Let's briefly look at the other two stages of the jellyfish lifecycle that lead to the medusa stage:
Planula: The eggs of the female jellyfish get fertilized when they come in contact with the male sperms that are released into the water. The embryos are stored either in the stomach of the mother or in brooding pouches along her oral arms. As the embryonic stage comes to an end, the embryos get transformed into free swimming planulae. These planulae are oval in shape and have small hair along the sides of the body. The jellyfish remains at this stage for a few days as the planule float at the surface of the water carried by the tides and currents of the water. After a few days of floating, the planulae sink into the water and begin the polyp stage.
Polyp: The sinking planulae attach themselves to a hard surface and remain immobile throughout this stage. This stage is therefore called the sessile stage in the life cycle of the jellyfish. In this stage, the polyp attaches itself to the hard surface at its base and has its mouth and a few tentacles at the other end. In time, new polyps emerge from the trunk of the original polyp and this process continues until a large polyp colony is formed. All the polyps are connected to each other with feeding tubes that ensure that nourishment is equally divided among them. This stage of the jellyfish can last for years and the colony can grow to huge proportions.
Ephyra: Under proper conditions, the polyps in the hydroid colony start developing horizontal grooves. As each groove reaches maturity, they get transformed into free swimming jellyfish called the ephyra and detach themselves from the colony. At this stage, the jellyfish is extremely tiny.
Medusa: This is the adult stage in the life cycle of the jellyfish. The medusa form consists of the fully developed bell, with thick oral arms along the sides of its mouth and tentacles on the margins of its body. It also has the elementary digestive system of gullet, stomach and gut in a small stomach cavity on the underside of the bell. The thick mesoglea of gelatin like substance forms the inner layer of the jellyfish. The gonads also develop during this stage and become visible through the translucent bodies in a number of species of jellyfish. In this stage, the development of the jellyfish is complete and the growth is only visible in the jellyfish's size. Jellyfish can only reproduce once they have reached the medusa stage.
Written by Nathan Dickerson and Sudarsana Sinha.