An aquatic scientist in Australia has filmed a video which shows thousands of giant spider crabs gathering on the shore of Port Phillip Bay near Melbourne to molt in a moving mass of shells and pincers.
The clip shows these marine creatures piled on top of one another and stretching for hundreds of meters in the sea.
Scientists says these crabs are most likely shedding their outer shells, and because they have now become soft and smaller in size, they are trying to seek safety in their groups.
Sheree Marris, the marine scientist who filmed the video describes watching the phenomenon as “spectacular”.
“I’ve seen the aggregation so many times but it never ceases to amaze me,” she said.
“This was by far the largest I have ever seen and it’s going to get bigger and better as the crabs are still on the march.”
According to scientists, these spider crabs are eaten by larger prey including seagulls, octopus, and larger fish. They move slowly on their six legs which are long and thin.
Victoria-based Marris says it usual for thousands of spider crabs to travel to southern Australia’s shores between May and July each year as the ocean waters cool.
“What I found really interesting about this aggregation is I’ve never seen so many before,” she told the ABC.
“I swam in a straight line for four-and-a-half 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.”
Ms Marris said she was excited to see the crabs piling on the top of one another.
“I was excited. I was like a kid in a candy shop. The ocean is my happy place,” she said.
“When the crabs have freshly molted, their bodies are soft, making them vulnerable to predators such as rays and sharks.”
“That’s why they commit to the shallows. For crustaceans, for them to grow, they need to shed their shell, which is really hard.”
“They get out of their old shell and they grow a new shell, which is really soft and takes time to harden.”
“So by being in this aggregation, it reduces their chances of being eaten. It’s like a case of safety in numbers.”
Another marine biologist Dr Julian Finn from Museum Victoria says it is difficult to know the exact number of crabs coming to Port Phillip Bay.
“But it is fantastic what is happening. It’s an amazing spectacle that people should go and see,” Finn said.
“We are really lucky that such an amazing thing happens near Melbourne.”
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