What marine life is dangerous to divers?

When you are diving in the open water you are diving in nature, in the wild, where it is an eat or be eaten world.  There are a number of predatory animals that live in the ocean.  Fortunately, the majority of these pose no threat to humans.  However, although rare, sharks, rays, and alligators have been known to attack a human.  Barracudas are also risky critters.  Aside from predators, there are a number of marine invertebrates that will harm a human who touches them; these include corals, sponges, sea cucumbers, jellyfish, anemones, hydroids, and fire coral.  The rule of thumb while diving is to keep your hands and the rest of your body to yourself.