The mid-ocean “twilight zone” holds the key to several tantalizing questions about the marine food web and the ocean’s carbon sequestration capacity. But studying this vast and remote region is extremely difficult. Many inhabitants of the twilight zone are easily destroyed during sampling – or quickly avoid disturbance – so it is difficult to sample them with traditional nets. Advances in acoustics have allowed more precise estimates of biomass, but questions about the diversity and distribution of species within this biomass remain unanswered.

This knowledge gap is starting to close, thanks to genetic material – scales, fecal pellets, or pieces of tissue creatures that move through water. The resulting environmental DNA track, or eDNA, gives researchers clues to the species present in that water and their relative abundance. In an article published on Thursday, October 28 in Scientific reports, researchers at the Woods Hole Oceanographic Institution found that changes in the concentration of eDNA in the ocean accurately reflect the movement of creatures as they move between the twilight zone and the surface.

“A major finding from our article is that the eDNA signal does not disappear immediately if the animal moves up or down in the water column,” said Elizabeth Andruszkiewicz Allan, WHOI postdoctoral fellow during the study and currently a postdoctoral fellow at the University. from Washington. “It helps us answer big questions that we can’t answer with net lines or acoustic data. Which species migrate? What percentage of them migrate every day? And who is an early or late migrant? “

Allan and co-author Weifeng “Gordon” Zhang, an OMSI physical oceanographer, used a computer model to simulate what happens to eDNA in the water column after it is removed by the animal host. They found that physical processes – currents, wind, and mixing – and particle sedimentation did not significantly impact the vertical distribution of eDNA. In fact, most eDNA signals have stayed within 20 meters (66 feet) of where they were first emitted, meaning that changes in eDNA concentration can be reliably used. to determine where certain species live at different times of the day, the time they spend at those depths, and the percentage of certain species that migrate from the twilight zone to the surface.

“Prior to this work, we couldn’t say for sure what happened to the eDNA released by the twilight zone species. But a very clear pattern appeared in the model, providing a basic understanding of the concentration of eDNA between the surface and deep layers over time, “said Zhang.” With this new knowledge, researchers in the field will be able to target where they take the precious water samples so they can identify migratory species and estimate the percentage of animals from each species group that migrate each day. “

As one of the first studies to model eDNA concentration, the researchers note that more field data is needed to help test the model. However, these promising results show how useful eDNA can be for studying animal migration and carbon sequestration in hard-to-reach parts of the ocean like the twilight zone.

“These modeling results provide a basis on which we can more effectively study the ecology of the twilight zone of the ocean,” said Annette Govindarajan, WHOI molecular ecologist and co-author of the article. “It sets up experiments for future directions, some of which we have already started working on.”

This research was supported by the Woods Hole Oceanographic Institution’s Ocean Twilight Zone Project, funded under the Audacious Project hosted at TED.

Key points to remember:

-Environmental DNA, or eDNA, is the genetic material that sea creatures release into water. The sampling and analysis of eDNA gives researchers clues about the species present and their relative abundance, without disturbing them.

-Using a model that incorporates biological and physical forces in the ocean, OMSI researchers found that eDNA does not move vertically more than 20 meters (66 feet) from its source.

-This finding gives scientists confidence that by measuring changes in eDNA concentration, they can determine where certain species live at different times of the day, the time they spend at those depths, and the percentage of certain species that migrate from the twilight zone. on the surface.

-Used in tandem with observation netting and acoustics, eDNA is an important tool for understanding the ecology of the vast and remote mid-ocean region known as the Ocean Twilight Zone.

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