The lifecycle of a Jellyfish is complex. During the life cycle, jellyfish reproduces both sexually and asexually. The jellyfish experiences an alternation of generations in which one generation called “Medusa” reproduces sexually and the next generation called “Polyp” reproduces asexually.
Let’s see the various stages in the life cycle of jellyfish:
EGG AND SPERM
An adult jellyfish is called as medusa. This is the familiar umbrella-shaped (bell-shaped) form which we see in the water. Jellyfish reproduces sexually so the adult jellyfish are either male or female. Both sexes have reproductive organs named as gonads. The gonads in males produce sperms and in females produce eggs. When jellyfish are ready to mate, the male releases sperm by opening its mouth located on the underside of its bell. The female releases eggs from their mouth. The fertilization of eggs in the female jellyfish depends on the species. In most species, fertilization takes place in the water but in few species the sperm swim up into the female's mouth and fertilize the eggs within.
After the fertilized eggs are done with embryonic development, they hatch and the free-swimming planula that emerge will then leave the female's mouth or brood pouch and set out on their own. The planula larva is the shortest stage in the jellyfish's life cycle. It is a tiny oval structure and its outer layer is lined with minute hairs called cilia. The cilia will beat together to push the planula through the water. This force of the cilia does not carry the planula far, rather the ocean currents are responsible for transporting planula for long distances. The planula will float for few days at the surface of the sea.
POLYP OR SCYPHISTOMA
After settling to the sea floor, the planula larva transforms into a polyp (scyphistoma) by attaching itself to a hard surface. This polyp stage is also called as the sessile stage in the jellyfish life cycle, because the polyp is stationary and it remains attached to a single spot on the sea floor. A polyp is cylindrical and it is stalk-like in form. At its base it looks like a disc that sticks to the substrate and its top is a mouth opening surrounded by small tentacles. It feeds by drawing food into its mouth. The polyp starts to grow and then it begins to bud new polyps from its trunk.
POLYP HYDROID COLONY
The polyp then develops into a polyp hydroid colony also called as strobilating scyphistomata. All the members of the polyp colony are linked together by feeding tubes. The complete polyp hydroid colony like the originating polyp is called as the sessile. The polyp colony will grow for several years. When polyps within the colony reach a competent size they will begin the next stage in the jellyfish life cycle.
EPHYRA AND MEDUSA
When the polyp hydroid colony is ready to transform, its stalk portion begins to develop horizontal grooves. These grooves will continue to deepen until the polyp resembles like a stack of saucers. The topmost groove matures fastly and will eventually buds off as a tiny baby jellyfish also known as an Ephyra. This budding process is asexual in which polyps release ephyra. The ephyra grows in size and then become as an adult medusa form of jellyfish.
Most jellyfish have a short life span. Medusa or adult jellyfish commonly live for few months depending on the species but some species can live for 2-3 years. Polyps can live and reproduce asexually for several years, or it can even decades. These are the stages in the lifecycle of Jellyfish.