Most of the world's harbors have been invaded by non-native sea Squirts.

http://en.wikipedia.org/wiki/Ascidiacea

Over the last few hundred years, most of the world's harbors have been invaded by non-native sea squirts that have clung to ship hulls or to introduced organisms such as oysters and seaweed. Several factors, including quick attainment of sexual maturity, tolerance of a wide range of environments, and a lack of predators, allow sea squirt populations to grow rapidly.