True jellyfish have a complex life cycle that allows them to quickly take advantage of favorable environmental conditions. This is achieved by alternating with each generation between reproducing sexually (with sperm fertilizing eggs) and asexually (by cloning themselves). What we think of as the ‘adult’ jellyfish that swims and stings is actually just one lifestage called a medusa.
1. In the Chesapeake Bay, sea nettle medusae will release eggs and sperm into the water throughout the late summer.
2. These fertilized eggs will grow into a planktonic spore called a planula. These planulae will drift around until they come into contact with a hard surface like a rock or oyster shell. They then attach to the hard surface and transform into a polyp called a scyphistoma.
3a. These scyphistomae resemble tiny anemones, are less than an inch long, and they feed by capturing small animals out of the water with their tentacles.
3b. When conditions are poor, such as when there’s high temperatures or low food, the scyphistoma can transform and shrink into a small, dome-shaped cyst. These podocysts are very hardy, and can survive for years until conditions improve. Once the environment becomes favorable again, the podocyst can return to its polyp form and continue growing.
4. While it’s in the scyphistoma stage, the polyp can reproduce by branching out and cloning itself.
5. Once the waters warm to about 62oF in the spring, the polyps will begin to grow and elongate into a chain or stack of tiny jellyfish clones called a strobila. Periodically, the strobila in the end of the chain will break off and swim away in a process called strobilation. https://www.youtube.com/watch?v=bcQs3dkrH0A
6. This tiny clone is now free-swimming and called an ephyra. These ephyrae quickly begin to feed on other zooplankton, and they continue to grow into medusae, where the cycle begins again.
In contrast to true jellyfish, the sea walnut only reproduces sexually, and almost constantly. As adults, each individual produces both sperm and eggs, and self-fertilization is the norm. These are released into the water, where the egg is fertilized and begins to grow. After about a day, the egg hatches into the larval comb jelly, which has two long, sticky tentacles to capture prey. Once the larval comb jelly grows larger than about 5mm, it begins to absorb its tentacles and grow out the lobes that it will use to capture prey as an adult. Once the larval comb jelly reaches about 15mm long, it has fully absorbed its tentacles and is now an adult.