Giant spider crabs swarm in Port Phillip Bay

But for the divers of Melbourne, the excitement is building. This is one of the great under-sea events of the year and it’s right here, in the shallow waters of our bay.

PT Hirschfield devotes hours per week to exploring the underwater worlds of Port Phillip Bay and is such a devotee of the giant spider crab migration that she set up an exclusive Facebook group for fellow enthusiasts, to share photos, sighting notes and ‘citizen scientist’ observations.

She first saw the crabs in about 2012. She’d heard the whispers, the stories, and once she laid eyes on them she was transfixed. But seeing the crabs is not a straightforward exercise. The diver grapevine can trill that the migration is happening off, say, Rye Pier, only for the crabs to be gone the next day. You have to be lucky, and you also need to know how to read the signs that an aggregation might be about to happen. A small cluster of spider crabs might show up, for example, possibly checking for adequate food sources, nobody is sure, before the main body of crabs arrives.

Marine biologist and local diving identity Sheree Marris says nobody knows exactly why thousands upon thousands of spider crabs decide to form one massive heaving seabed of crabdom. No scientists have adequately studied the event or the crabs. She says there are theories that it’s a mating ritual but adds she has never observed the crabs breaking off into thousands of couples.

A more likely theory is that the crab swarm offers protection when the spider crabs are at their most vulnerable. The crabs that arrive have dirty, brown, ageing shells, covered in gunk and grime. While swarming, they lose that exoskeleton and a new bright-orange shell hardens up. But while the new shell is soft, they are at the mercy of predators. Diving with the crab swarm means also diving with the smooth rays that prey on them, or octopi, or even aerial attacks by gannets, deep diving into the crab swarm.

Film crews and diving enthusiasts from across the world will gather to try and spend time with the crabs before they vanish again, to who knows where (probably into the deep waters of Port Phillip Bay, but nobody is sure). Sir Richard Attenborough’s documentary filmmakers are among the international crews that have shown up in the past.

And it’s easy to see why. As far as anyone can tell, this is close to a unique world event. Crabs have been observed swarming in other parts of the world, but only at depth. Anybody can watch our crabs, by snorkelling straight off the beach near Blairgowrie, Rye or Rosebud Pier, depending on where the spider crabs have decided to meet. Although you need to be smart, watch for boat traffic and be respectful of the crabs themselves. Swarming, vulnerable spider crabs, even giant ones, need their space. Even if we’re not sure why.

“It’s an incredible mystery,” Sheree says.

“I’ve dived in the Solomon Islands, Fiji, Vanuatu, all over the place, but nothing beats the Peninsula,” PT Hirschfield declares. “I can dive there five times a week and never get bored.”