The mesopelagic zone, stretching roughly from 200 to 1,000 meters below the ocean surface, is the largest habitat on Earth by volume and one of the least understood. Often called the twilight zone because only faint traces of sunlight reach it, this band of ocean holds staggering amounts of life, drives a significant share of the planet’s carbon cycling, and supports food webs that reach from deep-sea squid all the way up to tuna and dolphins. Yet researchers are still debating basic questions about how much biomass lives there and what would happen if humans started harvesting it.
Where the Twilight Zone Begins and Ends
The textbook definition puts the mesopelagic between 200 and 1,000 meters deep, but those round numbers are a convenience rather than a biological boundary. A more meaningful way to define it is by light. One research framework proposes marking the zone by the range of light intensities that mesopelagic animals can actually detect, from the visual threshold of lanternfishes at the dim end up to the tolerance of pearlsides, which have unusually light-adapted eyes, at the bright end. Under that definition, the daytime twilight habitat can extend deeper than 1,000 meters in very clear open-ocean water, while in murky coastal areas it may sit largely above 200 meters.1ICES Journal of Marine Science. Enlightening the ocean’s twilight zone The zone is not a fixed slab of ocean. It shifts with water clarity, season, and geography.
Temperature drops sharply through the mesopelagic, and dissolved oxygen thins out in many regions. Oxygen minimum zones, where microbial consumption of sinking organic matter depletes the available supply, intersect the twilight zone across large swaths of the tropics and subtropics. Research tracing the origins of these low-oxygen waters shows that roughly half of the water volume in oxygen minimum zones originates in high-latitude regions, but most of its oxygen gets consumed during remineralization before it arrives. The zones themselves are primarily oxygenated by tropical and subtropical waters formed in nearby regions.2Journal of Geophysical Research: Oceans. On the Origins of Open Ocean Oxygen Minimum Zones For the animals living at mesopelagic depths, oxygen availability is as defining a feature of their habitat as light is.
How Much Life Lives in the Twilight Zone
Estimates of mesopelagic fish biomass have been revised upward repeatedly over the past two decades, and there is still no consensus number. A major challenge is that the standard tool for counting fish at depth, trawl nets, misses a lot. Fast-swimming or net-avoiding species slip away, and the size-dependent efficiency of different nets can skew biomass estimates by a factor of five. Off southern California, combined acoustic and trawl-based methods estimated mesopelagic fish biomass at roughly 25 to 37 grams per square meter of ocean surface, a density comparable to that of inshore surface-dwelling plankton-eating fishes.3Deep Sea Research Part II: Topical Studies in Oceanography. Mesopelagic fish biomass in the southern California current ecosystem Extrapolate that kind of density across the global ocean and the numbers become enormous, though the uncertainty remains wide.
Fish are only part of the picture. The twilight zone teems with micronekton, the small free-swimming animals in the 2 to 20 centimeter range that include crustaceans, cephalopods, and gelatinous organisms alongside fishes.4Oceanography. Unraveling Major Questions in Micronekton Ecology and Their Role in the Biological Carbon Pump Through Integrative Approaches and Autonomous Monitoring Cephalopods are a good example of how sampling methods distort what we think we know. Off the western Canary Islands, a single research cruise captured over 3,700 cephalopod specimens spanning 17 families and 37 species, dominated by small, luminous squid such as Pyroteuthis margaritifera and Abraliopsis moriisi.5Regional Studies in Marine Science. New contribution to the knowledge of the mesopelagic cephalopod community off the western Canary Islands slope Yet in studies comparing sampling approaches, cephalopods are routinely underrepresented by acoustic algorithms and barely show up in environmental DNA analyses using standard primers, despite accounting for the majority of trawl-net biomass.4Oceanography. Unraveling Major Questions in Micronekton Ecology and Their Role in the Biological Carbon Pump Through Integrative Approaches and Autonomous Monitoring Crustaceans and gelatinous organisms are similarly overlooked by genetic methods. The upshot is that every estimate of twilight-zone biodiversity and biomass is a known undercount, and the degree of undercounting varies by method and by group.
