Although relatively sparse, a few species of jellyfish can be found in the waters of the coral reef. A jellyfish is a cnidarian that spends its entire life in the medusa stage with its tentacles and mouth hanging down. To move in the water, jellyfish contract and expand their umbrella-shaped bodies, actions that thrust the animals from one place to another.
The most commonly found jellyfish at a reef is the moon jelly (Aurelia aurita). Its bluish body, which measures about 10.5 inches (26 cm) across, is transparent, revealing cloverleaf- shaped reproductive organs inside. One to four long stinging tentacles hang down among several relatively short feeding ones. The long tentacles are armed with nematocysts that can deliver powerful stings.