How can DNA help monitor how species are responding to ocean warming?

Ocean warming has triggered a dramatic shift in the distribution of marine life.

Many species have expanded their range towards the polar regions as sea temperatures heat up. At the same time, there has been a decrease in populations closer to the equator. This mass movement of marine species is known as tropicalisation.

One way of monitoring this phenomenon is using environmental DNA. Known as eDNA, this is genetic material from animals and plants that is shed into the water. Like how forensics may sample a crime scene to look for DNA of a suspect, scientists can sample water or sediment looking for DNA which can tell them what species are present.

This revolutionary technique has been used in several ways, such as detecting if an invasive species is present in an ecosystem, and it could also be used to monitor the effects of tropicalisation.

But in a recent study published in the journal Diversity and Distributions, scientists warn that eDNA still needs to be used alongside other data and expertise to get a better idea of how species ranges are shifting in response to ocean warming.

Karolina Zarzyczny is a PhD student at the Natural History Museum and the University of Southampton who led the study.

“Although eDNA can be a fantastic tool with many applications where it can be used, we need to be cautious when we use it to document changes in species ranges. Otherwise, you could end up with the incorrect interpretation of the data.”

Why can’t we rely entirely on eDNA to monitor range shifts?

To see if eDNA data accurately reflects the distribution of species, the research team looked at a North American sea snail called the speckled tegula.

These snails are found close to the shore from Santa Barbara County in California and down along the coast of the Mexican State of Baja California. But over the last few decades the snail has disappeared from the southern part of its range.

Speckled tegulas are reasonably well studied, with lots of data available about their current and past range, making them an ideal species for this study.

Researchers took water samples from a location that was part of the speckled tegulas’ historical range, but from which the snail is now no longer found. To their surprise, they still managed to find traces of speckled tegula eDNA. The question now was why.

The team quickly ruled out the possibility of an unknown population of snails, as the molluscs require a rocky shore habitat that is uncommon in this part of the Baja California peninsula.

They could also dismiss the influence of ocean currents. Although it is possible that eDNA could be carried from the nearest population by currents, evidence suggests that it would have likely disintegrated long before it reached the site.

Instead, they looked to see if the DNA was related to larva dispersal. When sea snails reproduce, they release hundreds to thousands of offspring into the water. The sea snail larvae are tiny and can be easily transported through the ocean.

“We initially dismissed the idea that the DNA was caused by larvae because, to our knowledge, speckled tegulas spawn in the summer and we were sampling in the winter,” says Karolina. “But we realised that the published data on the spawning season came from a very northern population of the species.”

“We spoke to experts who had worked on this sea snail in Baja California, and it turned out that in the southern part of their range they spawn in the winter rather than the summer.”

This highlighted the importance of fully understanding a species distribution and behaviour, rather than basing everything solely on the eDNA. Karoline explains, “If we had relied entirely on the eDNA data, we would have assumed that a population of snails were living in this area and that the distribution of this species was much greater than it actually is.”

How can museum specimens help support eDNA data?

The latest study shows how if we want an accurate picture of how the ranges of marine animals are changing, then we need to use a diversity of data sources, including the knowledge and expertise from scientists in a range of disciplines.

As well as expertise on how to collect, prepare and study eDNA, figuring out the current situation with the speckled tegula also required detailed knowledge of the biology of the snails in different populations across their range. It also required experts in the regions geography and oceanography to understand how ocean currents might disperse larvae.

Museum collections can also play an important role as many hold specimens that were collected before human-driven climate change. Detailed records of where and when these specimens were collected can help to piece together how the range of this species has shifted over many decades.

Dr Phillip Fenberg, a co-author on the study who has extensive knowledge of snails across Baja California, says, “The important part about this paper is that we use many different methods to tell the full story of the distribution change of the speckled tegula.”

“We use field surveys, historical data, museum collections, eDNA and oceanographic modelling to document range contractions. So eDNA is important, but there are so many other parts to it as well. If you have all those data sources, you can tell a really interesting story.”

“Tropicalisation is happening across the world. What we hope is that other scientists will take this study and use it to see how ocean warming is also driving a range shift in other species.”