The Humble, Immortal Hydra


A plunge into the depths of the supposedly non-ageing Hydra – the superior cousin to the jellyfish – and their secret to non-senescence (or the phenomenon, anyway). This review article will explore the adaptive qualities of Hydra and how they have arrived at the Fountain of Youth before us.
Throughout history, humans have been searching for the key to eternal youth with little success. From emperors of ancient empires to the myth of the Fountain of Youth, nothing has pertained to the possibility of human biological immortality thus far. But what about other species?


Hydra is a genus of tiny, freshwater organisms featuring two main lower classifications: hydra oligactis (brown hydra) and hydra viridissima (green hydra). Their body is shaped like a tube, and on the end of one side is a ‘foot’, which is adhesive to stick to surfaces. The other end features a mouth surrounded by one to twelve tentacles, which are employed to catch prey. They belong to the phylum Cnidarians: the same group as the jellyfish. The commotion around these modest polyps is due to their regenerative ability (morphallaxis), which is the overall cause of their potential ‘immortality.’ Initial recognition of this was when Daniel Martinez claimed in a 1998 article in Experimental Gerontology that Hydra are biologically immortal.[2]
Said article by Daniel Martinez cites that at the end of a four-year experiment on 60 hydra, they looked as youthful as they did on day one; this is a surprising feat since the general rule of nature is that the smaller you are, the shorter lifespan you have. This is evident in many instances: insects only have a lifespan of weeks, while gargantuan species like the blue whale function for nearly a century. Hydra measure an average of 15mm, yet they are capable of surviving for a long time. It is unknown how long hydra can live for, but in most cases, they surrender to threats like diseases; however, the possibility of non-ageing implicates that there could be hydra still alive and kicking that have lived for 10,000 years.[3]
So how exactly do hydra have the capability for non-senescence?

The Key is in Stem Cells

Hydra can regenerate any part of their structure via a set of stem cells in its body. They are so potent that they can regrow significant chunks of the body in the case of an accident/emergency – – or if you cut it in half because you are curious. Thus, continual replacement of their bodies occurs once every few weeks, essentially discarding any aged or worn cells; there is no such thing as an infirm hydra.
Additionally, these regenerative abilities are crucial in reproduction. Archetypally, they do not need to undergo sexual reproduction, and instead grow buds; clones of themselves.
It takes three distinct stem cell populations to produce a copy of a fully functional specimen (ectodermal and endodermal epitheliomuscular cells and interstitial stem cells) and all three shares one protein in common: FoxO.[4][5] Despite the limited clarity regarding how this prevents ageing, it can be deduced that this probably is a factor in non-senescence. For example, humans that live beyond 100 years normally possess variants of this protein.[6]

Other Adaptations

Alongside its noteworthy time-defying trait, hydra additionally utilise simple (but astoundingly effective) methods to aid survival. One of these is their feeding mechanisms.
The tentacles on a hydra is specialised to catch unsuspecting prey. They extend their bodies and tentacles (which can reach to four to five times the length of the body) to the maximum length. When this trap is set up, the appendages are maneuvered around slowly, in wait of suitable prey. 
When prey strays into the contact of one of these tentacles, nematocysts on the surfaces are fired, which release neurotoxins to paralyse the meal. Other tentacles then join to move the victim near the mouth.[7]
A specialised group of cells stretch to make an opening, where the prey is engulfed and digested. From observations by biophysicists at the University of California, these cells change their shape rather than move around to form the mouth.[8]
These mechanisms show off the sophistication of a simple nervous system. As this demonstrates, there seems to be no end to how hydra can impress us.


Ultimately, hydra have proved themselves to be remarkably adapted to survive, to the point of being able to bypass the throes of time. Despite being small, they probably have the answer to biological immortality, which may assist scientists and researchers in unravelling the mysteries of non-senescence. Hydra could be considered one of nature’s finest!


  1. Hamrsky, Jan. 2018. \”Hydra | LIFE IN FRESHWATER\”. LIFE IN FRESHWATER. (accessed May 27, 2018).
  2. Martinez, Daniel. 1998. \”Morality Patterns Suggest Lack of Senescence in Hydra\”. ScienceDirect. (accessed May 27, 2018).
  3. Simon, Matt. 2018. \”Absurd Creature of The Week: This Amazing Little Critter Just Might Be Immortal\”. WIRED. (accessed May 27, 2018).
  4. Bosch, Thomas. 2018. \”Hydra and The Evolution of Stem Cells\”. (accessed May 27, 2018).
  5. Barras, Colin. 2018. \”The Animals and Plants That Can Live Forever\”. (accessed May 27, 2018).
  6. Willcox, Bradley J.; Donlon, Timothy A.; He, Qimei; Chen, Randi; Grove, John S; Yano, Katsuhiko; Masaki, Kamal H.; D. Willcox, Craig; Rodriguez, Beatriz; Curb, J. David. 2008. “FOXO3A genotype is strongly associated with human longevity”. PNAS. (accessed May 27, 2018).
  7. \”Hydra (Genus)\”. 2018. (accessed May 27, 2018).
  8. Cell Press. \”Inside the mouth of a hydra: Hydra rips its own skin apart just to open its mouth\”. ScienceDaily. (accessed May 26, 2018).

Sources (especially image sources) are listed throughout the article as links also.

About the Author

Colin is a Year 9 student attending Penwortham Priory Academy. He has set his eyes on biology as one of his favourite subjects and probably will pursue something down that path, although the specifics aren’t determined yet.

He is also a dedicated pianist, martial artist and a self-appointed workaholic at school!

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