The Lifecycle of Sea Jellies
- AJ
- Feb 25, 2025
- 3 min read
Updated: Mar 17, 2025
The way that Jellyfish reproduce is interesting in many ways. They use methods of both sexual and asexual reproduction in their lifecycle. In order to reproduce, male sea jellies release their sperm into the open ocean to fertilize a female's egg. When fertilization occurs, the egg is stored in the oral tentacles of the female where they develop into planula larvae. This planula swims for a short amount of time then attaches itself to a surface, the ocean floor, docks, shells, rocks, etc.
After attaching itself to a hard surface, the planula grows into a polyp. This is the asexual phase of the jellyfish reproduction process. These polyps act almost as jellyfish factories and are able to clone themselves. A jellyfish can stay in the polyp phase for years depending on the species. This allows the polyp to wait for the perfect conditions for the jellyfish to survive.
When conditions are good, the polyp begins to elongate and form segments, this process is called strobilation. These segments eventually become a stack of small jellyfish clones and begin to separate from the polyp. These small jellyfish are known as ephyra, they are no more than a few millimeters across but as they eat, they grow into a full sized medusa jellyfish (the classic jellyfish that we all know). The polyp is also able to stay alive for years after the jellyfish have passed and some species can even revert back into their polyp stage after growing into a medusa.
The lifespan of a jellyfish varies from a few days to decades. The lion's mane jellyfish (the biggest jellyfish we have found) lives for about a year. They can grow up to eight feet in diameter and over 100 feet long ! These jellyfish are often found alone, but when the food supply is plentiful and conditions are good they gather in large numbers to spawn, these groups of sea jellies are called jellyfish blooms.
Jellyfish Are Cool
The fact that one polyp can make 10 medusa jellyfish was unimaginable for me. It also sparked my interest that we know so little about these creatures. Scientists have only ever found about two dozen species of polyps in the wild, but there are more than 2,000 different types of sea jellies. The locations of the polyps seem to be the biggest unanswered question about jellies. Another burning question is, what did the ancient jellyfish eat? We know that jellies have been around for much longer than the food they eat today but no food has been found in the fossils of ancient jellyfish.
Due to jellyfish fossils being so rare, there is little evidence of ancient jellies. This lack of research and knowledge about jellyfish has made my curiosity grow. Keeping them alive in aquariums or labs is also very difficult due to scientists struggling to replicate the sea currents in jelly's tanks. Most people are also incredibly afraid of these animals because of their sometimes deadly sting. I have never even seen a jellyfish in the wild but, I know people who have been stung by jellies.
The species of jellyfish are so diverse. The fact that there are over 2,000 different species of jellies is astonishing. Due to the large diversity among different species, climate change and human impacts on the oceans will affect each sea jelly differently. I think that there is so much more we can learn about and from these magnificent creatures, especially since they have been on this Earth for so long. Their ability to adapt, reproduce as well as repairing and protecting their DNA is truly remarkable.
Thank you for reading !
-AJ
2/25/25
References:
Berwald, J. (2017c). If You Dare. In Spineless: The Science of Jellyfish and The Art of Growing a Backbone essay, Riverhead Books.
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