the moving crab blanket and see the phenomenon for themselves before the crabs scuttle home. We do not, however, recommend it for people with ostraconophobia, otherwise known as a fear of shellfish.