Killer Algae (Caulerpa taxifolia)
A strain of this green seaweed, native to the Indian and Pacific Oceans, escaped public and private aquariums in California, Japan, Australia, and Monaco. It has spread widely in the Mediterranean, replacing native plants (such as seagrasses) and depriving marine life of food and habitat. In California, it was eradicated at considerable cost using toxic chemicals. Read No Passport Required: Five Invaders You Should Know to learn about other marine invasive species.