The idea of immortality appeared, probably for the first time, thousands of years ago in the Gilgamesh saga, and it is then found across the Egyptian, Greek and Latin tradition. It went through the entire Middle Ages till present days, with the first Harry Potter’s novel centred around the philosopher’s stone, and the chance of using it to produce an eternal life elixir. Whether it depended on a stone, a drink or a magic stone, immortality was still a component of myths and tales.
And there it remained until 1992, when the first example of life-cycle reversal was discovered in the jellyfish Turritopsis nutricula McCrady (1859). The news, later confirmed and further investigated by other scientists, may not seem particularly relevant. To better understand it, and to understand how this jellyfish can be considered immortal, first we must briefly describe the life cycle of this organism
The jellyfish T. nutricula is the first discovered animal that potentially, is able to avoid death and live forever.
Jellyfish belong to the phylum Cnidaria. T. nutricula belongs to a group of cnidarians called Hydrozoa, characterized by small jellyfish, different from those we normally observe on a day at the seaside (those belong to another group, Scifozoa). The jellyfish, however, is only one side of the coin. Cnidarians are in fact characterized (in most cases) by a digenetic life cycle, where two distinct generations can be distinguished: the jellyfish and the polyp (not to be confused with the polyp, the mollusc with eight tentacular arms equipped with suckers).
The life cycle begins with a larva, called planula, hatching from a fertilized egg. The planula, after a short free-living pelagic phase, settles on the seabed and originates a polyp. The polyp generates other polyps via asexual reproduction (more or less like a plant that generates new individuals through stolons) in many cases, polyps can stay connected, forming a colony. At some point the polyp, instead of giving birth to other polyps, can generate a jellyfish. The jellyfish, one reached the maturity, will reproduce sexually, releasing eggs and sperm into the water, thus closing the cycle.
But how do polyp and jellyfish differ? A jellyfish is, essentially, an upside-down polyp that, instead of sticking to the seabed, floats free in the water column, carried around by the currents. Both have a more or less sack-like body, with a single opening, functioning both as mouth and anus. The mouth/anus is surrounded by the (unfortunately) well-known stinging tentacles. There are, of course, other, more subtle differences between the polyp and the jellyfish, linked to the morphology of the body and to the type of cells present, especially on the body surface.
Jellyfish are generally short-living animals and, after spawning, tissue degeneration and death come quickly. T. nutricula is not an exception, but the peculiarity of this species lies in the ability, once sexual maturity is reached, to reverse the normal biological cycle and go back to the polyp stage. Jellyfish of other species (e.g. those of the genus Cladonema) can also return to the polyp stage, but only before reaching sexual maturity. The case of T. nutricula can be considered a real backward metamorphosis: we can compare it to a butterfly that, once out of the cocoon, could go back in and turn itself into a caterpillar again.
The transformation from polyp to jellyfish takes place through the replacement of certain cell types, the re-generation of tissues and the re-organization of the body plan. The mechanisms by which this happens have not yet been fully understood. To make it simple, it can be said that this is due to the transformation of differentiated cells present in the jellyfish tissues into other cell types (those typical of the polyp).
During the transition from jellyfish to polyp, following tissue degeneration and cell transformation, the jellyfish temporarily achieves a planula-like body organization (like the newly hatched larva, which then turns into a polyp). But assuming that the jellyfish does not aim at immortality on purpose, such as Gilgamesh or Lord Voldemort, it remains to understand what drives such transformation. Reversing the biological cycle could be a response to environmental stressors (such as changes in the chemical-physical parameters of water, or food scarcity), which would push the jellyfish back to the polyp stage waiting for better conditions, favourable to reproduction. It can therefore be said that T. nutricula is the first discovered animal that, potentially, is able to avoid death and live forever.
Carlà E.C., Pagliara P., Piraino S., Boero F. & Dini L. (2003) Morphological and ultrastructural analysis of Turritopsis nutricula during life cycle reversal. Tissue and Cell 35: 213-222.
Bavestrello G., Sommer C. & Sarà G. (1992) Bi-directional conversion in Turritopsis nutricula (Hydrozoa). Scientia Marina 56: 137-140.
Piraino S., Boero F., Aeschbach B. & Schmid V. (1996) Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). The Biological Bulletin 190: 302-312.