Twilight Zone

Twilight Zone


The twilight zone, or mesopelagic zone, is the layer beneath the euphotic zone (sunlight zone) where the amount of sunlight reaching the waters is insufficient for photosynthesis. This zone can be found between the depths of 200 to 1000 meters and includes a variety of species from microscopic zooplankton to squids, as well as its famous, bioluminescent organisms. In addition, this zone facilitates the carbon transfer from the surface to the depths, acting as a critical link in the process of storing carbon for millions of years.

This zone contains 95% of the world’s fish by weight, but it is largely unstudied compared to the easily accessible sunlight zone above. As humans begin to venture down into the depths, factors such as the high pressure and ease of access present a barrier to research, but the lack of national jurisdiction over these waters allows for a global collective to work towards understanding these systems.

This image shows the diversity that lives in the twilight zone. Though there is much to be discovered!

Habitats & Adaptations

The world beneath the waves holds a multitude of mysteries and marvels, where organisms and ecosystems unfold their intricate dance in the depths. In this realm, two fascinating phenomena come to light – vertical migration and bioluminescence. In the quiet expanse of the water column, billions of microorganisms and smaller animals embark on a remarkable journey every day. This rhythmic movement, known as vertical migration, sees them ascending towards the surface during the night and descending to the depths by day. Their enigmatic dance is orchestrated by a blend of internal biological clocks and external cues like light intensity, enabling them to thrive amidst the challenges of predation and sustenance.

The phenomenon of vertical migration, discovered through serendipitous encounters like WWII sonar readings, unveils a strategic survival strategy that allows these minuscule creatures to evade their larger predators and secure abundant nourishment. Yet, the intrigue doesn't end with the sunlit depths. In the depths of the twilight zone, an ethereal glow illuminates the darkness – a phenomenon known as bioluminescence. Organisms dwelling in this deep abyss emit light through intricate chemical reactions, creating a captivating spectacle that serves purposes ranging from hunting and reproduction to predator evasion.

As we delve into the intricacies of vertical migration and the luminescent allure of the deep sea, we unravel nature's ingenuity at its finest. By exploring the underlying mechanisms, ecological impacts, and scientific endeavors to comprehend these phenomena, we gain a glimpse into the mesmerizing underwater world that continually astounds and inspires. 

Learning Objectives:

  • Explain the phenomenon of vertical migration in the water column, including the factors that drive this movement. 
  • Define and identify the purposes of bioluminescence.
  • Investigate the challenges scientists face when studying deep-sea organisms due to pressure differences.

Key Vocabulary

Biological clock
Vertical migration
Visible light
Twilight zone
Water column


Next Generation Science Standards

LS1.A - Structure and Function

All living things are made up of cells, which is the smallest unit that can be said to be alive. An organism may consist of one single cell (unicellular) or many different numbers and types of cells (multicellular).

LS2.C - Ecosystem Dynamics, Functioning, and Resilience

In any ecosystem, organisms and populations with similar requirements for food, water, oxygen, or other resources may compete with each other for limited resources, access to which consequently constrains their growth and reproduction.

Vertical Migration

In the water column, billions of microorganisms and smaller-sized animals migrate upwards to the surface during the nighttime and down toward the depths during the daytime. This 24-hour process is driven by both the organism’s own biological clocks and external factors such as light intensity. While this energy-intensive commute toward the surface may seem inefficient, the abundance of food on the surface and the safe refuge of the deeper waters provides a chance for these organisms to survive. During the day, many of the larger organisms would feed on the zooplankton and others would feed on them if they migrated during the sunlight-lit hours of the day. This cycle allows these tiny organisms to feed safely with an abundance of food.

Midwater organisms that may be found in the deep scattering layer (DSL). Left: an iridescent copepod, the underbelly of a fish, and a squid. Middle: a jellyfish and a ctenophore. Right: a rabbit fish and a pelagic sea cucumber.

