Many of us have felt the familiar sting of the jellyfish while in the ocean during a day at the beach. This usually results in a mass human exodus to the shore from the waters, since one jellyfish can mean hundreds. While the injury is usually minor and no worse than a mosquito bite, one would certainly prefer one sting to a dozen or more. It may seem difficult to fathom, if you avoid them in the water, that many people do keep jellyfish as pets in home aquariums. Some jellyfish owners, despite the short lifespan during maturity, even experiment with breeding. How do jellyfish reproduce? It is an interesting and many-tentacled process.
How Jelly Fish Reproduce
We think of jellyfish as ‘fish’ because they only survive in water and are, obviously, regarded as fish within their common name. This is quite a misnomer, as jellyfish are invertebrates with no bones, heart, spine, or brain. They are largely comprised of water, and do not move via fins, just the incredible locomotion of their bell or umbrella shape blooming and closing. It raises a number of questions, jellyfish lacking all of those typical creature features. But within their bodies they have a very intricate net of nerves that dictate their motivations and behaviors, including reproduction. A jellyfish is sensitive to light and water temperature, and it is suspected that they do have a sense of smell. A mature, or medusa, jellyfish has four stomachs, and sacs positioned around the bell of their body that helps them balance and float.
There are a few thousand species of jellyfish, but a lot of these reproduce in just a few similar ways. Eggs are released by the female jellyfish, and sperm is released by the male through the mouth. Sexual organs are restricted to gonads in jellyfish, regardless of gender. The gonads are located under the bell of a mature jellyfish, at the base of the tentacle. The sperm and the egg meet one another in the water, where the egg is fertilized by the sperm. The fertilized egg becomes free-floating larvae or planula, swimming around for a period before settling like so much sediment to the bottom of the ocean or aquarium setting, where they attach. In the wild, these planula can travel quite a distance due to the motion of ocean waters.
Other jellyfish reproduce a bit differently. The sperm, released through the male jellyfish’s mouth, fertilizes the eggs inside of the female’s mouth. This happens rather naturally when a female jellyfish is swimming about in waters where male sperm has been released. Certain species of female jellyfish may carry those fertilized eggs around with her until it becomes a larva and is released to commence the life cycle.
Once the larvae collect at the bottom of the environment, a new stage of life begins. The polyp stage of jellyfish reproduction is perhaps the most intriguing for a few reasons. One reason being, the organism is completely stationary at this time, staying rooted to the point where it settled. This is a tenuous task, as many other forms of aquatic life will feed on jellyfish polyps, from sea slugs to whales. As a surviving polyp feeds, it essentially asexually multiplies itself, with more polyps budding and adding to the original structure. From this one polyp attaching to the floor to grow, many jellyfish can spring. At this juncture, the form that may release multiple jellyfish is also known as a strobila, and it is comprised of stacked, immature jellies. The polyps may take years and years to develop. This makes it, by far, the longest stage of life for a lot of jellyfish species. Moon jellyfish are the most popular species for human ownership in aquariums, and this may be due in part to their more predictable life cycle. Moon jellyfish typically live for about one year, with the polyps breaking free to mature in the spring.
The jellyfish can split off of the strobila incrementally, or in rapid succession. Often, in the wild, thousands of baby jellies will mature and be released at once, as this is the optimal way to ensure a good survival rate and keep the species going. As soon as one leaves the stack, it is known as an ephyra, or an immature medusa. Ephyrae are newly individual little jellyfish shared like a disk, with eight limbs. In most species, this developmental stage is among the shortest in the lifecycle, being next to the final stage- medusa.
For the medusa, or adult jelly, reproduction means death is imminent. Yes, creating new life in order to continue the species is one of the final acts of a mature jellyfish. The jellyfish we think of are actually in the twilight of their lives by the time they are full-grown and floating around en masse in open waters.
Of course, most humans are ambivalent about how jellyfish reproduce, since while interesting, they do pose a threat of injury to us. Do we want to encounter more of them? Maybe not, but jellyfish can be an asset to the planet’s oceans. Small baby fish often travel with jellies, hiding among their ranks, and, astonishingly, never getting stung. This guards them from predators who would otherwise eat them, threatening the survival of that species. And while so many other species of fish are endangered and dying off due to overfishing, jellyfish are booming in numbers. Blooms, or schools of jellyfish, are appearing all over the world, clogging up fisheries and invading tourist destinations. This is likely a direct cause of human actions and climate change. The birds, fish, and sea creatures that eat jellyfish, hence controlling their population, are mostly endangered. The salmon, for example, is overfished and in demand for human consumption, meaning there are less of them eating up jellyfish in the oceans. So the next time you curse a bloom of jellyfish for putting a damper on your vacation, consider how your lifestyle has influenced the mass survival and reproduction of these marine animals.