Answer: There are several ways deep-ocean animals survive in such an environment.
First off, the deep ocean is dark because sunlight can’t penetrate very far into the water. Many animals make their own light, called bioluminescence, to communicate, find mates, scare predators, or attract prey. Bioluminescence looks a bit like the glow sticks you use on Halloween, but is made naturally by chemical reactions within an animal.
Most of the deep ocean is cold, usually lower than the temperature in your refrigerator. Deep-sea animals don't mind the cold at all, and many can only survive and grow in the cold.
Food is scarce in much of the deep sea, in part because photosynthesis only takes place at the ocean’s surface where there’s sunlight. Most animals cope with this by being very small and needing less to eat or by growing very slowly. Big animals that travel long distances to find food eat huge amounts and store food for many months between meals.
Light, pressure, temperature, and food are typical features of the ocean that remain stable over long periods, allowing animals time to evolve in order to survive. However, humans are changing the deep ocean at a rapid rate through oil and gas drilling, fishing, waste disposal, and mining. Animals may not have time to adapt to the changes brought about by these activities. For this reason, we must think about ways to protect deep ocean-dwelling creatures.
-- Lisa Levin, biological oceanographer, Integrative Oceanography Division