Turritopsis nutricula (Immortal jellyfish)
The Immortal jellyfish is one of the most unique animals not just within the species of jellyfish, but within the entire history of the animal kingdom. It has actually managed to accomplish the one feat that has been yearned by many and accomplished by none. It appears to have cheated death and hence transformed itself into a perpetually immortal being! Let me explain how this works.
Immortal jellyfish, like most other species of jellyfish are either male or female. They do not have a specialized reproduction system. The male releases his sperms into the column of water. They come in contact with eggs that are present in the stomach of the female jellyfish. During the embryonic stage, they are either settled onto the mouth or the oral arms of the female. After they have passed this stage, they transform into free swimming planulae and separate themselves from the body of their mother. They float along the surface of the water for a few days and then settle on a hard, stationary object like the surface of a rock.
At this stage, they become transformed into polyps. These polyps become stationary as well. They continue to feed on microscopic plankton and zooplankton at this stage. This polyp then begins to grow multiple identical polyps until it becomes a colony. This colony of multiple polyps is also stationary and attached to the hard surface at its base. All the polyps are connected with minute feeding tubes and they receive equal nourishment from their microscopic diet. The colony of polyp can remain in this stage for years at a time. When the condition is right, this colony of polyp begins to grow horizontal grooves. The groove at the top is the fastest to mature, and will soon free itself and become a free swimming jellyfish.
This process of reproduction is common to most species of jellyfish. What is unique about the immortal jellyfish is that after reproducing sexually, they are able to return to their polyp stage. When most fish die after their sexual maturity, immortal jellyfish get transformed back into a polyp and restart the process of asexual reproduction. In this way, the jellyfish is able to convert itself back into a polyp, start a polyp colony again and give birth to a number of new jellyfish.
How does the immortal jellyfish accomplish this feat? It is through cell development process of transdifferentiation. What this means is that it can alter the differentiated state of the cell and transform it into a new cell. In this process of transdifferentiation, the medusa of the immortal jellyfish is transformed into the polyps of a new polyp colony. First, the umbrella reverts itself and then the tentacles and mesoglea get resorbed. The reverted medusa then attaches itself to the substrate by the end that had been at the opposite end of the umbrella and starts giving rise to new polyps to form the new colony. Theoretically, this process can go on infinitely, effectively rendering the jellyfish immortal.