Microplastics make mussels weaker

Most of us saw Blue Planet Two and the impact that plastics in our oceans are having on the wildlife there. But it’s not just the obvious large-scale things, like plastic bags, that are causing most of the problems: it’s actually the microplastic particles that these form when they break down. These particles are invisible to the naked eye, but they are steadily working their way through all forms of life in the ocean and appear to be impacting the health and activity of at least some of them. Speaking to Chris Smith, Danni Green is from Anglia Ruskin University; she’s just published a study showing that plastic particles affect the ability of some shellfish to attach themselves to their surroundings...

Chris - So which shellfish have you been looking at?

Danni - So we looked at the blue mussel, Mytilus edulis. This is the mussel that you probably most commonly would eat, the mussel that you would see in on the shelves of supermarkets. Myself and my collaborators from Maynooth University, we wanted to understand how these mussels have been affected in terms of the health but also how they function as ecosystem engineers. These are really important organisms: they form quite complex reefs, and they support a lot of biodiversity of other animals.

Chris - How did you discover that they're struggling to cling on?

Danni - I set up an experiment using an outdoor laboratory system so it's kind of realistic without releasing microplastics into the environment, which would not be very popular with DEFRA, and we dose them with microplastics and run the experiment for quite a long time - 52 days - and after this time I tested their tenacity, which is their ability to hold on to the substrate and you measure the vertical force that is necessary to detach them. And I found that the control mussels that were not exposed to microplastics had double the strength than the ones exposed.

Chris - They make a sticky material that enables them to cling on don't they?

Danni - So they form what's called byssal threads, so it's little tiny threads; and you would have seen these on mussels either on the shore or on your plate and these actually attach onto the rock and allow them to stay in place. And the mussels that were exposed to microplastics, so, they had half the number that they actually produce - they produce less threads. Therefore the strength was reduced.

Chris - And the dose of microplastics that you're exposing them to, was that sort of ocean-realistic? In other words, if I were to go out into the field and take a sample of seawater where mussels would be eating and going about their business, I would probably see levels of plastic equivalent to your experiment or not? 

Danni - I hope not! At the current day! So, the amounts that were used is representative of what we might find in the future, in the next kind of 50 to 100 years depending on which sort of projection you use. But actually, our concentrations were one of the lowest used in experiments to date. So it is quite realistic. These effects aren't happening now and there's still time to prevent this happening. 

Chris - Which is good news isn't it? But how do you think the plastics are achieving this effect? What are they doing to the mussels to mean that the byssal threads are reduced by up to 50 per cent in the affected animals? 

Danni - So it's difficult to tell. It's either that they're making the animal feel full, and it's not getting the same nutritional value from its food because it feels like it's full. We did a technical shotgun proteomics, where you basically take a blood sample (in this case it's called haemolymph but it's the equivalent of blood in vertebrates), and we analysed all the proteins in the organism and found that a lot of immune response proteins were being expressed - stress response detoxification proteins, all these things that are indicating that this animal is trying to get something out of its system. And this was the same whether you had normal microplastics or biodegradable microplastics. So there's an effect from both of these types.

Chris - Do you think this is unique to mussels, or have you started to look at other shellfish that will also be exposed in the same way? They're filter feeders - they'll be drawing in water and therefore potentially bringing these things into their bodies in the same way that the mussels do...

Danni - So it's difficult to tell if the exact effects that we had were unique but from other studies that I've done I found that oysters have also been impacted: they've had alterations to their filtration rate, differences in their biomass, and other people have found that their reproductive output has also been reduced by microplastics. So there are definitely effects happening on other bivalves, not just mussels.

Chris - It's interesting you're seeing this effect with just - if you want - "naked microplastics", because one of the other things people talk about in this context is that these plastics tend to soak up a toxic cargo of other oil-loving chemicals in the sea. So it could be a double whammy for these animals then, because not only are they getting a direct impact physically from the plastic, they could then get the toxic burden that goes with it. So they could be impacted twice?

Danni - Yeah, exactly. So it could either be from the plasticisers that are on the plastic itself, or it could be from the persistent organic pollutants they absorbed from the water that they're in; it could also be biological - it could be microorganisms attaching to the microplastics and then having an effect. And in order to separate these, we would have to do some pretty complex experiments, which I know that these sort of things are being done to try to work out what is it really that's causing this effect.

Chris - And just finally, if this turns out to play out the way you think it is in your experiments, what would be the marine impact of this?

Danni - If mussels can't attach to the rock then they can't form complex reefs; they can live in really exposed environments where the waves are gonna wash them away. So there's a greater chance of them being dislodged and therefore unable to form these really important habitats, which support biodiversity. They're also economically important, and when we farm them as well they're often left out on these kind of pole and line sorts of systems, so they could be dislodged: you could be losing yield. There's economic and ecological consequences...