The Nightly Commute
Diel vertical migration is the single most conspicuous phenomenon in the mesopelagic, and by some measures the largest coordinated animal movement on the planet. Each evening, dense aggregations of fish, squid, crustaceans, and zooplankton rise hundreds of meters toward the surface to feed under cover of darkness, then descend again before dawn. The pattern was first noticed in the late 1940s, when navy sonar operators discovered a mysterious “deep scattering layer” that reflected sound and moved up and down each day. That layer turned out to be living animals.6International Journal of Oceanography. The Development of SONAR as a Tool in Marine Biological Research in the Twentieth Century
Light is the master trigger. Acoustic tracking of zooplankton across latitudes has shown that above about 650 meters, the animals’ day-length behavior tracks the solar day length precisely, meaning they sense and respond to sunlight. Below that depth, sensitivity to day-length changes drops off, suggesting that plankton deeper than 650 meters do not sense sunlight at all and rely on other cues.7PubMed Central. Diel Vertical Migration in Deep Sea Plankton Is Finely Tuned to Latitudinal and Seasonal Day Length Even in the Arctic during the polar night, when the sun stays below the horizon for weeks, zooplankton maintain distinct diel vertical migration cycles, apparently detecting and responding to extremely subtle changes in ambient light.8PubMed Central. Diel vertical migration of Arctic zooplankton during the polar night The sensitivity is remarkable: these animals are orienting their entire lives around light levels so faint that human instruments struggle to measure them.
Carbon Shuttling to the Deep
The mesopelagic zone plays a critical role in the biological carbon pump, the set of processes that move carbon from the surface ocean into the deep, where it can be locked away from the atmosphere for centuries. This happens through two main pathways: the passive sinking of dead organic matter (marine snow), and the active transport of carbon by migrating animals.
Marine snow is not a uniform substance. Particles come in distinct shapes, and those shapes strongly influence how fast they sink and how much of their carbon survives the journey. Sphere-shaped aggregates and clumps sink fastest and lose the least carbon as they descend, while flat flakes and strings sink more slowly and break down more quickly.9Biogeosciences. Marine snow morphology drives sinking and attenuation in the ocean interior Speed matters because the longer a particle drifts through the mesopelagic, the more time microbes have to consume it. At an oligotrophic site near Bermuda, average sinking velocities were about 49 meters per day and microbial respiration rates on particles were substantial, meaning heavy carbon losses in transit. By contrast, particles off the west Antarctic Peninsula sank at roughly 270 meters per day with nearly undetectable microbial respiration, delivering far more carbon to depth.10Global Biogeochemical Cycles. Effects of sinking velocities and microbial respiration rates on the attenuation of particulate carbon fluxes through the mesopelagic zone The efficiency of carbon transfer through the mesopelagic is therefore not a fixed number; it varies enormously by region.
The microbes doing the consuming are themselves stratified in interesting ways. Bacteria attached to fast-sinking particles maintain relatively high per-cell activity all the way from the surface down to 500 meters, while free-floating bacteria become far less active with depth.11The ISME Journal. Mesopelagic microbial carbon production correlates with diversity across different marine particle fractions Those particle-attached microbes are responsible for the final step of remineralization, converting particulate organic carbon into dissolved CO₂.12The ISME Journal. Mesopelagic microbial carbon production correlates with diversity across different marine particle fractions The depth at which that CO₂ is released determines how long it stays sequestered: carbon released in the upper mesopelagic can return to the atmosphere within decades, while carbon released below 1,000 meters may stay out of circulation for centuries.
What Migrating Animals Add to the Equation
Passive sinking is only half the story. The animals commuting up and down every day carry carbon with them. Zooplankton feed at the surface at night, then descend to mesopelagic depths during the day, where they respire, excrete dissolved organic matter, and produce fecal pellets. All three pathways deposit carbon at depth that would otherwise have stayed near the surface.13PubMed. Active Carbon Transport by Diel Vertical Migrating Zooplankton: Calculated and Modeled, but Never Measured
In the Southern Ocean, this active flux is substantial. A study combining migrating-biomass measurements with temperature-dependent metabolic models calculated that diel vertical migration transported roughly 39 megatons of carbon during summer, with a wide uncertainty range from 27 to 60 megatons. That represented about 13% of total summer carbon export at 200 meters depth in the Southern Ocean. At individual stations, the active flux ranged from barely 1% to over 130% of the gravitational particle flux, depending on local conditions.14Limnology and Oceanography. Zooplankton diel vertical migration drives a major, variable and underestimated carbon sink in the Southern Ocean The title of the paper calls this a “previously under-quantified component” of the carbon cycle, which captures the situation well: for years, models focused on sinking particles and largely ignored what swimming animals were doing.
