Alien invasions and environmental DNA

As crowds hit the beach this summer the newspapers heralded the arrival of the ‘Pacific death worm’. This alien nematode worm, dubbed ‘more toxic than cyanide’, could spell trouble for a family day out. So how far off the mark were the press, and should we be worried about new species visiting our coasts?

What is eDNA?

To understand what set things off we have to go back to 2007 when postdoctoral scientist Dr. Gentile Ficetola was looking for the American Bullfrog in the wetlands of France. Gentile found that he could use the DNA traces left in the environment to track the frogs, in a similar way to the police tracking criminals from DNA found at the crime scene. This trace DNA has since become known as environmental DNA (eDNA) and we have learnt that almost all aquatic organisms leave eDNA in the water bodies they live in. This trace remains detectable for several days, but varies depending on water conditions such as temperature and pH. We’ve since been able to detect all kinds of animals from microscopic plankton to whales, and eDNA surveys have been shown to be incredibly sensitive and useful in many different environments.

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Environmental DNA is made up of a range of different sized particles isolated from environmental samples. Illustration: Luke Holman
Alien Species

Here at the National Oceanography Centre, Southampton I’m working with Dr Marc Rius to learn how eDNA can help us to track non-native and invasive species in the ocean. Non-native species are those that have been transported from their native range to a novel area through the actions of humans, a subset of these are invasive and have damaging effects on biodiversity and ecosystem services. In the ocean this transport is often in the ballast water of ships, through unintentional release associated with aquaculture or by intentional release of pets into natural systems.

In our most recent study we used eDNA to survey for non-native species in four key areas across the UK. These surveys used a method called eDNA metabarcoding to sequence DNA isolated from seawater and sediment in next-generation sequencing machines. These instruments produce many millions of DNA sequences, we then compared these sequences to those on a public database, looking for matches to species from other countries. We found that water and sediment detected different species, and that both types of sample detected a number of non-native species in the areas we surveyed.

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Oil tanker discharging ballast water near Fawley Refinery, Southampton Water. Photo: Luke Holman
The Prime Suspects

There were two particularly interesting species detected in our eDNA surveys. The first was the Asian date mussel (Arcuatula senhousia), initially published on by Peter Barfield from the University of Portsmouth who found the species in the Solent near the mouth of Southampton Water in 2017. Our eDNA detection found the species further into Southampton Water. This is important as in some countries it can produce a cocoon of byssal thread that stabilises the sediment and creates a mat-like substrate altering the species that can live in the environment. Southampton Water and the Solent is a designated Special Protected Area and many birds rely on the rich variety of invertebrate species that currently live in the soft sediment. We should therefore be cautious and keep monitoring the spread of this species to see if it has any effect on the benthic environment.

The second species detected by the eDNA survey was the nemertean worm Cephalothrix simula. This worm was first detected in South West Cornwall and again at Poole, Dorset by marine recorder David Fenwick (who among other things produces www.aphotomarine.com – a website filled with amazing images of marine life). C.simula has been shown to contain a large amount of tetrodotoxin, a potent neurotoxin, in it’s home range in the North West Pacific. In this area the toxin found in a single worm could potentially kill a human if ingested. However in a study documenting the arrival of C simula in the UK no single worm had sufficient toxins to significantly harm anyone. It is important we continue to monitor this species as we don’t know how far the species has spread around the UK and there may be populations with more neurotoxin than those examined in the study.

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Cephalothrix simula individual. Photo : http://www.aphotomarine.com
What’s next?

Both these species have the potential to produce serious and wide reaching consequences for the UK. We were excited to see our eDNA work confirming earlier detections not only of the two new arrivals but also a vast suite of species documented by Dr John Bishop, Chris Wood and others from the Marine Biological Association. We hope that eDNA can become a part of routine monitoring for non-native species across the UK. Both as a method to detect a species upon arrival but also to test for invasions while in transit, for example in ballast water tanks or before leaving the port of departure.

We will continue our work into exploring how eDNA can help us learn more about species invasions. We’re currently working with a team from The Galway-Mayo Institute of Technology in Galway, Ireland who want to expand our initial work around the entire island of Ireland. More broadly we are expanding our use of eDNA surveys across the globe and are finding new and interesting ways to look at marine biodiversity through the scope of eDNA.

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Asian date mussels collected from Southampton Water. Photo: Luke Holman
Pacific Death Worm Invasion Cancelled? 

Hopefully it’s clear that the non-native species found in our study offer no threat to a day out at the beach. It’s important that we maintain routine monitoring for these species across the UK but most people shouldn’t worry about coming into contact with worrisome worms or malicious mussels. With new eDNA tools we’re in a better position than ever before to detect new species arriving in the UK and take action to present their spread and ensure they have minimal impact.

Luke Holman is a PhD student at the University of Southampton based at the National Oceanography Centre. He is supported by the NERC SPITFIRE doctoral training program. You can learn more about his work at https://sites.google.com/view/lukeeholman and keep up to date with his research on Twitter Twitter here https://twitter.com/luke_earl  
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