There’s a small, tentacled freshwater animal called a hydra, whose mouth disappears every time it closes.

I really mean that: it disappears. When your mouth closes, the two halves are still distinct. No matter how tightly you purse your lips together, they’re still separate bits of flesh. The same is true for the enormous mouths of whales and the tiny ones of mice, the beaks of birds and the expandable jaws of snakes. But not a hydra—its closed mouth fuses shut to form a continuous, sealed sheet. In the words of one scientist, “when a hydra closes its mouth, it obliterates it.”

Which means that whenever a hydra opens its mouth, it must tear itself apart.

In Greek mythology, Hydra was a venomous, many-headed snake monster with amazing powers of regeneration. Real hydras… aren’t actually that far off. They’re small and not that terrifying, but their shape—a tubular body crowned by wavy tentacles—recalls the beast of mythology. Like their cousins, the jellyfish, sea anemones, and corals, they’re armed with stinging cells that fire venomous harpoons. And they do regenerate with incredible skill; some biologists have even suggested that they’re effectively immortal. They are amazing creatures, which feature (along with their microbes) in my upcoming book, I CONTAIN MULTITUDES. But until recently, I had no idea about their weird mouths.

View Images Hydra viridissima. Frank Fox CC BY-SA 3.0

In the 1970s, scientists saw that hydras can open their mouths wider than the diameter of their own bodies, allowing them to swallow prey far larger than themselves. They also noticed that the mouth seemed to disappear whenever it closed. They couldn’t see it, not even with powerful microscopes.

In 1987, Richard Campbell from the University of California, Irvine discovered why: a hydra mouth is not a permanent opening. It constantly forms and vanishes. When it closes, a wide ring of cells around the edge of the mouth collapses into a small mound called a hypostome, with a rosette of 6 to 12 cells at its centre. These cells are stapled together by small junctions, so that not even a tiny pore remains between them. In many ways, Campbell wrote, closing the mouth is very much like healing a wound.

When a hydra opens its mouth, these rosette cells slowly stretch and flatten over a minute or so. Finally, a small rupture appears between them. As soon as this happens, the mouth quickly snaps open. Within half a second, a gap that extends between two or three cells becomes a gaping maw with hundreds of cells around its margin.

Campbell described this process in huge detail using careful microscopy but there was still a lot he didn’t understand. Now, three decades later, Eva-Maria Collins from the University of California, San Francisco has discovered more about this process, by studying genetically engineered hydra that have glowing molecules in their heads.

Hydra’s hypostome contains contractible filaments called myonemes, which are arranged in rings and spokes, much like a spider web. Collins proved that the mouth opens when the spokes contract, and presumably closes when the rings do. (A similar process dilates and constricts the pupils in your eyes.) Her team also showed that the cells around the opening mouth don’t rearrange themselves, as Campbell suggested. Instead, they just change shape, becoming longer and narrower to accommodate the creature’s widening gape.