The Immortal Jellyfish

The idea of ageing backwards might seem like the kind of far-fetched tale that only the likes of Brad Pitt could sell, but for some of our distant, under-water cousins, it’s just a really bad day. A few unusual jellyfish species have evolved the ability to completely rearrange the cells in their bodies and return to an earlier life stage. They use this curious talent to deal with times of stress; if adult jellyfish (medusa) find themselves in unfavourable conditions, they can simple revert to their juvenile form and wait it out until things improve. Scientists believe this may enable them to endure poor environments and ultimately spread across the globe into regions other jellyfish cannot reach.

Jellyfish are members of the quite bizarre group of animals called Cnidarians, primarily known for their stinging ‘Cnidocyte’ cells, which no other animals posses. Included in this group are jellyfish, corals and sea anemones. However, alongside their sting, Cnidarians are also the only group of organisms that can undergo reverse development. Several species have developed this talent, however only members of one genus (Turritopsis) are able to perform reverse development at any stage of their life. Starvation and physical damage are common triggers for this process, signalling to the jellyfish that conditions are not suitable for sexual reproduction and that it might be best just to sit this one out in an asexual form.

All jellyfish undergo the pretty remarkable process of metamorphosis, beginning life as groups of larval hydroids, which eventually release medusas for sexual reproduction. Development from an asexual polyp (larva stage) to a sexual medusa involves major cellular rearrangements and the formation of a third layer of cells called the entocodon. This is analogous to metamorphosis in insects, in which a larval form completely breaks down its cells and reforms as an adult. During this process differentiated cells revert to pluripotent cells (similar to stem cells), which can then develop into different cell types. Other cells die and their constituent parts may be reassembled into new cells. To us mammals, this process is dramatic and extreme. But it’s run-of-the-mill for many invertebrates. What’s not run-of-the-mill is the ability to perform this process in reverse.

Some cell types, such as the ectocodon, are lost entirely. This step may be the most difficult to achieve, as in several species reverse development is only possible prior to the appearance of this extra cell layer marking sexual maturity. Instead of releasing sexual gametes into the environment for fertilisation, the medusa becomes a cyst and eventually develops back into an asexual polyp form that releases thousands of new medusa into the environment. These medusas are clonal copies of the original adult, hoping to find more favourable conditions when they mature.

Recent research suggests that this unique capability may enable members of the Turritopsisgenus to hitch-a-ride on long-distance cargo ships, in which they can survive by reverting to their polyp stage. Genetics reveal that widely distributed members of one species found off the coasts of Japan, Panama and Spain are all very closely related, indicating rapid global expansion, a hallmark of an invasive species. Whilst the global invasion of Turritopsis might largely go unnoticed, the genus is attracting attention in the field of regenerative medicine. Understanding the process of cell transdifferentiation in jellyfish could ultimately aid the development of treatments for heart disease and even cancer.

Want to Know More?

• De Vito et al(2006) Evidence of reverse development in Leptomedusae Cnidaria, Hydrozoa): the case of Laodicea undulata Marine Biology 149: 339 – 346
• Piraino et al(2004) Reverse Development in Cnidaria. Canadian Journal of Zoology 82: 1748 – 1754
• Leda et al (2010) Direct reprogramming of fibroblasts into functional cardiomyocytes by defined factors Cell 142: 375 – 86