Only the shallow seas above the continental shelf receive much light. The zone from around 200m to 1km deep is known as the twilight zone, and below that it is completely dark because the sun’s rays cannot penetrate that far.
Some species like the giant squid cope with low levels of light by having enormous eyes, the size of a dinner plate. Other creatures, such as tripod fish, probably detect their prey by sensing vibrations instead of by seeing them.
Many species in the twilight zone and below produce their own light, called bioluminescence (living light), which they use to find potential food and mates. Bioluminescent light is a ghostly blue colour.
Without sunlight, photosynthesis and the production of new food, is impossible. So, deep sea creatures have to depend on food that falls from the sunlit surface waters above. This could include anything from 'marine snow' to the carcasses of entire whales.