Australian scientists need public to help solve spider crab mystery

Winter is coming. And with it come the armies… of crabs.

As the shallow waters off Australia’s southern coast begin to drop in temperature, native spider crabs (Leptomithrax gaimardii) congregate in numbers that tip the scale towards the thousands. They’re getting ready for the battle of their lives, one whose outcome is solely up to how quick-thinking they are.

The enemy? Themselves. Technically their shells, which seem to have shrunk in size as they age. In order to protect themselves from predators, spider crabs possess an exoskeleton which becomes too tight as the animals grow. Once a haven from the perils of the ocean world, the exoskeleton is now suffocating.

Once a haven from the perils of the ocean world, the exoskeleton is now suffocating.

They aren’t alone with this issue, as all who have come together face the same problem. And all are wanting to evade hungry predators while they shed their ‘old selves’ to make way for the new. This is an especially vulnerable state, their soft flesh exposed and not yet hardened to defend the crabs from anything, which is why they stick together and hope for safety in numbers. The crabs that have yet to moult stand guard on the outskirts of the group, while the recently-moulted individuals huddle in the middle.

Hundreds don’t make it. But for those who do, they continue the legendary legacy of spider crabs. And while this phenomenon has been broadcast worldwide through nature documentaries – bringing tourists from all over to get a glimpse of the battlefield – scientists actually know very little about these animals.

Believed to be a species that is widely found in deeper waters, humans rarely see them live outside of an aquarium exhibit. They are found in seaweed, reef, and sand habitats in south-eastern Australian marine waters.

They don’t hunt. These scavengers search for dead and decaying matter on the seafloor and have a diet of dead or decaying fish, invertebrates, and algae. Famous for gathering in shallower waters of Victoria and Tasmania during the winter, the aggregation seems to last a few weeks; sometimes they are seen during other times of the year, but it isn’t thought they are moulting in these gatherings. Each time they disappear into the deep as quickly as they emerged, the only evidence they were there the discarded shells from those who did and did not survive.

Their quick departure also leaves researchers with many questions. Just how many gather in these giant moulting parties? How many individuals make up the Australian population of spider crabs? How do they know where and when to go to these specific locations? Why do they sometimes not come? 

Each time they disappear into the deep as quickly as they emerged, the only evidence they were there the discarded shells from those who did and did not survive.

Australia is not the only country with these large aggregations of spider crabs. In 2021, the Cornwall Wildlife Trust confirmed an annual spectacle of Spiny spider crab (Maja brachydactyla), with thousands of male crabs filmed in cold, knee-deep waters. A common species in Cornish waters, they have recently thrived due to climate change and warming sea temperatures and the charity hopes the numbers being seen off the coast are an indication the population is healthy. They are asking members of the public to record any spider crab sightings via its app or online.

And a team of scientists hopes the Australian public can come in and help, too, because researchers only have so many eyes. “These aggregations seem to happen at random times,” says Deakin research fellow in spider crab ecology Dr Elodie Camprasse.

Although the main project is based at the Mornington Pensinsula, south east of Melbourne, there are many insights other citizen scientists can learn from the approach.

“Unlike some citizen science programs, we need our members to record both when they see spider crabs, but also when they don’t, so we can try and establish a pattern of when the aggregations happen,” says Camprasse. “That’s why we’re asking our helpers to keep an eye out so we can ascertain exactly what’s going on – and what’s not – right around Port Philip Bay.”

The Spider Crab Watch citizen science project, created by Camprasse with the support of funding from the Victorian Government, asks those who see spider crabs to report them. The process is simple (just type in your location, time, date, and a few more details) and allows you to make a report even if you didn’t capture any photos. Doesn’t matter to Camprasse how big or how many you see, log the details into the report. The scientists are even encouraging members of the public to log details of any discarded shells they might come across while walking along the beach, since it means a moulting event happened in the area not too long ago.

“Despite their winter aggregations being a world-renowned event, very little is known about the dynamics and ecological role of the aggregations.”

Camprasse

“Log your sighting and upload any pictures (video tutorial here). Both presence and absence information is important! In science, zero (0) is also a valuable result, so if you don’t see any spider crabs on your dive or snorkel, please log this result as well (absence video tutorial),” explains the website.

You may even spot a spider crab shell with a tag on it, as the team is also tagging individuals to see their movements and habitat use along their range. Underwater surveys and timelapse cameras, coupled with the public’s reports may shed some light on what a typical day (or couple of weeks, months, maybe even year) for a giant spider crab may look like.

“Despite their winter aggregations being a world-renowned event, very little is known about the dynamics and ecological role of the aggregations. If citizen scientists record not only when they see spider crabs, alone or in groups, but also if they spend a day on the bay and don’t see them at all, by recording that absence on the app, we will be more equipped to fill some important knowledge gaps,” says Camprasse.

If this project interests you, sign up for Spider Crab Watch updates to see how everyone can get more involved. For more information about the project, contact Dr Elodie Camprasse elodie.camprasse@deakin.edu.au. This project is funded by the Port Phillip Bay Fund and delivered by Deakin University in collaboration with the EcoCentre and ReefWatch.

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