Hydroids: Hydrozoa

Some medusae remain immobile in the water, their tentacles outstretched, ambushing their prey. Others cruise across the water to catch their prey. Polyps simply extend their tentacles to catch passing prey. They also form currents by moving their tentacles to direct food particles toward their mouths.

Polyps compete for space and defend their territory. Medusae are sharply individual but can be gathered by winds and currents to form large swarms. When hungry, both polyps and medusae are always in search of food. When the body cavity is full of food, the tentacles usually are contracted, showing that these animals probably can control their stinger discharge.

Most polyps have separate sexes. Males release sperm in the water, and the sperm swim toward eggs that are on the female's body. Fertilization (FUR-teh-lih-ZAY-shun), the joining of egg and sperm to start development, usually takes place inside the female's body, and she releases the larvae (LAR-vee) into the water, where they fall to the bottom and settle near the parent colony. Larvae are animals in an early stage that change form before becoming adults.

Among medusae, males release sperm and females release eggs in the water, where fertilization occurs and larvae develop. The larvae reproduce asexually by budding, or producing bumps that develop to full size and then break off to live as new individuals. Asexual (ay-SEK-shuh-wuhl) means without, and sexual means with, the uniting of egg and sperm for the transfer of DNA from two parents. Therefore, a single larva that develops from a bud produces a polyp colony that in its turn produces many adult medusae.