LIFE CYCLE AND REPRODUCTION.


The normal life cycle of hydrozoans involves the asexual budding of medusae from colonial polyps. That is, beginning with fertilised eggs, this leads to the formation of planula, to larval hydroids and then the transformation into their adult meduase form; and then the cycle repeats. However, Turritopsis are different. When they are damaged or starving, they can interrupt their normal hydrozoan life cycle, by reverting to their polyp stage via a process called "transdifferentiation." (5).


REPRODUCTION

As there hasn't been an extensive amount of research on this species of jellyfish, it's difficult to know for sure how Turritopsis dohrnii reproduces. However, Turritopsis nutricula, which is a part of the same class (Hydrozoa) also has the ability to revert, so it is possible that their reproductive system is similar (5).

Turritopsis nutricula develop fertilised eggs within their stomach, as well as in the planula. The eggs are then implanted on the seabed. After two days, the jellyfish hatches. They normally become sexually mature at around a few weeks old. This time frame can vary depending on the temperature. Turritopsis nutricula can asexually reproduce during their polyp stage (6).

However, there is also debate within the scientific community on whether nutricula and dohrnii are the same species or not. It's been suggested by many that they are different due to the fact that nutricula has been found in the Caribbean, whereas dohrnii is from the Mediterranean. But they've also been known to travel vast distances, hence the confusion and uncertainty of this claim. (7)


TRANSDIFFERENTIATION
As fascinating as this creature is, its ability to revert back to its infant self is still not fully understood. The reason for this is because the jellyfish has only been observed under a microscope a few times. 

According to nature.com, transdifferentiation is "the conversion of a cell type present in one tissue or organ into a cell type from another tissue or organ without going through a pluripotent cell state" (8).

In terms of this jellyfish, and other jellyfish species which may undergo a similar process, multiple cells are likely to undergo change. For example, muscle cells can transform into egg or sperm cells. 
However, this process can only take place once the jellyfish has reached adulthood. Many polyps do not make it to adulthood due to predators, illness or starvation. (9). This is another reason why this process hasn't been observed many times.

SOURCE: How It Works from https://www.howitworksdaily.com/cows-are-magnetic-and-4-other-weird-science-facts/

As seen in the diagram above, if an adult is suffering from illness, physical damage or starvation, the Turritopsis dohrnii can revert back to its polyp stage and its life continues as normal. It can make this transformation many times once it reaches adulthood.

This means that Turritopsis dohrnii is not really immortal by definition. However, it does have an extraordinary ability to revert its cells back to their infancy stage as many times as they care for/need to which is extremely unusual and quite impressive.



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