The mesopelagic zone, often referred to as the “twilight zone,” represents a vast and mysterious part of the ocean. This global layer extends approximately 200 to 1,000 meters (660 to 3,300 feet) below the surface. Situated between the sunlit surface waters and the perpetually dark abyss, the mesopelagic zone holds a unique position in the marine environment. It encompasses about 60% of the planet’s surface and roughly 20% of the ocean’s total volume, making it a significant portion of the Earth’s biosphere.
The Mesopelagic Environment
The mesopelagic zone presents a challenging habitat due to several distinct physical characteristics. Light is a primary defining feature of this “twilight” realm; some sunlight penetrates from above, but it is insufficient to support photosynthesis. Light intensity decreases significantly with depth, transitioning from dim illumination to near-total darkness. Temperatures in this zone are cold and relatively stable, ranging from about 4°C to 10°C.
Hydrostatic pressure increases considerably with depth. Food availability is limited, as most organic matter, known as “marine snow,” sinks from the surface epipelagic zone. Only about 20% of surface primary production reaches this depth.
How Animals Survive the Twilight Zone
Animals in the mesopelagic zone have developed a variety of biological and physiological adaptations to thrive in this low-light, food-scarce environment. Bioluminescence, the production of light by living organisms, is a widespread adaptation. This light is used for various purposes, including camouflage through counter-illumination, attracting prey, evading predators, and communication. Many species match the faint downwelling light from the surface, making their silhouettes less visible to predators looking from below.
Many mesopelagic animals possess large, sensitive eyes capable of capturing the minimal light available. Some species, like krill and certain squid, have tubular eyes directed upwards to detect silhouettes of prey against the faint overhead light. Another common strategy is diel vertical migration, where many species ascend to shallower, food-rich surface waters at night to feed and then descend back into the darker, deeper mesopelagic zone during the day to avoid visual predators.
Feeding strategies include adaptations for scarce food, such as large mouths, extendable jaws, and specialized lures. Organisms often have slower metabolisms to conserve energy in this nutrient-poor habitat. Many also have gelatinous bodies or weak musculature, which helps them save energy by reducing the need for strong swimming in the water column.
Who Lives in the Mesopelagic Zone?
The mesopelagic zone supports a diverse array of animal life, each with unique features enabling survival in this deep, dim environment. Mesopelagic fish are particularly abundant, with lanternfish (Myctophidae) being one of the most common groups, comprising over 245 species. These small fish, typically less than 15 centimeters long, are known for their photophores, light-producing organs that dot their bodies. Lanternfish undertake daily vertical migrations, moving to shallower waters at night to feed on zooplankton and returning to deeper waters during the day to avoid predators.
Bristlemouths (Gonostomatidae) are another prevalent family of mesopelagic fish, potentially representing the most abundant vertebrates on Earth. Hatchetfish are also found here, characterized by their distinctive body shapes and use of counter-illumination. These fish often have well-developed muscles and bones.
Various invertebrates also inhabit this zone. Squids and octopuses include species like the colossal squid and vampire squid. The colossal squid is one of the largest invertebrates on Earth, known for its massive eyes. The vampire squid is a small, soft-bodied cephalopod that feeds on detritus.
Jellyfish and siphonophores are common gelatinous organisms in the mesopelagic zone. Siphonophores are colonial animals, many of which are bioluminescent. Crustaceans, such as copepods and krill, form a significant part of the zooplankton community and serve as a food source for many larger animals.
Exploring the Mesopelagic Depths
Studying the mesopelagic zone presents considerable challenges due to its extreme conditions. The increasing pressure, low temperatures, and perpetual dimness make direct human exploration difficult. Scientists utilize specialized technologies to overcome these obstacles and gather information about this elusive environment.
Remotely operated vehicles (ROVs) and submersibles equipped with advanced cameras and sensors allow researchers to observe and collect samples without human presence at depth. Deep-sea trawls are also employed to capture organisms for study. Sonar technology has revealed the massive biomass within this zone, identifying the “deep scattering layer” which consists of millions of mesopelagic organisms undergoing vertical migration. Despite ongoing efforts, the mesopelagic zone remains a largely unexplored frontier, with new species continually being discovered.