What's down there? WHOI study shows environmental DNA is a reliable way
to learn about migration from the ocean twilight zone

Date:
November 1, 2021
Source:
Woods Hole Oceanographic Institution
Summary:
The mid-ocean 'twilight zone' holds the key to several tantalizing
questions about the marine food web and carbon-sequestering
capacity of the ocean. But studying this vast and remote area is
extremely difficult.

Many inhabitants of the twilight zone are easily destroyed
during sampling -- or are quick to avoid any disturbance -- so
it's difficult to sample them with traditional nets. Advances in
acoustics have enabled more accurate estimates of biomass, but
questions about the diversity and distribution of species within
that biomass remain unanswered.



FULL STORY ==========================================================================
The mid-ocean "twilight zone" holds the key to several tantalizing
questions about the marine food web and carbon-sequestering capacity
of the ocean. But studying this vast and remote area is extremely
difficult. Many inhabitants of the twilight zone are easily destroyed
during sampling- or are quick to avoid any disturbance-so it's difficult
to sample them with traditional nets.

Advances in acoustics have enabled more accurate estimates of biomass,
but questions about the diversity and distribution of species within
that biomass remain unanswered.


==========================================================================
That knowledge gap is beginning to close, thanks to the genetic material -
- from scales, fecal pellets, or bits of tissue-creatures shed as they
move through the water. The resulting trail of environmental DNA, or
eDNA, gives researchers clues about which species are in that water-and
their relative abundance. In a paper published Thursday, October 28th
in Scientific Reports, researchers at the Woods Hole Oceanographic
Institution found that changes in the concentration of eDNA in the ocean accurately reflects the movement of creatures as they travel between
the twilight zone and surface.

"A major finding of our paper is that the eDNA signal doesn't go away immediately if the animal moves up or down in the water column," said
Elizabeth Andruszkiewicz Allan, a WHOI postdoctoral fellow during the
study and currently a postdoc at the University of Washington. "That helps
us answer some big questions we can't answer with net tows or acoustic
data. Which species are migrating? What percentage of them migrate each
day? And who is an early or late migrator?" Allan and co-author Weifeng "Gordon" Zhang, a WHOI physical oceanographer, utilized a computer model
to simulate what happens to eDNA in the water column after it is shed
by the host animal. They found that physical processes - - currents,
wind, and mixing -- and settling of particles did not have a significant
impact on the vertical distribution of the eDNA. In fact, most eDNA
signals remained within 20 meters (66 feet) of where they were first
shed, meaning that changes in eDNA concentration can reliably be used
to determine where certain species live at different times of the day,
the amount of time they spend at those depths, and the percentage of
certain species that migrate from the twilight zone to the surface.

"Before this work, we couldn't confidently say what happened to the eDNA
shed by twilight zone species. But a very clear pattern showed up in the
model, providing a baseline understanding of the concentration of eDNA
between the surface and deep layers over time," said Zhang. "With this
new knowledge, field researchers will be able to target where they take
the precious water samples so they can identify the migrating species and estimate the percentage of animals in 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 just how useful eDNA
can be for studying animal migration and carbon sequestration in difficult-to-access parts of the ocean like the twilight zone.



========================================================================== "These modeling results provide a foundation for which we'll be able
to more efficiently study the ecology of the ocean twilight zone,"
said Annette Govindarajan, a WHOI molecular ecologist and co-author of
the paper. "It sets up some experiments for future directions, some of
which we've already started working on." This research was supported by
the Woods Hole Oceanographic Institution's Ocean Twilight Zone project,
funded as part of The Audacious Project housed at TED.

Key Takeaways: -Environmental DNA, or eDNA, is genetic material that
marine creatures shed in the water. Sampling and analyzing eDNA gives researchers clues about which species are present, and their relative abundance, without disturbing them.

-Using a model that incorporates biological and physical forces in the
ocean, WHOI researchers found that eDNA doesn't travel 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 amount of time they spend at those
depths, and the percentage of certain species that migrate from the
twilight zone to the surface.

-Used in tandem with observational net tows 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.

========================================================================== Story Source: Materials provided by
Woods_Hole_Oceanographic_Institution. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Elizabeth Andruszkiewicz Allan, Michelle H. DiBenedetto, Andone C.

Lavery, Annette F. Govindarajan, Weifeng G. Zhang. Modeling
characterization of the vertical and temporal variability of
environmental DNA in the mesopelagic ocean. Scientific Reports,
2021; 11 (1) DOI: 10.1038/s41598-021-00288-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211101141745.htm

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