Hydrozoa

Organisms of the Class Hydrozoa belong to the phylum Cnidaria. Hydrozoans are mostly marine, though a few (Hydra, Craspedacusta) are freshwater. Hydrozoans may be solitary or colonial, and are extremely diverse, both in life-cycle forms and morphology. Most hydrozoan species have both a polyp and medusa stage, but others make use of only one or the other. Like other Cnidaria, Hydrozoans keep form through the use of a hydrostatic exoskeleton.

Hydrozoa are cnidaria which have solid tentacles (as opposed to fluid filled tentacles). They can be divided into two groups, thecates and athecates. The thecates have a solid structure (theca) like a skeleton around the tentacles, into which the tentacles can retreat, the athecates have no such structure which makes them less rigid and more able to move to follow the tides.

The most widely-known and researched freshwater hydrozoan is Hydra, which is found in slow moving waters. Hydra has a pedal disc composed of gland cells that helps it attach to substrates, and like all cnidarians uses nematocysts, or "stinging cells," to disable its prey. Hydra eats small crustaceans (such as brine shrimp), insect larvae, and annelid worms. Hydra may reproduce sexually, through the spawning of sperm (and thus insemination of eggs on the female body column), or through asexual reproduction (budding).

Colonial hydrozoans typically have both a medusa stage and a polyp stage in their lifecycle. They have a base, a stalk, and one or more polyps. Hydrozoan colonies are composed of a number of specialized polyps (or "zooids") - including feeding, reproductive, and protective zooids. Reproductive polyps, known as gonozooids (or "gonotheca" in thecate hydrozoans) bud off sexually-produced medusae. These medusae mature and spawn, producing gametes. Zygotes become free-swimming planula larvae or actinula larvae that either settle on a suitable substrate (in the case of planulae), or swim and develop into another medusae or polyp directly (actinulae). Colonial hydrozoans include siphonophore colonies, Hydractinia, Obelia, and many others.

The medusa stage is typically the dominant sexually-reproductive phase in hydrozoans that alternate between a polyp and a medusa. The medusa often has a limited lifespan, though, and may die shortly after releasing gametes (as in the case of fire corals).

Some examples of hydrozoans are: Hydra, Obelia, Portuguese Man o' War (Physalia), Chondrophores, air ferns (Sertularia argenta), and Tubularia.