Not all organisms in the twilight zone migrate, but many do. What appeared to be a shifting seafloor turned out to be plankton, fish, and other twilight-zone animals making a nightly migration to and from the surface to find food. Navy sonar operators looking for enemy submarines were puzzled when sonar images seemed to show the seafloor changing its depth, from about a quarter mile down during the day, to near the surface after dark. Why do some twilight-zone animals migrate?ĭuring World War II, U.S. Some, for example, have a bioluminescent organ called an esca that dangles from a whisker-like barbel on their chins or backs like a kind of fishing lure to tempt potential prey. Consequently, some small prey fish have rows of organs called photophores along their bellies that emit light similar in intensity and color to the light of the surface water above, making them nearly invisible when viewed from below.Ĭonversely, predatory fish can also use bioluminescence-in their case, to attract prey to catch and eat. Since light in the twilight zone comes from above, many predators look up to search for prey silhouetted against the surface. To survive in such a low-light environment, many twilight-zone species-from microbes to jellies-produce their own light through a biochemical process known as bioluminescence to avoid being eaten and to attract prey.įor example, some fish use an adaptation called counterillumination to keep from being seen by potential predators. And there are countless species still to be discovered.
#TWILIGHT ZONE MIDNIGHT SUN FULL#
The bristlemouth-a small twilight-zone fish with a large jaw full of spiny teeth-is the most abundant vertebrate on Earth, possibly numbering in the quadrillions. But their size does not keep twilight-zone animals from being a powerful force in the ocean. Most mesopelagic fish are only a few inches long. With their strange shapes and behaviors, many of the twilight zone’s inhabitants seem like the stuff of fantasy or science fiction, but they are all uniquely adapted to life in a deep, dark, watery world where temperatures stay close to freezing and water pressure can reach 1,500 pounds per square inch. 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. What kinds of organisms live in the twilight zone? But some fishing interests are poised to begin extracting the biological resources of the twilight zone, with unknown consequences for marine ecosystems and Earth’s climate. So far, the twilight zone is largely unexplored and its rich biodiversity has remained mostly beyond the reach of commercial fishing-and the international laws that govern the high seas. Animals in the twilight zone help support the ocean’s food web and transport huge amounts of carbon from surface waters into the deep ocean, helping to regulate global climate. Some organisms spend their lives in its shadowy depths, while others travel to and from the surface every day in the largest animal migration on Earth.
Recent studies suggest that the biomass of fish in the twilight zone may be ten times greater than previously thought-more than in all the rest of the ocean combined.Īnimals in the twilight zone range in size from microscopic to among the largest on the planet. Also known as the midwater or mesopelagic, the twilight zone is cold and its light is dim, but with flashes of bioluminescence-light produced by living organisms. It lies 200 to 1,000 meters (about 650 to 3,300 feet) below the ocean surface, just beyond the reach of sunlight. The ocean twilight zone is a layer of water that stretches around the globe. Other Expeditions Highlighting WHOI Research.
Expedition to Stellwagen Bank National Marine Sanctuary.How do polynyas help feed emperor penguins?.Why is pressure different in the ocean?.What’s the difference between climate and weather?.
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