This phenomenon was first noticed in WWII when the newly created sonar system detected changing seafloor depths varying by the time of day. This created a false, higher seafloor that only appeared at night, which they later discovered was the many organisms during their nighttime vertical migration. The pack was so dense that their sonars reflected off, so they named it the “Deep Scattering Layer.” This was confirmed by Scripps Institution of Oceanography scientist Martin Johnson who collected copepods in a nighttime experiment in Point Loma, CA, discovering that these creatures of the deeper waters were on their daily migration.

However, this phenomenon isn’t limited to areas with sunlight as the main driving factor for the cycle. Scientists have studied the migration of organisms across different taxonomical groups from squids to plankton to sharks and found that the pattern is uniform across species and locations. In 2008, researchers studying the migration of zooplankton in the Artic waters used acoustic sampling over the course of many months to discover the pattern of these organisms. In this area of the world, sunlight is absent in many months of the summer, so the scientists collected data during this period and discovered that the migrations were driven by the intensity of moonlight instead of the sunlight.

In another study conducted over the northern Pacific Ocean, scientists discovered that the creatures adjusted their height in the water column based on the cloud cover and weather affecting the availability of light, further signaling their acknowledgment of this factor of light intensity. Without this dependence on the sun, creatures across the oceans follow a fully synchronous migration each day, moving up and down the water column.




In the dark world of the twilight zone, there is a surprising amount of light present. Creatures produce the light themselves through chemical reactions to emit visible light waves, thus being bioluminescent. When scientist Peter Wiebe descended into the depths of a single human submersible, he turned off all the lights of his vessel and saw what he coined “the night sky in the deep sea.” He describes the commonly known “deep dark sea” as “not so dark,” but rather illuminated in its own majestic way.

In this zone, creatures will illuminate themselves to find food, find mates, or avoid detection by predators. In the case of the Dreamer anglerfish, its iconic lamp-like bulb lures prey into its vicinity, where it quickly lunges forward for an easy meal. Some visually inclined creatures such as squids use complex lighting patterns to attract mates, although they reveal their position in the process, which is a trade-off they also consider. Most commonly, creatures such as lanternfish light themselves up to hide from predators through counterillumination since predators look upwards towards the light and target the darker areas. This is similar to countershading, where predators such as orcas have a darker back and lighter underside to remain undetected.

However, these creatures are often hard to study due to the environment they must be in to retain their natural characteristics. In the twilight zone, the pressure increases steeply until about 1000m, where the curve of the pycnocline flattens out. These creatures are meant to live under these high pressures, and when taken to the surface, their bodily forms may change, or sometimes even explode due to the pressure differences.

Ctenophores (Comb jellies)

Ctenophores or comb jellies are translucent creatures that use small beating motions of their cilia (small hairlike structures) to move and eat the bioluminescent krill in the twilight zone.


Anglerfish sport a glowing lure near their mouths, achieved through special cells called photophores. This natural light serves two critical purposes:

  • Luring Prey: In the dark abyss, the anglerfish's bioluminescent lure attracts unsuspecting prey, drawing them close for an easy capture.
  • Mating and Recognition: For certain species, the light helps anglerfish mates find each other in the vast darkness, ensuring successful reproduction.

Environmental Importance

The twilight zone, found about 200 to 1000 meters below the ocean's surface, is a vital part of ocean life. It helps control the carbon cycle by moving tiny living things up and down. At night, these creatures bring carbon from the surface down, which affects the amount of carbon dioxide in the air. The zone also matters because small animals living there are food for bigger ones. This keeps the ocean food chain balanced.

The twilight zone also plays a role in moving nutrients around. Creatures moving up and down bring nutrients to the surface at night and take them deeper during the day. This helps keep the ocean ecosystem healthy. The zone is full of marine species that have learned to live in the dark and under pressure because of unique adaptations to help them survive in these tough conditions.

Scientists are excited about exploring the twilight zone. It's challenging because it's so deep and dark, but it's a treasure trove of new knowledge. Studying the zone helps us learn more about how life works and how creatures adapt to different environments. Also, the creatures in the zone might hold secrets that can be useful in making new medicines or technologies. By understanding and taking care of this special part of the ocean, we can learn a lot and help protect our planet's health.

