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Rafting bivalves in Britain and Ireland

Anna Holmes, 20 April 2020

Chama sarda from the Caribbean, found in western Ireland

In the previous blog (What are Non-native (or alien) Species?) I talked about non-native species and how they are transported around the planet. In this blog I’ll tell you a bit more more about rafting bivalves that can cross the Atlantic on plastics and about some of the UK hotspots for these non-native (alien) hitchikers.


What is rafting?

Rafting has occurred throughout geological time, and it is how many terrestrial (land-dwelling) species colonised islands and other regions across the seas. A good example of this is the lemurs of Madagascar. 60 million years ago there were no lemurs on the island of Madagascar, but their ancestors were on the mainland of the African continent. So how did they actually get to Madagascar? Palaeontologists tell us that rafting is the answer. Back then, Madagascar was closer to the mainland and currents in the Mozambique Channel were much stronger towards the island than they are now. The lemurs’ ancestors must have found their way onto mats of vegetation or branches and by chance rafted to Madagascar. A completely fluke event! 


Violent storms assist the dispersal of non-native species on plastics

Today, our litter ends up in the oceans and provides unnatural vehicles for marine non-native species. Over the last decade more than 20 species of Caribbean bivalve shells have ended up on British and Irish beaches attached to plastic buoys, bait buckets, ropes and others items – even a piece of a car running board! The latter had three different types of Caribbean bivalves attached, one of which, the Bicolor Purse Purse Oyster, is an invasive species in Brazil. Violent storms help to throw the plastic objects high onto our shores and they are then found by beach cleaners, beachcombers and others on the strandline at the top of the beach. Many of the shells or photographs are sent to Amgueddfa Genedlaethol Caerdydd - National Museum Cardiff to be identified and hence to work out where they came from.


Conveyor belts and hotspots

General locations of rafting bivalve records in the southwest of Britain and Ireland

The rafting species that we are studying start off attaching to plastics in the Caribbean. These plastics eventually float into the warm ocean currents of the Gulf Stream, which originates in the Gulf of Mexico, and provides a conveyor belt to transport non-native species across the Atlantic Ocean to the British Isles. Once here, violent storms then deposit the plastics, with their hitchhikers still attached, onto our shores. 

The hotspots for non-native species records are in the southwest of England and western Ireland, but there are also records for western Scotland. Strangely enough there are no records for Wales, even though some of the beaches are prime, exposed shores similar to those in Ireland and England. 

I want to discover if there are any welsh hotspots for rafting bivalves, find out which beaches to search and to encourage people to go out to those locations to try and get some records for Wales. 


Why should we monitor these species?

It is important to establish points of entry for any non-native species, which is why we need to map where the rafting species are arriving so that we can monitor numbers of any species arriving alive that could possibly pose a threat. When a species moves to a new location, the species that already live there have to adapt to cope with the newcomer. This can put a strain on populations that use the same food source or habitat as the invading species, which is why we want to know where these rafting species are washing ashore. All the rafting species that we’ve encountered so far cannot reproduce in our waters as they need warmer sea temperatures of 20°C or more to breed. However, if sea temperatures continue to rise, climate change could aid more rafting species to create self-sustaining populations here which could become a real problem


The Bicolor Purse Oyster – an invasive species in Brazil

Byssus threads of Bicolor Purse Oyster from the Caribbean

Of the non-native rafting species found so far in the UK, the Bicolor Purse Oyster (Isognomon bicolor) is the one that has shown up in the greatest numbers. It was first described as a species in 1846 by C.B. Adams who collected it in Jamaica. It has been spotted around the coasts of Florida, Texas and Bermuda and several of the Caribbean islands all of which are considered its natural range. However, in 1970 it was recorded outside its natural range in the eastern state of Rio Grande do Norte in Brazil. It has since moved further southwards and is considered an invasive species in Brazil as it is competing with native oysters and mussels for space and is spreading quickly. 

The Bicolor Purse Oyster in Britain and Ireland

The Bicolor Purse Oyster has been found in Cornwall, Dorset and western Ireland by eagle-eyed beachcombers. They noted locations, objects that the oysters were attached to, and they collected the shells. The beachcombers are great photographers so we have a record of the variety of sizes, shapes and colours of the shells found here. The Bicolor Purse Oyster is small (up to 28mm), flattened and elongated. The outside is beige and white, sometimes with purple blotches and is smooth apart from being a bit flaky-looking. The front of the shell has byssus threads of pale to dark brown that protrude ready to attach to hard surfaces. In its natural range this species attaches to rocks and is commonly found in the Florida Keys. 


What next?

Although there are lots of records of rafting species in Ireland and England, there are none for Wales. Does that mean that they do not wash ashore in Wales? Doubtful! This is why I’ve set up a project to get people out onto beaches looking for any plastics that could be likely rafts. The project involves citizen scientists – volunteers from the general public – who can help to spot these rafting species in Wales.

To find out more about this part of the project see next week's blog entitled Rafting Bivalves - the Citizen Science project.

Anna Holmes

Curator: Invertebrate Biodiversity (Bivalves)
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Comments (1)

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Kathy Borsuk
31 May 2021, 15:36
Would you be willing to send me a high resolution version of the Cherry jewelbox shell (Chama sarda) image included here?
I would like to use it for an article in our magazine Times of the Islands, which is published in the Turks & Caicos Islands. We have an article about Taino children's possible "toys" and this would be a perfect illustration.
We have a very small circulation (about 5,000 copies), primarily distributed in-country.
You can see our website at:

Please let me know,