The most recognized image of a jellyfish is that of the adult jellyfish, with its bell-shaped body and long tentacles. But this image is just one of several stages in the life cycle of a jellyfish. Jellyfish (Class Scyphozoa) progress through a number of forms, including a tiny free-swimming planula, a small polyp that attaches itself to the sea floor, and a pelagic medussa.
During their life cycle, jellyfish experience an alternation of generations in which one generation (the medussa) reproduces sexually and the next generation (the polyp) reproduces asexually. The medusa form is the dominant and most recognized form of the jellyfish. Overall, the basic stages in the life cycle of a jellyfish include:
- egg and sperm
- planula larva
- polyp (or scyphistoma)
- polyp hydroid colony (or strobilating scyphistomata)
Egg and Sperm
Jellyfish reproduce sexually so adult jellyfish are either male or female. Both sexes have reproductive organs called gonads. The gonads in males produce sperm, in females they produce eggs. When jellyfish are ready to mate, the male releases sperm through its mouth opening located on the underside of its bell. The fertilization of eggs in the female jellyfish depends on the species.
In some species, the female's eggs attach themselves to brood pouches located on the upper part of her oral arms surrounding her mouth. Then when she swims through the male's sperm the eggs become fertilized. In other species of jellyfish, the eggs are retained inside her mouth and the male's sperm swims into her stomach where it fertilizes the eggs. The fertilized eggs later leave the stomach and attach themselves to the female's oral arms.
After the fertilized eggs have undergone embryonic develoment, they hatch and the free-swimming planulae that emerge then leave the female's mouth or brood pouch and set out on their own. The planula larva is a short-lived stage in the jellyfish's life cycle. A planula is a tiny oval structure whose outer layer is lined with minute hairs called cilia. The cilia beat together to propel the planula through the water, but the motion of the cilia does not carry the planula far, instead ocean currents are responsible for transporting planulae long distances. The planula floats for a few days at the surface of the sea. It then drops downward to settle on a solid substrate where it attaches itself and begins its development into a polyp.
Polyp (or Scyphistoma)
After settling to the sea floor, the planula larva attaches itself to a hard surface and transforms into a polyp (or scyphistoma). This polyp stage in the jellyfish life cycle is a sessile stage, so called because the polyp is stationary and remains attached to a single spot on the sea floor. A polyp is cylindrical and stalk-like in form. At its base is a disc that adheres to the substrate and its top is a mouth opening surrounded by small tentacles. The polyp feeds by drawing food into its mouth. It grows and begins to bud new polyps from its trunk. As it does, the polyp develops into what is called a polyp hydroid colony (or strobilating scyphistomata). Members of the polyp colony are linked together by feeding tubes. The entire polyp hydroid colony, like the originating polyp, is sessile. The polyp colony can grow for several years. When polyps within the colony reach an adequate size, they are ready to begin the next stage in the jellyfish life cycle.
Ephyra and Medusa
When the polyp hydroid colony is ready to transform, the stalk portion of its polyps begin to develop horizontal grooves. These grooves continue to deepen until the polyp resembles a stack of saucers. The topmost groove matures the fastest and eventually buds off as a tiny baby jellyfish also known as an ephyra. The budding process by which polyps release ephyra is asexual. The ephyra grow in size and become the adult (medusa) form of jellyfish.