Giant Spider Crabs

If you ever visit the ocean on the South East shores of Australia, you might catch a glimpse of Australia's giant spider crab. These amazing crabs live on the ocean floor and scavenge for food scraps that fall to the bottom. While they spend most of their lives alone, at certain times of the year they gather in enormous groups. While scientists have discovered some things about these crabs, there are still many unanswered questions and there is ongoing research to learn more about them.

Appearance

Giant spider crabs have their name due to their large size. Their body and shell can grow to be 16 cm across, about the size of an adult hand. Their long legs can grow to be 70 cm across, which is about the length of a bedroom pillow, or the size of a classroom desk! Giant spider crabs are arthropods, which means they have a shell on their body and jointed legs. Since they have a hard shell, every time they grow to be too big for their shell they must shed the old one and grow a new one. When it is time to do this, the crabs come together in huge groups in shallower waters.

March of the Spider Crabs

Once a year in the shallow waters of Australia, spider crabs gather together in groups that can grow to over 50,000 crabs. The crabs gather in large piles which can grow to be the height of a person. While in these large groups, the crabs molt. Molting is when the crabs shed their old shells to grow bigger shells. According to marine ecologist Dr. Elodie Camprasse, “while they do that, they’re soft, so they tend to be vulnerable to predators. So we think one of the reasons they come together in such big numbers is because they’re seeking protection from each other.” It takes a few days for the crab’s new shell to fully form and harden. The crabs stay together for up to a few weeks until all the crabs have molted. Afterward, they separate and return to deeper waters, leaving behind a huge pile of abandoned shells. This behavior is so amazing to see, and so unique, that it leaves scientists with many questions.

Ongoing Research

There are many unanswered questions about spider crabs that require further research. For one thing, it’s unclear how spider crabs know when and where they should gather. While most gatherings happen in winter, gatherings have been spotted at other times of year as well. Some scientists think that the water temperature may alert the crabs that it’s time to gather and molt, while others think it may have to do with the full moon. One of the challenges to answering questions like these is that spider crabs don’t consistently gather at the same place and at the same time. For that reason, Dr. Elodie Camprasse created the spider crab watch program. People who commonly fish, swim, snorkel, or go boating in South Eastern Australia may stumble across a gathering of giant spider crabs. If they do, they can take pictures and submit the location of the gathering to the Spider Crab Watch website. According to Dr. Elodie Camprasse, “by having people like divers and snorkelers logging their sightings and submitting photos when they have them, we can find out a lot more than with the limited time we have as scientists to spend in the marine environment.”

Spider crabs are amazing animals not just for their large size, but their ability to gather in big groups from otherwise solitary lives. Their behavior has sparked many questions for scientists. Why do they gather in big groups? How do they know when to come together? How do they contribute to the marine environment? While we don’t have answers to these questions, we can still appreciate the amazing adaptations that allow these crabs to survive in incredible numbers.

Informative | Bioluminescence

Bibliography

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Victorian Fisheries Authority. (2021, June 1). Giant Spider Crab Distribution and Biology. VFA. https://vfa.vic.gov.au/science-in-fisheries/spidercrabs/giant-spider-crab-distribution-and-biology

Jones, H. (n.d.). Spider Crab. Great Southern Reef. https://greatsouthernreef.com/spider-crab

Camprasse, E. (2022, July 15). Why are thousands of giant crabs gathering off Australia’s coast? this. https://this.deakin.edu.au/society/why-are-thousands-of-giant-crabs-gathering-off-australias-coast

Camprasse, E. (2023, November 1). Ecology Matters - Dr Elodie Camprasse. other, Ecological Society of Australia. https://www.youtube.com/watch?v=KHM1D_VW0TQ

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Informative | Bioluminescence