Young again: study shows that comb jellies can age in reverse

A new article published in PNAS reveals the unprecedented ability for reverse development in a ctenophore, also called comb jelly. The findings suggest that life cycle plasticity in animals might be more common than previously thought.

Animal life cycles typically follow a familiar pattern, declined in countless variations: they are born, grow, reproduce and die, giving way to the next generation. Only a few species are able to deviate from this general principle, the best-known example being the ‘immortal jellyfish’ Turritopsis dohrnii, which can revert from an adult medusa back to a polyp. This elusive group of animals with flexible life cycles now includes the ctenophore Mnemiopsis leidyi.

“The work challenges our understanding of early animal development and body plans, opening new avenues for the study of life cycle plasticity and rejuvenation. The fact that we have found a new species that uses this peculiar “time-travel machine” raises fascinating questions about how spread this capacity is across the animal tree of life”, said Joan J. Soto-Angel, a postdoctoral fellow in the Manet Team at the Department of Natural History at the University of Bergen.

Gelatinous time-travel

The remarkable discovery began by chance. While observing animals in the laboratory, Soto Angel noticed that an adult ctenophore had vanished from a tank, seemingly replaced by a larva. Curious to find out if these could be the same individual, he and Pawel Burkhardt, group leader at the Michael Sars Centre at the University of Bergen designed experiments to try and reproduce this potential reversion under controlled conditions. When exposed to the stress of starvation and physical injury, Mnemiopsis leidyi demonstrated an extraordinary ability to shift from its lobate form back to a cydippid larval stage. “Witnessing how they slowly transition to a typical cydippid larva as if they were going back in time, was simply fascinating”, Soto Angel recalled. “Over several weeks, they not only reshaped their morphological features, but also had a completely different feeding behavior, typical of a cydippid larva.”

An ancient model to study life cycle evolution

The study broadens the known developmental flexibility of ctenophores and positions Mnemiopsis leidyi as a valuable model for future research in developmental biology and aging. Since ctenophores are among the earliest animal lineages, the findings suggest that reverse development may represent an ancient feature in the animal kingdom. “This is a very exciting time for us”, Burkhardt said. “This fascinating finding will open the door for many important discoveries. It will be interesting to reveal the molecular mechanism driving reverse development, and what happens to the animal’s nerve net during this process”.

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The Department of Natural History at University of Bergen, holds internationally recognized scientific collections of more than two million specimens of animals, fungi, plants and minerals, as well as living collections at the University Gardens. At the department, the Manet Team is led by Associate Professor Aino Hosia. Their ongoing work includes studies on integrative taxonomy and systematics, life cycles, faunistics and biogeography, species interactions, and eDNA for gelatinous zooplankton.

The Michael Sars Centre at the University of Bergen, Norway is an international community of scientists using advanced technologies to study the unique molecular and cellular biology of marine organisms in a changing environment. As one of the first EMBL partners, the Michael Sars Centre is rooted in the Bergen academic community and serves as a national strategical asset for Norwegian marine life sciences. The Centre aims to establish, strengthen, and leverage local, national, and international networks through specific activities, including collaborative research, joint training, and scientific exchange.