Gelatinous organisms add another overlooked channel. When populations of jellyfish, salps, and ctenophores collapse after blooms, their carcasses sink extremely fast, measured at 850 to 1,500 meters per day depending on the species, with salps around 800 to 1,200 meters per day and ctenophores reaching 1,200 to 1,500.15Limnology and Oceanography. Jelly biomass sinking speed reveals a fast carbon export mechanism Those speeds are far faster than typical marine snow, meaning jelly carcasses bypass much of the mesopelagic microbial gauntlet and deliver carbon to depth with high efficiency. Global estimates suggest that from an upper-ocean production of about 0.038 petagrams of jelly carbon per year, roughly 59 to 72% reaches 500 meters and 46 to 54% makes it all the way to 1,000 meters.16Global Biogeochemical Cycles. Sinking of Gelatinous Zooplankton Biomass Increases Deep Carbon Transfer Efficiency Globally
Camouflage in Near-Total Darkness
The dim light of the mesopelagic has driven the evolution of remarkable visual and camouflage systems. The dominant strategy is counterillumination: animals produce bioluminescence on their undersides to match the faint downwelling light from above, erasing their silhouette when viewed from below. This tactic is widespread across mesopelagic fishes, sharks, crustaceans, and squid.17PubMed. An Investigation into the Mechanism Mediating Counterillumination in Myctophid Fishes (Myctophidae)
The sophistication goes beyond simply glowing. Two species of mesopelagic squid have been shown to dramatically shift the color of their bioluminescence depending on water temperature, which changes as they migrate vertically. Because the color of downwelling light shifts between their daytime deep habitat and their nighttime shallow habitat, the squid adjust their glow to match whichever light field they are in.18PubMed. Bioluminescence in mesopelagic squid: diel color change during counterillumination In bioluminescent sharks, the same basic counterillumination machinery has been co-opted for additional purposes. Species facing moderate predation risk from below have repurposed some of their light-producing organs for signaling, leading to the diverse and species-specific photophore patterns seen across the group.19Scientific Reports. Iso-luminance counterillumination drove bioluminescent shark radiation
Lanternfishes, the most abundant vertebrates in the mesopelagic and possibly on the planet, have eyes that have been pushed to their sensitivity limits. The ambient light in the twilight zone is extraordinarily faint, and the bioluminescent signals produced by other animals are generally dim and brief. Visual systems in the mesopelagic operate at the extreme edge of what photoreception can achieve.20PubMed Central. Seeing in the deep-sea: visual adaptations in lanternfishes
Who Eats What, and Why It Matters at the Surface
The mesopelagic is not an isolated ecosystem. It feeds a remarkable range of surface and near-surface predators. A broad diet analysis found mesopelagic fishes to be important prey for multiple phyla of predators. Among dolphins, the frequency was striking: 89% of Northern right whale dolphin diet samples and 86% of short-beaked common dolphin samples contained mesopelagic fish. Pelagic squids rely on them heavily as well, with armhook squid at 75% and Humboldt squid at 52%. Among commercially important fish, bigeye thresher sharks and broadbill swordfish had mesopelagic fish in about half their diet samples.21Frontiers in Marine Science. Mesopelagic fishes are important prey for a diversity of predators
Carbon isotope analysis in the northwest Atlantic put finer numbers on the connection. Bigeye tuna sourced an average of about 62% of the carbon in their livers from mesopelagic communities. Yellowfin tuna averaged around 46%, and swordfish about 28%. The variability among individual fish was large, ranging as high as 93% mesopelagic carbon in some bigeye tuna specimens.22ICES Journal of Marine Science. Evaluating the importance of mesopelagic prey to three top teleost predators in the northwest Atlantic Ocean The twilight zone, in other words, is not just feeding deep-sea oddities. It is fueling some of the most commercially valuable fisheries in the world.
Should We Fish the Twilight Zone?
The enormous biomass in the mesopelagic has drawn interest from fishing industries looking for new sources of fishmeal and fish oil, particularly as many surface fish stocks are already fully exploited. Some countries have begun exploring pilot harvests of lanternfish and similar species. The basic economics look viable: modeling suggests that mesopelagic fishing could be profitable for harvesters and that adding mesopelagic catch to global fishmeal production would reduce fishmeal prices, benefiting aquaculture while potentially easing fishing pressure on surface forage-fish stocks.23Natural Resource Modeling. Assessing the potential economic effects of mesopelagic fisheries as a novel source of fishmeal
The ecological risks, however, are where the picture darkens. An economic analysis of four European Union pelagic trawling fleets found that while a mesopelagic fishery would be profitable from a private industry perspective, the climate impacts from disrupting the carbon pump could outweigh those private economic benefits.24ICES Journal of Marine Science. Climate damage from fishing the mesopelagic zone exceeds its economic benefits A separate ecological simulation for the California Current found that most modeled predators of mesopelagic fishes did not suffer large biomass declines even under high harvest rates, but the changes were small in both directions and the authors stressed the need for more research into the mesopelagic zone’s various roles before opening it to commercial exploitation.25PubMed Central. The economic tradeoffs and ecological impacts associated with a potential mesopelagic fishery in the California Current The honest state of knowledge is that we do not yet understand the system well enough to confidently predict what large-scale harvesting would do to carbon cycling, food webs, or the surface fisheries that depend on mesopelagic prey.
