By Renato Nagata
English edit Carla Elliff
In the summer of 1988, Christian Sommer, a German marine biology student in his twenties, and his girlfriend were on holiday in the small town of Rapallo, in the Italian Riviera. While on vacation, Christian was also conducting some studies involving small jellyfish (or medusas). He would dive in the turquoise blue waters of the sea, between the Portofino cliffs, in search of these creatures, rummaging on the seafloor and using very fine nets to collect medusas that are almost imperceptible while diving.
Among the different species he came across, Christian found a tiny jellyfish species called Turritopsis dohrnii, which is purple in color, has 15 to 30 tentacles and measures only a few millimeters in size. After a few days of observation, Christian forgot about his small medusas, but instead of them dying, they shriveled up and transformed into a younger stage. Investigating this phenomenon more closely, Christian and other Italian scientists in Genova realized that, unlike other jellyfish that normally grow and develop into adults, Turritopsis seemed to develop like the character in the “Curious Case of Benjamin Button”, a story portrayed in a movie in 2008, based on a novel by F. Scott Fitzgerald in 1921, in which this character grew younger as time went by.
Some years later, Christian and these other scientists published a study demonstrating the ability of this species to develop in two directions, either growing and becoming older, or regressing to younger stages, the opposite of what would be the usual direction of development. This regression to younger stages can be followed by normal growth again, which would effectively make Turritopsis dohrnii a potentially immortal species.
As with any other animal, most of the 1,000 known species of jellyfish go through life following the natural path. They grow swimming freely following the sea currents, they feed off other planktonic animals (read more here), they develop over a series of stages until they become sexually mature adults. They can then reproduce (by the way, they have very similar eggs and sperm to ours!) and, after that, they die and disintegrate. This reproduction generates a small larva called planula, which goes down to the bottom of the ocean and transforms into a tube-shaped creature with tentacles around its mouth called polyp. Polyps grow stuck to the seafloor and can divide themselves to form large colonies with many clone polyps, which by definition have identical DNA. After a while, individual polyps (or colonies of polyps, depending on the species) produce new medusas that will grow, thus closing the “life cycle” of these species.
The problem is that among jellyfish the exception seems to be the rule, and the diversity of lifecycle patterns is the greatest in all the animal kingdom. One of these alternative routes is the ability to develop backwards, called “reverse development” or “ontogeny reversal”, which is known to occur in very few species of jellyfish (but may possibly be more common than originally thought). The medusa, usually still young stops eating, its tentacles retract, and it goes down to the seafloor, where in a few hours it becomes a small mass of tissue. This mass of tissue can “rest” for a long time and later transform back into a polyp, resuming the usual development cycle. The big difference with Turritopsis dorhnii, however, is its ability to make this reversion even after becoming an adult and doing so during any stage of its life, as pointed out by another Italian researcher who has studied this phenomena since the 1990s. Reversing after full adult development would be like an adult butterfly instead of dying, simply being able to transform back into a caterpillar.
But what is the secret to the potential immortality of this species? In the case of Turritopsis, scientists have observed that various stressful conditions, like lack of food, very low temperatures, or even the mechanical destruction of the medusa, can trigger reverse development. These alterations activate certain gene groups that cause changes to the organization of cells.
All animals have the famous stem cells (or pluripotent cells), which differentiate into several other specialized cells such as muscle cells, neurons, and other tissues. However, Turritopsis dohrnii has cells that can do the inverse path, meaning that already specialized cells can return to a non-differentiated state (of a stem cell) or even directly transform into other types of cells. And that is the secret of Turritopsis! This cell transforming ability is called “cell transdifferentiation” and allows this medusa to reorganize the specialized cells in its body, losing “unnecessary” cells and reversing growth. If a single medusa could do this reversion indefinitely, it would potentially make it immortal. However, no medusa has been followed-up for very long and we still do not have the means to measure the age of jellyfish in the wild to know how many times it is possible for them to do this reversion. Only the Japanese scientist Shin Kubota, from the University of Kyoto, was able to monitor Turritopsis dorhnii for 2 years, and observed this growth and reversion cycle happening 10 times in a row.
From right to left: Hydractinia carnea, Nausithoe aurea and Laodicea undulata. These three jellyfish species also have reserve development!
Immortality can in fact be undesirable, like in Greek mythology when gods curse mortals that have tricked them with eternity. Or in the novel “Death with Interruptions” written by José Saramago in 2005, in which Death puts its job on hold, condemning humans to live forever. If this jellyfish was indeed immortal in nature, the ocean would soon be full of this species, which would cause mayhem. However, fortunately, ontogeny reversal does not stop the destiny of these medusas of becoming food for fish, sea turtles, or other marine animals. Moreover, as any other living being, Turritopsis is not immune to diseases, such that there are ways to interrupt the life of this species.
Reverse development is a survival strategy: if the species finds an unfavorable environment, internal mechanisms make it go back to a previous stage. Meaning, if things go south, the medusa interrupts its growth, goes back to the bottom of the sea and starts all over. Other species of medusa and even polyps can also transform into masses of tissue protected by a sort of capsule and stay in this form for years, waiting for conditions to improve for their development. Even so, the cell transdifferentiation capacity of Turritopsis dorhnii can represent a big step in the comprehension of many other cell aging mechanisms, regeneration of tissues and even rejuvenation in the animal kingdom. Who knows, one day it might reveal some secrets about immortality (see Youtube link – Google Zeitgeist: Dr. Shin Kubota and the immortal jellyfish).
Suggested reading:
Bavestrello, G., Sommer, C., and Sarà, M. (1992) Bi-directional conversion in Turritopsis nutricula (Hydrozoa). In Aspects of hydrozoan biology. Edited by J. Bouillon, F. Boero, F. Cicogna, J.M. Gili, and R.G. Hughes. Scientia Marina 56: 137–140.
Kubota S. (2011). Repeating rejuvenation in Turritopsis, an immortal hydrozoan (Cnidaria, Hydrozoa). Biogeography 13: 101–103.
Piraino, S., Boero, F., Aeschbach, B., and Schmid, V. (1996) Reversing the life cycle: medusae trasforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). Biological Bulletin 190: 302–312.
Piraino S, De Vito D, Schmich J, Bouillon J, Boero F (2004) Reverse development in Cnidaria. Canadian Journal of Zoology 82:1748–1754
Rich, N. Can a Jellyfish Unlock the Secret of Immortality? The Ney York Times, 28 Nov 2012. http://www.nytimes.com/2012/12/02/magazine/can-a-jellyfish-unlock-the-secret-of-immortality.html?_r=0 accessed 30 May 2015.
About the author:
As many other people, I have always been attracted to the sea, a strange and unfamiliar environment for someone born and raised in the interior of the state of Paraná, Brazil. While I was studying for my undergraduate biology degree at UFPR, I had the opportunity to do an internship on jellyfish. I could not have imagined the vast universe of so many possibilities that I was diving into. Not only are jellyfish beautiful, I came to notice that they were abundant in beaches around the world. This triggered many questions in my mind: when do they occur? What are their effects on man and other components of the ecosystem? I decided to become a marine biologist specialized in jellyfish. I did my master’s degree (UFPR) and doctorate (USP) in zoology and I am currently a professor at the Oceanography Institute of the Federal University of Rio Grande (FURG). I am interested in questions that can only be investigated when we look at live and pulsating animals: How is the growth and development of these animals? How do different species carry out basic functions like swimming and getting food? In addition to my research, I also feel the need to share a bit of what I do. This is why I immediately accepted the call for a chat with Neptune, to tell some tales and bring some curious facts about these fascinating ocean-dwellers!