It’s an army like no other and they march not in unison, but in an organised chaos on eight long legs. Their weapons: swollen claws. Their mission: seek shelter in the safety of the sandy shallows and avoid encounters with the enemy … winged giants fashioned with mouths like crushing plates and sharp-beaked birds that swim.
What might sound like the beginning of a sci-fi alien war is, in reality, something much more spectacular: the march of giant spider crabs, one of the world’s greatest marine migrations. And it happens in the waters of Victoria’s Port Phillip Bay.
As the name suggests, these crabs are massive. Their legs are over 70cm long with a body measuring nearly 17cm wide. As the water cools from the sweltering summer heat these soldiers, with their orange armour-plated uniforms, emerge from the depths and advance on the sandy shallows. Their numbers swell into the tens of thousands. The result is a moving mountain of crabs that can be 2m high and extend over 1km. It transforms the turquoise waters of the Mornington Peninsula into a moving sea of orange that blankets the shallows.
This natural phenomenon is believed to be linked to the crabs’ annual winter moult. Like most crustaceans, a hard suit of armour protects these soldiers but, while it protects the crabs, it doesn’t allow them to grow. So the crabs need to get rid of the old armour and grow a new bigger one.
To do this, they secrete a special enzyme that separates the old shell from the underlying skin, while a new soft, paper-like shell is developing beneath the old one. The crabs then absorb seawater and swell up, causing the old shell to come apart – the soldier’s shell opens up like a lid and the crab extracts itself.
Once one crab starts to moult, it sets off a chain reaction, causing the rest of the orange-clad army to moult almost simultaneously. While moulting allows the crabs to grow, it also helps rid them of parasites and other creatures growing on their shells, including bacteria that can weaken and erode their protective uniform.