These Sea Anemones Grow Limbs When They Eat

To many humans, the popular proverb “you are what you eat” is a reminder of the benefits of healthy eating. For some sea anemones, however, it’s actually quite literal.

When starlet sea anemones eat more food, they sprout extra arms, according to a study published last week in Nature Communications. They are the first known species to grow entirely new limbs in response to consumption of food.

Cnidarians—a group of animals that includes sea anemones, jellyfish and corals—are highly adaptable. They diverged from other animals more than half a billion years ago. Unlike most animals, whose physical structures are determined by genetics, anemones have been shown to change in body size, reproductive strategy and venom composition as they mature, reports Cara Giaimo of the New York Times.

The starlet sea anemone is a tiny invertebrate that lives in shallow, salty lagoons. This species is most commonly found with 16 tentacles, though their arm count can range from as few as four to as many as 24 appendages.

To understand why this is, researchers from the European Molecular Biology Lab Heidelberg fed a test population of anemone varying amounts of brine shrimp. For more than six months, the team studied more than 1,000 fingernail-sized growing polyps, a type of Cnidaria that attaches to a surface. They discovered that when they fed the anemone more shrimp, they sprouted more tentacles.

“Sea anemones show us that it is possible that nutrients are not converted into excess fat storage – as it is the case in all mammals – but instead transformed into a new body structure,” Aissam Ikmi, a group leader at the European Molecular Biology Lab Heidelberg and lead author of the new paper, says in a statement.

Though they develop differently, the adult-stage tentacles and larval tentacles have the same structure. “There is not one recipe to build a tentacle,” Ikmi tells the New York Times.

Anemone start with four buds near their mouths, which develop into full tentacles. When they are well-fed, scientists observed that it took the anemone five days to sprout new buds and five more for those to develop into full-sized tentacles, reports Rasha Aridi for Science magazine.

Like some other animals, plants and yeasts, anemone have cells that send signals to trigger growth when there is an abundance of food. The team identified certain proteins and molecules that are responsible for triggering limb development in response to food.

The starlet sea anemone is the only species known to produce tentacles this way, but there is evidence that food intake effects the development of other cnidarian species.

A study published last year found that when food is scarce, the Aiptasia anemone produces offspring with small tentacles or none at all. When nutrient starved, developing anemones stopped growing tentacles, and fully developed individuals produced tentacle-less and asexual offspring.

The reason for these adaptations may have to do with the anemones’ stationary lifestyle over their long lifespan. Some anemone species have lifespans of more than 65 years, so “they need to continuously adapt their body to changing environmental conditions,” Jake Warner, a developmental biologist at the University of North Carolina, Wilmington, who was not involved in the study, tells Science magazine.

“Controlling the number of tentacle arms by food intake makes the sea anemone behave more like a plant developing new branches than an animal growing a new limb,” Ikmi says in the EMBL statement.

Scientists have yet to verify another species for which increased food intake causes limb growth. But, according to the New York Times, because food availability is a commonly found trigger, it’s possible that it is a phenomenon shared by other species.

Claire Bugos | | READ MORE

Claire Bugos is a journalist and former print intern at Smithsonian magazine. She is a recent graduate of Northwestern University, where she studied journalism and history.