The twilight zone is a layer of water depth that is penetrated by significantly less light than what can be found closer to the water’s surface. For this reason, the twilight zone is cold and quite dark, making it unsuitable for most photosynthetic plant species. Twilight zones can be found around the world and are not unique to any specific body of water. According to National Oceanic and Atmospheric Administration, the twilight zone can be found at a depth of about 200 meters to 1000 meters (650 to 3,300 feet) beneath the water’s surface. This layer range is below the water’s photic layer- the sunlit area, and just above the midnight range.
While some species spend their lives in undisturbed depth range known as the twilight zone, many animals move in and out of it. Species of fish, squid and plankton likely swim in darkness to find food or to keep away from predators. These traveling organisms can potentially carry environmental DNA signatures with them.
A new study by researchers, Elizabeth Andruszkiewicz Allan, Michelle H. DiBenedetto, Andone C. Lavery, Annette F. Govindarajan , and Weifeng G. Zhang simulates the physical conditions that cause environmental DNA samples to move through the twilight zones.
Their conclusion: environmental conditions like currents, wind, and mixing do not significantly impact the vertical distribution of DNA samples. To be precise, their computer generated model demonstrates that eDNA samples didn’t move beyond a 20 meter range of where it was released into the environment. If this model reflects the actual conditions of marine ecosystems in twilight zones, perhaps changes eDNA concentrations can be used to determine which fish species are present at a sea depth or how long species spend at varying depths. This has groundbreaking implications for tracking marine species travel patterns and migration more generally in aquatic ecosystems.
Vast populations of unexploited fish and unexplored habitats can be found in twilight zones, also known as disphotic zones or mesopelagic zones, which make these aquatic regions extraordinarily interesting to marine researchers. Environmental DNA may prove useful for learning about organisms that live down in ocean twilight zones and how these species travel. Also, using environmental DNA for sampling can protect the ecological processes and species that inhabit these middle ocean zones.
There is still much to learn about the carbon sequestration potential, ecological processes and biological diversity profiles of middle ocean twilight zones. Ecosystems must be protected during sampling missions and disturbed as little as possible. Sampling techniques like trawling, bait camera trapping and other forms capture carry ethical concerns which could hamper further research efforts.
Twilight zones likely provide ecological services to the network of species that migrate in and out of them, and more permanent inhabitants. In order to preserve full ecological function and avoid disturbing species, researchers will have to prioritize more minimally invasive sampling techniques. Sampling approaches that are minimally invasive to species and ecosystems are more likely to win over public approval.