Green Crabs in Australia
An interview with Marine Biologist & Artist René Campbell
Q: Hi René great to chat with you! Do you mind telling us a bit about yourself?
A: G’Day! I am René Campbell - I am a marine ecologist (Bsc Honours in Marine Biology at Flinders University) and professional visual artist and scientific illustrator based in Adelaide, South Australia. I have just submitted my PhD thesis for examination which focused on the ecology and population biology of green crabs (Carcinus maenas) in South Australia and have researched this species for five years. I am interested in community ecology, marine bioinvasions, and issues relating to coastal and natural resource management of our marine ecosystems. Thanks for having me - I am a big fan of Greencrab.org!
Q: How did you get involved with/ learn about green crabs?
A: I started researching green crabs in 2015 as part of my Honours research project (which is essentially a Bachelor’s thesis). My research looked at the population structure and predatory effects of green crabs in South Australia, including impacts on native and commercially important bivalves (which has since been published).
I was not actually aware of green crabs until my Honours project. My project supervisor, Professor Sabine Dittmann, had informed me about green crabs in South Australia. That is when my awareness for marine invasive species started. Here in Australia, a lot of our focus has been on terrestrial and freshwater invasive species, but I think we have a way to go for raising awareness about marine invasive species.
Q: You’re both a visual artist and marine ecologist. How do you integrate these two identities?
A: I have been creating art and maintained an interest in ecological sciences since early childhood. The tough part was choosing which career to focus on: art or science. I ended up pursuing marine biology but maintained my art on the side, taking commissions and eventually starting my freelance business. When scientists I worked with saw my work, my artistic skills started to become integrated into science quite naturally. Commissions I have worked on include scientific artworks for books, scientific diagrams and illustrations, and graphic design materials. Now I consider myself an artist and scientist. I was quite shocked to find that visual science communication and illustration is in demand, and I am now actively sought over by scientists globally to illustrate research. Some of my works can be found here.
Q: Could you share some of the main takeaways of your thesis “Morphological variation, reproductive biology, and genetic structure of an invasive marine crab, Carcinus maenas”?
A: My PhD thesis researched aspects of Carcinus maenas biological traits and population biology that drive invasion success of this species, with a focus on the South Australian green crab population. Some main takeaways were that the crabs show morphological variation in their body shape depending on habitats they are found in, which indicates a plastic response to the environment. Green crabs in southern Australia are the only populations found among mangrove habitats anywhere across the green crabs’ global distribution. Being able to respond to local habitats and environment conditions is essential for introduction and persistence of invasive species.
Another key aspect was the reproductive biology of female green crabs: I discovered that females will spawn nine months of the year when water temperatures decrease below 18 Degrees Celcius, and on average produce 200,000 eggs per brood! Large females (65 mm carapace width or more) can produce up to 500,000 eggs per brood. Reproduction is an important aspect for maintaining population densities and range expansion.
Finally, I assessed the genetic structure of green crabs in South Australia and contrasted my results with the global genetic structure of this species. The South Australian population is genetically distinct, driven by geographic isolation from southeast Australia and has high genetic diversity. High genetic diversity is frequently observed in marine invasive species as it assists with overcoming challenges during introduction, known as founder effects. Genetically, the South Australian population appears to be the result of multiple introduction events from Europe and southeast Australia.
Research findings from my thesis are currently being prepared for publications so when they are published I would be happy to let you know!
Q: How and when were green crabs introduced to Australia?
A: Green crabs were introduced to Port Phillip Bay, Victoria (Australia) in the late 1800s and were transported to Australia via the dry ballast of wooden vessels of early European colonisers. Green crabs spread from Victoria into southern New South Wales around 1971, South Australia in 1976, and Tasmania in 1993 (Thresher et al. 2003). The Tasmanian population is actually a secondary introduction from mainland Australia (Victoria and New South Wales) driven by interstate shipping, aquaculture transfer and/or natural larval dispersal. Meanwhile the South Australian population appears to be part of the mainland introduction. There was also an individual green crab found in Fremantle, Western Australia in 1965, but no establishment has yet occurred in Western Australia. While ballast water was identified as a major vector it is now well managed in Australian waters, and now biofouling and aquaculture transfer are major concerns for green crab transport.
