Sexual exchanges shuffle the genetic deck, usually just before winter sets in. Fertilized eggs withstand freezing, produceing a new hydra in the spring. The fertilized eggs produce a larva that settles to become the first polyp of a new colony. Two more hydrozoan jellyfish common in bays, Gonionemus and Polyorchis , are shown. Note the large stinging capsules, which are very dangerous to swimmers.
Most scyphozoans originate from small attached reproductive polyps.
The most familiar members are sea anemones and corals. Temperate zone anemones crowd the intertidal rocks. Their powerful stinging cells can capture crabs and small fish. Cloning is practiced by the rock covering that spreads as individuals simply pull in two. Tropical anemones often are accompanied by small anemone fish that avoid triggering its host's stinging cells.