Dr Elodie Camprasse, Deakin University

A few invertebrate aggregations considered “natural wonders” take place in coastal and marine environments in various places around the world. These aggregations see vast numbers of cnidarians, crustaceans and molluscs come together, mostly to mate or moult. Some of these aggregations unravel annually in and around Australia and include gatherings of giant Australian cuttlefishes, great spider crabs, ornate rock lobsters, and Christmas Island red crabs. 

As a Melbourne-based scuba diver and marine ecologist, I am always puzzled and sad when I think about how little we know about the beautiful, biodiverse and productive marine ecosystems we have right on our doorsteps, particularly on the Great Southern Reef. This part of the Australian coast, which stretches from NSW all the way to Western Australia, is home to incredible and unique species that are found nowhere else in the world, and the first two of the spectacular aggregations mentioned above.

Finding out more about great spider crab mysteries had become an obsession since I realised years ago how scarce information on this iconic species and their aggregations was in the scientific literature. I still can’t quite wrap my brain around how such an amazing natural phenomenon, which has taken place in various places along the Victorian and Tasmanian coast for decades, and has been featured in the press and documentaries locally and internationally, remains so understudied. 

Spider crabs come together in the winter months to seek safety in numbers at a very vulnerable stage of their life cycles. They need to moult in order to grow bigger, and after painstakingly extracting themselves from their old shells, dodging predators on the hunt for an easy meal of soft crabs remains their main challenge. The formation of huge piles of crabs in their thousands offers some much-needed protection for the vulnerable crustaceans.

Where do spider crabs come from before gathering in shallow areas and where do they go afterwards? How do they choose where to go and to aggregate? What signals do spider crabs use to start their annual aggregation? How many are there in the population and how many hang out together during the aggregations? Does these numbers vary from year to year and why? How important is the annual phenomenon in keeping the whole ecosystem and predators, such as rays, sharks and seals, healthy? 

Thanks to funding from the Victorian government, our research team at Deakin University are finally working to put together the pieces of the puzzle and answer those questions using a mix of traditional and citizen science. 

Marine lovers – anglers, boaters, swimmers, divers, snorkellers, beach goers – across the Great Southern Reef are invited to log information when they come across spider crabs, whether alone or in groups, on the iNaturalist Spider Crab Watch project. Spider crabs may also be found aggregating outside the winter months for unknown reasons, so any time is a good time to look out for spider crabs!

Spider crab photos are always welcome, but people who didn’t have the opportunity to take photos are still encouraged to submit sightings. Indicating the date, time, location and answering a few short questions about the observation is all that’s required to log a sighting. Historical sightings are most welcomed too, and we would love to see sightings from people that have come across spider crab aggregations in the past. Lastly, we are interested in sightings of moults – the spider crabs’ discarded shells. Moults can be seen underwater or on the beach and a high number of moults in one place is a good sign that an aggregation has taken place nearby. 

This information will allow our team to have a more precise idea of when and where spider crabs gather, and for how long. We are also interested in correlating these sightings with environmental variables to figure out what trigger the spider crabs’ mass gatherings. 

Timelapse cameras have been deployed under piers on the Mornington Peninsula in Victoria, where the most renowned spider crab aggregations are taking place. These cameras will help document spider crabs and marine life at this time of year. The images obtained will be uploaded to a web portal for citizen scientists to analyse at the end of the season, which could last until June or July. We’re hoping that this data will help us understand how spider crab numbers may vary during the winter months and how the presence and type of predators vary according to spider crab activity. 

This work will complement surveys in Port Phillip Bay, including baited camera surveys to investigate predator activity, underwater surveys to sex and measure spider crabs and understand what the aggregations are made of, as well as acoustic tagging to work out the movements of spider crabs post migration. 

Interested in finding out more about the research program? Sign up to the newsletter to get Spider Crab Watch updates straight to your inbox!

Images:

1) Cuttlefish aggregate to mate in the Upper Spencer Gulf in South Australia. Credit: Elodie Camprasse

2) Spider crab aggregations on the Great Southern Reef are a delight for marine enthusiasts. Credit: Elodie Camprasse

3) Protection against predators such as rays, sharks and seals is thought to be the main reason why spider crabs aggregate in such vast during moulting events. Credit: Elodie Camprasse

4) People who come across spider crabs are invited to log sightings to help our team understand spider crabs and their aggregations better. Credit: Elodie Camprasse

5 & 6) Spider crab moults can be found underwater or on the beach when an aggregation has taken place nearby. Credit: Mandy Robertson, Dolphin Research Institute