Generalized life cycle of scyphozoan jellyfishes (e.g. Aurelia)
Scyphozoan jellyfish have four distinct stages to their life cycle:
Planula
Female jellyfish produces eggs and the males produce sperm which combine to produce a larva, called a 'planula' (plural = planulae). A planula is a tiny oval structure whose outer layer is lined with minute hairs called cilia. The cilia beat together to propel the planula through the water, but the motion of the cilia does not carry the planula far, instead ocean currents are responsible for transporting planulae long distances. The planula floats for a few days at the surface of the sea. It then drops downward to settle on a solid substrate where it attaches itself and begins its development into a flower like 'polyp'.
Polyp (or scyphistoma)
The planula metamorphoses into a sessile (i.e. fixed-position), usually benthic (i.e. bottom dwelling) polyp called a 'scyphistoma'. A scyphistoma has an anemone like body with a stalk attached to the substrate. At the top of the stalk there is a mouth surrounded by stinging tentacles which the polyp uses to feed. The polyp feeds by using its stinging tentacles to catch microscopic organisms from the water column which it passes to its mouth and ingests.
Polyps can multiply by producing buds or cysts that separate from the first polyp and develop into new polyps to form a polyp colony also know as a polyp hydroid colony (or strobilating scyphistomata). Members of the polyp colony are linked together by feeding tubes. These colonies can live for years and each polyp is capable of producing thousands of jellyfish in its lifetime. When a polyp is has built up enough energy from feeding and it feels the environmental conditions are right the stalk like structure begins to develop into a larval stage (the strobila), which resembles a stack of saucers. One at a time, starting form the top, the saucers separate to become 'ephyra'.
Ephyra
The process by which new medusae are produced is called 'strobilation' and involves metamorphosis of the end of a scyphistoma into an 'ephyra', an immature medusa, that subsequently detaches and swims away. Depending on the species, a single polyp may produce one or many ephyrae all at once, over a period of time, or at different intervals.
Medusa
The ephyra subsequently develops into a mature medusa over a period of weeks to months. This is the stage most people recognize as a 'jellyfish'.
This is one of our videos showing the various stages of a moon jellyfish (aurelia aurita). The video starts at the polyp/scyphistoma stage.
