The deep sea is permanently dark below the reach of sunlight, so many animals there make their own light.

The longer version

Sunlight fades quickly once it enters water; red vanishes within the first few tens of metres, and true darkness arrives long before the deepest floor. Without light there can be no photosynthesis, so food drifts down as a slow rain of particles, faecal pellets and carcasses from above.

Life responds by economising: slow metabolisms, soft bodies, wide mouths and stretchable stomachs for meals that may come rarely. Pressure climbs by roughly one atmosphere every ten metres, constraining chemistry and design. In such permanent night, emitting light replaces seeing colour, serving as lure, warning and signal.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.