Negligible senescence

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Some tortoises show negligible senescence.

Negligible senescence is the lack of symptoms of aging in a few select organisms. Negligibly senescent organisms do not have measurable reductions in their reproductive capability with age, or measurable functional decline with age. Death rates in negligibly senescent organisms do not increase with age as they do in senescent organisms.

Negligibly senescent organisms have no "post-mitotic" cells; they reduce the effect of damaging free radicals by cell division and dilution. Another related mechanism is that of planarian flatworms, which have “apparently limitless telomere regenerative capacity fueled by a population of highly proliferative adult stem cells.”

There are many examples of species for whose organisms scientists have not detected an increase in mortality rate after maturity. In other words, they are equally likely to die at any given age after maturity; or, alternatively, it could be that the mean lifespan of the organisms is so long—multiple millennia—that researchers' subjects have not yet had the chance to live up to the time when a measure of the species' longevity can be made.

Study of negligibly senescent animals may provide clues that lead to better understanding of the aging process and influence theories of aging. The phenomenon of negligible senescence in some animals is a traditional argument for attempting to achieve similar negligible senescence in humans by technological means.

There are also organisms that exhibit negative senescence, whereby mortality chronologically decreases as the organism ages, for all or part of the life cycle, in disagreement with the Gompertz–Makeham law of mortality (see also Late-life mortality deceleration). Furthermore, there are even more peculiar examples, those of species that have been observed to regress to a larval state and regrow into adults multiple times; e.g., Turritopsis dohrnii.

Recent studies have indicated a connection between phenomena related to negligible senescence and the general stability of an organism's genome, specifically transcription processes, over its lifetime.

In vertebrates

Some fish, such as some varieties of sturgeon and rougheye rockfish, and some tortoises and turtles are thought to be negligibly senescent. The age of a captured fish specimen can be measured by examining growth patterns similar to tree rings on the otoliths (parts of motion-sensing organs).

In plants

In plants, aspen trees are one obvious example of biological immortality. Each individual tree can live for 40–150 years above ground, but the root system of the clonal colony is long-lived. In some cases, this is for thousands of years, sending up new trunks as the older trunks die off above ground. One such colony in Utah, given the nickname of "Pando", is estimated to be 80,000 years old, making it possibly the oldest living colony of aspens.

The world's oldest known living non-clonal organism is the Methuselah tree of the species P. longaeva growing high in the White Mountains of Inyo County in eastern California, aged 4856–4857 years.

In bacteria

Among bacteria, individual organisms are vulnerable and can easily die, but on the level of the colony, bacteria can live indefinitely. The two daughter bacteria resulting from cell division of a parent bacterium can be regarded as unique individuals or as members of a biologically “immortal” colony. The two daughter cells can be regarded as “rejuvenated” copies of the parent cell because damaged macromolecules have been split between the two cells and diluted. See asexual reproduction.

Maximum life span

Some examples of maximum observed life span of animals thought to be negligibly senescent are:

Rougheye rockfish	205 years
Aldabra Giant Tortoise	255 years
Lobsters	100+ years (presumed)
Hydras	Observed to be biologically immortal
Sea anemones	60–80 years (generally)
Freshwater pearl mussel	210–250 years
Ocean Quahog clam	507 years

Cryptobiosis

Some rare organisms, such as tardigrades, usually have short lifespans, but are able to survive for thousands of years—and, perhaps, indefinitely - if they enter into the state of cryptobiosis, whereby their metabolism is reversibly suspended. It's hypothesized by advocates of cryonics that the human central nervous system can be similarly put into a state of suspended animation shortly before brain death to be revived at a future point in the technological development of humankind when such operation would be possible.

See also

• Biological immortality
• Category:Senescence in non-human organisms
• DNA damage theory of aging
• Indefinite lifespan
• Maximum lifespan
• Strategies for Engineered Negligible Senescence