Jellyfish are able to reproduce both sexually and asexually during different stages of their lifecycle which may go a little way to explaining how they have survived on earth for 650 million years. They predate and have out lived dinosaurs. Different species of jellyfish occur in all of the world's oceans, there are even some freshwater varieties.
Jellyfish look so graceful in water that it belies the painful sting they give when disturbed. Interestingly, one species of jellyfish called the Box Jellyfish actually kills more people that any other marine creature.
When we think of Jellyfish we think of them in their adult stage, otherwise known as the Medusa stage. This is when they have the bell shaped body, long tentacles and propel themselves gracefully through the water like Ballerina's on a stage.
While in the Medusa stage there are both male and female Jellyfish. The male squirts his sperm into the water and some females squirt their eggs into the water to be fertilized. In other species of Jellyfish, for example the Moon Jelly, the eggs are held under their bell and the sperm swim to them.
When the eggs hatch they reveal the Jelly larvae. With the Moon Jelly, and other species who hold the eggs under their bell, the larvae will hang off of the bell for a little while before being released to their next stage of life. The larvae will then float around the water seeking out a solid surface to attach to.
After the larvae have attached themselves to a solid surface they become polyps. Polyps have a body that is attached to the surface and tentacles that stick up collecting food rather like sea anemones. It is during the polyp stage that asexual jellyfish reproduction occurs.
Polyps are neither male nor female. They produce other polyps in what is called budding. They continue to bud and produce other polyps until the polyp colony is formed. A polyp colony looks like a stack of plates, and is actually several polyps joined together with feeding tubes.
As budding continues flat segments will peel off of the top of the colony. These segments are called Ephyrae. Ephyrae are the teenagers of the Jellyfish lifecycle.
Ephyrae are the young, still developing, adult Jellies. They will feed off of plankton and continue to develop until they "come of age" as adult medusas. The Jellyfish lifecycle will then begin all over again.