New citizen science program to unlock mystery of giant spider crabs

The Citizen Science program is seeking the help of everyone, from recreational fishermen, divers, charter tour operators and boating enthusiasts to join 'Spider Crab Watch' so that data can be collated to work out not only why the amazing natural phenomenon happens, but also where and when it's happening.

Deakin research fellow in spider crab ecology Dr Elodie Camprasse is leading the program and said spider crabs and their aggregations in Port Phillip Bay had long been surrounded by mystery, with little scientific data collected so far.

"These aggregations seem to happen at random times," said Dr Camprasse. "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.

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."

In what is known scientifically as 'aggregation', giant spider crabs come together in shallow parts of the bay in huge numbers at various times of the year, but also during the cooler months when they get more attention, including a regular aggregation close to Rye and Blairgowrie pier, as well as St Leonards and Queenscliff.

The winter phenomenon is thought to be associated with moulting and perhaps reproduction, with thousands of crabs moving from deeper waters where they are usually more widely dispersed. But more evidence is needed to understand the purpose of aggregations at other times of year and the drivers of all aggregations.

Now, with the support of funding from the Victorian Government, 'Spider Crab Watch' includes an iNaturalist app for citizen scientists to report back sightings of the crustaceans.

"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," said Dr Camprasse.

Collecting information via the Spider Crab Watch iNaturalist app is the first step in a series of activities community members can  get involved with as part of this program.

Later in the year, citizen scientists will have the opportunity to analyse images of the spider crabs, including those gathered by timelapse cameras, and help researchers count spider crabs during aggregations, as well as report on the presence of predators.

The citizen science program, designed in partnership with the EcoCentre and Reefwatch, will be complemented by more traditional scientific research, including:

* acoustic tagging to understand spider crab movements in and out of Port Phillip Bay;

* leveraging a recently established network of acoustic listening stations;

* baited underwater cameras to gather information on the presence of predators during aggregations, as well as information on fish diversity across Port Phillip Bay; and

* underwater diver surveys to document the species abundance at aggregation sites.

Spider Crab Watch · iNaturalist