The oceans are far deeper than the land is high. Stack Mount Everest, Ben Nevis and the world’s tallest building on top of each other at the bottom of the Mariana Trench in the Pacific, and just the top 28m of the Burj Khalifa’s spire would be above water.
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Cold, dark and deprived of oxygen and nutrients, the bottom of the deepest sea is another world compared to the surface layers. The pressure exerted by the water above is 1,000 times greater down there. Denizens of the deep cannot survive being brought to the surface and, likewise, surface dwellers that descend too far soon get the life squeezed out of them. To illustrate how conditions change with depth, marine scientists divide the oceans into five zones.
What are the five ocean zones?
The five ocean zones are the epipelagic (sunlight), mesopelagic (twilight), bathypelagic (midnight), abyssopelagic (abyssal) and hadalpelagic (hadal).
What is the sunlight zone?
The top 200m of water in the ocean comprises the epipelagic (or sunlight) zone. This is the layer that the sun’s rays can penetrate. The majority of marine life-forms are found here: photosynthetic plants and algae, and herbivorous and carnivorous animals. Coral reefs, seagrass beds and kelp forests thrive in the shallows of continental shelves.
What is the twilight zone?
Just one per cent of sunlight penetrates to 200m, the depth that marks the boundary with the mesopelagic (or twilight) zone. That’s not enough light to power photosynthesis, so in the absence of plants, mesopelagic animals must find alternative food sources. These include the planktonic organisms that spend the night feeding in the epipelagic zone and descend to the mesopelagic by day to evade predators that hunt by sight.
Another is the organic matter that falls from above. The vampire squid traps this ‘marine snow’ using sticky tentacles and possesses the largest eyes, relative to body size, of any animal on Earth to make the most of what little light is available. The barreleye fish has bizarre tubular eyes that point straight up to detect the faint silhouettes of prey swimming above.
Why is it called the twilight zone?
While there's no definitive explanation of the meaning of 'twilight zone' in relation to ocean zones, it's generally thought that the zone got its name from its liminal state: a murky space where the last rays of sunlight can reach. This meaning later entered popular vocabulary as describing something as inbetween, undefined or mysterious.
What is the midnight zone?
No sunlight at all reaches the bathypelagic (or midnight) zone, which starts a kilometre down. And yet even here it’s not entirely dark. Many fish, cephalopods and invertebrates produce their own light by chemical means in specialised bioluminescent organs, for the purposes of communication, hunting or deterring predators.
What is the abyssal zone?
At about 4,000m, the bathypelagic zone gives way to the abyssopelagic (or abyssal) zone, a realm of constant dark and cold. The animals that live here are generally small and slow-growing because of the low temperatures and dearth of nutrients. But we’re not at the bottom yet.
What is the hadal zone?
Life is at its most sparse in the ocean trenches of the hadalpelagic (or hadal) zone at 6,000-10,994m – the deepest part of the ocean. Invertebrates dominate here, though a species of cusk eel has been trawled from a depth of 8,372m in the Puerto Rico Trench.
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Main image: Getty