Environmental DNA is a dependable method for learning about migration from the ocean's twilight zone


Environmental DNA, or eDNA, gives researchers clues about which species are in that water—and their relative abundance. A WHOI-led study finds that changes in eDNA concentration reveals details of a creature's movement to and from the ocean twilight zone. Credit: Natalie Renier / Woods Hole Oceanographic Institution
Environmental DNA, or eDNA, gives researchers clues about which species are in that water—and their relative abundance. A WHOI-led study finds that changes in eDNA concentration reveals details of a creature's movement to and from the ocean twilight zone. Credit: Natalie Renier / Woods Hole Oceanographic Institution

The mid-ocean "twilight zone" offers the key to several enticing issues regarding the marine food chain and the ocean's carbon-sequestering capacity. However, researching in such a huge and distant area is incredibly challenging. Many twilight zone occupants are quickly destroyed during sampling or are quick to evade any disturbance, making standard net sampling challenging. Acoustic advances have permitted more accurate biomass estimations, but issues about the variety and distribution of species within that biomass remain unsolved.


The genetic material from scales, fecal pellets, or fragments of tissue that organisms shed as they travel through the water is helping to close that knowledge gap. The ensuing trial of environmental DNA, or eDNA, provides researchers with information about which species are present in the water–as well as their relative abundance. Researchers from the Woods Hole Oceanographic Institution discovered that fluctuations in the concentration of eDNA in the ocean properly mirror the movement of animals as they traverse between the twilight zone and the surface in an article published Thursday, October 28 in Scientific Reports.


A noteworthy discovery of our work is that the eDNA signal does not go away instantly if the animal goes up or down in the water column, according to Elizabeth Andruszkiewicz Allan, a WHOI postdoctoral fellow during the study and currently a postdoc at the University of Washington. This lets us answer some big questions that net tows or acoustic data can't address. Which animals are migrating? How many of them migrate every day? And who is a late or early migrant?

Allan and his co-author, WHOI physical oceanographer Weifeng "Gordon" Zhang, used a computer model to simulate what happens to eDNA in the water column after it is shed by the host animal. They discovered that physical phenomena such as currents, wind, and particle mixing and settling had no effect on the vertical distribution of eDNA. In fact, most eDNA signals remained within 20 meters (66 feet) of where they were first shed, indicating that changes in eDNA concentration can be used to reliably determine where certain species live at different times of the day, how long they spend at those depths and the percentage of certain species that migrate from the twilight zone to the surface.


Before this study, we couldn't state with certainty what happened to the eDNA shed by twilight zone animals, according to Zhang. However, the model revealed a very distinct pattern, offering a baseline understanding of the concentration of eDNA between the surface and deep levels throughout time. With this new information, field researchers will be able to pinpoint where they collect valuable water samples in order to identify migrating species and estimate the percentage of animals in each species group that travels each day.


The researchers add that, as one of the first studies to model eDNA concentration, more field data is required to properly test the model. However, these encouraging findings demonstrate how important eDNA can be for investigating animal movement and carbon sequestration in difficult-to-access areas of the ocean, such as the twilight zone.


According to Annette Govindarajan, a WHOI molecular ecologist and co-author of the paper, these modeling results give a platform for which we will be able to explore the ecology of the ocean twilight zone more efficiently.  It establishes various experiments for future directions, some of which we have already begun to work on.


Journal Information:  Elizabeth Andruszkiewicz Allan et al, Modeling characterization of the vertical and temporal variability of environmental DNA in the mesopelagic ocean, Scientific Reports (2021). DOI: 10.1038/s41598-021-00288-5

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