List of Long-living Organisms - Biological Immortality

Biological Immortality

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. Coral colonies and aspen trees are the clearest examples. Some large trees may routinely grow in size for decades, while their mortality rates decrease. Some sources say that sharks, too, grow larger in size while their mortality rate decreases, for long periods of their lives.

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". There are many examples of species for which scientists have not detected an increase in mortality rate after maturity. An alternative explanation for this phenomenon may be that the mean lifespan of the species is so long that the modern scientific study of longevity and senescence has not yet matured enough itself to measure longevity in the species.

• Sanicula is an herb, native to Europe and the Americas, which lives about 70 years in the wild. Old saniculae do not die at a higher rate than younger ones.

• Sea urchins, lobsters and some clams have relatively high rates of mortality in the ocean, but mortality does not appear to increase with 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 nutricula 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 been made 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.