The Hydra Does Not Age and Scientists Are Studying Why

Hydra are freshwater polyps about 15 millimeters long, with a tubular body and tentacles for catching prey. They have no brain, no organs in the conventional sense, and no apparent aging. In 2015, Daniel Martínez and colleagues at Pomona College published a study in which they tracked 2,256 hydras over eight years under controlled lab conditions. The mortality rate didn't go up as the animals aged. No measurable senescence.

The reason seems to be the animal's cellular composition. A hydra's body is continuously renewed by three lineages of stem cells that divide without stopping. The animal essentially replaces itself on a cycle of about 20 days — the whole body turns over, with old cells shed at the ends of the tentacles and the base. There's no tissue that accumulates damage the way vertebrate cells do under the standard model of aging.

In 2012, researchers at Kiel University studying both hydras and humans identified FoxO — a transcription factor gene — as the key regulator. When FoxO was suppressed in hydras, their stem cells stopped dividing properly and the animals aged normally. The same FoxO gene family is associated with longevity in humans: centenarians tend to have specific variants of it. The discovery didn't imply that hydras had unlocked human immortality, but it suggested that the molecular machinery for extended lifespan is conserved across hundreds of millions of years of evolution.

Hydras reproduce mostly by budding — a genetic copy pinches off from the parent's side — which means a well-fed hydra in a stable environment has no obvious biological reason to die at all.