In some ways, jellies are a lot like people– they grow from fragile embryos to mature adults, and most species require a male and female to reproduce– but in other ways, jellies are very very different. Jellies live complicated lives.
The life of a jelly begins when male and female jellies release eggs and sperm into the water– that’s right, they don’t have copulatory organs or intercourse, they just release their eggs and sperm into the water around them– the egg and sperm fuse, developing into a small larva that looks like a fuzzy pill. This pill is called a planula. The planula swims around the ocean until it finds the right kind of home, usually a rock or shell. Once it’s in a nice spot, it sticks itself to the surface, and turns into a little ball of cells. This ball of cells grows tentacles, a gut and mouth, and soon begins to resemble a minute sea anemone. This sea anemone-looking creature is called a polyp. It’s capable of wiggling its tiny body, catching food with its tentacles, and eating and digesting this food.
Now the plot thickens. At this point different jellies tend to do things differently. We’re going to look at a type of jelly called a hydrozoan. There are over 2,000 described species of hydrozoan; most of these jellies are small, clear and do not sting people. When conditions are right, the polyp grows a small lump on the side of its body, which at first looks like a bubble or blister. This bump is called a jelly bud.
As the bud gets bigger it begins to take on structure. Small tentacles can be seen inside. With time, the bud develops into a tiny jelly, which eventually begins to pulse on the side of the polyp. With enough movement the jelly detaches itself, and swims away to begin its jelly life. One polyp can produce many jellies, which partly explains why an empty bay can be filled with jellies in only a matter of months. These jellies eat plankton, grow, and eventually begin releasing eggs and sperm into the water, starting the cycle anew.