The Hydra is best known as an ancient fearsome beast, a terrifying, multi-headed monster belonging to the mythical realm. Biologists, however, know it as a real-life tentacle-wielding organism that is arguably immortal, one that can genetically modify its cells in order to survive.
A new study in Biophysical Journal only serves to make this bizarre critter even stranger. Although it’s long been known that it “opens” its mouth by ripping apart its own skin, precisely how it does this on a cellular level remained a mystery. Now, a team of researchers from University of California, San Diego have discovered that instead of just rearranging its cells to accomplish this unusual feat, the cells themselves stretch in order to self-destruct.
The Hydra belongs to a large grouping of animals (phylum) called the cnidarians, which includes jellyfish, sea anemones and corals. Each share a set of specialized prey capture cells known as cnidocytes, but Hydra have a set of characteristics that easily make them stand out from the pack. In particular, thanks to their incredible regenerative capabilities, they can theoretically live forever. In fact, they can grow back from nothing more than a fragment of a tentacle.
They also spend most of their life motionless, moving rapidly – using somersaults, no less – only when their planktonic prey drift past. It’s been observed that when they’ve got a grip on their hapless prey, they tear open their outer layer in order to fashion a mouth. When they’re done consuming their miniature meal, their new mouth closes up, healed by the Hydra’s remarkable cellular regeneration.
Previously, scientists have thought that in order to create this temporary mouth, they would have to rearrange the cells between its tentacles in order to create an area that could expand into an opening. This new study’s research team decided to use genetically modified specimens of the Hydra vulgaris species in order to find out once and for all how it “opens” its mouth.
The Hydra vulgaris opening its mouth. Its ectoderm is glowing green, while its endoderm is highlighted in magenta. The scale bar is 200 micrometers long. Carter and Hyland et al./Biophysical Journal
These GM Hydra have two fluorescent “tags,” one for the outer layer (ectoderm) and one for the inner layer (endoderm). This tagging allows the researchers to see how the two cell layers move with respect to each other, as well as letting them see how individual cells are behaving.
By looking at how these tagged Hydras feed, they realized that no cellular rearrangement was occurring. Instead, parts of the animal’s ectodermal cells called “myonemes” act as muscles, stretching part of the ectoderm until the cells tear themselves apart. This process can take only 13 seconds to complete, which is relatively rapid for a creature of such a diminutive size.
Hydras' ability to regenerate is considered to be extremely ancient, something that first appeared when more specialized cells (those responsible for vision, for example) hadn’t yet evolved. Essentially, these aquatic curiosities have remained physiologically simple in order to keep their ability to rapidly heal, without which their self-destructive feeding mechanism would probably not exist.
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