From BBC Blue Planet
This documentary explores the unknown depths of the ocean.
Over 60% of the sea is more than a mile deep and it forms the planet's most mysterious habitat.
A sperm whale descends 1000 metres to look for food and is followed.
On the way down, a number of unusual creatures are witnessed, such as transparent squid and jellies, whose photophores give pulsating displays of colour.
In such dark places, both being able to see (or sense movement) and the means of quick concealment are equaly desrirable.
To that end, some use bioluminescence as a means of detecting food or evading predators.
A descend to the very bottom of the ocean - some 4,000 metres - reveals life even at such cold temperatures, much of it new to science.
It is dominated by echinoderms that sweep the sea bed; however, there are occasional large hunters, such as chimaera.
Life in the Deep Oceans (part I)
The term deep sea refers to organisms that live below the photic zone of the ocean.
These creatures must survive in extremely harsh conditions, such as hundreds of atmospheres of pressure, small amounts of oxygen, very little food, no sunlight and constant extreme cold.
Most creatures have to depend on food floating down from above.
These creatures live in very harsh environments such as abyssal or hadal zones, which, being thousands of meters below the surface, are almost completely devoid of light.
The water is very cold (between 3 and 10C) and has low oxygen levels.
Due to the depth, the pressure is between 20 to 1000 atmospheres.
These animals have evolved to survive the extreme pressure of the sub-photic zones.
The pressure increases by about one atmosphere every ten meters.
To cope with the pressure, many fish are rather small, usually not exceeding 25 cm in length.
Also, scientists have discovered that the deeper these creatures live, the more gelatinous their flesh and more insignificant their skeletal structure is.
Lack of light
Because there's no light most animals have very large eyes with retinas constructed only of cones, which increases sensitivity.
Many animals have also developed large feelers to replace peripheral vision.
To be able to reproduce, many of these fish have evolved to be hermaphroditic, eliminating the need to find a mate.
Many creatures have developed very strong senses of smell to detect the chemicals released by mates.
Since at such deep levels, there is little to no sunlight, photosynthesis is impossible as a means of energy production, leaving some creatures with the quanday of how to produce food for themselves.
For the giant tube worm, the answer comes in the form of bacteria that live inside of it.
These bacteria are capable of chemosynthesis and live inside of the giant tube worm, which lives on hydrothermal vents.
These vents spew high amounts of chemicals that these bacteria can transform into energy.
These bacteria can also grow freely of a host and create mats of bacteria on the sea floor around hydrothermal vents, where they serve as food to other creatures.
Bacteria are a key energy source in the food chain.
This source of energy creates large populations in areas around hydrothermal vents, which provides scientists an easy stop for research.
Whales can dive to about 1.200m deep in search of their prey.
The giant squid is one of the very few deep ocean creatures that can visit the ocean surface.
The viperfish have long sharp clear teeth that they use to catch their prey.
The hatchet fish has a light that attracts their prey, gulper eels have huge heads and mouths so they can swallow their prey easily.
They also have elastic stomachs, which allows them to eat fish larger than themselves. Anglerfishes use a light on top of their head to catch their prey, the rattail fish detects its prey with a whip like tail, a sea pen is like a worm like creature that lives and crawls on the ocean flood.
Many fish larger than the sea pen make it their lunch.