'Army' of thousands of crabs stacked under the sea

Image of spider crab off the coast of Australia

This year, the army of giant crabs gathered in the two states of Victoria, Tasmania and in the west of Port Phillip Bay (Australia).

These fascinating crustaceans are on a risky mission: to shed their shells to become adults, according to The Conversation.

Crabs cannot simply grow like humans and other mollusks. They have to break out of the old shell to develop the soft flesh and form a new hard shell.

All molting in the water to avoid predators.

The spider crab has just molted next to the old shell

This molting process makes the crab clumsy and weak. That's why they gather in such large numbers to keep each other safe.

South Australian spider crabs are typically orange to reddish brown in color. They can grow up to 16cm on the carapace and 40cm on the legs, and are commonly known as the great spider crab.

After gathering together, spider crabs take about an hour to shed their old shells. The crabs stay together until their new hard shell forms, which can take several days.

Crab gathering time can last a few weeks. The molt crabs take refuge among the crabs, protected by the crabs that have not yet molted.

The spider crabs then return to deeper waters to live their solitary lives, leaving the seabed covered with crab shells.

Close-up of a spider crab

Many mysteries about this crab species that scientists have not yet answered, such as: What signals do crabs know to come together at the same time? Why do crabs gather in one place for many years and then don't come back?

To better understand spider crabs, a scientific project called Spider Crab Watch has been launched, which aims to identify suitable habitats and conditions for spider crab agglomeration. This project is funded by the Victorian Government.