Climate Change and the Shrinking Habitat
The mesopelagic is not buffered from a warming planet. Ocean warming reduces dissolved oxygen, and expanding oxygen minimum zones squeeze the habitable space for twilight-zone animals. Modeling projects that species richness in the mesopelagic declines by roughly 2.3% per degree of global warming, a slightly steeper drop than in surface waters.26Biogeosciences. Impact of deoxygenation and warming on global marine species in the 21st century The interaction of oxygen and temperature on metabolism is crucial: as water warms, animals need more oxygen, but less is available.27Integrative and Comparative Biology. Hypoxia Tolerance and Metabolic Suppression in Oxygen Minimum Zone Euphausiids: Implications for Ocean Deoxygenation and Biogeochemical Cycles
There is a precedent in the fossil record. During ancient sapropel events in the Mediterranean, when deep waters turned anoxic, lanternfish populations collapsed. Their habitat between about 250 and 600 meters became uninhabitable as the oxygen minimum zone expanded. Shallower-dwelling species like silvery lightfish and European anchovy, whose habitat sat above the expanding dead zone, survived.28Communications Earth & Environment. Ocean deoxygenation linked to ancient mesopelagic fish decline That ancient pattern offers a warning: if oxygen minimum zones expand significantly under current warming trends, the core inhabitants of the mesopelagic may lose their habitat from the inside out.
Microplastics at Depth
Pollution has already reached the twilight zone. A study of mesopelagic fishes in the southwestern tropical Atlantic found microplastics in 67% of specimens examined, with fibers being the most common shape and polyamide, polyethylene, and polyethylene terephthalate the dominant polymers. Fish caught in the upper mesopelagic (200 to 500 meters) were more contaminated than those from the lower mesopelagic (500 to 1,000 meters), suggesting the particles concentrate in the shallower portion of the zone.29Environmental Pollution. The role of mesopelagic fishes as microplastics vectors across the deep-sea layers from the Southwestern Tropical Atlantic Because mesopelagic fish are consumed by commercially important predators, the contamination does not stay at depth. Microfibers ingested by a lanternfish can pass up the food chain when that fish is eaten by a tuna or a dolphin.30Frontiers in Marine Science. Synthetic and Semi-Synthetic Microplastic Ingestion by Mesopelagic Fishes From Tristan da Cunha and St Helena, South Atlantic The mesopelagic’s role as a trophic highway means it is also becoming a contaminant highway.
Why the Twilight Zone Is So Hard to Study
One reason the mesopelagic remains poorly understood is that it is fiendishly difficult to sample. Trawl nets, as noted above, undercount fast or fragile organisms. Acoustic surveys can detect the deep scattering layer but cannot always distinguish what species are producing the signal. Environmental DNA offers a promising alternative, picking up genetic traces shed by animals in the water, and has shown potential for detecting species that traditional nets miss entirely.31Frontiers in Ecology and Evolution. Exploring the use of environmental DNA (eDNA) to detect animal taxa in the mesopelagic zone But eDNA has its own blind spots: gelatinous organisms and crustaceans may go undetected depending on the genetic primers used, and cephalopods are chronically underrepresented.4Oceanography. Unraveling Major Questions in Micronekton Ecology and Their Role in the Biological Carbon Pump Through Integrative Approaches and Autonomous Monitoring No single tool gives a reliable census. The field is moving toward integrating multiple methods, combining acoustics, nets, eDNA, and in situ optical instruments, to triangulate a more complete picture.
Depth as an Engine of New Species
The mesopelagic is not just an ecological layer; it is an evolutionary arena. In the open ocean, where there are no obvious physical barriers like mountains or rivers to separate populations, depth itself appears to drive the formation of new species. A study of Pacific rockfishes found a strong signal of speciation along the depth gradient, with species evolving traits that adapted them to different depth ranges.32PubMed Central. Speciation along a depth gradient in a marine adaptive radiation Among deep-sea copepods, closely related species that are otherwise similar in their biology and ecology tend to occupy different depth layers, suggesting vertical partitioning of the water column is a key mechanism separating lineages.33Molecular Phylogenetics and Evolution. Evolution in the deep sea: Biological traits, ecology and phylogenetics of pelagic copepods
The genetic evidence is striking even in young lineages. Atlantic rockfish, a comparatively recent group, show sharp depth-associated divergence at the rhodopsin gene, which encodes a light-sensitive protein critical for vision. This parallels patterns seen in the older, more species-rich Pacific rockfish clade, and suggests that natural selection on visual sensitivity at different light levels is actively driving populations apart.34PeerJ. Divergence by depth in an oceanic fish The mesopelagic’s steep gradient in light, temperature, and oxygen creates selection pressures that can sort populations by depth, eventually producing distinct species. It is, in effect, an invisible mountain range, and animals evolve on its slopes much as they do on terrestrial ones.