Life in the Mesopelagic Zone of the Ocean

The Ocean's Twilight Zone

Ocean Zones
This image shows the ocean zones.

Encyclopaedia Britannica/UIG/Getty Images Plus

The ocean is a vast habitat that is divided into several regions inclusive of the open water (pelagic zone), water near the ocean floor (demersal zone), and the ocean floor (benthic zone). The pelagic zone consists of the open ocean excluding areas near the coasts and sea floor. This zone is divided into five major layers marked by depth.

The mesopelagic zone extends from 200 to 1,000 meters (660-3,300 feet) below the surface of the ocean. This area is known as the twilight zone, as it sits between the epipelagic zone, which receives the most light, and the bathypelagic zone, which receives no light. The light that reaches the mesopelagic zone is dim and does not allow for photosynthesis. However, distinctions between day and night can be made in the upper regions of this zone.

Key Takeaways

  • Known as the "twilight zone," the mesopelagic zone extends from 660-3,300 feet below the surface of the ocean.
  • The mesopelagic zone has low levels of light that make it impossible for photosynthetic organisms to survive. Light, oxygen, and temperature decrease with depth in this zone, while salinity and pressure increase.
  • A variety of animals live in the mesopelagic zone. Examples include fish, shrimp, squid, snipe eels, jellyfish, and zooplankton.

The mesopelagic zone experiences significant temperature changes which decrease with depth. This zone also plays an important role in the cycling of carbon and maintenance of the ocean's food chain. Many of the mesopelagic animals help to control the numbers of upper ocean surface organisms and in turn serve as sources of food for other marine animals.

Conditions in the Mesopelagic Zone

The conditions in the mesopelagic zone are more harsh than those of the upper epipelagic zone. The low levels of light in this zone make in impossible for photosynthetic organisms to survive in this ocean region. Light, oxygen, and temperature decrease with depth, while salinity and pressure increase. Due to these conditions, little resources for food are available in the mesopelagic zone, requiring the animals that inhabit this area to migrate to the epipelagic zone to find food. 

Thermocline
The red line in this illustration shows a typical seawater temperature profile. In the thermocline, temperature decreases rapidly from the mixed upper layer of the ocean to much colder deep water in the thermocline (mesopelagic zone). National Oceanic and Atmospheric Administration

The mesopelagic zone also contains the thermocline layer. This is a transition layer where temperatures change rapidly from the base of the epipelagic zone through the mesopelagic zone. Water in the epipelagic zone is exposed to sunlight and rapid currents that distribute warm water throughout the zone. In the thermocline, the warmer water from the epipelagic zone mixes with the cooler water of the deeper mesopelagic zone. The thermocline depth varies yearly depending on global region and season. In tropical regions, thermocline depth is semi-permanent. In polar regions, it is shallow, and in temperate regions, it varies, usually becoming deeper in summer.

Animals That Live in the Mesopelagic Zone

Angler Fish
Anglerfish (Melanocetus murrayi) Mid-Atlantic Ridge, North Atlantic Ocean. Anglerfish have sharp teeth and a luminescent bulb that is used to attract prey. David Shale/Nature Picture Library/Getty Images

There are a number of marine animals that live in the mesopelagic zone. These animals include fish, shrimp, squid, snipe eels, jellyfish, and zooplankton. Mesopelagic animals play an important role in the global carbon cycle and ocean's food chain. These organisms migrate in massive number to the oceans surface at dusk in search of food. Doing so under the cover of dark helps them to avoid daytime predators. Many of the mesopelagic animals, like zooplankton, feed on phytoplankton found abundantly in the upper epipelagic zone. Other predators follow zooplankton in search of food creating a vast ocean food web. When dawn arises, the mesopelagic animals retreat back to the cover of the dark mesopelagic zone. In the process, atmospheric carbon obtained by consumed surface animals is transferred to the ocean depths. Additionally, mesopelagic marine bacteria also play an important role in global carbon cycling by capturing carbon dioxide and converting it to organic materials, such as proteins and carbohydrates, that can be used to support marine life.

The animals in the mesopelagic zone have adaptations to life in this dimly lit zone. Many of the animals are capable of generating light by a process called bioluminescence. Among such animals are jellyfish-like creatures known as salps. They use bioluminescence for communication and to attract prey. Anglerfish are another example of bioluminescent deep-sea mesopelagic animals. These strange looking fish have sharp teeth and a glowing bulb of flesh that extends from their dorsal spine. This glowing light attracts prey directly into the mouth of the anglerfish. Other animal adaptations to life in the mesopelagic zone include silvery scales that reflect light to help fish blend in with their environment and well developed large eyes that are directed upward. This helps fish and crustaceans to locate predators or prey.

Sources

  • Dall'Olmo, Giorgio, et al. "Substantial Energy Input to the Mesopelagic Ecosystem from the Seasonal Mixed-Layer Pump." Nature Geoscience, U.S. National Library of Medicine, Nov. 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC5108409/. 
  • "New Research Reveals Sound of Deep-Water Animal Migration." Phys.org, 19 Feb. 2016, phys.org/news/2016-02-reveals-deep-water-animal-migration.html. 
  • Pachiadaki, Maria G., et al. "Major Role of Nitrite-Oxidizing Bacteria in Dark Ocean Carbon Fixation." Science, vol. 358, no. 6366, 2017, pp. 1046–1051., doi:10.1126/science.aan8260. 
  • "Pelagic Zone V. Nekton Assemblages (Crustacea, Squid, Sharks, and Bony Fishes)." MBNMS, montereybay.noaa.gov/sitechar/pelagic5.html. 
  • "What Is a Thermocline?” NOAA's National Ocean Service, 27 July 2015, oceanservice.noaa.gov/facts/thermocline.html.