It has long been suspected that hydra, small freshwater animals, are immortal in that they do not suffer degenerative aging. In practice this means that no changes in mortality rate, reproduction rate, and measures of cellular metabolism are observed over time. This is a highly regenerative species, with individuals capable of rebuilding themselves from fragments, and it may be the case that their constant regeneration is the source of their agelessness. Regarding that agelessness, the challenge for researchers is that verifying the lack of aging in a species is a slow statistical business of wait and see, and one can always suspect at the end of any given study that the authors did not check rigorously enough for signs of aging. Perhaps it is there, just too slow to show up over the time allotted. Certainly there has been some back and forth debate over the last twenty years regarding what the data does or does not support. This latest research provides a set of much more robust evidence in support of hydra agelessness:
The common perception that the bodies of all living beings age, is wrong. This has now been proved by a long-term experiment with the freshwater polyp Hydra, a microscopic animal. Observing many hundreds of them for almost ten years, they calculated that Hydra's mortality permanently stays constant and extremely low. For most species, including humans, the probability of dying within a specific year rises with age. Scientists regard this as an indicator of the decay of the aging body. For Hydra, evolution seems to have found a way to escape the mechanisms of the physical deterioration of getting older. For humans the probability of dying within one year is reaching levels as high as 50 percent for advanced ages. For Hydra, however, it remains constant at a low 0.6 percent. Humans only experience such small values when they are between 20 and 30 years old. Additionally, Hydra's reproduction rate did not diminish with age, instead the small animals continued to breed. In this sense the Hydra stayed forever young.
In a unique long-term experiment researchers created artificial conditions for the tiny water animals with their flimsy tentacles, which were free of fatal natural threats like predators. For almost ten years they have cared for of about 1,800 of the Hydras. Overall, the team has counted 3.9 million observation days of individual Hydra. The number of natural deaths per year, however, can be counted on one hand. On average there have been only five. When a Hydra passed away it was mostly due to laboratory accidents, such as a polyp sticking to the lid of its bowl and then drying up or simply having been dropped on the floor. From of the few natural deaths that remained researchers calculated Hydra's mortality. It is so low that even several lifetimes of researchers would not suffice to observe the end of the lifecycle of the polyps. Even after 500 years five percent of a cohort will still be alive. For two out of twelve of the Hydra cohorts under investigation, the risk of death was actually so small, that it will take 3,000 years until only five percent of the polyps remained.
"Hydra apparently manages to keep its body young because it does not senesce by accumulating damages and mutations, as most other living beings do. Hydra are probably able to follow a special self-preservation strategy, as its body and cellular processes are rather simple." For instance, Hydra are capable of completely replacing parts of the body that are damaged or are somehow lost. It can even fully regenerate if its body is destroyed almost completely thanks to a high number of stem cells. Stem cells are capable of developing into any part of the body at any time. Additionally, as Hydra replaces all of their cells within only four weeks, it regularly and quickly expels all cells that have been changed genetically by mutations. Thus, damages have little chance to accumulate.