The Ocean Twilight Zone

Updated: Oct 15, 2021

What is the Ocean Twilight Zone?

The ocean twilight zone is a layer of water that expands all around the globe. Also known as the midwater or mesopelagic, the twilight zone lies 200 to 1,000 meters below the ocean's surface and it is just beyond the reach of sunlight.

The animals that live in the twilight zone range in size from microscopic to among the world's largest. Some species live their whole lives in its murky depths, while others migrate to and from the surface on a daily basis in the world's greatest animal migration. Animals living in the twilight zone contribute to the ocean's food cycle by transporting massive amounts of carbon from surface waters to the deep ocean, which aids in climate change.

As of now, the twilight zone is mostly unexplored, and its rich biodiversity has stayed beyond the reach of commercial fishing. However, some fishing interests are preparing to begin exploiting the twilight zone's biological riches, with uncertain effects for marine ecosystems and the Earth's climate.

Life in the Twilight Zone

Microscopic bacteria and tiny organisms known as zooplankton, as well as larger crustaceans, fish, squid, and a variety of gelatinous species, live in the twilight zone.

Many of the twilight zone's inhabitants appear to be the stuff of science fiction, yet they are all specially suited to exist in a deep, dark, environment. Temperatures revolve around freezing and water pressure can reach 1,500 pounds per square inch. The majority of mesopelagic fish are only a few inches long. Despite their small size, twilight-zone species are a formidable influence in the water. The bristlemouth, a small twilight-zone fish with a big jaw full of spiky teeth, is the most abundant vertebrate on the planet, with perhaps quadrillions of other individuals. There are even more creatures to be discovered in the twilight zone.

To avoid being eaten and to lure prey in such a low-light environment, many twilight-zone organisms, from microorganisms to jellies, make their own light through a biochemical process known as bioluminescence. Some fish, for example, use a defense mechanism known as counterillumination to avoid being observed by predators. Many predators glance up in the twilight zone to look for prey silhouetted against the surface because the light illuminates from above. As a result, certain small prey fish have rows of photophores down their belly that generate light identical in strength to that of the surface water above, rendering them practically invisible from below.

Why don't we know more?

Using acoustic imaging, nets, and submersibles, scientists have already learned a lot about the twilight zone. However, since the region is located below the ocean's surface, it is not easily researched with ship-based sonar and cannot be scanned with satellite technology. It covers such a big area, and changes quickly as the water and animals move. Organisms in the twilight zone are widely dispersed and adept at escaping shipboard nets or underwater vehicle cameras. The myriad gelatinous species found in the twilight zone, such as jellyfish, salps, and siphonophores, are particularly challenging to study since they tend to come apart in nets and require special lighting to photograph or film.


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