Invasive Comb Jellies Might Overproduce Babies in Summer to Eat Them in Winter

Comb jellies might have evolved to eat their young when prey runs out, but some experts are skeptical of the strategy

Comb Jelly
Comb jelly larvae, highlighted by red arrows, shown inside an adult. Jamileh Javidpour/University of Southern Denmark

Comb jellies don’t look like much—they’re usually transparent, though they also glow in the dark. They’re native to the western Atlantic but have expanded their range into other waters where they’re wreaking havoc on the food chain. Now, researchers have a new and brutal suggestion as to how comb jellies are so successful.

A new study published on May 7 in the journal Communications Biology presents evidence that the warty comb jelly, Mnemiopsis leidyi, eat their young to survive the winter. The invertebrates produce thousands of larvae each summer and the booming population vacuums up all of the tiny prey available. By winter, the buffet is empty—except for each other. The researchers suggest that adult comb jellies’ offspring are nutrient stores for adults come winter.

The insight could help conservation efforts to fight comb jellies in the Black Sea, where they are an invasive species.

“They are very prolific. An adult lays up to 12,000 eggs in two weeks,” ecologist Thomas Larsen of the Max Planck Institute for the Science of Human History tells the Guardian’s Nicola Davis. “The Black Sea just became this gelatinous ocean.”

The comb jellies, also called ctenophores, munch on tiny plankton along with fish eggs and larvae. The population booms in late August, so the researchers gathered jellies daily for two months, Inside Science’s Katherine Gammon reports. They counted the adult and young comb jellies, and when the adults’ food ran out, the population of young began to drop. But the young jellies probably weren’t starving, their preferred prey was still around.

To test whether the adult jellies were eating their larvae, the researchers took their collected jellies to the lab. They fed a set of young jellies with algae containing an extra heavy form of nitrogen. As the jellies grew up, the nitrogen stayed in their systems. Then, after leaving one adult comb jelly without food for a day, the researchers put ten young comb jellies in its tank. The researchers then calculated how much mass the adults gained after eating the young based on how much of the heavy nitrogen they had after the 36-hour feeding experiment.

The study concludes that the comb jellies in the wild eat their larvae when their own prey run out, making the population explosion in August a way of “building up resources for the winter,” Larson tells Science News’ Erin Garcia de Jesus.

“The authors present a new insight into how a non-native and invasive species can survive and become established in environment far away and with very different environmental conditions from its original conditions,” marine scientist Sophie Pitois of the U.K. Centre for Environment, Fisheries and Aquaculture Science, who wasn’t involved in the research, tells the Guardian.

Pitois adds, “To the best of my knowledge, these findings are new and provide an opportunity to rethink and design appropriate conservation strategies in controlling the spread of non-native invasive species by taking into account the entire range of an animal behaviors that allow it to adapt and thrive in new environments.”

However, some experts question the study’s conclusion. Monterey Bay Aquarium Research Institute marine biologist Steve Haddock points out that the strategy is unsustainable because a lot of energy from the prey is lost when it is eaten by larvae. When the larvae move and grow, they expend energy that the adults could have gained by eating the prey directly.

"If the parents ate that same prey item, they would get 10 times the return on investment," Haddock tells Inside Science. "I haven't seen evidence that this is a general pattern for ctenophores at large, nor that the larval ingestion is a strategy rather than incidental."

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