So it's not exactly shocking that these strange swimmers can split themselves open and plop their skin back together without missing a beat. But in the new study, published Tuesday in the Biophysical Journal, scientists were able to observe and describe the process step-by-step at the cellular level for the first time. They did this using genetically modified Hydra vulgaris with green and red fluorescent proteins tagging two different skin layers.

It turns out that the cells of the hydra actually change their shape, stretching like contracting muscles before they split.

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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-Maria Collins, a biophysicist at the University of California, San Diego, said in a statement. "When you watch the shapes of the cells, it looks like even the cell nuclei are deformed."

The process is a lot like the stretching of eye muscles to open the iris, the researchers say — and when they gave hydra muscle relaxants, their mouths stayed shut.

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

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The researchers point out that their study is a great example of hydra's usefulness in the lab: They're small, tidy, resilient, and it's easy to genetically modify them to give certain cells a fluorescent pop. The creature can be used to study how groups of simple cells are able to develop and grow into complex, mouth-splitting tissues.