Of the estimated 200 species of sea jellies, approximately 70 are known to sting humans, with varying degrees of severity. In addition to their tentacles, jellies also have round, umbrella-like bodies or bells for propulsion, and oral arms or flaps that are used to consume prey or brood their young. They eat plankton, shrimp, plants, fish, worms, and even other sea jellies. They are without eyes, but have simple sensors around the rims of their bell-shaped bodies to let them know if they are headed up or down, into the light or away from it. In contrast to their structural simplicity, sea jellies have highly complex reproductive strategies.
Most jellies propagate asexually, a process that typically involves four distinct stages.
The eggs remain on the mother's oral arms and grow into round, flat larvae, or planula. They attach to a solid surface and grow until they resemble a tiny flower, or polyp, a process that may last a few months or several years. A polyp reproduces by cloning and eventually transforms itself into a stack of exact replicas. However, in controlled conditions, such as an aquarium environment, sea jelly life expectancy can be extended up to five years. The Portuguese call them aguas vivas - living water.