An invasive marine creature whose anus is disappearing and which has a tendency to eat its own young may have another trick up its sleeve.
According to Norwegian scientists, when a sea walnut’s life is not going well, it shrinks into a larva shape and stays that way until things improve again.
“We found that under certain conditions, these animals can essentially regress to their stage of life,” said Pawel Burkhardt, an evolutionary biologist at the University of Bergen in Norway. As it happens guest host is Susan Bonner.
“This was completely unknown for this group of species. It was a wonderful, wonderful discovery.”
The study authors call this “reverse development” — or, more simply, aging backwards. If they are right, the sea walnut joins an elite group of organisms known to have this extremely rare ability, along with Turritopsis dohrniior the immortal jellyfish, and Echinococcus granulosusa type of parasitic tapeworm.
Some scientists, however, criticize the study’s conclusions, arguing that the ability of sea walnuts to change shape and size in response to external stimuli is already well documented and that this does not mean they can develop vice versa.
The results were as follows: published this week in the preprint journal BioRxiv and have not yet been peer-reviewed.
Responding to stress
Mnemiopsis leidyiAlso known as the sea walnut or comb jellyfish, it is not a jellyfish but rather a type of transparent, lobed marine invertebrate known as a comb jelly
“They live in the ocean and they look like walnuts, and their light is mesmerizing,” Burkhardt said. “Every time you see one, you’ll remember it for the rest of your life. It’s a beautiful, beautiful sight.”
They occur naturally on the Atlantic coast but have become an invasive species in the oceans of Europe and Asia, traveling around the world by arriving in ships’ ballast tanks.
To find out if sea nuts could undergo reverse development, Burkhardt and his colleague, marine biologist Joan Soto-Angel, subjected them to two major stressors: starvation and trauma.
For two weeks, they stopped the jellyfish from eating and performed lobectomy, which means cutting off their gelatinous lobes. Burkhardt admits the methods seem harsh, but says sea nuts experience similarly harsh conditions in the wild.
The hungry and injured sea nuts, he says, shrank to about the size they were in the larval stage.
What’s more, 13 of the 65 grew pairs of tentacles and used them to catch microscopic prey, something they normally do only in the larval stage. When the sea nuts become adults, their tentacles shrink so much that they become nonfunctional, and instead they use their lobes and other limbs to hunt.
“Basically, from an adult, they can regress to a younger stage,” Burkhardt said.
Once the creatures were fed again, they returned to their normal size and shape.
Reverse development or simple metamorphosis?
Burkhardt believes this ability may give sea nuts an evolutionary advantage and help explain how they can survive in ships’ ballast water for weeks without food and thrive in oceans around the world.
“If you’re a big, proper adult, you need a lot of food. And if you have those conditions, you thrive in those conditions and you live like a happy, big, big sea nut. If you don’t, you have to come up with a strategy, right?” he said.
“The sea nut may have found a way to become smaller, to revert to a different, younger stage. And during that time it may be able to take in food in much lower doses and thus survive the difficult conditions.”
Jamileh Javidpour, who has studied sea nuts but was not involved in this study, says she has also seen the creatures shrink in size in the lab when there was no food, but the growth and use of the larvae’s antennae make these findings novel.
“These findings suggest that reverse development may play a key role in how marine organisms adapt to changing environments, especially under pressure from human activity,” Javidpour, an assistant professor of ecology at the University of Southern Denmark, told CBC in an email.
However, not everyone is convinced.
Mark Martindale, director of the Whitney Laboratory for Marine Bioscience in Florida, told CBC that anyone who has worked with comb jellyfish knows they can shrink and regenerate, so the study’s results “aren’t that important.”
Sidney Tamm, a biologist at Boston University who has studied the species, agrees.
“The regression in the starved and lobectomed animals … is certainly not a reversal of developmental processes and should not be called reverse development,” Tamm told CBC in an email.
Tamm says it would be more accurate to say that the sea nuts “regenerate and morph” to “simulate” the shape of their larval form.
He said if comb jellyfish were indeed returning to an earlier stage of life, they would see a reduction in the number of cilia, or thread-like organelles, on parts of their bodies called comb plates.
“The resulting phenotype may superficially look like a … larva, but it’s not a larva,” he said. “So I think the authors are kidding themselves that what they’re seeing is a reversal of development to an earlier stage of the life cycle.”
However, the authors maintain their conclusions.
Soto-Angel says the sea squirts in the study had smaller comb plates and smaller sizes. But the key evidence, he says, is the growth and use of the larvae’s antennae.
“Reverse development is about returning to a previous stage of the life cycle, and that’s what it’s all about. Mnemiopsis leidyi “Yes,” he said.
