The cnidarians include jellyfishes, sea anemones, corals, and hydrozoans. Of the roughly 11,000 living cnidarian species, all but a few live in the oceans. The smallest cnidarians can hardly be seen without a microscope. The largest known jellyfish is 2.5 meters in diameter, and some colonial species can reach lengths in excess of 30 meters.
The life cycle of many cnidarians has two distinct stages, one sessile and the other motile, although one or the other of these stages is absent in some groups. Obelia (a hydrozoan) is an example of a cnidarian with a life cycle alternating between polyp (sessile, with asexual reproduction through budding) and medusa (motile, with sexual reproduction) generations.
In the Obelia colony, the polyps take on different functions. Some have stinging tentacles that are specialized for capturing prey and feeding, while others are specialized for reproduction.
The reproductive polyp produces—by asexual means—tiny medusa body forms. These medusae grow and bud off from the parental tissue. The medusae nearest the tip of the polyp are the most mature and are the first to leave the parent animal. Because they can swim, the medusae help disperse the species in the sea.
A medusa may be male or female. Male and female medusae have gonads, which produce sperm and eggs, respectively. The medusae reproduce sexually by releasing their sex cells into the water. The sperm cells meet and fertilize the eggs, which become the single-celled zygotes of the next generation.
A zygote begins to divide and develops into an immature larval stage called a planula. A planula, which is covered with cilia, swims to a substrate and settles there. It grows and develops into the polyp body form. A single polyp of Obelia becomes a colony of polyps through asexual budding. Some of the polyps differentiate into the reproductive forms that, in turn, bud off medusae. The medusae will swim away and begin another round of the cnidarian life cycle.
Obelia belongs to the cnidarian group Hydrozoa. Life cycles are diverse among the hydrozoans, but most of the cycles include a generation in which an animal buds repeatedly to form a colony of polyps. Perhaps the best known hydrozoan is the Portuguese man-of-war. Although this animal resembles a single, large medusa form, in reality it consists of a free-floating colony of polyps. The polyps hang from a gas-filled float and, like the polyps of Obelia, they are specialized. Some are reproductive, some capture prey (these polyps have tentacles that may hang as far as 50 meters into the water), and yet others specialize in digesting prey.