Today, we share a notion that most of the earth’s greatest wild frontiers have been thoroughly explored and that limited mysteries remain to be discovered. With such a range of technological and transportation advancements, we feel as if we have reached everywhere in the world which offers significant unknowns.
However, as we descend only 200 meters down into the global oceans, we enter possibly the least explored wild region on earth, the mesopelagic, known as the twilight zone.
As the depth spans from 200-1000 metres from the surface, we enter a dark region of immense pressures, extremely low temperatures and misunderstood importance.
Creatures in the twilight zone have to cope with a severe lack of light inhibiting any possible plant growth at these depths. It’s vastly different to the plant and algae abundance seen in shallow ocean waters above, where photosynthesis can take place.
Within the twilight zone, organisms have evolved to utilise the most bizarre colours and alien-looking physical features to survive in the murky depths. Many of the organisms within this region rely on producing light themselves- bioluminescence- to help them survive.
The twilight zone spans across the globe from the artic to the southern ocean making up 20% of the ocean's contents. With more exploration into this segment of our seas, the benefits which it could hold for the population, in regards to food, medicine and climate change combatting, are becoming more apparent. Unfortunately, with our developing understanding of the ecological role that the twilight zone provides, we are more importantly viewing the alarming threats that this unique ecosystem faces.
Through studying this layer of the sea, which is found between the sunlit surface waters and the deep ocean beneath, remarkable mysteries have already been found. Supporting creatures longer than blue whales and providing the habitat for the most abundant vertebrate on the planet, the twilight zone is one of the most biodiverse regions we’ve seen and home to the largest animal migration anywhere on earth.
The large ctenophore, Leucothea, produces rainbow-like colours due to a chemical reaction involving a light-emitting enzyme.
PHOTOGRAPH BY Steve Haddock
Scientists have utilised deep penetrating sonar in order to track the biomass of creatures seen in the deep reaches of the twilight zone. As the day progresses, the depth level of the biomass varies becoming shallower in the day and deeper at night creating a wavelike motion once graphed. These depth changes show the diel vertical migration, the largest animal migration on the planet.
Every day around the world, this mass migration takes place, sweeping through the global oceans as an imposing living wave. Organisms such as fish, squid and plankton travel hundreds of meters to surface waters to feed at night, before descending back down to deeper and safer waters during daylight hours.
In completing their individual journeys, the animals play a vital role in moving carbon from the surface waters down into the deeper layers of the ocean to be stored. This mass movement is a critical part of carbon sequestration and a vital step in the marine food web.
The cranchid squid, Leachia pacifica, inhabits the twilight zone moving to shallower waters daily to feed.
PHOTOGRAPH BY Sonke Johnsen
Carbon dioxide is removed from the atmosphere by small microscopic organisms such as phytoplankton. As they carry out photosynthesis, the carbon dioxide is converted to organic products which pass down the food chain before eventually being dispatched to the ocean's depths. The twilight zone is fundamental in the workings of the ocean’s biological carbon pump.
Migrating organisms from the twilight zone feeding in surface waters create marine snow, ultimately this is responsible for moving carbon to the sea floor. Marine snow is a constantly falling debris comprised of a mixture of dead sea creatures such as plankton and algae. As it falls, the marine snow provides a fundamental food source to many creatures dwelling in the twilight zone. Appearing small and rather insignificant in size, the debris over time compacts into carbonate structures forming geological landscapes such as the iconic white cliffs found in Dover.
Most importantly, the biological carbon pump aids to prevent climate change. Locking up carbon stores for tens to hundreds of thousands of years, it regulates our atmospheric temperatures. Scientists are trying to determine what exact fraction of carbon is transported through the twilight zone, on its way to the deep sea, to fully grasp its importance in controlling carbon dioxide levels.
An array of organisms in the twilight zone are changing perceptions of how the ocean ecosystem functions, such as salps. They look like transparent gelatinous tubes, similar in appearance to jellyfish, but are actually more closely related to us. Salps are non-selective filter feeders. By contracting bands of muscles, they constantly move, drawing in water through their bodies to provide them with food such as bacteria and phytoplankton. Salp's abundance is what makes them greatly influential. Food is converted into faecal pellets that sink down to deep seas, again storing carbon.
This is a chain of the salp, Cyclosalpa polae. Salp's are seen to be one of the fastest growing multicellular animals. This enables them to take advantage of algal blooms by increasing their population size to feed on the abundance of food.
PHOTOGRAPH BY Sonke Johnsen
An alluring question is which large shallower sea predators rely on the twilight zone to feed, and to what extent? Through satellite tagging of blue and great white sharks, it has enabled scientists to track their routine movements showing remarkable habits.
