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Heredity Fieldwork Grant from the Genetics Society to research the range expansion of hermit crabs

Last Summer, Christophe Patterson was awarded £1500 from the Genetics Society to conduct fieldwork in Brittany, examining the origins of a range-expanding hermit crab to the UK. Here's an article he wrote for the Genetics Society originally published in their newsletter in July. The original article can be downloaded here.

In recent decades, species have begun shifting their ranges away from the equator and towards the poles. This is due to the changing climate brought on by the increased concentration of CO2 in the atmosphere and the oceans and has the potential to have profound impacts on ecosystems across the world. Species may begin interacting with novel competitors or be freed from conflict as other species disappear. Those unable to tolerate changes to local temperature and/or precipitation.

One such species that is range expanding northwards is the warm water hermit crab Clibanarius erythropus. This species lives throughout the Mediterranean Sea, as well as southwards down to Morocco and northwards to the coast of Brittany in France. The species did make a short appearance in the UK in the 1960s but was wiped out by the Torrey-canyon oil spill with only a few individuals surviving at two unaffected sites. Apparently unable to reproduce or sustain the population, those remaining disappeared sometime around the 1980s. However, in 2016 the species reappeared across the south-west of the UK, in Cornwall and Devon.

C. erythropus’ is notably different from the other species of hermit crab found in the rockpools of the UK. It is rusted red, with electric blue highlights on its legs and claws. These claws are equally sized, opposed to other UK hermit crabs which are asymmetrical. These distinctive features, its high density at some sites, and its behaviour of “sunbathing” mean its appearance is unlikely to have gone unnoticed for long. Thus, the 2016 resurgence is likely a genuine colonisation event. The species has been nick-named the St Piran’s crab, after the patron saint of Cornwall who was supposedly washed up on the coast of Cornwall in the 5th Century.

In 2016, recordings of the species occurred across the coastline seemly all at once, which makes its appearance different from other organisms that have colonised the UK. Most have been translocated by humans actively or by accident. These, such as the Pacific oyster and the seaweed Sargassum, appeared at a single site close to an area of intensive human activity then spread outwards. The reappearance of C. erythropus at multiple sites, all at once, indicates that the species has colonised the UK by the natural transportation of larvae from further south.

C. erythropus may be an exciting new addition to the UK’s rocky shore, but it could also have an impact on the UK’s native hermit crab, Pagurus bernhardus: the two compete for empty gastropod shells which they both use for protection against predators and desiccation. The recent arrival of C. erythropus may therefore suggest we should seek to understand species changes on the UK’s rocky shore. There is an ongoing shift in the community structure of the UK’s rocky shore being driven by a mixture of natural range shifts (such as the Common Topshell which has been progressing northwards from the South-West of England upwards into Wales) and the introductions of non-native species. How rapidly we can expect the UK’s rocky shore community structure to change due to climate-driven colonisations will largely be determined by how often and easily colonisation events can occur. Therefore, discovering where the source population of C. erythropus larvae arrived from will allow us to understand and potentially predict how climate change will bring more species to the UK’s rocky shore.

"Over the past year, I have been working to create the geographic phylogeny of C. erythropus by conducting fieldwork in the UK and from requesting samples from across Europe. This is with the aim to discover the approximate spawning location for the UK’s population.

With generous financial help from the Genetics society's Heredity fieldwork grant I was able to expand my fieldwork to Brittany (the most likely source of Cornwall’s C. erythropus population) and, this April, I spent two weeks travelling from Roscoff, southwards along the coast, towards Lorient. There I was able to visit the Parc Naturel Régional d'Armorique, Pointe du Raz, and other coastal bays and peninsulas searching for the species.

With the sun shining (nearly) every day I was able to find the hermit crabs at all the sites I visited. Sampling can be done non-lethally by cracking open the hermit crabs’ shell using a bench vice. This leaves the hermit crab exposed, if a little startled, and the lower two segments of one of its walking legs can be removed. Each hermit crab can then be placed in a Tupperware box with a collection of new empty shells for it to choose from. They quickly take these up with only a few having to be encouraged to move into more appropriately sized shells.

Progressing southwards, the community structure of the surrounding rocky shore began to change. I began finding new species appearing amongst the more familiar, those I am used to finding in the UK. For example, an abundance of intertidal Sea urchins and Sea hares triple the size of those typically found in Cornwall. If the waters of the UK continue to warm which of these will follow C. erythropus to reach the UK next?

I have now returned to the UK and over the next few months will begin working in the laboratory in order to start DNA extraction. COI sequences will be extracted from the muscle tissue of the pereopods by high salt isolation.

I would like to thank my supervisor and co-supervisor, Dr Regan Early and Dr Chris Lowe, for their ongoing guidance with the project. As well the continued support of Dr Chris Laing who has continued to provide invaluable support for project beyond my undergraduate dissertation. With the kind support of the Genetics society, I hope that this work will expand our knowledge of how the UK’s biodiversity will begin to change over the next few decades."

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