Listen to part of a lecture in a marine biology class.

(female professor)
A lot of plants and animals live near the surface of the ocean, and that means there’s a lot of food near the surface, because there’s lots of plants and animals to eat. But if you go down to the deepest parts of the ocean, it’s cold and dark,and there’s not a lot of living down there, so food is very scarce. So organisms that live down at these great depths have developed special adaptations to help them survive in this environment where food is so hard to find.

For example, many deep-sea organisms have body features that enable them to eat prey that are larger than themselves. A good example, um, there’s a species of eel that has an enormous mouth and a large stomach that’s capable of expanding. And these unusual features allow this eel to eat prey larger than itself. That’s a big advantage because if the eel eats something big, that’s a lot of food, a lot of nutrition. So the eel can go for quite a while before it has to find food again.

Another helpful adaptation in some deep-sea organisms is the ability to generate light. And some organisms use that light to help them capture food. For example, there’s a kind of fish called the angler fish . And on its head, this fish has a little structure that produces light that glows in the dark. This little structure, this little light, is positioned close to the fish’s mouth. Other fish are attracted to this light, they think it’s something small they can eat, so they swim straight toward it. And that brings them close enough for the angler fish to capture them and eat them.