When The Going Gets Tough, The Tough Become Babies

We celebrate the New Year as a time of rebirth, renewal, and do-overs. We join gyms, swear off our bad habits, and promise to be better people. This is especially true for those of us that have had a rough 2012… Our 2013-version-of-us has got to be better, right? But what if you could get a real do-over? What if you could be a kid again, grow up again, and become a brand new person? As far-fetched as it may sound, some animals do exactly that.

Cnidarians (the “C” is silent) are a huge group of aquatic animals that includes jellyfish, corals, and anemones (like the one Nemo lived in – Yeah, that tentacled home was a living animal). They are named after prickly plants known as nettles, or cnides in Greek, and if you touch one you will quickly know why. Cnidarians, armed with stinging cells called nematocysts, sting at the slightest touch.

Jellyfish make up many of the cnidarian species, and they have been found in every ocean and at every depth. Some even live in freshwater. The “typical” jellyfish life cycle starts when eggs and sperm are released into the water and find one another. When they do, they form larvae, which you can think of as baby jellyfish. The larvae sink and settle on a hard surface, where they mature into polyps. These polyps are jellyfish in a juvenile stage. The polyps elongate and begin to bud off adult medusa, which are the bell-shaped blobs with tentacles that most of us think of when we think of a jellyfish. Medusa mature to become reproductive adult jellyfish.

The jellyfish life cycle by Zina Deretsky at the National Science Foundation (NSF). Image available at Wikimedia.

Larval and polyp jellyfish are much more resistant to harsh conditions then are medusa jellyfish. When life gets hard for a jellyfish, perhaps because of starvation, physical damage, temperature changes or salinity changes, those that are in the larval or polyp stages can often shrink and rest in a hibernation-like state while they wait for more favorable conditions. But in some species, young adult medusa can even regress back to the juvenile polyp stage. By reverting back to a juvenile stage, they have more protection from the challenging world around them.

In most cases, this reversal to a juvenile state can only happen in young medusa that have not yet developed their gonads. Thus, the onset of sexual reproduction (puberty, if you will) might be regarded as the point of no return in development. However, one species, called the immortal jellyfish, has shown that this rule can be broken.

As an adult medusa, the immortal jellyfish is a pea-sized jellyfish with a round bell, bright red stomach and anywhere from 8 to 90 tentacles. It is currently the only known animal that can regress from a fully reproductively mature adult into a juvenile polyp. If exposed to dangerous conditions, immortal jellyfish medusae completely reduce all of their medusa-specific organs and tissues and develop new polyp-specific tissues, essentially becoming kids again!


This figure from the Piraino et al. 2004 paper at the Canadian Journal of Zoology shows the life stages of the immortal jellyfish. The adult medusa is in panel (a). Panels (b) and (c) show the medusa tranforming to a ball-like blob as it reverts to a juvenile stage. The green stain in these panels shows the cells initiating this transformation. Panel (d) shows the remnant of a medussa, and the black arrow shows the stalk that is common in the polyp stage. Panel (e) shows the resulting juvenile polyp.

But wait! It gets better! Theoretically, if an animal can revert to a juvenile stage at any point in its adult life, it could attain immortality. But if that were true, they would have the classic immortality problem: These animals would reach such high populations they would saturate the world’s oceans…And this may actually be happening.

Immortal jellyfish are thought to originally be from the Caribbean, but they have since been discovered worldwide and their populations seem to be growing. Likely, they are hitching rides in the ballast water that is sucked into cargo ships to provide stability. If this is true, the immortal jellyfish polyps could be attaching to the ships’ hulls and settling in for a long voyage to a new home.

We don’t yet know if the immortal jellyfish are actually immortal, but it is fun to consider that they might be (although they can still be killed by predators or viruses, so they’re not invincible). And we can take inspiration from them: When the going gets tough, try reverting to your more resilient juvenile self, but be thankful you don’t have to go through middle school again!

Happy New Year!

Want to know more? Check these out:

1. Piraino, S., De Vito, D., Schmich, J., Bouillon, J., & Boero, F. (2004). Reverse development in Cnidaria Canadian Journal of Zoology, 82 (11), 1748-1754 DOI: 10.1139/z04-174

2. Miglietta, M., & Lessios, H. (2008). A silent invasion Biological Invasions, 11 (4), 825-834 DOI: 10.1007/s10530-008-9296-0