How Hydra Rip Open New Mouths at Every Meal

Scientists finally figured out how the tiny aquatic creature opens its mouth to eat

Transgenic Hydra
Scientists studied Hydra vulgaris to determine how the animal opens its mouth Callen Hyland, UC San Diego

Hydra are infamous for their ability to regenerate tissue after being torn apart. But one mystery about these tiny tentacled creatures that has dogged scientists was: How do Hydra open their mouths?

Biologists have long known that Hydra do not have a permanent mouth, writes Rachel Feltman for The Washington Post. Each time the animal needs to feed, its skin cells separate to form an opening. Soon after ingesting its dinner, the proto-mouth closes back up.

But because the opening forms so quickly, researchers had trouble observing the changes on a cellular level. They could only guess how the process works. Now, researchers at the University of California, San Diego think they have the answer for the tiny Hydra vulgaris—and they recently published their results in the Biophysical Journal.

To observe how the skin cells move in real time, researchers tagged different layers of cells with colored proteins—the outer layers in green and the inner layers with red, according to a university press release.

Hydra Opening Its Mouth, Sequence
Callen Hyland, UC San Diego

While some researchers expected the cells to rearrange themselves to create the mouth opening, the imaging showed quite a different process. Elements called “myonemes” in the innermost layer of cells act like muscle fibers and contract, deforming the cells. This creates an opening in a process similar to how an iris expands and contracts in the human eye.

The team confirmed their results by giving the animals magnesium chloride, a muscle relaxer. Even when stimulated to open a mouth, the hydra stayed shut.

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

Even so, the researchers don’t have strong reasoning for why the animals have such unusual mouths or what the evolutionary advantage could be, Feltman points out. It just adds to the mystique of these enigmatic creatures.