Tiny freshwater animals known as hydra must rip open their mouth skin in order to eat, and now new research shows how they manage to do this.

The unusual process involves changes to the hydra’s mouth skin cells that allow the creature to stretch and split apart these cells in a dramatic deformation. The findings are published in the latest issue of the Biophysical Journal.

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“The fact that the cells are able to stretch to accommodate the mouth opening, which is sometimes wider than the body, was really astounding,” senior author Eva-Marie Collins said in a press release. “When you watch the shapes of the cells, it looks like even the cell nuclei are deformed.”

Collins, who is a biophysicist at the University of California, San Diego, and her team genetically engineered hydra so that the animals’ two layers of tissues would light up, revealing how their mouths and bodies work on a cellular level.

Hydra measure less than one half an inch long. They look like a column with a ring of tentacles at the end. In the wild, the other end adheres to a rock or some other kind of surface, keeping the hydra in place while it waits for unsuspecting prey to swim by.

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When a potential victim, such as a small shrimp, brushes against the hydra’s tentacles, the predator shoots out poisoned barbs to sting and paralyze its prey. Once the hydra contracts its tentacles, a special group of cells splits apart to display the black interior of its mouth, and the hydra sucks the prey in.

When the meal is digested, the hydra rips open its mouth to spit out any leftover materials. It then seals it back up into a continuous sheet of tissue, and waits for the next shrimp or other victim to swim by.

In the below video, a hydra in a lab eats a small crustacean, showing what the hunting and feasting look like from farther away:

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The lit tissues of the genetically engineered hydra showed that the animal’s cells change their shape, rather than move around, when the mouth opens.

“We can try to understand what look like very complicated processes in the living animal with relatively simple physics,” Collins said.

She and her team next determined that once triggered, radially oriented fibers in the hydra’s tissue contract to stretch the cells apart, similar to how muscles in the iris contract to open our pupils. When the researchers added magnesium chloride to act as a muscle relaxant, the hydras couldn’t open their mouths at all.

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No other species on Earth is known to feed in such an unusual manner.

Collins asked, “Evolutionarily, why do these animals have this weird mechanism for feeding? We don’t really have an answer for that, but it’s a really interesting question.”

The scientists are trying to answer it now by investigating what physiological consequences the shape-changing might have for hydra’s cells.