To date, there is only one species that has been called "biologically immortal": the jellyfish Turritopsis dohrnii. These small, transparent animals hang out in oceans around the world and can turn back time by reverting to an earlier stage of their life cycle. However, they can still be consumed by predators or killed by other means. Other animals that offer tantalizing glimpses into an ageless existence include:

• Hydra: They don’t go through senescence at all. Instead of gradually deteriorating over time, a Hydra’s stem cells have the capacity for infinite self-renewal. This seems to be thanks to a particular set of genes called FoxO genes, which are found in animals from worms to humans and play a role in regulating how long cells will live for.

• Bdelloids: They are a group of freshwater rotifers (zooplankton) that can be considered “immortal” in the sense that they can tolerate extreme conditions like high heat, low temperature, and extreme pressure. Furthermore, they can also enter a state of “hibernation” and come back when they wish to do so.

• Bowhead Whales: They are not technically immortal animals, but they are the oldest living mammals. Several species of whales have been known to live for more than 70 years, though in the 1990s scientists discovered scars on the massive animals that had been made by weapons from the 1800s. The oldest known bowhead lived to be 211 years old.

• Deinococcus radiodurans: This is a poly-extremophilic bacterium that isn’t only radiation-resistant. These immortal animals can also die and come back to life thanks to their incredible DNA repair response.

• Tardigrades: Anything that tries to kill a tardigrade will quickly realize they encountered practically immortal animals. These tiny creatures can survive extreme conditions like high heat, cold, and radiation.

It is important to note that while some animals can stave off death for thousands of times longer than they would have existed "naturally," no animal is truly immortal.