How Tiny, ‘Immortal’ Hydras Regrow Their Lost Heads

A long green tubular animal with tentacles at one end against a black backdrop
The hydra’s unusual ability to regenerate parts of its body makes the creatures biologically immortal.  Choksawatdikorn / Science Photo Library via Getty Images

Thanks to new research, scientists are closer to understanding how “immortal” aquatic animals called hydra regrow their lost heads.

Hydra are inch-long freshwater creatures that resemble a floating tube with a grasping appendage at one end and a cluster of tentacles at the other. They belong to the phylum Cnidaria, a group of symmetrical invertebrates that includes jellyfish, sea anemones and corals, per Mindy Weisberger for Live Science. The hydra is best-known for its unusual ability to regenerate parts of its body, making the creature biologically immortal. 

“It’s one of these organisms that’s thought to never die unless you try to kill it or, you know, starve it to death,” says study coauthor Ali Mortazavi, a developmental biologist at the University of California, Irvine, to Sam Jones for the New York Times.

Unlike most animals that must worry about old age, disease, or losing a limb, a hydra can constantly replace damaged parts of itself. In certain cases, an entirely new animal can grow from a detached chunk of hydra tissue. On average, they replace all their cells every 20 days, reports Discover magazine’s Katharine Gammon.

Though earlier studies have uncovered some of the secrets behind hydra tissue regeneration, researchers are still looking for answers about how the animal directs its cells to sprout a new head where one was lost. Learning more about the process of regeneration in animals like hydra could potentially lead to new insights about human development, too. In an effort to understand the genetic basis behind the ability, Mortazavi and his colleague Aide Macias-Muñoz looked at which genes are switched on and off during head regeneration and how those genes are controlled.

“We wanted to know…what is happening at the genome level that’s telling these cells to grow or stop growing, and how does that compare to normal development?” says Macias-Muñoz, an evolutionary biologist who undertook the research while at the University of California, Irvine, to Kate Baggaley for Popular Science.

In their study published last week in Genome Biology and Evolution, Macias-Muñoz, Mortazavi, and their team first identified more than 27,000 genetic elements that play a role in hydra head regeneration. They analyzed tissue samples taken from hydra undergoing both head regeneration and budding, a form of asexual reproduction where a hydra grows a polyp that is essentially a duplicate of itself. Budding requires the growth of a second head, but the team found that a budding head forms differently than a head regrown after injury—meaning different genes were behind the two regeneration processes.

In budding, genes are slowly and constantly increasing over time, “but in regeneration, we noticed these sharp turns,” Macias-Muñoz says to the Times. “Even though the end result is the same, it looks like the trajectory is actually very different.”

It’s the first evidence that hydra’s genetic instructions for head regrowth varies depending on budding or regeneration, leading scientists to suspect the animals could have entirely different sets of head-growing blueprints for each process.

“Clearly, there’s more than one way to make a head,” Mortazavi tells the Times.

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