Cavalcade of Mammals

Turritopsis nutricula is jellyfish that can develop back into a polyp from the jellyfish form, with cells differentiating from adult cells types to polyp types. This is quite unusual, only known to occur in a few jellyfish species.

This reversion to an immature form may allow it to become effectively immortal, renewing itself by passing through the polyp stage again. The ‘effectively immortal’ aspect hasn’t been examined–no one has followed one of these jellyfish through multiple jellyfish-polyp-jellyfish cycles or observed cycling jellyfish living longer than they otherwise would. That is, the reversion to the polyp state may have this effect but it hasn’t been demonstrated yet.

Articles describing this are getting linked around, but the research was first described in 1996. And this phenomenon has now been described in a couple jellyfish, and is not unique as the articles say. This article has lots of good pictures. This link to Development Biology, 8th ed. by Scott F. Gilbert has good background info.

Scientific articles:

• Piraino et al. (1996) Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa).
  –The original article.
• Carla et al.., (2003). Morphological and ultrastructural analysis of Turritopsis nutricula during life cycle reversal.
  –Nice photos.
• Schmich et al., (2007). Induction of reverse development in two marine Hydrozoans.
  –Two other jellyfish that undergo reverse development. With photos.