Individual Jellyfish are either male or female. The eggs and sperm develop in very colourful special areas called ‘Gonads’ inside the body wall. When all of the eggs and sperm are fully developed, they are released into the stomach and then through the mouth into the sea. In most cases, to reproduce, a male jellyfish releases his sperm into the surrounding water. The sperm then swims into the mouth of the female jellyfish, allowing the fertilization of the ova process to begin. Moon jellies, however, use a different process: their eggs become lodged in pits on the oral arms, which form a temporary brood chamber to accommodate fertilization.
When the eggs are released into the sea they are fertilized by the sperm and continue to develop. As in all many-celled animals, the microscopic fertilized eggs begin a series of cell divisions which finally result in an embryo. However, the embryo does not develop directly into a baby jellyfish, but becomes a tiny, flattened creature called a ‘Planula’.
The Planula, which is covered with rapidly beating hairlike cilia (thin, tail-like projections), is able to swim and may be carried a considerable distance by ocean currents during its short swimming period, lasting from a few hours to several days. The tiny Planula then makes its way toward the sea bottom where it actively looks for a suitable place to attach itself. At this point, a amazing series of events take place.
Most jellyfish pass through two different body forms during their life cycle:
The tiny Planula attached to a hard underwater object such as a rock or shell immediately begins to grow into a Polyp. The Polyp can live like this for several years capturing passing small shrimp-like organisms and other tiny marine animals to feed upon. At this stage, the polyps mouth and tentacles are facing upwards.
Each Polyp produced from a Planula is able to reproduce new Polyps by a process called budding.
Budding is the development of a completely new Polyp that comes from the body wall of the original creature, much like a branch growing from the side of a tree. These become detached as separate small Polyps. This is a type of asexual reproduction, since one Polyp gives rise to several ‘daughter’ polyps without eggs and sperm having to come together. This happens usually in the spring. Each Polyp begins a different sort of asexual reproduction.
At the second stage, a series of grooves appear in the body of the Polyp, which gradually become deeper. Finally, the grooves become so deep, that they cut all the way though the Polyp body. Following this, a pile of disc-shaped structures resembling a stack of coins are formed. Each of the coins is an individual baby Jellyfish of the Medusa type. It can now break off from the stack and swim away to begin its own life as a floating Medusa. The Medusa has an umbrella shaped body called a bell and their tentacles hang from the border of the bell. This is the form most people recognise as a Jellyfish.
Below is a diagram showing the process of transformation from a Planula to a Jellyfish’
The tiny Planula floating in the sea.
Searching for a place to attach itself.
Once attached, it feeds on passing, floating food.
At this stage, the Polyps mouth and tentacles are facing upwards.
Grooves appear and become deeper, eventually cutting through the Polyps body.
A pile of disc-shaped structures emerge and break away. Each being a baby Jellyfish.
At this stage it is called a Medusa and is the Jellyfish form we recognise in our oceans.