Once, I have had the opportunity to see moon jellyfish migrating across the shallows of Puget Sound. By the thousands their glittering bells bobbed as they followed the sun and its warmth. Dipping, rising separately then suddenly synchronized they reminded me of butterflies in migration. I did not know then that the shapes which mesmerized me were only one form in the lifecycle of this creature. Similar to a butterfly in the air, a jellyfish in the water has four distinct appearances. Both jellyfish and butterflies, like all animals start from a fertilized egg. But in the jellyfish lifecycle this is followed by a planula larva, then a polyp and finally an ephyra juvenile which grows to a full size medusa jelly.
The bell shaped jellyfish (or sea jelly) often shown in photos, movies or cartoons is in its medusa stage. This is the end stage of the jellyfish life, when male and female jellies release their sperm and eggs to create a new generation.
The fertilized egg grows into a planula larva, an asexual bean-like blob covered with small cilia or vibrating hairs that can propel it through the water. It may hover under its mother's bell along with dozens of its siblings, such as with the moon jelly species, or it might float around in a herd of larvae, ingesting microscopic marine life. It drifts until it sinks to the floor of the sea or reef, looking for a spot where it will attach itself in preparation for the next stage of life.
It now becomes a stationary polyp, At this point the jelly would appear to be upside down. Its leg-like tentacles point up and its tubular body is at the bottom, firmly stuck to a rock, a bit of coral or even seaweed. It captures small crustaceans and other animals as prey while it becomes larger, and may produce additional polyps as it matures.
Meanwhile, eight legged discs grow like a stack of plates from the free end of the polyp. The topmost disc comes off, and then the next and the next as each disc matures and free-floats off again. This disc, the ephyra larva, will be a male or female. Its mission is to grow in size and shape through this juvenile stage becoming a full-fledged adult medusa. Whether swimming by pulsating or floating freely, the medusa has a sense of orientation and direction based on light.
The medusa, like every stage of the jellyfish in its lifecycle, is a carnivore. It doesn't actively pursue prey, but "bumps" into it, then stings its victim into submission before ingesting it. Medusas swim in large groups and may follow a daily migration pattern in conjunction with their prey, both following the path of sunlight and warmth across the water's surface.
Medusa jellies are also dependent on groups of large numbers for mating, since sperm is released by male medusas directly into the water and must come into contact with a female's egg for fertilization. When the female's eggs are fertilized inside her body by the male's sperm from the water nearby, the cycle of life begins again.