Helping solve spider crab mysteries, one citizen scientist at a time

Spider crab aggregation in Port Phillip Bay

About nine years ago, I arrived in Melbourne to start my PhD in seabird ecology. Before this, I had the great pleasure of working in Vietnam as a dive instructor. I used to teach Australian customers during this time, and I was always very excited to tell them that the next step of my journey was going to take me to Melbourne. In response, I used to get confused looks and questions about my motivations. I was told by some of those customers, including Melburnians, that there was no diving in Melbourne. Though diving wasn’t the reason I had picked Melbourne as my next destination, I was certainly hoping that I could enjoy some great dives close to home.

When I finally arrived in Melbourne and got to dive in Port Phillip Bay, I was relieved and pleasantly surprised – there were indeed heaps of breathtaking underwater landscapes to submerge myself in and a suite of unique, weird-looking, and fascinating creatures to hang out with.

When Australians are asked to portray beautiful, productive and biodiverse marine ecosystems, the first thing that would probably come to the mind of most is the Great Barrier Reef, or tropical reefs elsewhere in the country. It saddens me that the beautiful temperate ecosystems on our doorsteps, and indeed in other places in the world, get so overlooked. A lot of Australians are unaware of the beauty of the Great Southern Reef – a series of interconnected reef systems dotted along the southern coast of Australia, from NSW all the way to Western Australia. The Great Southern Reef supports local peoples and the economy, and a high proportion of its species are found nowhere else in the world.

Incredible natural spectacles that only happen in this part of the world also unfold annually on the Great Southern Reef – like the Giant Cuttlefish aggregation in South Australia, or the Great Spider Crab aggregations in Port Phillip Bay.

Giant cuttlefishes in Whyalla
Giant cuttlefishes in Whyalla. Image: Elodie Camprasse

The Great Spider Crab aggregation has captured the imagination of locals and tourists from interstate and overseas. Even Sir David Attenborough featured the annual natural phenomenon on BBC Blue Planet II. And yet, despite all this attention, there is very little scientific information on Leptomithrax gaimardii, the mighty Great Spider Crabs that gather en masse in shallow areas of Port Phillip Bay, and other locations like Tasmania, in winter. Marine scientists like me have so many questions, as do the local community who await with excitement the chance to experience the aggregations for a few days to a few weeks every year!

Spider crabs are arthropods – animals with jointed legs like insects and other crustaceans – and because of their hard, inflexible shells, they need to moult in order to grow. As spider crabs extract themselves from their old shells, expand their soft flesh and grow a new, shinier and bigger shell, they are vulnerable to hungry predators. Rays, seals and sharks alike can’t resist a meal of soft-shell crab! The main purpose of the crabs’ annual gathering is to seek safety in numbers – reduce their individual risk of being eaten by joining thousands of other moulting crabs. Whilst these winter events, which usually take place on the Mornington Peninsula, get most of the attention, spider crabs are known to gather at other times of year and in other places for unknown reasons – yet another mystery to solve!

Spider crab moulting
Spider crab moulting. Image: Elodie Camprasse

Where do they 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? How important is the annual phenomenon in keeping the whole ecosystem and predators healthy?

To answer these questions, we need your help!

If you live in an area of the Great Southern Reef and get out and about to explore the marine environment, we’d love you to report where and when you see spider crabs (and also when and where you don’t!).  My Deakin University team and I have created a new citizen science program – Spider Crab Watch – to solve the mysteries surrounding the lives of spider crabs and their aggregations.

All it takes to join the fun is to create an account on iNaturalist. If you’ve been lucky to come across spider crabs, alone or in groups, on your adventures on or in the water, all you need to create a sighting is the location, date and time. If you have got photos taken underwater or from the surface, that’s fantastic, but you don’t have to have photos to create a sighting when you have seen spider crabs.

We’re also asking people to tell us when they have been out exploring – on a dive, a snorkel, a fishing session from a pier, a kayaking trip, etc – and have NOT seen spider crabs. In order to figure out what kind of habitats spider crabs like and don’t like, knowing where and when spider crabs are not sighted is also really important.

Spider crab in Port Phillip Bay
Spider crab in Port Phillip Bay. Image: Elodie Camprasse

In wintertime, we’re also asking people to look out for moults – the old shells that the spider crabs leave behind after moulting, which can wash up on the beach and tell us that an aggregation happened under the surface nearby.  If you see a moult, please create a sighting to help us get a more complete picture of when and where spider crabs aggregate.

If you’re ever unsure that what you’ve photographed (live animals or moults) is indeed a spider crab, do submit a sighting and experts will review the photos and confirm the species for you – that’s the beauty of the iNaturalist platform, which allows for quality control.

Spider crab moults.
Spider crab moults. Images: Mandy Robertson, Dolphin Research Institute

We’re also getting ready to deploy timelapse cameras at known aggregation sites during the next spider crab gathering in Port Phillip Bay. The images obtained will be uploaded on a portal and we’ll call out on anyone in Australia (or the world!) to help us analyse the images to count spider crabs and help us identify the predators that are around at that time. Sign up for Spider Crab Watch News and stay tuned to see how you can get involved!


Find out more about Spider Crab Watch

Watch the introductory video

Learn how to record an observation where you have seen spider crabs

And how to record an 0bservation where you haven’t seen any spider crabs

Banner image courtesy of Elodie Camprasse.

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