In the clear turquoise waters of the Mediterranean, there pulsates a transparent creature with a glowing red core. The ruby red is reminiscent of the legendary Philosopher’s Stone, the source of the elixir of life. This only comes to mind because this particular jellyfish, Turritopsis dohrnii (formerly T. nutricula) has the ability to shrivel into a ball when it is injured, and then revert to its juvenile form, maturing into an adult all over again. This means it is effectively biologically immortal at only the size of an adult fingernail!
How does this unassuming creature pull this off?
First, a bit more on jellyfish. Jellyfish belong to the phylum Cnidaria (the same as corals). The phylum is named because of the ubiquitous presence of a cell called a cnidocyte. which produce nematocysts. This explosive organelle (organ of a cell) contains a spring-loaded harpoon that embeds its victim with poison when disturbed; incidentally, these are the what's responsible for jellyfish stings. The animals in this phylum go through a sessile phase called a polyp phase, where they are attached to a substrate, and a motile phase as a medusa, where they are free-floating. (Corals are predominantly sessile organisms, although they start off as free-floating larvae, which eventually settle and form the gorgeous reefs we are all familiar with, whereas all jellyfish have a medusa stage. For this reason, they are grouped into the subphylum Medusozoa, which is where our little friend belongs.)
An animal with a bell shaped body, a fringe of tentacles, and oral arms around its mouth that create the characteristic ribbons one imagines, is a true jellyfish. It is a simple animal, with two cell layers, the outer layer containing a nerve net, and the inner layer containing its digestive system, bound together by the mesoglea (similar to the mesophyll in sponges I discussed in a previous post). It cannot control its motion, and generally drifts around on ocean currents.
However, T. dohrnii is not a true jellyfish.
It is a hydrozoan. Another highly recognizable member of this class is the Portuguese Man o’ War, which you may recognize from this FTDM post. Hydrozoans differ from jellyfish in that they can control their movements. Cells are generally absent in their mesoglea, but a circular membrane called the velum under their umbrella helps propel them through the ocean. As they are generally much smaller than true jellyfish, they are not always visible to the naked eye.
Their basic life cycle is very similar to other jellyfish. Adult T. dohrnii females spawn mature eggs into the water, where they are fertilized by sperm released by male medusae. The infant larvae produced by this are called planula, which is a term used to refer to the larval form of most cnidarian species. Medusazoan planula are ciliated, bilaterally symmetric, and unable to feed. As this planula settles down, it forms a colony which kind of looks like a tree (or coral branch). This structure, known as a hydroid, buds off tiny medusae called ephyrae, which resemble their adult forms, but are only 1 mm in diameter. They only have eight, evenly spaced out tentacles, and all of these jellies are genetically identically to the hydroid. These jellies mature and reproduce, with adult medusae possessing upwards of 80 tentacles, and this life cycle continues.
Unless the jellyfish is exposed to stress, or falls sick, or gets attacked.
Then things get really cool: as long as it retains some of its specialized cells from its epidermis (outer layer of cells) and part of its gastrovascular system (the inner layer of cells) it undergoes a process known as renovation and reverts to polyp, or hydroid, form. This hydroid then continues to produce genetically identical jellyfish, which can also revert to infant form when they need to, although they still often succumb to predation in the wild. Renovation is achieved by transdifferentiation of the cells.
What is transdifferentiation?
Each biological cell in our body is known as a somatic cell (as opposed to our reproductive cells, called gametes). As a cell develops, it specializes: it can become part of your eye, or perhaps your brain. In most animals, this specialization is permanent. In T. dohrnii, somatic cells can change from one type of cell to another. When the jellyfish reverts into a blob-like cyst form, its cells dedifferentiate and then redevelop. Nerve cells can become sperm or eggs, muscle cells can become cnidocytes. So, although the jellyfish is genetically the same, it is not the same jelly that was injured. This is why the distinction of biologically immortal is important: although they may be able to live forever, each jellyfish is not the same jellyfish that shriveled up and formed a new colony.
Although human beings have been fascinated by immortality since time immemorial, questing for the ‘Elixir of Life’, our definition of immortality, as humans, is a little bit more complicated than that: we would like to retain our personalities, and our memories. This hasn’t stopped scientists from trying to find the fountain of youth in the genetic code of a marine invertebrate. Specifically, one eccentric Japanese scientist believes that:
Although his claims seem a little fantastic, he is the only scientist to successfully culture T. dohrnii in a laboratory, reporting that his colony re-birthed itself ten times in two years. He is completely dedicated to this animal, to the point where it spills over into his personal life: after he is done with his research for the day, he performs karaoke songs about jellies that he has written himself, while wearing a jellyfish hat.
Although one would like to imagine that a tentacled researcher from Japan can singlehandedly halt aging, the reality of the research surrounding this fascinating creature is very different. Their remarkable age reversal hints at very effective cellular repair mechanisms, and although they may not hold the key to eternal youth, they might be able to help with something else entirely. According to a biologist at the University of Salento in Italy,
This is very similar to a disease prevalent in our species: cancer. By studying this mechanism, we may be able to elucidate how the disease spreads, and maybe even figure out a better cure for it!
Annam Raza is a recent graduate of University of California, San Diego with a BSc in Environmental Systems- Ecology, Behavior and Evolution, with a focus on marine science.