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?
Box jellies are one of the coolest animals in the world. A set of eyes and light sensors on each side of the body.
The first sea jelly-like creatures that come to mind in fiction – because I just read it – are the air jelly things in the Vorkosikan Saga. They float through the air with sacs of hydrogen and land on their prey to suck blood through their tentacles.
In the Outcast in another World series there is a race of sapient, psychic, sea jelly-like people who also float through the air.
The Netch in the Elder Scrolls game series are kind of like a cross between flying jellies and small whales. A little too meaty and violent when attacked to really qualify.
Navigational Entanglements by Aliette de Bodard prominently features a jellyfish alien, and gets deeply nerdy about jellyfish biology.
Oooo! (rushing to move it to the top of the TBR Mountain)
Oooh, for once I know one! Sheri Tepper’s book, Shadow’s End is the one with jellyfish aliens if I remember correctly. Lots of other weird aliens too.
A lot of sf stories involving gas giant planets like to imagine that the native fauna are jellyfish-like “floaters”, such as Arthur Clarke’s “A Meeting with Medusa” (not too subtle!).
My go-to work on jellies is Spineless by Juli Berwald, her account of her worldwide travels in search of the cause(s) and meaning of recent jellyfish “blooms” in multiple different oceans. Not quite as affectionate a look as Sy Montgomery gives her octopuses, but very intriguing nonetheless.
How about Arthur Conan Doyle’s aerial jellyfish and their high-altitude predators in The Horror of the Heights?
Eleanor Aranson’s “Ring of Swords” features a species if jellyfish aliens.
Frank Herbert had some sort of giant airborne jellyfish in The Jesus Incident.