Q: What impact do green crabs have on ecosystems, species, and fisheries in Australia?
A: Compared to North America, the ecological and commercial impacts of green crabs in Australia are not as well researched or documented. In Tasmania, a study by Walton et al. (2002) found that green crabs had higher predation rates on native mud cockles (Katelysia scalarina) than other native predators, and fed extensively on juvenile cockles. Reduced cockle densities could impact recruitment of cockles for the local fishery in Tasmania.
Our study (Campbell et al. 2019) looked at South Australian green crab predation on commercially important cockles (Katelysia peronii) and habitat-forming mussels (Xenostrobus inconstans) using choice and non-choice predation experiments. Male and female green crabs significantly preferred the mussels over the cockles in the choice and non-choice experiments. We then tested the shell strength of these bivalves and found that the shells of cockles were significantly harder than mussels. Hard-shelled prey are more difficult for green crabs to break and open and can damage their claws, so they maximise their time by feeding on smaller and/or softer-shelled prey. We tested shell strengths of some other bivalve species in the area and found that most bivalves are susceptible to green crab predation based on their claw-crushing strength.
The overall ecological and commercial impact is not known, but our findings are of fishery management concern. Soft-shelled mussels are important habitat engineers in these intertidal ecosystems and could be impacted by green crabs, especially at large densities. Meanwhile, Katelysia cockles are commercially harvested and are part of a AUD $1.2 million fishery in South Australia. While large cockles are generally safe from predation, small cockles are still susceptible and so this could impact cockle recruitment. Finally, an AUD $20 million oyster reef restoration project has begun in southern Australia, including South Australia, Victoria, Tasmania, and New South Wales where green crabs are established. Large oysters are unlikely to be impacted by green crabs, but predation on juvenile oysters during the seeding process is of concern in some habitats.
Competition with native crustaceans is not well documented either. Commercially important blue swimmer crabs (Portunus armatus) are much larger and more aggressive than green crabs and may be able to withstand green crab competition, however blue swimmer crabs prefer subtidal habitats and so these species may not interact frequently. Other native crabs, like reef crabs (Ozius truncatus) have been photographed feeding on green crabs, including a female with eggs, which is a positive sign! Work by Garside et al. (2014) in New South Wales documented potential predators feeding on green crabs such as octopus and several fish species; while these predators could never feasibly be used as a management tool, it is very positive to see that native predators can recognise green crabs as prey.
Q: Where in Australia are populations of green crabs densest?
A: Density estimates of green crab populations in Australia is limited. I have heard anecdotal accounts that some parts of Victoria (i.e. Gippsland Lakes) and Tasmania have high densities, while densities in estuaries and enclosed bays throughout New South Wales and South Australia do not appear to be very high. What we do know is that a) the densities are not as high as those seen in North America; and b) densities fluctuate across years and between populations. For example, here in South Australia we can catch as many as ten-twenty green crabs per trap depending on year and season, but usually we tend to find less than five in a trap (Dittmann et al. 2017).
Q: Do you worry that green crabs could become a problem in Queensland and Western Australia?
A: Green crab establishment in Western Australia is a big concern for the Commonwealth Federal Government. In 1965, one individual green crab was located in Fremantle, Western Australia. While surveys have not located green crab establishment in Fremantle (Wells et al. 2010), the shipping activity, water temperatures and enclosed bay habitats of this area are suitable for green crab invasions. Queensland is not as much of a concern: it is possible that green crabs could establish in south Queensland but warmer water temperatures are likely limiting expansion into Queensland. Queensland is subtropical and tropical and green crabs have not yet established anywhere in the tropical zones even though single records throughout the equator are confirmed. While green crabs have a high thermal tolerance, egg development does not occur above ~18 Degrees Celcius and larval development is also stunted. Modeling of green crab expansions in the future limit most of their distribution within temperate and subpolar latitudes. It remains to be seen how climate change could alter this distribution in the future, but so far management concern is for Fremantle, Western Australia, which is at a similar latitude to South Australia.
Q: Do any current fisheries or markets exist for green crabs in Australia?
