Invaders: From Gulf of Mexico, to Lincolnshire!

04 July 2017

Interview with

David Alridge, University of Cambridge


We’ve looked at species that should live on the beach, but what about the ones that shouldn’t? Increasingly, what are called “invasive species” are turning up in many countries. These are animals and plants that are not naturally found in a particular area but have been introduced there and, because there are no natural predators and very little to compete with them, their population explodes and they drive out the native species.

In the last year or two, scientists have discovered a species of clam that normally lives in the Gulf of Mexico - on the other side of the Atlantic Ocean - thriving in some waterways in Lincolnshire on the UK’s east coast. Izzie Clarke joined the field team who are studying its impact. Marine Ecologist, Danni Green, then explained the global issue of invasives to Chris Smith.

Izzie - You might not associate grassy banks with the coast, but estuaries can take many forms. It’s in this transitional zone where a river meets the sea and provides vital nesting and feeding habitats for many aquatic plants and organisms.

I joined David Aldridge and his team from the University of Cambridge as they waded through the South Forty Foot drain, an estuary in the Lincolnshire fens all in search for the rather mysterious Gulf wedge clam…

David - Today we’re trying to understand some pretty fundamental stuff about an organism we know next to nothing about. The Gulf wedge clam was discovered in the drain here in 2015 but, unusually, we have very little information about the impacts of this organism.

Izzie - Generally, how do coastal invasive species go from one place to another?

David - Certainly globally in brackish water systems like this, one of the major pathways is through ballast water of ships. That’s where water is picked up into the bottom of a ship to stabilise its weight when it goes across the ocean. It goes to a new port and takes on its cargo, and then releases that ballast water because it’s now balanced by the weight of the cargo. But, therefore, you get organisms transported from a brackish water, or freshwater system across the ocean in the hull of a ship and then down into a new freshwater location, and you can transport lots of organisms that way, thousands of miles.

Izzie - We started the day in Hubberts Bridge, and area with low salinity, to have a look at these invaders…

David - It is clearly wedge shaped. It’s actually quite beautifully golden, particularly in this dark black, oozy mud at the bottom of the drain. It’s like panning for gold a little bit when we’re searching for them! They get quite big; they’re 5 or 6 centimetres long some of the biggest ones and quite a hefty weight to them.

Izzie - What do they feed off and what are they surviving on out here?

David - These are bivalve molluscs, and what bivalve molluscs do is that they filter huge volumes of water. By doing that they can trap small suspended particles of algae out of water, which is what they feed on. So potentially the invaders here, the Gulf wedge clam, could be removing food which otherwise could be available for the native species. We know with other invasive bivalves that they can increase the rate at which the rivers clog up with sediments because they’re stopping the process of the silt out to sea.

Izzie - So that could have quite a large effect on flooding and affecting local ecosystems? Is that all just from this one new species coming into this environment?

David - We know relatively little about it, and one of the things we’re starting to look at lab is how efficient the Gulf wedge clam is in filtering particles out of the water compared with the native species. Also, by estimating the abundance in the drain here, we can work out how important these new arrivals are in the functioning of the entire ecosystem.

Izzie - Across the day, David and his team were working along the estuary checking the number of Gulf wedge clams at different salinities. With their nets in hand, Justin Kemp who’s doing a Masters in invasive species explained what they’re looking out for.

Justin - We’re just trying to do as many sweeps and counts as we can. Literally just counting to make sure they’re alive and they're closed and you can feel there’s a clam inside and it’s not just an empty shell.

Izzie - Are these going to be going back to the lab with you or you going to just do your measurements and then throw them back into the estuary?

Justin - These will be coming back to the lab with us to run further experiments on.

Izzie - What will those experiments involve?

Justin - For me, they’ll be feeding rate experiments to see how fast these clams are able to filter out food of a known volume of water over about two hours. Then we’ll be comparing that to the native species and see how the feeding rates differ, and that will allow us to make some predictions about the potential impact they’re having here.

Izzie - As the team continued their measurements it was clear that they were not expecting the Gulf wedge clam to be so abundant…

David - I’ve just pulled out maybe 40 or 50 in one sweep of the net and these are all big golf ball-size things, and I’m quite surprised.

Izzie - It seems that the Gulf wedge clam was not the only intruder at the site.

David - This is a carothium, so it's like a crustacean. It’s rather large, so it’s something that I’m quite interested in getting back to the lab and double checking that it’s not another species of concern that’s just turned up.

Izzie - When David was out of the water and all dried off, I asked him if there was anything we could do to prevent these invasive species?

David - We have the technology, which we know works against some other invasive bivalves, something called the “biobullet.” It’s very simple but it’s very effective. What we do is we take a product which is toxic to the clams and we encapsulate it in a tasty coating. You remember I said that these clams can filter large volumes of water, well they swallow this poison pill without realising they’ve taken the poison in and they die straight away. What’s particularly interesting is that our native bivalves recognise the biobullets as non-food and they spit them out, so they’re totally protected from them. All the other native organisms that we’ve tested the biobullets against seem totally unharmed, so it actually offers the potential eradication tool which is really remarkably specific.

Chris - And David Aldridge, whom you just heard there, will be testing the clams they collected against their “biobullets” over the next few weeks… We’ll let you know how they get on. Danni, what do you think about this question of invasives because it’s not just confined to a few shellfish round a few estuarine locations? This is big business worldwide isn't it - it’s a serious problem?

Danni - Yeah. My PhD was looking at invasive bivalves. I was looking at Crassostrea gigas which is the Pacific oyster, which comes from Asia. I found, initially, their effects were positive. They increased biodiversity because they’re providing a physical structure, lots of little nooks and crannies that other animals can live in. But once they got to beyond 50% cover, they ended up decreasing biodiversity, and they also stopped important nutrients from being recycled up out of the sediment into the water column which could have a knock-on effect to (08.18) activity. It’s really important stuff that he’s doing.


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