Hydrozoa is a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater.

These organisms are related to jellyfish and corals and belong to the phylum Cnidaria.

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

Hydra, a freshwater genus

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 eat 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

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).

External links

• J. Bouillon, M.D. Medel, F. Pagès, J.M. Gili, F. Boero and C. Gravili. 2004. Fauna of the Mediterranean Hydrozoa. Scientia Marina, 68 (Suppl. 2).
• Hydroids from Reunion Island and Indian Ocean
• http://zygote.swarthmore.edu/intro6.html
• Puget Sound Online
• Aquascope
• The Hydrozoa Directory
• Hydrozoa Picture Gallery