Column SFF Bestiary The Lore and the Weird Magic of the Jellyfish Even as sea creatures go, the jellyfish is especially alien… By Judith Tarr | Published on November 24, 2025 Photo by Katarzyna Urbanek [via Unsplash] Comment 0 Share New Share Photo by Katarzyna Urbanek [via Unsplash] Even as sea creatures go, the jellyfish is weird. It’s 95% water. It has no bones, no brain, no blood, no heart. It’s been swimming through the sea for hundreds of millions of years. At the rate it’s going, it may be the last thing left after everything else has succumbed to climate change and plain old entropy. I’ve talked about the jellyfish that may be immortal, but that’s only one of hundreds of different varieties across two phyla, Cnidaria and Ctenophora. They come in all sizes from a tiny pinprick to a monster almost three meters across. What they all have in common is their general shape: a bell or a box with trailing tentacles, nearly always equipped with stinging cells that stun and capture prey. Jellyfish stings are a notorious problem for humans. Most jellies are not lethal, but their stings can hurt like fury, and it’s often impossible to know they’re there until you step or swim into them. Even if you can see the bell, the tentacles can stretch far beyond it. The most venomous animal in the world is the Australian box jellyfish. Unlike most jellies, which aren’t particularly strong swimmers and mostly drift on the currents, this jellyfish can swim against the tide, and it has eyes. It can see you. It’s not just scary, it’s deadly. And it’s big: its tentacles are up to 3 meters (10 feet) long. What we recognize as a jellyfish, or a sea jelly as they’re more properly called, is only one of several life stages: the medusa. That’s the pulsing, graceful, sometimes deadly creature that’s found in every ocean. Under the right conditions it can congregate in groups as small as a meter across and as large as hundreds of meters: a phenomenon called a bloom. This is a supremely simple animal with a complex life cycle. It consists of a bell or box with an orifice that serves as both mouth and anus, a nerve net that’s capable of detecting levels of light, temperature, and chemical changes in the water (i.e., a sort of sense of smell), and the tentacles I mentioned above, which can trail for many meters behind the bell. They eat plankton and small sea creatures and sometimes fellow jellies. They are eaten in turn by other animals including sea turtles, whale sharks, and humans—some species are a delicacy in Asia. Jellies reproduce in two stages, one sexual and one asexual. The medusa releases either eggs and sperm into the water. They fertilize there (or sometimes in the mouth of the female) and develop into free-swimming larvae, which eventually attach to a surface and grow into polyps. These polyps in turn reproduce asexually by budding off ephyrae or infant jellies. Those that survive to adulthood become medusae. And so the cycle begins again. Medusae tend to be short-lived, as little as a few months, but polyps can live for years or even decades. And then of course there’s that wonder of the world, the immortal jellyfish, which can reverse the aging process and transform back into a polyp. Then, when conditions are right, it matures again into a medusa. It’s a complicated process, but it works. Jellies have outlasted most other species that have existed in the world. They’re invaluable to science not just for their longevity and persistence, but for what they can tell us about conditions in the ocean. Jellyfish blooms serve as indicators of climate change, as the water temperature rises and their reproductive cycle speeds up. Warmer water, more jellies, bigger blooms. This affects shipping and fisheries, as well as swimmers and beachgoers. Jellies have been ringing another set of alarm bells for the scientists who study them. As they feed, they accumulate microplastics. When other animals prey on them, the plastics move further on up the food chain. They’re a clear indicator of a growing problem. But there is a possible upside to this. Jellies, and the mucus some of them generate as a means of trapping prey, can actually trap microplastics. It may be possible to use them as a filter, to help clean up the ocean. That would be a serious benefit to the whole planet, not just to humans. While I’m contemplating the strangeness and possible usefulness of the jellyfish, I’m trying to remember where I’ve read of jellyfish-like aliens in science fiction. Anne McCaffrey’s Thread, for example is a kind of medusa-less jellyfish tentacle: all sting, no pretty. Where else do we see this particular kind of beautifully weird alien?[end-mark] The post The Lore and the Weird Magic of the Jellyfish appeared first on Reactor.