Sea Walnut (Mnemiopsis leidyi)

A photo of a sea walnut, a ctenophore.
(Marco Faasse, World Register of Marine Species)

This ctenophore (a stingless jellyfish-like animal) is native to the east coast of North and South America. In 1982, it was discovered in the Black Sea, where it was transported by ballast water. It subsequently spread to the Caspian Sea. In both places it multiplied and formed immense populations. The sea walnuts contributed to the collapse of local fisheries because they feed on zooplankton that the commercial fish also consume. Mnemiopsis leidy has also been discovered in the Mediterranean, Baltic, and North Seas. Read "No Passport Required: Five Invaders You Should Know" to learn about other marine invasive species.