Researchers have discovered that tiny crabs about a third of an inch (one centimeter) long take care of a huge job. They actually help keep coral reefs alive. And that's important, because more than nine million species depend on coral reefs around the world for food and shelter.

These tiny crabs, called trapeziid crabs, accomplish a lot despite their size. Particles of dirt, or sediment, are bad for living coral.

"If sediment builds up on the coral, the weight of it can damage the coral," explains researcher Hannah L. Stewart of the University of California Santa Barbara's Marine Science Institute. "Sediment can also block the light healthy coral needs."

Like many living things, the crab and the coral have a symbiotic relationship; that means they each help the other. The coral provides a home for the crab, and the crab protects the coral.

"An organism doesn't just survive on its own," says Stewart. "There are a lot of interactions that go into the success of every organism. That's the web of life."