Learning Objectives:

  • Explain how the twilight zone contributes to the carbon cycle by describing the vertical movement of tiny organisms that transport carbon from the surface down and its impact on carbon dioxide levels in the atmosphere. 

  • Describe the twilight zone's contribution to the regulation of atmospheric carbon dixoide levels and its impact on global climate. 

Next Generation Science Standards

LS2.A - Interdependent Relationships in Ecosystems

Organisms, and populations of organisms, are dependent on their environmental interactions both with other living things and with nonliving factors.

LS2.B - Cycling of Matter and Energy Transfer in Ecosystems

Matter cycles between the air and soil and among plants, animals, and microbes as these organisms live and die. Decomposition eventually restores (recycles) some materials back to the soil.

Key Vocabulary

Calcium carbonate
Carbon dioxide
Carbon cycle
Carbon pump
Food chain
Marine species
Ocean ecosystem

Marine Snow

Although largely unstudied, the twilight zone is the key component to carbon sequestration in the ocean. This occurs through the vertical migration of organisms, but a larger contributor is the falling debris of dead organisms and feces, also known as “marine snow.” The constant shower of organic material provides sustenance for many creatures in the twilight zone and appears to be a flurry of white, powdery snowflakes.

But this organic rain serves another purpose, moving the carbon taken from the atmosphere and storing it in the deeper parts of the ocean for thousands of years. As the creatures in the twilight zone consume the material, and eventually fall to the seafloor, they become a part of the ooze of the seafloor which stores the carbon. These layers cover the seafloor in white peaks, with the boundary being the carbonate compensation depth (CCD), or the depth at which the rate of the carbonate material dissolves faster than it accumulates. The snowline varies based on the input of calcareous material and acidity of the water, as a lower pH will dissolve the calcium carbonate material faster.

What is Marine Snow?

Marine snow is tiny particles that fall from the upper layers of the ocean to the deeper parts. These particles can include dead plant and animal matter, as well as minerals and other small particles. As these particles sink, they provide food and nutrients to organisms in the deeper ocean layers, contributing to the marine ecosystem's health and balance.

What other factors drive the carbon pump?

Besides marine snow, vertical migraters themselves consume organisms living above and migrate back towards the depths, creating a net carbon movement downwards. But currents in the ocean also play a large role in the net movement of carbon too, bringing the dissolved carbon from the surface into the deeper ocean. These currents circulate a giant underwater path for hundreds of years before returning to the surface, thus keeping the carbon out of the atmosphere for a long period. The increased carbon lowers the water’s pH increasing its acidity as well. In places such as coral reefs or other sensitive communities, a small change could harm the lives of many marine organisms, especially ones made of a carbonate structure.

Carbon Flow

Carbon dioxide from the atmosphere dissolves into the ocean's surface waters, where it becomes part of the marine ecosystem. As organisms, like phytoplankton, take up this carbon, they contribute to the ocean's vibrant life. Some of this carbon, represented by downward arrows, sinks deeper into the ocean, gradually reaching the twilight zone where it becomes marine snow, nourishing the depths and impacting global carbon cycles. Later, this deep carbon may resurface, represented by upward arrows, as a result of ocean currents, completing the carbon pump's journey, and playing a crucial role in the planet's delicate balance. 

The Changing Climate

Rising ocean temperatures caused by climate change are leading to shifts in the distribution of species within the twilight zone. Some marine organisms that are adapted to specific temperature ranges may find it challenging to thrive as conditions change.

Moreover, changes in ocean currents and circulation patterns, often influenced by climate shifts, can affect nutrient availability in the twilight zone. These alterations can disrupt the normal flow of nutrients to the surface during nighttime vertical migrations. This could potentially impact the food web dynamics and nutrient cycling within this zone, subsequently affecting the organisms that depend on it for sustenance.

Furthermore, increased carbon dioxide levels in the atmosphere, driven by human activities, result in higher concentrations of carbon dioxide dissolving into the ocean's surface waters. This has the potential to affect the pH of the water, making it more acidic. Ocean acidification can impact the ability of marine organisms, including those in the twilight zone, to build and maintain their shells and skeletons. Such changes in ocean chemistry may have cascading effects on the entire ecosystem.

