Passage V

The Curious Case of Turritopsis Dohrnii

For decades, the diminutive jellyfish Turritopsis dohrnii—mere millimeters wide as an adult—did not unveil much notice from scientists. But in 1988, marine biology student Christian Sommer observed T. dohrnii doing something astonishing. Reverting from mature jellyfish to hydroid colonies, an earlier life stage. In a sense, they grew younger.

T. dohrnii has no brain or heart. After fertilization, a T. dohrnii egg develops into a free-swimming, ovoid larva. In time, the larva settles on the ocean floor and transforms into a mound of cells—a hydroid colony. Buds grow on the colony and develop into young jellyfish with the familiar bell-like shape and tentacles. These jellyfish then detach from the colony, and drift away reaching maturity in a few weeks.

After Sommer’s discovery, studies confirmed that T. dohrnii’s life cycle is not a one-way street. An adult T. dohrnii, if stressed—by injury, disease, or even just old age, has the ability to revert to a hydroid colony. That colony can then create new jellyfish, which in turn can also revert to hydroid colonies. There’s no apparent limit to these perpetual cycles of metamorphic transformation.

In the 1990s, journalists nicknamed T. dohrnii the “immortal jellyfish.” It is true that scientists have replicated all stages of T. dohrnii’s life cycle, but each individual T. dohrnii isn’t immortal. Although the cells of the adult are essentially recycled during its transformation, all the new jellyfish grow into separate organisms—albeit genetically equivalent ones. In a way, the new jellyfish are clones amidst the originating adult.

Scientists are still studying how T. dohrnii achieves its transformations. At the cellular level, what the jellyfish does, which is called transdifferentiation, wherein one type of cell becomes another type. (74) Understanding the mechanisms of T. dohrnii’s particular kind of transdifferentiation may yield insights into aging and disease in all animals, including humans.