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Beneath the ocean's crushing blackness lies an ecosystem that has learned to thrive without sunlight or warmth.
It's a world full of organisms with unique adaptations to such a challenging environment. Welcome to the abyss.
Depth: 400m to 1km
The viperfish (pictured above) is a fierce predator, with fangs so big that its mouth can't close over them. Instead, they slide up the front of the fish's face when it shuts its mouth.
Its flexible neck allows it to bend its head back and stick out the lower jaw to reach out and grab fish, squid and crustaceans.
Depth: 500m to 1km
Many deep-sea creatures give out blue light called bioluminescence - but the stoplight loosejaw emits red light as well.
This light is invisible to both prey and predators, and probably acts like a torch for searching out shrimps and small fishes. It may also be used to communicate with other stoplight loosejaws.
The fish's jaw and neck are hinged so its mouth can open mouth wide and impale prey on needle-sharp teeth.
Depth: 200m to over 1km
This species is about the size of a football and covered in spines. It also has branches that extend from its head and end in tiny glowing lights.
These lights attract prey which are then grabbed by the footballfish's mouth crammed with sharp teeth.
Depth: 250m to 2km
Females have big teeth and a light-organ on a stalk between their eyes. Males are much smaller and have enormous nostrils they use to sniff out their mates in the dark.
Once a male finds a female, it attaches itself to the female's underside with its teeth. It remains there as a parasite, feeding from the female until it is needed to fertilise the eggs.
Depth: 22m to 2.2km
This fish normally lives close to the seabed, in the relatively shallow waters nearer the coast.
It is highly sensitive to vibrations in the water that could be made by prey such as shrimps or small squid and fishes.
Depth: 150m to 400m
The coelacanth is the most famous living fossil. It was once thought to be a missing link between fishes and amphibians because of its leg-like lobed fins.
Small populations live in the Indian Ocean along the east coast of Africa and near Sulawesi in Indonesia.
Although living coelacanths have only been known to science since 1938, local fishermen have been aware of them for much longer - but considered them worthless because of their unpleasant taste.