The fourth of the five ocean zones – the abyssopelagic (abyssal) zone – begins at an astonishing 4,000 metres deep and stretches down to around 6,000 metres.
To put that in perspective, if you plonked Japan’s tallest mountain – the 3,776-metre Mount Fuji – into the ocean with its peak grazing the waves, its base wouldn’t reach the abyssal zone.
And if you plopped Mount Kilimanjaro at the bottom of the abyss, its 5,895-metre summit wouldn’t be visible above the waves.
- What are the different ocean zones?
- “The crushing pressure can be anywhere between 100 to 400 atmospheres, which would kill a human instantaneously.”
By the time you reach the cold, black abyss, you’ve descended through the epipelagic (sunlight), mesopelagic (twilight) and bathypelagic (midnight) zones.
In most regions of the ocean, you hit the bottom in the abyssal zone but in some places, you can find even deeper oceanic trenches (like the Mariana Trench, where the deepest part of the ocean – Challenger Deep – can be found) in the hadal zone.
The abyss is made up of flat abyssal plains that can stretch for miles. These featureless muddy landscapes are sometimes broken up by underwater mountains, canyons, hydrothermal vents and methane seeps. The abyssal plain is the world’s largest habitat.
Here, there is no light and, so, no photosynthesis. But remarkably there is life. Fish, squid, jellyfish, sea cucumbers (including adorably ugly sea pigs) and crustaceans, among other organisms, all live here but it’s out of reach to the world’s deepest diving mammals: the deepest recorded dive of Cuvier’s beaked whales is less than 3,000 metres.
In these dark waters, many abyssal animals rely on bioluminescence (creating their own light) to communicate or find food.
How have animals adapted to the abyssopelagic zone?
Animals down here have found unique ways of surviving the harsh conditions.
First, they must deal with the intense pressure. With so much seawater weighing down from above, the pressure would crush a human in an instant.
But deep-sea creatures have flexible skeletons and gelatinous bodies without air cavities such as lungs or swim bladders (which would be crushed by the pressure).
To withstand the near-freezing temperatures (the water lingers around a frigid four degrees celsius), they slow their metabolism and some have antifreeze-like blood.
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A slow metabolism is also important because food is so scarce at these depths. The animals that live down here must wait for food to drift down from the surface in the form of marine snow.
Falling from the surface, marine snow brings carbon down to the deep sea and locks it away in the seabed – making the deep ocean an important carbon store.
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Sometimes, the creatures of the deep are rewarded with a much meatier feast in the form of a whale fall. The nutrients from the deceased whale’s body can feed the whole ecosystem for years.
Nothing is wasted – even their bones are consumed (by the terrifyingly-named bone-eating zombie worms, Osedax). Large blooms of algae or plankton that sink to the seabed in huge rafts can also provide these deep-sea banquets.
There is still much to learn about the abyss: only around one quarter of the seafloor has been mapped in detail but researchers are learning more about it every day through remotely operated vehicles, submersibles and other technologies.
The abyss may be far from the lives of humans but it is threatened by our activities, including deep-sea fishing, mining and pollution.
Top image: tubeworms at hydrothermal vents. Credit: Krista Few/Getty Images
