The ghostly Rhizostoma luteum is a jellyfish so elusive that decades often pass without a single sighting. The creature was first discovered by naturalists in 1827, but it wasn't until 2013 nearly 200 years later   that the species was confirmed to be real.

Not surprisingly, we know very little about this enigmatic animal, but now, thanks to a collaboration between Jellyfish Research South Spain (JRSS) and Austria's Zoo Vienna, that's about to change. For the first time, Rhizostoma luteum has been bred in captivity.

While conducting a survey off the Spanish coast, JRSS biologist Karen Kienberger had a chance encounter with the rare jellyfish. Eager to learn more about the species, she brought a large adult specimen back to her lab, where she discovered the animal was sexually mature. 

"She collected the [jellyfish's] larvae and sent them to us, in hopes that we could breed and raise them to adulthood," explains Zoo Vienna staffer Johanna Bukovsky. "It was a challenge, even for the expert aquarists we have here."

Despite the recent desktop-tank craze, caring for jellyfish is extremely difficult. The tendrilled invertebrates feed on live prey, and are sensitive to changes in their environment. As you can imagine, attempting to raise a species we know very little about was a daunting task for the team. 

"You don't know anything about the needs of the animal," says Bukovsky. "What water temperature do they need? What do they eat? What current?"

Image: Zoo Vienna/Used with permission
Kienberger dives with an adult R. luteum. Image: JRSS

The animals were fed a mixture of phytoplankton and brine shrimp, a cue taken from other, better-known jellyfish species. 

"We managed to raise 30 babies from larva, to polyp, to jellyfish," says Bukovsky. "By keeping them in captivity, we are able to collect critical scientific data, while Karen continues to do research in the wild." The team even managed to document each step in the jellyfish's development something that would otherwise be impossible in such a rarely seen animal.

Jellyfish are related to anemones and coral (family Cnidaria), and at certain stages of life, you can see the resemblance. During development, each larvae will hook itself to a rock and grow into a polyp a stationary factory that produces baby jellyfish, each one a clone of the original larva. Eventually, the tiny jellies growing within bud off the top, and pulse their way out to sea.

Scyphozoan polyp releases baby jellyfish. Image: Alvaro Migotto/YouTube

The babies might be small (my squishy), but the bell of an adult R. luteum can reach lengths of 60cm (2ft). It's far from the biggest jellyfish in the world (just look at this ten-metre wonder), but staff are expecting these youngsters to be somewhere in the region of 40kg (88lbs) when they're fully grown.

"The bigger of the babies already have a sting," adds Bukovsky, but as in other jellies in the order Rhizostomae, that sting doesn't come from where you'd expect. The billowing structures you see in the photos aren't actually tentacles – they're oral arms. These fleshy, mouth-covered folds of tissue help with filter feeding in the open ocean. The sting comes from the jellyfish's thin thread cells, which are hidden below the bell.

With jellyfish "starter kits" already sent to Germany and Japan's Kamo Aquarium (and some heading to France soon), Rhizostoma luteum might not be a mystery for long.