A: I am commonly asked if eating green crabs or starting a fishery is a viable management tool when I discuss my research. I actually have a slide in nearly all my presentations that features “The Green Crab Cookbook” in anticipation for this question. The answer is that it really depends on the demand. Currently, there does not appear to be demand for green crab consumption here in Australia as blue swimmer crab, mud crab and sand crab dominate the crab fishery market (they have a lot more meat). Uses for bait or compost have also been suggested, but again, the market needs to be there first. And that market starts with awareness. I genuinely think some crab fishermen have come across green crabs at my field sites but thought they were a native species!
Green Crab Illustration by René Campbell
Q: What management approaches do you think would be the most effective for mitigating the impacts of invasive green crabs in Australia and globally?
A: The management approaches depend a lot on the stage of invasion. As introduced species establish, the cost and difficulty of managing that species increases. Established species with self-sustaining populations (such as green crabs in Australia) are unlikely to ever be eradicated. Therefore, management strategies for established species shifts to protecting assets (i.e. shellfisheries) or preventing range expansions and population density increases by managing vectors (i.e. shipping, aquaculture).
Nearly all control methods for green crab in Australia have limited success. Physical removal (collection by traps or by hand etc.) has been useful for surveys but is time intensive and you won’t collect all crabs this way. It can be useful for early phases of introduction just prior to establishment and for research purposes, such as developing environmental DNA (eDNA) surveillance methods. Biological and chemical controls also have limited success: these methods are often not species-specific, meaning many native and non-target commercial species are impacted (and possibly at greater detriment than what green crabs cause themselves). Castrating parasites that can “sterilise” male green crabs are also not host-specific and currently non-viable as a management tool.
An interesting control method is the use of heated water baths on aquaculture mussel seed to kill green crabs between mussel transfer. It worked really well in a study by Best et al. (2014) in Canada but would be dependent on the bivalve species used in aquaculture, and is really only useful for mussel seed transfer. This would be difficult to pull-off in a natural, open system like a harbour. The study also only looked at juvenile green crabs, so how adults react to heated water baths is currently unknown.
A lot of focus on green crab management is based on reducing range expansion. Surveys using physical and molecular methods are often routinely undertaken to help determine the presence/absence of marine invasive species in ports. Australia has implemented rather rigorous ballast water management throughout the country, and now management focus is directed at biofouling. I am definitely supportive of research and development in areas of marine pest and biosecurity management, and hope that I can assist in this area more in the future!
References and links for further reading :
Best, K, McKenzie, CH and Couturier, C 2014, ‘Investigating mitigation of juvenile European green crab Carcinus maenas from seed mussels to prevent transfer during Newfoundland mussel aquaculture operations’, Management of Biological Invasions, vol. 5, pp. 255-262.
Campbell, RT, Baring, RJ and Dittmann, S 2019, ‘Cracking the cuisine: invasive European shore crabs (Carcinus maenas) select a menu of soft-shelled mussels over cockles’, Journal of Experimental Marine Biology and Ecology, vol. 517, pp. 25-33
Dittmann, S, Baring, RJ, Jessup-Case, H and Campbell, RT 2017, ‘Monitoring and morphometric studies of the European shore crab (Carcinus maenas) in Gulf St. Vincent, 2016/2017’, Report for the Adelaide and Mount Lofty Ranges NRM Board, Flinders University, Adelaide SA.
Garside, CJ., Glasby, TM., Coleman, MA., Kelaher, BP. and Bishop, MJ. 2014. Putative predators of Carcinus maenas in eastern Australia’, Estuaries and Coasts, pp. 1-12.
Thresher, R, Proctor, C, Ruiz, G, Gurney, R, MacKinnon, C, Walton, W, Rodriguez, L & Bax, N 2003, 'Invasion dynamics of the European shore crab, Carcinus maenas, in Australia', Marine Biology, vol. 142, pp. 867-876.
Walton, W. C. MacKinnon, C. Rodriguez, L. F. Proctor, C. Ruiz, G. A. . 2002, Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia), Journal of Experimental Marine Biology and Ecology, vol. 272, pp. 171-189.
Wells, FE, McDonald, JI & Travers, MJ 2010, ‘Absence of the European shore crab, Carcinus maenas, from the Fremantle marine area, Western Australia’, Records of the Western Australian Museum, vol. 25, pp. 378-381.