After Santa gave some to my daughter, we learned the moon jellyfish is capable of going from its adult stage back to its polyp stage and then back to adult again, making it effectively immortal.

For Christmas my daughter requested, and received from Santa, a somewhat unusual gift: a jellyfish tank, along with three moon jellyfish.

Yes, my daughter must have been pretty good all year to get such a gift from Santa. I, however—as the one who’d be charged with taking care of them—wondered if I’d done something to earn placement on the naughty list. Still, it was some consolation to learn that moon jellyfish have little, if any, stinging power.

My only experiences with jellyfish had been at large aquariums, watching hypnotic displays of the translucent creatures against a backdrop of slowly changing colored tank lights, and at the Jersey shore, where occasional increases in their numbers caused them to wash up on the beach in massive hordes. As I read the instructions on how to introduce the overnight-shipped jellyfish to their new home, along with the proper ways to maintain and test for salinity… and ammonia… and nitrates… and nitrites… the magnitude of the task prompted me to wonder “How long do jellyfish live, anyway?”

Once they were safely inside their 2-gallon tank (complete with very cool multi-color LED options), I decided to look for answers. By coincidence, Discover magazine’s December 2017 issue contained an article describing a species of jellyfish, Turritopsis, that was able to “reverse-age”—that is, go from its adult (medusa) stage back to its polyp stage, and then back to medusa again, making it effectively immortal. The article said that the reverse metamorphosis had also been demonstrated in another species—the common moon jellyfish.

In the wild, moon jellyfish might fall prey to any number of hazards—eaten by birds or fish, torn apart by curious or hysterical swimmers. But in a well-cared for, controlled environment, they could apparently live forever. I hadn’t really considered the jellyfish endgame, but figured it would involve a toilet, not including them in my will. The Wikipedia entry on moon jellyfish confirmed the immortality phenomenon, but said that the average lifespan in captivity was a more modest “several to many years,” still vague enough to be worrisome.

Our three jellyfish—Katniss, Peeta, and Rue, named for characters from The Hunger Games—seemed happy enough, I suppose, doing what jellyfish do…mostly floating about. I’d read that they shrink when stressed, and they did indeed reduce to about half their original size over the months we had them. I don’t know what they were so stressed about—I was the one feeding, cleaning, and monitoring them, not vice versa.

Then one day, I noticed an absence—one of them seemed to have disappeared. The next day, another one was gone. Did they get sucked into the filter somehow? Had the bigger one eaten the others? Was there some Finding Nemo-like conspiracy among them to escape from the tank, one by one? This was most interesting thing they’d done in months.

When they didn’t reappear promptly, I called the JellyArt help line, described the situation, and was told that because they’re 95 percent saltwater, when jellyfish die they just dissolve into their surroundings.

“Kind of like Obi-Wan Kenobi in Star Wars,” I observed.

“Um… yeah,” the guy on the other end replied, sounding unconvinced.

Denied a traditional home marine life funeral (ashes to ashes, flush to flush), we briefly mourned our losses, mostly absent of ceremony. Although it had been sort of relaxing to watch them, jellyfish aren’t really the kind of creature you bond with.

Unless you’re a 10-year-old girl, that is. At my daughter’s tearful urging, I set about purchasing replacements. I ordered two “medium” sized moon jellyfish, looking to match the size of the remaining one. Instead, the replacements were massive in comparison, well over the promised two inches in diameter. These were vital, healthy specimens—until I got hold of them. I switched all the water in the tank, intending to start fresh, but due to my misread of the replacement water’s salinity, the new jellyfish quickly developed gaping holes and began to slough off parts of their bodies. By the time I realized the problem and fixed it, Katniss II and Rue II were in pretty bad shape, each looking like a barely-connected bunch of tiny old rags. The surviving jelly from our original trio, Peeta, seemed to be humming along just fine, apparently unfazed by inconsistent salt levels and the survival struggles of his companions.

It turns out that in addition to their potentially indefinite lifespan, jellyfish have amazing recuperative abilities, which I had inadvertently put to the test. As they stabilized, the newcomers began, over several days, to re-form the familiar dome shape of the jellyfish “head,” with one slight adjustment: they were inside out. They looked like the cheap umbrellas sold by street vendors in many cities—after an encounter with gale-force winds. The tentacles still trailed behind, so now, instead of looking like jellyfish, the two newbies looked more like hollow, decapitated fantail goldfish.

As more days passed under now-stable conditions, the two new jellies began to show the familiar four horseshoe-shaped gonads that mark a thriving individual. But they went even further, developing a second set of gonads. The flesh connecting the two halves grew thinner, until it looked like the new part would cleave off and form a new, independent jellyfish. When the separation seemed immediate, I left my tank-vigil to get my phone and take a picture. When I came back, we had not three jellyfish, but four.

Soon it was five—all swimming about, seemingly healthy. The two newest ones, dubbed Haymitch and Gale, had long tentacles and small hoods, while the three others had more typical hoods and small tentacles. I tried to figure out exactly what had happened, consulting various scientific sources, but it seems that despite their being on the planet for well over a million years, there’s an awful lot we don’t yet know about jellyfish.

Will this cloning/self-replication happen again, for example? Was this a one-off fluke, or do I have the aquatic equivalent of rabbits on my hands? The next time you’re at the beach and see a moon jellyfish floating along, take a moment to appreciate their beauty, and their general weirdness. Or just come over to my house… I might have some extras for you to take home.