A June 2008 reports claims that the species Turritopsis dohrnii is silently invading the world's oceans with the help of a unique reproduction mechanism. Under normal circumstances, it breeds the old-fashioned way via sperm/egg union. When threatened by starvation or injury, however, it reverts to a younger stage of development, completely transforming itself in the process, and allowing it to spawn hundreds of identical copies of itself. Though the "immortal" jellyfish was first discovered in the Mediterranean in 1883, it's unique reproductive ability was not discovered until the 1990's. A fully-grown adult is roughly the size of a human's pinky fingernail.
http://news.nationalgeographic.com/news/2009/01/090130-immortal-jellyfish-swarm.html
