*This author wrote the paper as a part of FIYS121: Making Sense of Aging under the direction of Dr. Smith. 

T. nutricula is unique because of its ability to revert to an earlier stage in its life cycle by reactivating specific genes that were only used for that stage. This reverse metamorphosis is produced by a variety of cell processes such as transdifferntiation and apoptosis(Piraino et al., 2004). Transdifferentiation is when an already mature, specialized cell redifferentiates into another type of differentiated cell without going through an intermediate stage (Piraino et al., 2004). Ontogeny reversal has been observed in multiple Cnidarians, but this is always before maturation and sexual reproduction can occur (Piraino et al., 2004). T. nutricula is the only organism that is able to perform ontogeny reversal even after sexual reproduction occurs. In addition, T. nutricula exhibits the rare ability to have potential immortality in its solitary state, which is a medusa. Many colonial organisms have a potentially unlimited lifespan because even though individual organisms within the colony die, the entire group as a whole continues on. All the stages of a medusa have the ability to transform back into a stolon or polyp, allowing it to have a potentially unlimited lifespan Therefore, transdifferentiation plays a key role in the transformation process.

Selective excision experiments were conducted by removing specific tissues from medusae to see how it would affect medusae transformation. Results showed that in order for medusae to revert to the stolon or polyp form, differentiated cells of the epidermis and the gastrovascular cavity were required(Piraino et al., 2004). Interestingly, medusae have special muscle cells, nerve rings, and sensory organs that are not present in polyps. These anatomical differences between the forms of T. nutricula demonstrate that tissue substitution, regeneration, and reorganization must occur, and epidermal and cells of the gastrovascular cavity play a crucial role in this process. This is done through the process of transdifferentiation, where previously differentiated cells return to an undifferentiated state or spontaneously change their specialization through gene expression alterations(Piraino et al., 2004). Although stem cells most likely are involved in the process, it was initially thought that stem cells were responsible for ontogeny reversal. However, other hydrozoans that contain large amounts of replicating stem cells cannot revert back to their polyp stage(Piraino et al., 2004). Based on the data, it appears that transdifferentiaton is the main process in which ontogeny reversal is accomplished, not stem cells.

Another experiment was conducted where groups of medusae in various stages of their development were exposed to a variety of stressors to trigger ontogeny reversal (See Figure 2). The factors were starvation, reduction in the salinity of the environment, change in the temperature of water, or mechanical damage to the bell structure of the medusae. All immature medusae, when subject to these different variables, regressed to the polyp or stolon stage after going into a cyst-like stage. The control group that was exposed to no abnormal conditions did not transform. When medusae were exposed to all of the stressors during sexual maturation, 20-40% of medusae reverted into the stolon or polyp stage without entering a cyst-like stage that immature medusae reverted to(Piraino et al., 2004). This most likely means medusae that spontaneously transformed into a polyp stage were already sexually mature because all of the mature medusae skipped the cyst stage as well(Piraino et al., 2004). Reverse metamorphosis is a defense mechanism used when the organism is not in an optimal environment. By reverting to an earlier state, T. nutricula is able to wait longer to reproduce in expectation that the environment will be better suited for reproduction. Although it is not completely understood how T. nutricula reacts in this manner, studies have revealed the conditions that trigger physical changes in the hydrozoan.