Spider crabs: no rest for the wicked

Migration – when people hear this term, they usually picture herds of mammals (including people) or flocks of birds en route to places where they can find better conditions for living. However, did you know that Melbourne has a migration of its own in its underwater backyard? Giant Spider Crabs (Leptomithrax gaimardii) put on a show every winter in Port Phillip Bay. If you let me, I’d like to take you on one of the most amazing dives where I was witness to this amazing event. Don’t worry though – you won’t have to get wet!

Giant Spider Crab (Leptomithrax gaimardii). Image: Elodie Camprasse

It is a chilly and dark Friday night and I’m meeting with a bunch of very enthusiastic divers. We have all been looking forward to experiencing the annual Giant Spider Crab migration. It is a first for me and even facing the cold, 12°C water of Blairgowrie could not take the excitement away. A nice, easy beach entry allows us to progressively get used to the water temperature before starting our actual dive under the pier.

The first things we bump into are a tiny seahorse and a few stingarees, which I take as being good signs of the fascinating dive that is to come. As we swim towards the end of the pier, we cross paths with squid swimming in mid-water, porcupinefishes, big Leatherjackets, colourful sea slugs and, as our torches reveal, the vibrant colours of sponges. I am already in awe when we reach, at last, the spider crab aggregation. Here they are, hundreds of them, hanging out in about four to five metres of water. Most of them sport broken limbs and dull, old shells that they will need to get rid of within the next few weeks in order to grow bigger, this being the purpose of their migration. Crabs like to blend in and they tend to pick up bits and pieces from the sea floor and stick them on their shells in order to increase their camouflage. A few of them even “wear” sponges on their heads and backs, ironically making them stand out even more as I try to spot the odd ones out to create better shots. As I focus on taking photos, a curious octopus cruises by to check me out.

Marching spider crabs in Blairgowrie. Image: Elodie Camprasse

This is only one of a few dives I was lucky enough to take part in during this yearly phenomenon. The other dives allowed me to witness the crabs’ movements and behaviours further, at various times of day and in different light conditions. Additionally, I was able to discover bizarre-looking critters, including Tasseled Anglerfish and stargazers, to name a couple. I was amazed to observe the big piles that spider crabs often form during this season, sometimes reaching more than a metre in height as they seek safety in numbers.

Spider crabs forming aggregations. Image: Elodie Camprasse

Towards the end of the migration, the last crabs switch from the sandy sea floor to the pylons and sponge gardens on the walls of the pier. This provides the now more isolated crabs with better protection. When they moult, the crabs release a specific scent that predators such as rays, seals and seabirds are able to pick up and follow to gorge themselves on freshly moulted and hence very soft individuals. The crabs need to wait a few days for their new, brighter-coloured shells to harden in order to become less vulnerable and leave the shallows to resume their solitary life in different parts of the Bay. Watching the Smooth Stingrays circling in between the pylons and waiting for clumsy, freshly moulted crabs to fall within their reach is quite a show. So too is observing the crabs extracting themselves from their old shells: a process that seems exhausting and can take up to approximately half an hour; it almost felt like I was watching a creature being born, right in front of my eyes.

They usually free-fall in the water once they have managed to extract themselves from their old shells, rolling around the sea floor and looking stunned for a few minutes. Only when they are quick enough to recover their senses and climb back to the pylons are they able to escape the hungry rays. Nevertheless, more discrete critters join the frenzy. Small shrimps and seastars hang around the crabs to scrape the last bits of meat off the old shells. By this time, the bottom is strewn with discarded shells, adding to the already apocalyptic atmosphere.

Spider crab removing itself from its old shell. Image: Elodie Camprasse

There is still a lot of mystery revolving around the annual migration, which seems to happen at different sites in different years. Spider crab aggregations have been observed at Rye, Sorrento and St Leonards in the past. Water temperatures or moon cycles might play a role in triggering the phenomenon, although this is only an educated guess. People used to believe that mating occurred after the crabs moulted; this, however, is not supported by observations.

There is not a lot of information about where these usually solitary animals spend the rest of the year and there have been aggregations in the form of pyramids sighted at other times of year, the purpose of which is unknown. Clearly, we still have a lot to learn about these migrations and we will surely gain more information as enthusiastic divers and snorkelers continue to get in the water and share their sightings and behavioural observations on social media. So let’s hope that next year and in years to come, there will be no rest for these wicked crustaceans.