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Real-life hydra creates a new mouth for every meal

Freshwater hydras aren't nearly as fierce as their mythological namesakes. But they might be even weirder.

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    A transgenic Hydra vulgaris is shown with its two tissue layers labeled: the outer layer, ectoderm, in green; and its inner layer, endoderm in magenta. In transgenic engineering, at least one has gene has been transferred from another species.
    Courtesy of Callen Hyland
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On almost any measure, the multi-headed Hydra monster of Greek mythology is more fearsome than its real-life namesake: a genus of tiny freshwater polyps that snack on shrimp and other small invertebrates. But the less than one-inch creatures do have a fearsome ability that might wow even the original Hydra: each time they eat, the tiny freshwater-dwelling hydra must rip apart and sew up their own mouths. 

The opening on their tubular bodies isn't just closed; their mouths really disappear until it's time to digest dinner, which they catch with poison-barbed tentacles. Once they're done, the opening is sealed up again with a layer of tissue. But how?

Scientists at the University of California's San Diego and Irvine campuses used color-coded, transgenic proteins to help track cells around the hydras' mouth areas for the first time. Their research suggests that the openings are temporarily created by changing cells' size, not rearranging them, as was previously thought. 

"We can try to understand what look[s] like very complicated processes in the living animal with relatively simple physics," lead author Eva-Maria Collins, a biophysicist at UC San Diego, said in a press release. The trick, however, was finding a way to watch the cells in vivo, which no one has previously been able to do. 

Co-author Robert Steele, from UC Irvine, created transgenic hydra years ago: hydra which include some cells transferred from another species. In this case, he transferred green and red proteins into other proteins in the epithelial cells in the hydra's inner and outer tissue layers, both of which adjust to create the mouth each time.

Cells surrounding the center of the mouth don't reposition themselves, as researchers had previously thought; instead, the UC team discovered, they radically change shape. One clue was the speed with which the mouths reappear and disappear, growing even wider than the hydra's normal size in less than a minute. Mouth sizes vary based on the size of prey, but the rate of growth and shrinking was constant, they found, suggesting that it is governed by the nervous system. 

"The fact that the cells are able to stretch to accommodate the mouth opening, which is sometimes wider than the body, was really astounding," said Dr. Collins. "When you watch the shapes of the cells, it looks like even the cell nuclei are deformed."

In a paper published Tuesday in Biophysical Journal, the team argues that "myoneme" fibers in cells around the mouth area are triggered to contract, dramatically shrinking the size of each cell and producing a huge opening. If given the equivalent of a muscle relaxant to block those contractions, hydra can't create new mouths.

Questions remain about why, evolutionarily, hydra need to create and re-create their mouths. But the UC team believes their work will contribute to research beyond hydra, while also highlighting promising new research methods. 

"This work illustrates that the structural simplicity and the availability of in vivo labeling techniques make Hydra an excellent model for studying fundamental biomechanical processes on both the cellular and tissue levels," the researchers report.

The processes that help hydra reassemble after being torn apart time and again could also help understand two fundamental systems found in all creatures: tissue formation and patterning. 

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