Under deadly conditions, these sea creatures can age in reverse
For most animals, aging is a one-way street. But not in the sea walnut, a type of comb jelly about the size of a mango that’s native to the eastern Atlantic Ocean. When the going gets tough, the transparent invertebrate ages in reverse, regressing to a tentacled larval form. Then, when conditions improve, it matures back into an adult, biologists reported last week in a preprint posted to bioRxiv. This remarkable ability may explain how this comb jelly, Mnemiopsis leidyi, survived cross-oceanic trips trapped in ship ballast for weeks on end with little food to become an invasive nuisance, first in the Black Sea and now in parts of Europe and Asia. And this invader may now help researchers understand aging better.
“The study [is] a testament to how much we still can learn from invertebrates,” says Maria Miglietta, a marine biologist at Texas A&M University. She studies the so-called immortal jellyfish Turritopsis dohrnii that was the first—and for a long time considered the only—species discovered to age in reverse.
Six years ago, marine biologist Joan Soto-Angel and evolutionary biologist Pawel Burkhardt were struggling to keep M. leidyi specimens thriving in their lab at the University of Bergen. “When [the comb jellies] are having a bad day, they basically disintegrate,” Soto-Angel recalls.
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Eventually, they learned the right temperature and housing arrangements to keep the animals going. Once they succeeded in completing their initial studies of the species’ nervous system, the team began to push the creature to its limits—depriving it of food or amputating lobes of gelatinous tissue that surround the mouth and make up much of the 2-centimeter-long adult body.
Those starved or injured animals shrunk into blobs just a few millimeters in size, but didn’t die. When the duo began to feed them again, a few came back to life. Thirteen out of 65 animals tested grew two tentacles—a characteristic the animals have in their larval stages. The revived animals used these appendages to hunt microscopic plankton floating by, Soto-Angel and Burkhardt report. It’s possible that the animal in this larval stage has an advantage when it comes to capturing food resource, the authors note, which may explain its invasive success, though that remains to be tested. With enough sustenance, the comb jellies eventually regrew their lobes and even started to reproduce again.
Until now, biologists had found this ability to revert to an earlier life stage and regrow in only two animals: the immortal jellyfish, a type of Cnidarian, and a species of tapeworm, Echinococcus granulosus. Finding it in a third animal, the comb jelly—which despite sharing a similar name with jellyfish, belongs to a different phylum known as Ctenophores—“was quite a surprise,” Soto-Angel recalls.
Despite both the immortal jellyfish and these comb jellies possessing the ability to grow young again, the animals go about it in different ways. Jellyfish have distinct life stages: a sexually reproducing free-floating bell-shaped medusa and a stationary “polyp” that resembles a sea anemone and can bud off more polyps. As it reverse ages, its cells lose their identities and merge into a “cyst” that redifferentiates into a polyp. “It’s not the exact same individual which transforms back to the preceding stage,” Soto-Angel says.
But that cellular disintegration doesn’t happen with comb jellies. With them, Soto-Angel notes, “it is clear that it is the exact same individual which rejuvenates, like going back in time.”
Yoshinori Hasegawa, a zoologist at the Kazusa DNA Research Institute who was not involved with the work but who has studied the jellyfish, says it’s an important finding but questions whether these comb jellies are truly turning back their biological clocks, or simply shrinking instead. “It looks like an imperfect rejuvenation.”