In summary, the changing climate is causing shifts in temperature, nutrient availability, and ocean chemistry within the twilight zone. These changes can impact the distribution of species, alter the nutrient dynamics, and potentially disrupt the delicate balance of this critical oceanic layer.


Learning Objectives:

  • Explain how shifts in temperature influence the distribution of species within the twilight zone. 

  • Describe how changes in ocean currents and circulation patterns, influenced by climate shifts, can impact nutrient availability within the twilight zone. 

  • Describe the potential impact of ocean acidification on the pH of the water and its effects on marine organisms, including those in the twilight zone. 

Next Generation Science Standards

ESS2.D: Weather and Climate

Use a model to describe how variations in the flow of energy into and out of Earth's systems result in changes in climate. 

LS2.B: Cycling of Matter and Energy Transfer in Ecosystems

Evaluate the claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable, but changing conditions may result in a new ecosystem.

ESS2.C: The Role of Water in Earth's Surface Processes

Plan and conduct an investigation of the properties of water and its effects on Earth's materials and surface processes. 

CO2 in the Water

Carbon Dioxide (CO2) is easily soluble in water, which makes the ocean a large sink for the carbon in the air, but this carbon combines with the water to create carbonic acid. Carbonic acid, or H2CO3, is the product of a synthesis reaction between CO2 and H2O. Then, carbonic acid’s H+ ion dissociates and forms HCO3-(bicarbonate). The issue with this is that the excess hydrogen in the water will bond with CO3 2-, reducing the availability of this molecule for calcareous organisms to form tests or shells made of CaCO3 (calcium carbonate). In addition, the H+ ions themselves decrease the acidity of the water, weakening organisms with carbonate body parts.

The Sea Butterfly

A common species of the twilight zone is thecosomata, a group of little-winged gastropods known as the “sea butterfly.” These creatures move around with little flapping motions, but their body is encased within an aragonitic shell. Aragonite is a carbonate material, which means it is sensitive to ocean acidification. Continued decreasing pH levels would weaken the shells of the creature, and the decreasing availability of calcium carbonate would make shell formation more difficult.

Twilight Coral Reefs

Recently, scientists have discovered coral reefs right above the twilight zone where they have been thriving undetected. These communities are host to a diverse group of creatures of both the deep and ones seeking refuge from the deteriorating reefs above.

Social Cost of Carbon 

The Social Cost of Carbon can be defined by the estimate of damage from emitting one ton of CO2 into the atmosphere. Scientists can use this metric to show policymakers the effects of CO2 in a more easily understandable way, often in the context of climate change. With this in hand, policymakers can conduct a cost-benefit analysis on issues such as net-zero calculations and city infrastructure plans, which has been required by US federal regulations since 1981.

This method has also been used to calculate net-zero carbon emissions for shipping companies. In the case of shipping companies, many have been reaching a number closer to net-zero at a surprising rate. This isn’t due to the use of lower-carbon fuels, but rather increased speeds and efficient ship design to carry the cargo at a faster rate, which leads to the questioning of the viability of the metric. Goals and standards are being produced to be completed by the years 2030 and 2050.

Based on estimates provided by WHOI, the twilight zone provides ~$300 to $900 billion towards society each year, a staggering amount that is the last buffer to increasing ocean temperatures. As corporations and scientists work together to combat climate change, this metric will need to be updated to adapt to current conditions.

Social Cost of Carbon

Human activities affecting the mesopelagic ocean zone could impact the social cost of carbon (SCC), the economic damage caused by carbon emissions. Changes in the zone's ability to sequester carbon, disruptions to its ecosystems, alterations in climate regulation, scientific advances, technological interventions, and regulatory actions could all influence the SCC related to this zone.


