Hydras are tentacled, freshwater animals that vaguely resemble tiny sea anemones. They're rather unremarkable except for one trait: if they're torn into tiny pieces, they can reassemble themselves.
At the Conversation blog, Rebecca Helm, who studies jellyfish and other sea life at Brown University, has instantly won my admiration by describing the Hydra as the "Dr. Manhattan of the animal kingdom." Just as Watchmen physicist Jon Osterman's consciousness was able to pull his body back together, atom by atom, after he was blown away in a lab accident, so too can the Hydra reconstitute itself after being ripped apart:
To see their true power, all you have to do is blend a Hydra to hamburger meat and swirl the puree to the bottom of a bowl.
Slowly, the disembodied pieces will begin crawling together, rising like tiny volcanoes from the sea of shredded remains. Forms will begin to take shape. Mouths and skinny tentacles stretching out into the water, and suddenly little bodies everywhere have regrown.
Helm points us to a recent paper published by scientists at the University of California at Berkeley, which explains this extraordinary ability. Hydras don't have brains, but their heads contain specialized "command cells" that continuously send signals to other cells, instructing them what to do.
So they key to a Hydra's survival is keeping at least part of its head intact. After it has been torn apart (or pureed) it only needs between 5 and 20 command cells to issue orders to the rest of the cells to come back together, forming an amorphous mound that begins reassuming its original shape in less than 96 hours—eventually growing into a new body. And, since the Hydra has more than 20 command cells in its head, it can potentially grow multiple bodies.
Most likely, this ability allows the Hydra to survive predator attacks. If even a few pieces of it remain, it can live again.