It’s seen that species of both sharks seek out and often ride large swirling, warm ocean currents known as eddies to enter the twilight zone. These currents offer a direct safe path for sharks to feed in this region of the sea which has the largest biomass of fish. We just don’t know yet how vital these currents and food stores are for large predators such as blue and great white sharks.
Deep dives from these species of sharks are seen to be less frequent at night as many twilight zone animals take part in their daily migration to the surface to feed. It’s energetically a significant output for sharks to descend to these depths with the distance and the cold, so it’s only worth making it if the prey is there. Once the migrating creatures descend down again during the day, the dives continue.
Other large predators such as sperm whales, tuna and swordfish have all been monitored and seen to also dive into the twilight zone to feed. Alternatively to diving, spinner dolphins trap lanternfish from the twilight zone as they migrate to shallower waters to feed and spawn. We have to ask if these large species could survive without these abundant food sources and if they depend on them more for feeding than shallower water species.
Large predators such as the great white shark frequently dive into the depths of the twilight zone to feed on the abundance of prey available.
PHOTOGRAPH BY Gerald Schömbs
Creatures in the twilight zone show a mysteriously different mechanism when it comes to pursuing prey and camouflage survival for the hunted. Many organisms have the ability to produce their own light or have large sensitive eyes to make use of the limited light reaching the twilight zone.
The production of light however is utilised in extraordinarily varied ways. Predators find it hard to look downwards and observe any prey as a result of the darkness, but due to the faint light entering the twilight zone from above, looking up enables silhouettes of prey to be detected.
The cock-eyed squid has a noteworthy variation in the size of its eyes. The large eye looks upwards searching for silhouettes of potential prey whereas the smaller eye looks downwards.
PHOTOGRAPH BY Sonke Johnsen
The barreleye spookfish, Opisthoproctus soleatus, has a transparent head allowing its eyes to look directly upwards making use of the limited light to hunt for prey.
PHOTOGRAPH BY Sonke Johnsen
Small fishes in the twilight zone have developed a genius method of camouflage. Lantern fish possess small light-producing organs on the side of their bodies called photophores. These fishes can dictate the level of bluish light emitted from the photophores to exactly match the surrounding light, in a method known as counter illumination. It’s lighting up to blend in.
Lantern fish produce bluish light via light-producing organs called photophores.
PHOTOGRAPH BY Noé Sardet
Producing a wavelength of light is also important in how predators operate. The stoplight loose jaw fish also has photophores. It possesses the same organ but has it situated under its eye to emit a red wavelength of light instead. As red light loses its intensity quickly in water, it never reaches down to the twilight zone and so most mesopelagic fishes don’t see it. Ultimately, this gives the fish the ability to swim right up close to prey undetected.
With larger species constantly being found during diving expeditions into the twilight zone, microbial life has rarely been touched upon. We rely on all the ocean's microbial life to produce up to 70% of the earth’s oxygen but how dependent is that upon the twilight zone’s activity?
Certain microbes such as species of light-producing photobacterium have evolved to form relationships with fish in order to survive and aid the fish in attracting prey. Anglerfishes, made famous by Finding Nemo, have their ‘lure’ (known as an esca) inhabited by photobacteria. The luminous bacteria attract prey to the anglerfish’s jaws whereas the fish provides the bacteria with a safe location to live and nutrients as it swims along. It’s a beneficial relationship for both.
The humpback angler fish, Melanocetus johnsonii. Notice the long lure which glows due to the relationship with photobacteria.
PHOTOGRAPH BY Sonke Johnsen
Bioluminescence is even seen to be used by a colonial organism longer than a blue whale: the Praya dubia. It’s made up of a collection of different small animals, with each animal that is part of the organism being called a zooid. These zooids rely on a mutually cooperative relationship, symbioses, to survive and create the Praya dubia. Production of a blue bioluminescence light attracts prey which are snagged in its 40 meter tentacles. It is puzzling to understand why creatures in these depths are this size and how many more remain to be discovered.
This is a siphonophore called Vogtia sp found in the mesopelagic zone. It has the same order as Praya dubia so is closely related, but far different in physical size and shape.
PHOTOGRAPH BY Sonke Johnsen
Possibly the biggest mystery in my view is the sheer abundance of biomass that this zone contains. No longer than 30 centimetres, the small bristlemouth fish are the most abundant vertebrate found on the planet. Scientists estimate that their numbers could topple 1 quadrillion. Unfortunately, this vast abundance doesn’t go unnoticed, attracting growing interest from the fishing industry.
Due to the twilight zone spanning across the globe, there has to be variation in the biomass of animals from area to area. It’s challenging to calculate an exact figure, but volumes of fish in the twilight zone range from 1 to 20 billion compared to the overexploited 1 billion fish seen in surface waters. In such difficult expansive conditions, how do we explore the validity of these numbers and address the ecological mysteries of the twilight zone?
