The ocean is a weird place. Today’s proof? The video above, in which hundreds of thousands of crabs weave themselves into a vast living carpet.

The stacking of the crabs is an annual event in Melbourne’s Port Phillip Bay. The gargantuan crabs can reach nearly 30 inches across from claw to claw. With huge claws and thick carapaces, you’d think they’d be all set for self-defense. But once a year, during molting season, the giant spider crabs become very vulnerable.

Like lobsters, crabs continually grow inside their shells, but at a certain point, their shells stop growing. To keep from being crushed inside their own exoskeletons, crabs have to shuck their hard shells and grow new ones. But the moment they climb out of their old armor into the water, the newly de-shelled crabs lose all their defense mechanisms while, unfortunately for them, retaining their deliciousness.

Here’s where the carpet comes in. Relying on the principle of safety in numbers, the crabs converge to form a dense, clacking mat of bodies. The crabs have no concern for personal space, and will stack themselves up to 10 crabs high in a mass that stretches hundreds of meters across the sand. In the hour it takes them to molt and the period of tenderness that comes after that, the crabs can rest assured that no predators will eat them all.

This may seem uncomfortable and kind of creepy to us, but the alternative, as you can see below, is much worse.

Images from YouTube // The Nature of Science