Why is it so important to understand life in the ocean twilight zone?

How much life is in the ocean twilight zone?

The twilight zone is home to more fish than the rest of the ocean combined. Most of these fish—and other organisms that live in the zone—are tiny, measuring just a few inches long or less. But some, like gelatinous siphonophores, can form chains that extend as much as 130 feet, making them among the biggest animals on Earth. Even the smallest twilight zone inhabitants can be powerful through sheer number, however. A tiny but fierce-looking fish called a bristlemouth is the most abundant vertebrate on the planet—for every one human, there are more than 100,000 bristlemouths. 

How does life survive in this extreme environment?

The twilight zone’s inhabitants are the stuff of fantasy or science fiction. They range from whimsical to terrifying, and are all uniquely suited to life in the cold, deep darkness, where temperatures remain near-freezing and water pressure can reach 1,500 pounds per square inch. Whether they’re microbes, plankton, jellies, or fish, everything living in the zone has evolved incredible adaptations that let them survive under harsh conditions. Many animals, for example, can produce their own light—a trait called bioluminescence—that they use to camouflage themselves, to scare off predators, or even to attract prey.

Closeup of long chain of Salp zooids. Photo by Larry Madin, Woods Hole Oceanographic Institution.
Bristlemouth. The most abundant fish species in the ocean twilight zone. Photo by Paul Caiger, Woods Hole Oceanographic Institution.
The gulper or pelican eel, (Eurypharynx-pelicanoides. Photo by Paul Caiger © Woods Hole Oceanographic Institution

How does life in the twilight zone affect global climate?

By migrating to and from the surface, eating, being eaten, dying—and even by pooping—organisms in the twilight zone transport huge amounts of carbon from surface waters into the deep ocean. That process, called the biological pump, plays an important role in regulating Earth’s climate. In sunlit surface waters, tiny plantlike organisms called phytoplankton use energy from the sun to transform carbon dioxide into the food that allows them to grow. Those phytoplankton are eaten by tiny animals called zooplankton, which in turn are eaten by jellies, fish, and other animals—some of which carry that carbon into the twilight zone as part of their daily migration.

Clumps of dead plankton, shells, fecal pellets, bacteria, and other carbon-rich particles also provide food for twilight zone animals. About 90 percent of that is eaten, but a small portion (about 200 million tons of carbon per year) sinks down deeper, and can remain locked away on the seafloor for millenia. In the process, it keeps heat-trapping carbon dioxide out of the atmosphere.

Confetti squid. Photo by Paul Caiger, Woods Hole Oceanographic Institution.
Dreamer anglerfish, (Oneirodes sp.). Photo by Paul Caiger, Woods Hole Oceanographic Institution.
Glass squid. Photo by Paul Caiger, Woods Hole Oceanographic Institution.

Is the twilight zone a major source of food for humans?

Humans are unlikely to eat anything caught from the twilight zone—the animals there are mostly small, bony fish and gelatinous organisms. However, some governments and corporations are planning to harvest animals from the twilight zone to provide fish meal for aquaculture operations. These in turn can produce seafood for humans. Worldwide, aquaculture is expected to grow 37% by 2030 to help meet the protein demand of a growing human population and reduce pressure on fisheries that are increasingly over-fished. 

How is the twilight zone connected to the surface ocean?

Perhaps most remarkable, many twilight zone organisms participate in the largest migration on Earth. It happens around the globe, every day, sweeping across the world’s oceans in a massive, living wave. Every night, fish, squid, plankton, and other mid-ocean dwellers begin their journey up to surface waters to feed. By daybreak, they will be gone again, headed back to the relative safety of deeper, darker waters. Large, surface-dwelling predators such as sharks, swordfish, and whales routinely make the reverse trip, diving into the twilight zone to feed on the abundant life there.

Lanternfish Viperfish otoliths. Otoliths (ear bones) help fish orientate themselves and maintain balance, acting like our middle ear. Photo by Paul Caiger, Woods Hole Oceanographic Institution.
Strawberry squid, (Histioteuthis-reversa). With have huge eyes and built-in headlights, the squid is well equipped for life in the dark depths. Photo by Paul Caiger, Woods Hole Oceanographic Institution.
Bobtail squid (order Sepiolida) are a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. Photo by Paul Caiger, Woods Hole Oceanographic Institution.