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What is the Earth's Twilight Zone?

VO: Noah Baum WRITTEN BY: Caitlin Johnson
Did you know that Earth has an official 'twilight zone'? It's a strange and mostly uncharted, ghostly environment, full of mysterious glowing animals so unique that they might as well be alien... And, it hosts some of nature's greatest ever mysteries!

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What Is the Earth's Twilight Zone?

Is it possible that Earth has a hidden place just out of sight, brimming with remarkable and unobserved species? Filled with strange, glowing animals that even our wildest imaginations couldn’t conjure up, creating an ecosystem so elusive and unique it might as well be alien? Yeah, it is.

This is Unveiled, and today we’re answering the extraordinary question; What is the Earth’s twilight zone?

Found between 650 and 3,000 feet below the ocean's surface, the twilight zone - also called the “mesopelagic” or “middle pelagic zone” - stretches from the point where sunlight begins to disappear down to the depths where all light stops completely. It is possible to dive deep enough to enter it, but it’s barely explored by humans at all - mostly because it’s out of reach for non-specialist scuba diving expeditions, but it isn’t quite deep enough to be a primary target for the costly submersibles that we send to the sea floor. So, the twilight zone is a neglected middle-ground, but there’s now a greater push for further exploration.

In terms of animals that live there, it’s a region of the sea packed with mysterious, often-luminescent creatures. Small fish, like hatchetfish and lanternfish, use glowing internal organs - or photophores – as a form of camouflage called “counter-illumination”. It disguises them to look like stray photons of sunlight still breaking through the water, reducing their silhouette and keeping them safe from predators. Other creatures use their glow for different reasons, though - including the nightmarish angler fish (which employs a glowing lure as bait), or the ominous-sounding vampire squid (which squirts glowing liquid as a defence mechanism). And then there are the many deep-sea dwellers which use their luminescence to attract mates - like birds and their feathers. And, significantly, the twilight zone is also home to the tiny bristlemouth - a fish which is actually the most abundant vertebrae species on the planet, with population numbers reaching into thousands of trillions.

It may be mostly unexplored, but it’s clearly a massive environment and an integral part of marine ecology. Only recently we’ve discovered that large ocean predators, like whales and sharks, regularly dive down to the twilight zone when they feed. There’s a vast krill population down there, with krill sitting right at the bottom of the food chain. So, the twilight zone is a crucial base for all of sea life and, by extension, all life on earth.

Similarly, this untapped ocean world could hold major influence over our climate. Given the gargantuan number of animals that inhabit it, the twilight zone also hosts a huge, daily mass migration - with thousands of creatures ‘visiting’ the surface and essentially removing carbon from the atmosphere. Take salps, for example, which are strange, goo-like blob creatures you may’ve seen washed up on beaches before. They’re nicknamed “the ocean’s vacuum cleaners”, because they feed on algae (which feeds on carbon), before digesting and redepositing it deeper in the sea. Though we don’t really know what happens to that carbon afterwards, salp poop is seen as one of mother nature’s many ways of self-cleaning the environment.

But, the problem with analysing the ocean’s twilight zone is that a lot of what we know is still built on speculation. The zone was only discovered in the 1930s by the US navy, who reportedly mistook it for the ocean floor because it’s so densely populated with billions of tiny fish. Standard sonar technologies aren’t capable of charting the individual species, but some estimates claim there might be more fish biomass in the twilight zone than in the rest of the ocean combined.

It’s a mysterious world waiting to be explored, but it’s also under increasing threat. As trawlers and fishing vessels hunt deeper and deeper in search for what they need (including krill for krill oil), we could see the twilight zone diminished before we’ve even started to properly study it. According to biologist Heidi Sosik, we’re on the brink of a “twilight zone goldrush” - forcing dramatic and disastrous consequences for the ocean as a whole. Pair over-fishing with increased sea pollution, which prompts entire species to relocate, and entire ecosystems are under pressure to adapt or perish.

An uncharted realm almost completely alien to science, but one that could well hold secrets to understanding the wider natural world, and all of life on Earth - if we work to preserve it…. That’s what the ocean’s twilight zone is.

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