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Here’s how the hydra rips its own body open to eat a meal

Hydra opens its mouth over the course of 13 seconds. (UC San Diego)

It's hard out there for a hydra. Scientists from the University of California have detailed the process by which the tiny freshwater creature Hydra vulgaris opens its mouth, and it ain't pretty: It has to rip a hole into its own body every time it consumes food.

Scientists already knew that the tiny aquatic creature Hydra vulgaris had to split its skin open every time it opened its mouth. Now, using a genetically modified organism made with contrasting fluorescent tissues, they've watched that process play out in detail for the first time. It turns out that the hydra distorts and stretches its cells to open its gaping maw. (Video: Carter and Hyland et al/Biophysical Journal 2016)

That's not actually the news here — scientists already knew about the whole the-mouth-is-actually-just-a-gaping-self-inflicted-wound thing. In fact, hydra are known for their incredible regenerative capabilities. The tube-like, tentacled critters usually grow less than half an inch long (and some are much smaller) but some scientists think that they age so little they must be functionally immortal.

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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.

"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."

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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."

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.

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