 |
Some giant clams can grow to 1.5 meters (4 1/2 feet) and may live up to 200 years. They reproduce by broadcast spawning, ejecting huge masses of eggs like undersea geysers. Contrary to myth, they do not seize divers. |
Building a reef together as a colony has many benefits, providing a stable, sunlit, massive structure that no single animal could create alone. But being rooted in one place also creates challenges. Unable to move about to seek food or mates, corals require special adaptations to survive.
How do corals eat?
How do they reproduce?
A Close Partnership
Reef-building corals get most of their energy from algae that live inside their bodies. These tiny one-celled dinoflagellates use sunlight for photosynthesis, creating nutrients that are digested by the coral.
 |
When coral spawn, eggs and sperm mix and fertilize to create swimming larvae. After drifting for days, a fortunate few settle on the sea floor to start new colonies.
© 2003 Greg Bunch / GB Undersea |
Corals can capture small animals with their tentacles, which shoot out poison darts when touched. Some coral species lack internal algae and get all their food this way. But nearly all shallow-water corals obtain most of their energy from symbiotic algae. The algae also benefit by getting a sheltered, sunlit home.
Coral polyps are connected so they can share nutrients throughout the colony. Corals grow best in clear, warm, shallow water, and most grow upward to get even more sunlight for their algal partners. Their soft bodies are transparent, enabling sunlight to reach the algae inside.
Synchronized Spawning
 |
You will need to have the Real Player installed on your computer to view this video.
|
Coral colonies grow and spread by asexual reproduction—that is, splitting or cloning themselves. But they can also reproduce sexually, combining sperm and eggs from different animals.
Corals can't move around to find mates, so they have evolved a strategy called broadcast spawning, in which millions of eggs and sperm are released into the water. The large number of eggs ensures that at least some will escape the mouths of hungry predators. Many other marine animals also utilize broadcast spawning.
To increase the chances that their eggs and sperm will meet in the water, thousands of corals spawn at the same time, following cues in the environment. In Australia's Great Barrier Reef, for example, the mass spawning peaks three days after a full moon in November.
BY THE NUMBERS: Coral Reef Facts
Reefs cover only 0.2 percent of the ocean's area, yet they are home to one-third of all known marine fish species and tens of thousands of other species. Porites corals are among the oldest living animals on Earth. Some colonies are thought to be 1,000 years old. Staghorn corals can add 15 cm (6 inches) to their branching tips in one year. Broadcast spawning is sometimes timed to within 1 hour so that eggs and sperm will meet in the water.
|
 
|
