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Original Papers: Ma, X. and J. E. Purcell. 2005. Temperature, salinity, and prey effects on polyp versus medusa bud production by the invasive hydrozoan Moerisia lyonsi. Marine Biology 147: 225−234.

Schmich, J., Y. Kraus, D. De Vito, D. Graziussi, F. Boero and S. Piraino. 2007. Induction of reverse development in two marine Hydrozoans. International Journal of Developmental Biology 51: 45−56.

In many ways, Turritopsis dohrnii is similar to many other marine cnidarians: It’s a small (5 mm in diameter) hydrozoan that drifts freely around warm ocean waters. It begins its life as a motile planula larva and grows into a sessile polyp when it finds a suitable substrate. Once it reaches adulthood, it develops into a motile, bell-shaped medusa capable of sexual reproduction. However, adults of T. dohrnii are capable of undergoing something strange: they can revert back to a younger version of themselves, rejuvenating and becoming “immortal jellies” (Figure A).

Individuals of T. dohrnii in the medusa stage undergo this reversal by reactivating genetic instructions from earlier in their life cycles to develop polyp cells. These polyp cells develop into sessile, asexual polyps, which then produce new motile, sexual medusae. Scientists curious about what triggers this unusual phenomenon hypothesized that some cnidarians may “throw their life cycle in reverse” if starving, injured, or facing stressors in their environment.

To test their hypothesis, scientists simulated unfavorable environmental conditions by incubating animals for 3 hours in different concentrations of cesium chloride (CsCl), which causes chemical disruptions in the function of various essential medusa cell types. Each treatment group included 30–50 specimens. All developmental assays, which categorize the life stages of specimens (medusa, reducing medusa, polyp), were performed multiple times to confirm the results. (A “reducing medusa” is a medusa that has started to revert back into a polyp.) Each treatment group showed a significant increase in the possibility of reverse development, compared with the control group (Figure B). Bars show 95% confidence intervals.

Questions

Question 1

Based on these data, the chemical disturbance of CsCl induced reverse development as compared with the control population. Increasing concentrations of CsCl resulted in stronger reverse development, as evidenced by the higher percentages of animals in polyp and reducing medusa stages in response to increasing concentrations of CsCl. Reverse development allows for some cnidarians to survive periods that are not suitable for their survival and reproduction as medusae.

Question 2

The medusa form is motile and sexual, while the polyp form is sessile and asexual. Disadvantages associated with reverse development include decreased outbreeding opportunities, decreased dispersal, and investment in reverse development that could otherwise be devoted to maintenance or reproduction as a sexual medusa. These negative trade-offs are tolerated because reverse development allows survival in an environment that is not suitable for medusae.

Question 3

Yes, you would expect the two populations to look different. Polyps promote survival and maintenance of local populations because they can reproduce without a mate, but they limit the ability to disperse. Medusae may enhance dispersal probability and/or population expansion and increase genetic diversity, but medusae are more energetically expensive, and developing and maintaining reproductive structures can be costly. So when conditions are good (high food abundance), it is favorable to be a sexual, motile medusa, and when conditions are poor (low food abundance), it is favorable to be an asexual, sessile polyp.