Fish filmed spitting ostracod 'fireworks'
One of nature's most dazzling underwater displays, bioluminescence, has been captured as a form of self-defence by a BBC film crew.
Ostracods are one of the ocean-living animals which give off light when they are disturbed.
They produce the chemicals luciferin and luciferase, which emit light when mixed together. The process is called bioluminescence, an adaptation that allows animals to be seen in the complete darkness of the deep sea.
To demonstrate why organisms are able to generate light, ostracods were put into a tank containing cardinal fish, which eat plankton.
Lights, camera, action!
When an ostracod is swallowed, it emits a burst of light, making the cardinal fish spit it out.
Physicist and BBC presenter Helen Czerski said: "It's like a little fish firework."Biggest eye
The ostracod uses bioluminescence to illuminate when threatened - the light deters predators such as the cardinal fish as they do not want to be seen by other, larger predators.
"Their dazzling glare acts like a security light," Dr Czerski added.
Most marine bioluminescence is blue, the colour on the visible light spectrum able to travel the furthest through water.
In the deep ocean it is the only light there is.
And according to the filmmakers, it is also why the world's biggest eye has evolved there.
Giant squid's eyes are the size of a human head and are able to detect the faint glow of bioluminescence hundreds of metres below the ocean's surface.
It protects it from it predator, the sperm whale, which moves bioluminescent-producing organisms as it approaches.
"The giant squid's huge eyes allow it to peer further through the gloom and detect these traces of blue bioluminescent light as the sperm whale approaches. It's this spectacular eye that has allowed them to survive in this inky black world," Dr Czerski said.
Super Senses: The Secret Power of Animals begins on BBC Two at 21:00 BST on Tuesday 19 August (20:00 BST for viewers in Scotland).