The abundant life in the twilight zone is specially adapted tonear-freezing temperatures, crushing pressures, and almost complete darkness. All of these animals, from predator to prey, are interconnected in a food web that moves energy across different levels of the food chain. Photosynthetic plankton form the base, producing food and energy from sunlight and carbon dioxide. Understanding food webs reveal key information about ecosystems: species interactions, community structures, and the general dynamics of energy transfer.
or click the buttons to learn more
Life in the OTZ
Phytoplankton are the primary producers in the open ocean, serving as the base of the food chain and the fuel that powers a majority of ocean life. Because they get energy from carbon dioxide and sunlight, these tiny, plant-like organisms are limited to the upper 100 meters of ocean, where sunlight can still drive photosynthesis. Small zooplankton are the major consumers of phytoplankton at the surface, and then larger zooplankton and small fish eat the small zooplankton, which are in turn eaten by larger fish.
These non-migrating fish may only be a few inches long, but they populate the twilight zone in staggering numbers, possibly in the quadrillions (1,000,000,000,000,000).
Migrators, like this krill, feast on plankton at the surface and rapidly transport that carbon deep when they swim down for the day.
Migrators & Non-migrators
Each night, massive schools of small fish, crustaceans, and jelly-like creatures migrate from the twilight zone to the ocean’s surface to feed at the surface. When the sun rises, these migrators return to the twilight zone with their bellies full. Eventually, they poop, depositing carbon in deeper water, away from the atmosphere. A separate population of organisms doesn’t migrate. These creatures likely feed on drifting bits of organic matter known as marine snow and other animals. Teasing apart these two distinct food webs is critical to understanding the entire twilight zone ecosystem.
Much of the life in the twilight zone is gelatinous. These organisms do well in the deep ocean because their bodies are mostly made of water, which enables them to withstand crushing pressures. A variety of gelatinous organisms inhabit the twilight zone. They may look similar, but the salp, pictured here, is more closely related to humans than the Atolla jellyfish or siphonophore.
Many larger twilight zone animals do not participate in the nightly migration. Because they live in constant darkness, these creatures’ eyes tend to be large, allowing them to detect as many photons of light trickling down from above as possible. The strawberry squid has one large eye facing up to detect prey’s silhouette against the brighter waters above, and a smaller eye facing down for glimpses of bioluminescence.
The abundant life in the twilight zone is specially adapted to just-above-freezing temperatures, crushing pressures, and near-complete darkness. All of these animals, from predator to prey, are interconnected in a food web in which organisms share energy across different levels of the food chain. Photosynthetic plankton form the base, producing energy from sunlight and carbon dioxide. Food webs reveal key information about ecosystems: species interactions, community structures, and the general dynamics of energy transfer.
Animals at the very top of the food chain, like sharks, swordfish and whales, are apex predators. Food in the open ocean is sparse, so the twilight zone is a critical source of food for these large creatures. Many top predators use their own body’s energy to keep warm while diving deep to feed, while others, like blue sharks and swordfish, ride warm water eddies to maintain their body temperature in the cold twilight zone.
In the vast, cold, dark water of the ocean twilight zone, you’d better be prepared to eat whatever you find, whenever you find it. Many fish have adapted to have giant teeth in order to be sure they capture their unlucky prey as it swims too close, because you never know when your next meal will swim by. The aptly-named fangtooth has the largest tooth-to-body ratio of any fish in the ocean. Their super chompers help this frightful-looking fish entrap fish larger than themselves for a hearty meal.
My, What Big Teeth You Have!
Numerous twilight zone species have evolved ways to produce their own light through a biochemical process called bioluminescence, which they use for mating, hunting, or protection. Some species, like lanternfish, fire up light-producing organs on their bellies to blend in with the weak light coming from above. This “counter-illumination” strategy keeps predators hunting from below from seeing their silhouette. Others, like the anglerfish, seen here, dangle a bioluminescent organ called an “esca” in front of their mouths to attract prey.
A Phronima, or pram bug, creates a safe nursery for its young by hollowing out a gelatinous salp and laying her eggs inside. She protects the eggs, pushing them around in their salp “pram” and providing them with food as they develop into larvae.
To mate, a small male anglerfish latches onto a male anglerfish parasitizes latches onto a much larger female and releases sperm to fertilize her eggs when she releases them. Over the course of an anglerfish’s lifetime, several males can physically fuse with a female, connecting to her skin and bloodstream and losing their own eyes and organs.
In a place as vast and dark as the twilight zone, finding a mate and protecting young is difficult. Many animals have special physical and behavioral adaptations for successful reproduction in this tricky environment.