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.
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,”