Meet the hydra: Masters of immortality - Raymond Adkins

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In 1740, while Swiss naturalist Abraham Trembley was tutoring the sons of a nobleman, he had scooped up some pond water for them to study when he found something shocking inside. He couldn't identify the odd little thing or even

tell if it was a plant or an animal. Its way of moving and consuming things seemed animal-like. But when he cut parts off, or even split it in two, it regenerated, which was decidedly plant-like, in keeping with its

green hue. Trembley noted the organism’s similarity to the Hydra, the monster from Greek mythology, and the name stuck. Today, we know hydras are animals capable of some incredible feats, apparently including immortality. They comprise a genus of tiny aquatic

animals in the broader phylum Cnidaria. Some cnidarians, like jellyfish, have a stationary polyp phase, then a free-swimming Medusa stage. But hydras and some of their relatives stay polyps their whole lives. Hydras have relatively simple cylindrical body plans, one

end forming the foot, the other the mouth, framed by fanning tentacles. They’ll eat just about any critter they can snag. And although they can scoot around if needed, most of the time, they remain stuck to things using a

gluey substance secreted from their foot. They can reproduce sexually, but they more often do so asexually, budding new growths off their bodies. Where exactly they hunker down in the freshwater ponds and slow-moving streams they populate varies by species,

which likewise differ in shape, size, and hue. Green hydras, for one, bask in sunny spots to feed the photosynthetic algae they host, which lend them their color and provide sugars in return for other nutrients. This explains some of

Trembley’s “animal-or-plant” confusion. Meanwhile, other species, like brown hydras, stick to the shade and hunt more actively. Hydra tentacles are covered with stinging cells containing little capsules of pressurized, harpoon-like structures. When triggered, they fly out at explosive speeds, immobilizing

small prey. This adaptation doesn’t protect hydras from larger predators— but what they lack in defenses, they make up for in their unparalleled regenerative abilities. Not only can hydras regrow lost appendages, they can also reassemble themselves after going through

a blending process of sorts, where they're completely broken down into individual cells. To do this, these cells first aggregate into a sphere, then they begin to sort themselves, gradually hollowing out and elongating as they group into external and

internal layers. Soon, one end sprouts a few buds, which become the tentacles, while the other end develops into the foot. In a matter of days, the hydra is back in one piece, bopping around and catching worms like nothing

happened. In labs, this process has helped shed light on embryonic development, because some of the stages and proteins are common among all animals— though the ability to perform it willy-nilly, as hydras do, is decidedly not. Interestingly, if multiple

hydras go through this shearing process, the resulting cells will form a single hydra. This is probably because their immune systems don’t distinguish between self and others. And this even holds true for interspecies mixtures. However, hydras made from multiple

species will fall apart because their cell types can’t adhere properly. But when they're all in one place, they're kind of unstoppable. All animals have stem cells, which can divide and differentiate into specialized cell types. But for humans, they

decline after embryonic development and become limited to specific parts of the body. Meanwhile, hydra stem cell levels remain constant and can help replace any part of their body and seemingly divide forever. In a roughly 20-day span, a hydra

has all new cells. This perpetual turnover prevents the buildup of age-related cellular damage, while enabling apparently boundless regeneration and reproduction. They aren’t indestructible. But so long as nothing kills them, they can theoretically live forever, without aging. And many

have indeed been persisting in labs for years on end, teaching us about development, regeneration, mortality, and tiny, tentacled freshwater life. All of this from some supposed pond scum.