The puzzle of longevity is not why we die so soon but rather why we live so long. ~ English ecologist Jonathan Silvertown
Negligible senescence characterizes organisms which lack aging symptoms.
Microbes asexually reproduce via fission: a cell divides into 2. The split is not of identical halves. One half gets older because it is laden with defective cell material while the other half is kitted out with new workings. By this microbes produce offspring younger than the parent.
Some organisms can exist for millennia. They are metabolically active but in stasis, with less energy than we thought possible of supporting life. ~ American microbiologist Karen Lloyd
Fission yeast were discovered in East African millet beer in 1893. In a stressful environment, these yeast suffer the pangs of senescence. Contrastingly, when life is good, fission yeast do not age: mother and daughters are equally well equipped for immortality.
Hydra are small, simple predators of other aquatic invertebrates. They appear immortal.
Turritopsis nutricula is an exemplary hydrozoan which can revert adult cells back to their childhood vigor. In essence, T. nutricula can reincarnate itself.
Some fish, including varieties of lake sturgeon and rougheye rockfish, are thought to be negligibly senescent.
Though the ocean is a rough neighborhood, with morbid mortality rates, sea urchins, lobsters, and certain clams are not known to die of old age. At the least, they may live well over a hundred years.
Sometimes bandied as living forever, there are no reliable reports of turtles or tortoises having negligible senescence.
An endolith is an organism that lives a sheltered life, inside rock, coral, or animal shell. Most are autotrophic extremophiles. The roster of endoliths includes certain viruses, archaea, bacteria, fungi, lichen, amoebae, and lichen.
Microbes living 2 km beneath the ocean floor have been found that appear to be millions of years old. Their reproductive cycle is on the order of 10,000 years.
The larvae of carrion beetles can developmentally retrogress when starved. They then regrow back toward maturity when food is available. This cycle is repeatable.
Bristlecone pines are long-lived. Individuals in the White Mountains of eastern California are ~5,000 years old.
Some plants approximate immortality via clonal colonies. Among these are Neptune grass, creosote bushes, and aspen trees.
Neptune grass is a slow-growing seagrass endemic to the Mediterranean Sea. This flowering and fruiting seagrass forms expansive underwater meadows which are crucial to the habitat of other species. Under favorable environmental conditions, Neptune grass may live 200,000 years.
Creosote bushes have a hard time in their early years: the hot, parched desert takes its toll. But once established, creosote bush colonies can live over 10,000 years.
An individual aspen tree may live 40–150 years above ground but its root system is a long-lived colony which may last thousands of years. One colony in Utah is 80,000 years old.