Far below the sunlit surface lies a vast, dimly lit realm known as the “twilight zone.” This layer of the ocean is bathed in perpetual dimness, making it a challenging environment for a diverse array of life.
What is the Ocean’s Twilight Zone?
The ocean’s twilight zone, scientifically known as the mesopelagic zone, extends approximately from 200 to 1,000 meters (660 to 3,300 feet) below the surface. Only about one percent of sunlight penetrates this depth, diminishing to near darkness at its lower boundary. The term “twilight zone” describes this region as a transition between the sunlit surface waters (epipelagic zone) and the completely dark deep ocean (bathypelagic zone).
Life in this zone faces environmental challenges beyond limited light. Temperatures typically range from over 20 degrees Celsius (68 degrees Fahrenheit) at its upper reaches to around 4 degrees Celsius (39 degrees Fahrenheit) at its deeper boundary. Water pressure also increases considerably with depth, reaching up to 1,500 pounds per square inch. Despite these conditions, the mesopelagic zone supports a significant amount of biomass, with some estimates suggesting it may hold more fish than the rest of the ocean combined.
Adaptations for Life in Dim Light
Animals inhabiting the twilight zone have evolved adaptations to survive in its conditions. Many possess specialized vision, such as large, highly sensitive eyes designed to capture the faintest traces of light. Some species have tubular eyes oriented upwards to detect silhouettes against the dim downwelling light.
Bioluminescence, the ability to produce light through chemical reactions, is a widespread adaptation in this zone, present in nearly 90% of marine creatures below 1,500 feet. This light serves various purposes, including attracting prey, evading predators through counter-illumination, and facilitating communication. Counter-illumination involves emitting light from the underside of the body to match the faint light filtering from above, camouflaging the animal against predators looking upwards.
To conserve energy in an environment with limited food resources, many twilight zone inhabitants exhibit slow metabolic rates. Buoyancy control is achieved through various means, such as having large, oily livers or gelatinous bodies, which reduce the need for energy-intensive swimming. A common feeding strategy involves vertical migration, where many species ascend to shallower, food-rich surface waters at night to feed under the cover of darkness and then return to the deeper twilight zone during the day. This daily migration represents the largest animal migration on Earth.
Remarkable Residents of the Twilight Zone
Lanternfish
Lanternfish (family Myctophidae) are among the most abundant vertebrates on Earth and a dominant fish group in the mesopelagic zone. These small fish are known for their light-producing organs called photophores along their bodies, which they use for camouflage, attracting mates, and communication. Many lanternfish also participate in the daily vertical migration, ascending to feed at night and descending during the day.
Viperfish
Viperfish (genus Chauliodus) are predators characterized by their long, needle-like teeth that are too large to fit inside their mouths, and hinged lower jaws. They use a bioluminescent lure on a long dorsal spine to attract prey. These fish have elastic stomachs and bodies, allowing them to consume prey larger than themselves. A low metabolic rate enables them to survive for days without food.
Bristlemouths
Bristlemouths (family Gonostomatidae) are considered the most abundant vertebrates on the planet, with population estimates potentially in the quadrillions. These small fish have large mouths with bristle-like teeth and possess bioluminescent organs along their undersides, which help them camouflage and communicate. Many bristlemouth species do not undertake daily vertical migrations, possibly due to their fat-filled swim bladders, which differ from the air-filled bladders of other migrating fish.
The Loosejaw Dragonfish
The Loosejaw Dragonfish (Malacosteus niger), also known as the stoplight loosejaw, is a predator with a jaw structure that lacks a floor, allowing it to open its mouth to an extreme degree to capture prey. This fish produces both red and blue bioluminescence. Most other deep-sea organisms cannot perceive red light, giving the dragonfish a distinct advantage by allowing it to illuminate prey without being detected.
Giant Squid
Giant Squid (Architeuthis dux) are residents of the twilight zone, typically found at depths between 300 and 600 meters (1,000 and 2,000 feet). These cephalopods possess the largest eyes in the animal kingdom, which are crucial for detecting faint light in their dim habitat. Their size and adaptations highlight the diversity supported by the ocean’s twilight zone.