#2: The Clibanarius conundrum
It is often said that St. Piran’s hermit crabs (Clibanarius erythropus) are the bashful beauties of our local rocky shores. While this may be true during the day, in my experience, when the water temperature is 15C or below, these crustaceans are anything but shy. As soon as the sun goes down, especially in the summer when the intertidal waters range from 17 to 21C under the cover of night, these crabs come out in force, foraging and exploring for the snail shells they use to protect their squishy abdomen.
There is no doubt that these are charismatic crabs. With the speckled black and white eyes, rust-coloured antennules and antennae, and blue and red-striped legs, these hermits are stunning. However, despite being named for the patron saint of Cornwall, the St. Piran’s crab is not indigenous to the Cornish coast. Current research shows that this is a warm-adapted species, with a Mediterranean origin. It is found along the North coast of Africa, around the Greek Isles, up and down the coast of Italy and throughout the Côte d’Azur into Spanish and Portuguese waters.
Clibanarius erythropus
Mediterranean hermit crab
While beautiful, this is a nonnative species that may be causing local populations of native hermit crabs a bit of chaos.
Since the crab is a Mediterranean species, why is it called the St. Piran’s hermit crab? Well, it didn’t used to be called that. In 2016, BBC Springwatch held a competition to name this little invader, which won out. It seems to have the unfortunate effect of making the crab seem more local than it is. This would not be a problem if the crab had no ill effects. However, ongoing research by Ari Drummond and other members of The Crab Lab reveals that this crab is taking valuable shell resources from native species, especially the common hermit crab, Pagurus bernhardus.
If all that wasn’t bad enough, recently, the crab was nominated by The Guardian as Invertebrate of the Year. It goes to show that there is a politics to naming with winners and loosers, impacting in how the wider public perceive our natural world. It clearly pays to be a pretty species and have a name that makes you seem like you belong.
The Guardian article (linked above) and other anecdotal sources frequently state that this hermit crab died out because of an environmental disaster and subsequent clean-up efforts. I have been unable to find any scientific, research-based evidence that supports this claim. While the Torrey Canyon spill was an ecological catastrophe, this is not likely why C. erythropus disappeared from the local shores. Instead, after arriving on unusually warm currents in the late 1950s, this hermit spread along the coast until a decrease that returned water current temperatures to normal slowed the spread down. Now, record-breaking increases in water temperatures are coinciding with break-neck expansions. In some areas, this species has all but displaced the native common hermit crab [ongoing research]. Work comparing metabolic rates between the two species [in progress] shows early evidence in support that as warm water stresses the native hermits, the Mediterranean species thrives.
Clibanarius erythropus foraging
In the summertime, when the seas are warm this Mediterranean hermit crab comes out in force to find shells and forage.
The story of this Mediterranean crab shows how we need to be cautious when reporting and interpreting evidence about the spread or disappearance of a species. It is far too easy to find correlations rather than causations and report on these as fact rather than hypotheses. Additionally, before celebrating the arrival of a new species, we should assess its impact on native wildlife. As the climate warms and ranges of southern species expand northwards, we may see other new arrivals to the rock pools of Devon and Cornwall. It is essential that we recognise this as part of a global climate change, something that should concern us, and cause us to reflect on what these habitats will look like if the new arrivals drive the native inhabitants of these rockpools to local extinction.
Referenced material:
Barkham, P. (2024). St Piran’s hermit crab – an opportunist with stunning eyes. The Guardian. [online] 7 Apr. Available at: https://www.theguardian.com/environment/2024/apr/07/st-pirans-hermit-crab-an-opportunist-with-stunning-eyes [Accessed 19 Apr. 2024].
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Ramos, J.E., Pecl, G.T., Moltschaniwskyj, N.A., Semmens, J.M., Souza, C.A. and Strugnell, J.M. (2018). Population genetic signatures of a climate change driven marine range extension. Scientific Reports, [online] 8(1). doi:https://doi.org/10.1038/s41598-018-27351-y.
Southward, A.J. and Southward, E.C. (1977). Distribution and ecology of the hermit crab Clibanarius erythropus in the western Channel. Journal of the Marine Biological Association of the United Kingdom, 57(2), pp.441–452. doi:https://doi.org/10.1017/s0025315400021858.
Southward, A.J. and Southward, E.C. (1988). Disappearance of the warm-water hermit crab Clibanarius erythropus from south-west Britain. Journal of the Marine Biological Association of the United Kingdom, 68(3), pp.409–412. doi:https://doi.org/10.1017/s0025315400043307.