Most people are only familiar with full-grown jellyfish-the eery, translucent, bell-like creatures that occasionally wash up on sandy beaches. The fact is, though, that jellyfish have complex life cycles, in which they go through no less than six different developmental stages. In the following slides, we'll take you through the life cycle of a jellyfish, all the way from fertilized egg to full-grown adult.
Eggs and Sperm
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Like most other animals, jellyfish reproduce sexually, meaning that adult jellyfish are either male or female and possess reproductive organs called gonads. When jellyfish are ready to mate, the male releases sperm through the mouth opening located on the underside of its bell. In some jellyfish species, eggs are attached to "brood pouches" on the upper part of the female's arms, surrounding the mouth; the eggs are fertilized when she swims through the male's sperm. In other species, the female harbors the eggs inside her mouth, and the male's sperm swim into her stomach; the fertilized eggs later leave the stomach and attach themselves to the female's arms.
After the eggs of the female jellyfish are fertilized by the male's sperm, they undergo the embryonic development typical of all animals. They soon hatch, and free-swimming "planula" larvae emerge from the female's mouth or brood pouch and set out on their own. A planula is a tiny oval structure the outer layer of which is lined with minute hairs called cilia, which beat together to propel the larva through the water. The planula larva floats for a few days on the surface of the water; if it isn't eaten by predators, it soon drops down to settle on a solid substrate and begin its development into a polyp.
Polyps and Polyp Colonies
After settling to the sea floor, the planula larva attaches itself to a hard surface and transforms into a polyp (also known as a scyphistoma), a cylindrical, stalk-like structure. At the base of the polyp is a disc that adheres to the substrate, and at its top is a mouth opening surrounded by small tentacles. The polyp feeds by drawing food into its mouth, and as it grows it begins to bud new polyps from its trunk, forming a polyp hydroid colony in which the individual polyps are linked together by feeding tubes. When the polyps reach the appropriate size (which can take several years), they begin the next stage in the jellyfish life cycle.
Ephyra and Medusa
When the polyp hydroid colony is ready for the next stage in its development, the stalk portions of their polyps begin to develop horizontal grooves, a process known as strobilation. 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, technically known as an ephyra, characterized by its arm-like protrusions rather than full, round bell. The free-swimming ephyra grows in size and gradually turns into an adult jellyfish (known as a medusa) possessing a smooth, translucent bell.