Tardigrades, also known as water bears, are nearly indestructible. The microscopic animals can survive boiling water, extreme cold, and even a trip to space. Tardigrades can even be frozen for a year, or 10, and return to life when they thaw. But these tough little animals are still surprising scientists. When scientists sequenced the genome of water bears, they found that 17.5 percent of the animals' DNA came from other species. We had no idea that an animal genome could be composed of so much foreign DNA," study co-author Bob Goldstein said in a news release. "We knew many animals acquire foreign genes, but we had no idea that it happens to this degree." Tardigrades have some 6,000 foreign genes, the scientists report in a paper published Monday. Foreign DNA appears in an organism's genome through a process called horizontal gene transfer. In that process, species swap genetic material directly, instead of exclusively inheriting DNA from the organism's parents. Dr. Goldstein and colleagues ... think the tardigrade's defense mechanism for extreme circumstances actually opens the door for this foreign DNA. When the water bears are under extreme stress they curl up, expel their water and appear dead. The scientists think the animal's DNA splits into tiny pieces during this process. When the animal starts to come back to life by rehydrating, their cells become leaky and can absorb molecules around the animal. As the animal stitches its own DNA back together, the foreign pieces can get woven in too.