SPIDER CRAB watch
Decoding the Spider Crab Conundrum
What is it that drives people each year to plunge into the cold waters of Port Phillip Bay at the beginning of winter? Is it the glory of being the first to know that the annual army of crabs has arrived? The pride of documenting their chosen aggregation site? Or the hope that photos and observations might unlock the secrets behind this bizarre annual spectacle?
Dr. Elodie Camprasse, leader of the citizen science program Spider Crab Watch, isn't sure exactly why people do it, but she is eternally grateful for the dedicated volunteers who contribute to the project. Their efforts are vital, given how little we still know about these iconic animals beloved by the Great Southern Reef community.
The Annual Gathering
Every winter, native spider crabs gather in the shallow waters of Port Phillip Bay, amassing in their tens of thousands to complete their moulting process. Giant spider crabs, like all arthropods, have jointed legs and a hard shell called a carapace. To grow, they must shed their old shells, expand their bodies, and harden new shells. This process leaves them incredibly vulnerable to predators, which is why scientists believe they aggregate in such large numbers. The changing aggregation sites and timings each year add to the mystery.
Moulting: A Vulnerable Transformation
Spider crabs are arguably one of the most iconic and mysterious species of the Great Southern Reef. This species can reach up to 70cm across the legs, although most are not this big. While they are found right throughout the Great Southern Reef with anecdotally documented aggregations in Tasmania and South Australia, Port Phillip Bay is the only place so far where aggregations are documented to occur on an annual basis. However, the timing and location of these gatherings remain highly unpredictable.
Citizen Scientists
So little is known about the phenomenon that researchers are turning to citizen scientists for help. Scientists and researchers at Deakin University, with funding from the Department of Energy, Environment and Climate Action (DEECA), have launched Spider Crab Watch, which is currently supported Parks Victoria. This program allows citizen scientists to collect valuable data through the iNaturalist app.
"These aggregations seem to happen at random times," said Dr Camprasse. “Up until recently, we thought that moulting was a phenomenon limited to winter (May-June mostly), but recent anecdotal information suggests that moulting could also be happening earlier in the year.”
Understanding the basics
“Our understanding of the species is so basic. We don’t know how the spider crabs mate and when they mate. We don’t have population numbers. We don’t know how many crabs come together during moulting time, and whether that varies through time naturally or as a result of human activities. Through being in the water doing scientific surveys and the citizen science aspect of it as well…we could be discovering just about anything.”
There are still many questions that scientists are trying to investigate such as:
• Where do the spider crabs that form these aggregations come from and where do they go afterwards?
• How many spider crabs come together during the moulting aggregations?
• What is the size and sex of the individuals during aggregations and does this vary between aggregations?
• What is the role of the aggregations in keeping the broader environment healthy, particularly the predators that hunt for soft crabs at this time of year?
Ecosystem Implications
Scientists also want to know if the large gatherings of spider crabs help predators maintain healthy populations. Large stingrays, seals, seabirds and sometimes sharks can be spotted near the aggregation sites. The aim of the research goes beyond the spider crabs and hopes to answer questions about how crab aggregations affect the animals at the top of the food chain and what role it plays in the marine environment.
Predation images by Rita Hencke
Modern Research Techniques
In addition to the data that citizen scientists collect, scientists and researchers are also using traditional methods like sexing and measuring spider crabs in aggregations, performing underwater transects to estimate spider crab densities, deploying acoustic tags on spider crabs after they have moulted and even putting listening stations scattered around the coast in order to sneak on spider crabs to know where they go after coming together to aggregate in the Bay.
How to Get Involved
Logging a sighting is a quick process. Just create an iNaturalist account and search for the Spider Crab Watch project to add your observation. You only need to report the date, time and location of the spider crabs, and answer a few questions. If you have photos that’s even better, but not necessary. Every sighting helps this project gain valuable information and insight to this natural spectacle.
Not a diver? You can still help. Since 2022, thanks to funding from DEECA and Parks Victoria, Spider Crab Cams have been deployed in winter at three ‘spider crab hotspot’ locations in Port Phillip Bay. These images helped document marine life and spider crab activity at a time where the spider crab aggregations were expected to occur.
With this data, researchers are aiming at understanding how long aggregations may last and what role they play for other species present in their environment. The Spider Crab Watch program makes this imagery available to anyone interested in the aggregation who could potentially identify predators and count the spider crabs through the Zooniverse platform. The team is currently running a survey to understand citizen scientists’ Zooniverse experience and make improvements ahead of releasing new Spider Crab Cam images, which will be collected this winter.
📷: Denis Lushch
Cracking the Crab Code
At the end of the day, there just aren’t enough marine ecologists and resources to keep an eye on the spider crabs across their range so the researchers at Deakin University are asking for all of the Great Southern Reef lovers out there to help! The contributions of citizen scientists are crucial. By providing data on sightings of spider crabs, volunteers help researchers piece together the puzzle of these extraordinary gatherings. Through collective effort, we can hope to uncover the secrets of the spider crabs' annual pilgrimage to Port Phillip Bay, and perhaps, in time, predict and protect these remarkable events.
If you’re interested in spider crab and spider crab research, you can sign up for Spider Crab Watch updates here. The research team also released a report with the findings of the research carried out in 2022.
📷: Imagery on this page supplied by Elodie Camprasse, Emily May and Rita Hencke.
