Jellyfish play an important role in the undersea food chain. Although they are relatively small and simple creatures, they're responsible for helping maintain balance in the delicate ecosystem of the ocean.
Jellyfish are a form of plankton, which means that their delicate bodies have a hard time resisting currents in the ocean. They tend to drift along in life, so to speak, and go with the flow. They can't effectively hunt for food, because they can't swim aggressively to catch it.
Since jellyfish are a kind of plankton, they have a direct effect on the food supply in the deep blue sea - many times, they are the ones on the menu. They often make tasty treats for sea turtles, sunfish, and spadefish. Crabs and other large crustaceans snack on jellies, too. In some Asian countries, even people eat jellyfish.
When jellyfish get hungry, they can eat quite a bit. Inside their body is a large stomach compartment that needs to be filled often. Since jellyfish aren't capable of hunting, they've got a passive approach to fueling up - they wait until something floats by and pull it in with their tentacles. Those flowing tentacles are actually equipped with stingers that stun their prey. Once their prey is out for the count, the jellyfish grab it up and have their own sushi dinner.
Small jellies eat little organisms, like algae and tiny zooplankton. Large jellyfish eat bigger food, though - they've been known to snack on shrimp and other forms of plankton. They don't attack prey that's larger than they are; instead, they prefer smaller, more manageable food. Since they don't have teeth (or any other bones, for that matter), they have to munch on things that they can swallow whole. By eating other sea life, jellyfish help to control the population of these creatures and maintain a healthy sub-marine balance.
After jellyfish eat, they have to eliminate waste just like every other creature on the planet does. The interesting thing about jellies is that they expel waste from the same place it came in as food! Since they're such a simple organism, they don't have the same equipment as more complicated animals - so their mouth doubles as their waste-disposal system. Jellyfish feces sink to the bottom of the sea and help fertilize other life down there (both plants and tiny sea creatures); this, in turn, helps grow new things that will eventually be eaten.
Some baby fish actually hide in jellyfish tentacles. Since many ocean predators know all about the jellies' painful sting, babies from species who are immune to their poison hang out beneath the flowing appendages until they're big enough to defend themselves. This helps other species of fish stay alive long enough to grow up and make small fry of their own.
Jellyfish perform some of the thankless tasks that are essential to the ocean's food chain; by both finding food and being food, they help continue the circle of life in the deep blue sea.