Asked by: Sebastian Dill, Bermuda
There are a few jellyfish species that receive sperm through their mouths to fertilise eggs inside the body cavity, but most jellyfish just release sperm or eggs directly into the water. Under favourable conditions they will do this once a day, usually synchronised to dawn or dusk.
The fertilised eggs hatch into tiny free-swimming flatworms called planulae, which either develop directly into adult jellyfish or settle on rocks to form an intermediate polyp stage. The polyps can then reproduce asexually by budding off tiny jellyfish one or two millimetres across, which feed on plankton and gradually grow into full-size adult jellyfish.