It is often described as a “living fossil” because of its resemblance to extinct,

 Paleozoic sharks. They’re rarely seen alive because of their preference for deep water.

  A dying one was captured near the surface in Japan in 2007

which let’s face it, look like their brains are on show. Originally from China, they are popular aquarium pets today.

 As you can see, this thing looks like it has freakishly human looking teeth.

 They’re actually just flaps of skin, but they’re still pretty unnerving! It’s a species of promachoteuthid squid

 and only one specimen has been found to date. It was captured in the Southern Atlantic Ocean at a depth of 2,000m in 2007.

 They lack both red blood cells and hemoglobin, and so have white blood. They have translucent bodies,

  and absorb oxygen directly from the water around them.

  that overshadows nearly all other sounds throughout the world’s oceans,

 including the calls of some whales. In fact, they are so loud, their snapping sounds interfere with military

and scientific sonar (so much so, that hostile submarines have used large colonies of pistol shrimps to hide!).

 They have been swimming in arctic waters since before the dinosaurs

 (over 650 million years ago) and are among some of the oldest surviving species in the world.

 The bell can grow up to 8 feet in diameter with hundreds of tentacles up to 120 feet long. Remarkably,

 only 6% of the jellyfish is solid matter; the rest is water.

 Their bright blue feet are a sexually selected trait. The brighter a males feet, the more attractive he is to a female. 

 To attract a female, they have an elaborate dancing ritual to display their feet, first lifting one foot and then the other.

and male sexual characteristics. As different sexes are differently coloured, each half of the bird is a different shade.

Ironically, the Mexican walking fish isn’t a fish at all, but an amphibian.

It’s official name is axolotl and as the name suggests, it’s found in Mexico.

Their int! ernal viscera, including the heart, liver, and gastrointestinal tract are all completely visible.

 grows to around 15cm long. Discovered at a depth of 2,200 metres (7,200 ft),

 it lives on hydrothermal vents. Based on both morphology and molecular data, the organism was deemed to form

a new biological family, which was named Kiwaidae. A second species in the family was described in 2011.

The “hairy” claws of the yeti crab contain filamentous bacteria, which the creature may use to convert carbon

 molecules emitted by the hydrothermal vents into nutrients. This is a process known as chemosynthesis.

  It may also consume bacteria.

When the alarm jellyfish is attacked, it flashes brightly using bioluminescence in an attempt to attract other animals.

The idea is to encourage confusion and fights between predators, while the jellyfish can swim away.

They grow to about 50mm in length and have black bodies with a strange luminous green “trim”.

 Like most nudibranchs, they deliver a painful sting. They do not produce the stinging cells themselves

but incorporate them into their own tissues from their prey, stinging jellyfish.

 the leatherback doesn’t actually have a shell. What looks like a shell is in fact a leathery skin supported

  by small bones. This gives it a flexibility that a solid shell would not provide, allowing it to dive to astonishing depths.