If the mortality rate of a species does not increase after maturity, the species does not age and is said to be biologically immortal. There are many examples of plants and animals for which the mortality rate actually decreases with age, for all or part of the life cycle.
If the mortality rate remains constant, the rate determines the mean lifespan. The lifespan can be long or short, even though the species technically "does not age".
Hydras were observed, in a study published in the journal Experimental Gerontology, for four years without any increase in mortality rate.
There are stranger examples of species that have been observed to regress to a larval state and regrow into adults multiple times:
The Hydrozoan species Turritopsis dohrnii is capable of cycling from a mature adult stage to an immature polyp stage and back again. This means that there may be no natural limit to its life span. However, no single specimen has been observed for any extended period, and it is impossible to estimate the age of a specimen.
The larvae of carrion beetles have evolved to undergo a degree of "reversed development" when starved, and later to grow back to the previously attained level of maturity. The cycle can be repeated many times.
Revived into activity after stasis
Various claims have been made about reviving bacterial spores to active metabolism after millions of years. There are claims of spores from amber being revived after 40 million years, and spores from salt deposits in New Mexico being revived after 240 million years. These claims have been made by credible researchers, but are not universally accepted. In a related find, a scientist was able to coax 34,000 year old salt-captured bacteria to reproduce and his results were duplicated at a separate independent laboratory facility.
A seed from the previously extinct Judean date palm was revived and managed to sprout after nearly 2,000 years.
Silene stenophylla was grown from fruit found in an ancient squirrel's cache. The germinated plants bore viable seeds. The fruit was dated to be 31,800 years old ± 300 years.
In 1994, a seed from a sacred lotus (Nelumbo nucifera), dated at roughly 1,300 years old ± 270 years, was successfully germinated.
Clonal plant colonies
As with all long-lived plant and fungal species, no individual part of a clonal colony is alive (in the sense of active metabolism) for more than a very small fraction of the life of the entire clone. Some clonal colonies may be fully connected via their root systems, while most are not actually interconnected, but are genetically identical clones which populated an area through vegetative reproduction. Ages for clonal colonies, often based on current growth rates, are estimates.
Pando is a Populus tremuloides (Quaking Aspen) tree or clonal colony that has been estimated at 80,000 years old. Unlike many other clonal "colonies" the above ground trunks remain connected to each other via a single massive underground root system. Whether it is to be considered a single tree is disputed, as it depends on one's definition of an individual tree.
The Jurupa Oak colony is estimated to be at least 13,000 years of age, with other estimates ranging from 5,000 to 30,000 years.
A huge colony of the sea grass Posidonia oceanica in the Mediterranean Sea is estimated to be between 12,000 and 200,000 years old. The maximum age is theoretical, as the region it occupies was above water at some point between 10,000 and 80,000 years ago.
King's Lomatia in Tasmania: The sole surviving clonal colony of this species is estimated to be at least 43,600 years old.
Old Tjikko, a Norway Spruce in Sweden, is a tree on top of roots that have been carbon dated to 9,550 years old. The tree is part of a clonal colony that was established at the end of the last ice age. Discovered by Professor Leif Kullman, at Umeå University, the tree is located in the county of Dalarna in Sweden. Old Tjikko is small, only 5 metres (16 ft) in height.
A box huckleberry bush in Pennsylvania is thought to be perhaps 8,000 years of age.
Some endoliths have extremely long lives. In August 2013 researchers reported evidence of endoliths in the ocean floor with a generation time of 10,000 years. These are slowly metabolizing, not in a dormant state.
Llangernyw Yew, the oldest individual tree in Europe and second or third oldest individual tree in the world. Believed to be aged between 4,000 years and 5,000 years old, this ancient yew (Taxus baccata) is in the churchyard of the village of Llangernyw in North Wales.
Fortingall Yew, an ancient yew (Taxus baccata) in the churchyard of the village of Fortingall in Perthshire, Scotland; one of the oldest known individual trees in Europe. Various estimates have put its age at between 2000 and 5000 years, although these days it is believed to be at the lower end of this range.
The tuatara can live well above 100 years. Henry, a tuatara at the Southland Museum in New Zealand, mated for the first time at the age of 111 years in 2009 with an 80-year-old female and fathered 11 baby tuatara.
A female Blue-and-yellow Macaw named Charlie was reportedly hatched in 1899, which would make her 111 years old, as of 2010. Her age has not been independently confirmed and the claim may not be reliable. She is claimed to have formerly belonged to Winston Churchill, but Churchill's daughter denies the claim.
The oldest living horse on record was named Ol' Billy. Bill was allegedly born in the year 1760 in London, England. Bill died in 1822 at the age of 62. Henry Harrison, an occupant of London during the time, had also allegedly known Ol' Billy for 59 years until Bill's death.
Creme Puff, a cat owned by Jake Perry of Austin, Texas, was born on 3 August 1967 and died on 6 August 2005 - a total of 38 years and 3 days.
Turritopsis dohrnii, the immortal jellyfish, is known to be the longest-living creature which could live on forever without dying of old age.
Some species of sponges in the ocean near Antarctica are thought to be 10,000 years old.
Specimens of the black coral genus Leiopathes are among the oldest continuously living organisms on the planet: around 4,265 years old.
The giant barrel sponge Xestospongia muta is one of the longest-lived animals, with the largest specimens in the Caribbean estimated to be in excess of 2,300 years.
The black coral Antipatharia in the Gulf of Mexico may live more than 2000 years.
A specimen, "Ming" of the Icelandic Cyprine Arctica islandica (also known as an ocean quahog), a mollusk, was found to have lived 507 years. Another specimen had a recorded life span of 374 years.
Some koi fish have reportedly lived more than 200 years, the oldest being Hanako, who died at an age of 226 years on July 7, 1977.
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