The familiar form of the jellyfish is the medusa; the smaller polyp form is restricted to the larval stage. Jellyfish reproduce sexually and individuals are either male or female. The reproductive organs develop in the lining of the gut. During reproduction, the male releases sperm through its mouth into the water column. Some of the sperm are swept into the mouth of the female, where fertilization occurs. Embryonic development begins either inside the female or in brood pouches along the oral arms. Small larvae (planulae) leave the mouth or brood pouches and enter the water column. After several days the larvae attach themselves to something firm on the sea floor (rocks, shells, piers, boats, etc.) and gradually transform into flower-like polyps (scyphistoma). Polyps can multiply by producing buds or cysts that separate from the first polyp and develop into new polyps.
“A colony of polyps can reproduce asexually and give rise to other polyps and this stage, can in theory, go on indefinitely,” said Crossley.
When conditions are right, fully developed polyps eventually produce a larval stage (the strobila), which resembles a stack of saucers. Each saucer develops into a tiny jellyfish (ephyra stage), which separates itself from the stack and becomes free swimming. In a few weeks, the ephydra will grow into an adult jellyfish, the medusa, thus completing the life cycle.