Elongated bristlemouth fish, Gonostoma elongatum. Bristlemouths are believed to be the most abundant vertebrate on the planet with estimated numbers of up to one quadrillion.
PHOTOGRAPH BY National Oceanic and Atmospheric Administration
The U.S Navy during world war two used sonar to test the depth of the ocean in certain regions whilst primarily searching for enemy submarines. Initially, their sonar images appeared to be skewed. Due to the diel vertical migration, it appeared as if the sea floor changed depth throughout the day as the biomass of creatures migrating was so colossal. Once they figured out what was causing this phenomenon, the US military attempted to use the abundant layers of fish as camouflage to shield their submarines from enemy sonar.
Technology since 1942 is now clearly a lot more advanced and adequately equipped to explore the deep seas. Recently designed, the mesobot has the ability to follow mesopelagic animals whilst they complete their daily migration to the shallower seas. With inbuilt cameras and a reliable operating time, the mesobot will help us understand which species migrate and what they feed on over time. The robot also has a pumped filter in order to capture microscopic life to study the microbial makeup of the twilight zone.
In order to further understand the biological carbon pump, floats called MINIONS are being used. They can be set to a certain depth in the sea and monitor the fall of marine snow. It is helping to define the carbon cycling processes between the surface ocean and the twilight zone below.
The twilight zone is now believed to accommodate larger biomass of fish than all of the oceans combined. With such a substantial untapped resource, questions are being asked relating to if this food source could have the capability to help sustainably feed the world.
As surface water species decline, sustainably dipping into this misunderstood biomass is inviting to reduce the pressures. Most of the fish species that permanently reside in the twilight are incredibly small, so if fished they would be used as a supply of protein to feed the rapidly growing aquaculture industry, rather than ending up on our plates.
Fish in the twilight zone also have a very high oil content containing beneficial omega-3 and fatty acids. This could rapidly expand the nutraceutical businesses and aid aspects of human health but at what cost?
A rather uncertain benefit that the twilight could hold for humans is the uncharted genetic resource it offers. Through taking ocean samples 70-95% of the virus genes being discovered in the sea have never been witnessed, we don’t know what they do or how they interact. There is the possibility of a huge genetic resource capable of aiding the creation of cures for certain human diseases. The prospects are vastly unknown but the potential it holds is massive. Already studies have been undertaken into using oceanic viruses to feed on bacteria that infect corals and cause bleaching.
If the nutraceutical business becomes more economically viable due to the huge stocks of fish in the twilight zone, over exploitation could quickly become a very present problem.
PHOTOGRAPH BY Steve Buissinne
Sadly, with the benefits that the twilight zone can offer, come the threats as the result of greedy exploitation and uneducated misuse. The stocks of fish and krill to be potentially used in the nutraceutical and aquaculture industries are commonly found outside of most countries' exclusive economic zones. An EEZ is an area of sea prescribed to each country to be used for fishing practices. As a result of the depth of the twilight zone, the region is often found out of international jurisdiction making it incredibly vulnerable to human exploitation.
Physically, deep ocean fish grow slowly and commonly are late to mature sexually leading to slow reproduction rates. Accompanied with the potential overexploitation, deep sea fish would struggle to recover. The exploitation of the twilight zone with limited knowledge could severely disrupt the complex marine food web. We have seen that large animals such as blue sharks and sperm whales rely on the food availability provided but not to what extent. It risks harming the large ocean animals which we characterize as a sign of a healthy functioning ecosystem.
Ecotourism could also suffer as these animals disappear or alter migration patterns due to lost food. This would directly destroy an industry that helps fund and educate people about the protection of our seas. We also threaten our most important climate regulator. Reducing the number of creatures that carry out the largest animal migration on earth, will lower the levels of carbon dioxide removed from our atmosphere. Disrupting this ecosystem will result in carbon dioxide dwelling in our atmosphere for prolonged periods.
Nutrient upwellings due to oceanic currents create hotspots for lanternfish. Once these hotpots are identified plans to exploit them are inevitably devised. One of which is seen in the Gulf of Oman which has been debated for the past few years to start fishing practices.
Scientists are studying the effects that fishing has on whale migration patterns. As krill is caught to create fishmeal and krill oil, ecotourism could be hugely impacted along with whale numbers.
PHOTOGRAPH BY Alan Bedding
With all the mysteries present, there is an urgency to explore and discover to provide hope for the twilight zone inhabitants and the earth’s wellbeing. As gaps are filled in our knowledge, hopefully we can gain more respect for our oceans. However, without a full-hearted urgency to explore and learn more, the twilight zone may be exploited before we fully understand what it gives to our own existence.
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