Shipping and Navigation

Navigation and exploration of the twilight zone, despite presenting challenges, offer valuable opportunities for scientific advancement and responsible resource management. Through careful navigation practices, we can gain insights into the zone's unique biodiversity and ecosystem dynamics, contributing to breakthroughs in medicine, biotechnology, and ecological science. Additionally, understanding the twilight zone's role in carbon sequestration can inspire innovative solutions for climate change mitigation, such as carbon capture and storage technologies. These efforts can also lead to well-regulated deep-sea mining practices, fostering economic growth while minimizing environmental impacts.

Furthermore, responsible navigation in the twilight zone enables the collection of crucial oceanographic data, enhancing climate models, weather predictions, and navigation safety. This data-driven approach can also support the development of advanced underwater communication networks, benefiting various industries and disaster response initiatives. By fostering education and public engagement, navigation activities promote awareness about ocean conservation, influencing sustainable practices and policies. In essence, careful navigation of the twilight zone holds promise for both scientific understanding and the balanced pursuit of economic and environmental goals.

Learning Objectives:

  • Understand the importance of responsible navigation and exploration in the twilight zone.
  • Explore the positive impacts of twilight zone navigation on scientific understanding and sustainable resource management. 

Next Generation Science Standards

LS2.C: Ecosystem Dynamics, Functioning, and Resilience

Ecosystems are dynamic in natural their characteristics can vary over time. Disruptions to any physical or biological component of an ecosystem can lead to shifts in all its populations.

ESS3.C: Human Impacts on Earth Systems

Human activities have significantly altered the biosphere, sometimes damaging or destroying natural habitats and causing the extinction of other species. But changes to Earth's environment can have different impacts on different living things. 

Key Vocabulary

Environmental impact
Precious metals
Resource management
Supply Chain
Sustainable practices

Increased Pressure From Fishing

Scientists have estimated the amount of biomass in the twilight zone to contain 90% of the world’s total fish, meaning that this is the largest source of unexploited fish stock in the world. In this dark area, little light has been shined into the depths of the region, but from the few surveys conducted by the Institute of Marine Research in Norway have resulted in dense catches teeming with fish. This area could cause a “gold rush” for the fishing industry, however, the consequences of it are unknown.

The fish that some companies are targeting are the glacier lanternfish and Mueller’s pearlside, which both have high oil contents of 20-30%. These fish are unknown to most consumers and industry corporations, so the producers are creating new solutions to market these products. Some examples are fish meal, fish meal, or medicine, which will all help the growing world population. But the main appeal is as a food source since the unimaginable number of fish in this zone provides a chance to create a new market. To make the fish more appealing, researchers have begun testing different combinations of enzymes to alter the taste. These experiments could lead to consumer-pleasing results, and thus create a market for these previously undesirable fish.

Deep Sea Mining

As the demand for precious metals grows, companies have begun to seek out a new frontier for their excavations, and the seafloor is ripe for picking. On the direct surface often lies polymetallic nodules, also known as manganese nodules, contain manganese, cobalt, copper, and nickel, all of which can be used in batteries, smartphones, and other electronic components. With supply chain issues, trade deals, or global restrictions, each country would be incentivized to create its own supply using offshore waters under international jurisdiction, or within their own national waters presents an opportunity.

But the lack of knowledge about the twilight zone prevents lawmakers from creating sustainable practices, so they launched the Joint Exploration of the Twilight Zone Ocean Network (JETZON) to study the area. With this new research, scientists hope to create a sustainable future where the use of resources can be balanced with minimal environmental impact. Possible harm to the twilight zone could be the discharge from the mining operations or the sound from the machinery, both which have harmed sea life near the shore in previous operations.

Glacier Lanternfish

The Glacier Lanternfish (Benthosema glaciale) is a small bioluminescent fish found in the ocean's twilight zone (200-1000 meters deep). It uses light to communicate and hide, feeds on small creatures like zooplankton, and migrates vertically to feed at night and avoid predators during the day. It's a crucial part of the twilight zone's food web.

Polymetallic Nodules

Polymetallic nodules, also known as manganese nodules, are rock-like formations found on the deep ocean floor. They contain valuable metals like manganese, cobalt, copper, and nickel, which are used in batteries and electronics. These nodules form over long periods and have attracted attention for their potential as a resource, but their extraction presents technical and environmental challenges.


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