Twilight zone

• The ocean twilight zone is a layer of water that stretches around the globe. It lies 200 to 1,000 meters (about 650 to 3,300 feet) below the ocean surface.
• Also known as the midwater or mesopelagic, the twilight zone is cold and its light is dim.
• Life in the twilight zone includes microscopic bacteria and tiny animals known as zooplankton, along with larger crustaceans, fish, squid, and many kinds of gelatinous animals.

Adaptations exhibited by Twilight zone animals.

• The bristlemouth, a small twilight-zone fish with a large jaw full of spiny teeth, is the most abundant vertebrate on Earth.
• To survive in such a low-light environment, many twilight-zone species, produce their own light through a biochemical process known as bioluminescence.
• Predatory fish also use bioluminescence to attract prey to catch and eat. Some, for example, have a bioluminescent organ called an esca to tempt potential prey.

Importance of the ocean twilight zone

• The ocean twilight zone provides important ecosystem services, including supporting ocean food webs and commercial fisheries.
• Dead animals, clumps of dead plankton, bacteria, fecal pellets, and other particles rich in organic carbon sink from surface waters through the twilight zone to the deep ocean, providing food for twilight-zone animals. 
• Satellite tagging has revealed that whales, tuna, swordfish, sharks, and other top predators dive deep down into the twilight zone to feed. Since humans value those predators for their ecological, commercial, and nutritional benefits, we indirectly depend on the twilight zone.

How does twilight zone help keep carbon dioxide outside the atmosphere?

• The twilight zone plays an important role in transferring carbon from surface water to the deep ocean, preventing it from returning into the air as a heat-trapping greenhouse gas. 
• The multistep process is often called the ocean’s “biological pump.” 
• In surface waters where there is plenty of light, tiny plantlike organisms called phytoplankton use energy from the sun to transform carbon dioxide into the energy and matter that allows them to grow. 
• Phytoplankton, in turn, become food for small animals known as zooplankton, which are then eaten by fish and other animals.
• Some of the carbon in surface waters becomes part of a kind of underwater blizzard known as marine snow. 
• That “snow,”consists of clumps of dead plankton, bacteria, fecal pellets, and other particles rich in organic carbon, which provide food for twilight-zone animals.
• Another fast track for carbon into deeper water is through the daily migration of twilight zone animals that feed near the surface at night then bring the carbon in their food back down into the twilight zone during the day.
• About 90 percent of the carbon that gets into the twilight zone remains there, but a small percentage of it sinks to down into the deep ocean. Once there, it can remain isolated from the atmosphere for hundreds or even thousands of years.

Why in news?

• Global warming could curtail life in the twilight zone by as much as 40% by the end of the century, according to new research.

https://www.bbc.com/news/science-environment-65460128