Myths and legends abound concerning magical waters that restore youthful appearances and sometimes might increase longevity. Over two thousand years ago, an historian of ancient Greece wrote about such a miraculous pool, and later European writers carried on the tradition. Similar stories of wonderful springs or poolsare reported from elsewhere, too; miracle waters have been thought to occur in many places, from Japan and India to North America.
Some mineral springs may have healing and soothing properties, but the hope of actual rejuvenation remains in the realm of myth (barring genetic engineering). So humans, at least for now, are stuck with myths and false hopes and the superficialities of face-lifts and cosmetics.
However, in the rest of the animal kingdom, there are thousands of species that have evolved life histories reminiscent of the fountain-of-youth myth, in which juvenile morphology is retained throughout the life of an animal. Technically, this is called paedomorphosis. Paedomorphic individuals never achieve what would be the “expected” adult morphology, as seen in related species. Although they keep their youthful appearance, paedomorphic animals mature sexually and reproduce; their lifespans are not necessarily extended.
One example is the tiger salamander of North America. Some populations have the normal life history, in which the juvenile phase is aquatic, with frilly external gills; the juvenile metamorphoses into a terrestrial adult, losing the external gills. But in some populations, the juveniles never metamorphose at all; they mature sexually in their juvenile, aquatic form. Tiger salamanders are closely related to the Mexican axolotl, in which an adult, terrestrial form is unknown. Axolotls are native only to the region of Mexico City and are critically endangered from loss of habit to urbanization and its effluents.
Paedomorphic species are found among fishes, too. Some gobies, for instance, keep their larval form and habits and become sexually mature. But paedomorphic species do not occur in reptiles, birds, or mammals, which develop directly from hatchling to adult, without a distinct juvenile stage.
In the world of insects, paedomorphosis occurs regularly in a variety of flies and beetles, as well as in hundreds of species in a taxonomic group called ‘twisted-wings’, which are parasitic in other insects. In all of these cases, paedomorphosis in known only in females, which do not complete the customary process of metamorphosing from larva to pupa to winged adult but, instead, remain in larval form. Researchers suggest that females have made an evolutionary trade-off: instead of spending energy on the costly process of metamorphosis, they devote that saved energy to making eggs. But the ecological conditions that favored that trade-off in those particular species are not known.
In many, probably most, of the paedomorphic insect species, paedomorphosis is obligatory for females—all the females do it. But in some cases, paedomorphosis is ‘facultative’, meaning that it happens or doesn’t happen, according to circumstances. For example, the larvae of many gall-midges feed on fungal mycelia (the fine filaments that comprise the main body of fungi), and the life history of females depends on what the larvae feed on: a diet of older mycelia induces complete metamorphosis and leads to sexual reproduction but, on a diet of young mycelia, female larvae do not metamorphose, and these paedomorphic females mature very quickly, producing live, all-female offspring from unfertilized eggs. Why diet should trigger such a difference in life history is unclear.
Not to be outdone, various other invertebrates exhibit paedomorphosis too. For example, there’s a sand crab in which tiny larval males attach themselves to the legs of adult females and produce sperm (later, these “youthful” hitchhikers transform into females). Males are paedomorphic in some parasitic isopods, too. And there’s a whole group of tunicates known as larvaceans, which closely resemble the larvae of other tunicates but are sexually mature in that seemingly larval form.
Perhaps the prize-winner among the “fountain-of-youth” invertebrates is the so-called immortal jellyfish named Turritopsis dohrnii. As in typical jellyfish, the adult form is a tiny, free-swimming medusa, a bell-shaped body with dangling tentacles. From a fertilized egg, a swimming larva (called a planula) emerges, settles down on a rock or a shell, and begins its development as a polyp. When a polyp matures, it buds off new medusae. But here is the possibly unique and fascinating thing: if a one of those little medusae is injured, starved, or stressed by abnormal temperatures or salinity, it can revert to the polyp stage. The bell and tentacles deteriorate and the individual again settles onto a substrate, back-transformed into a polyp, which grows up and produces more medusae.* The cycle of medusa to polyp to medusa can be repeated many times, leading to the idea that this jellyfish is potentially immortal. The process has only been observed in laboratory conditions and not in the wild, which is not surprising, given that the jellyfish is so tiny and the transformations can happen very quickly–and furthermore, many of the medusae are probably eaten by predators.
Just as paedomorphosis does not necessarily engender longevity, extended longevity does not necessarily involve paedomorphosis. Individuals of many organisms, both animals and plants send their own unique genetic makeup into the future by cloning—and thus, in a sense, extend their own lives as individuals without showing signs of youthfulness. Some, such as aphids, produce offspring asexually, and the offspring are genetically identical to the mother. Some plants propagate by means of runners, stolons, and rhizomes (e.g., strawberries and aspens), creating genetically identical ‘satellite’ individuals. Still others, such as some lichens, can produces copies of themselves by fragmentation.
(*Somehow, the cells of this organism have undergone ‘transdifferentiation’, by which mature cells have become reprogrammed to another kind of cell altogether, an extraordinary feat that is not well understood.)