Zebra mussels in the Great Lakes. Mitten crabs in San Francisco Bay. Transplanted species have permanently altered their new habitats. Now a hardy, unwelcome seaweed threatens California waters.

An aquarium variety of Caulerpa taxifolia, infamous for smothering nearly 15,000 acres of sea grasses in the Mediterranean, was recently discovered in Agua Hedionda Lagoon, just north of San Diego. It can tolerate cold and nutrient-poor water, and its ability to grow in sand or mud and on rocks and sea grasses makes it an exceptional invader, able to virtually carpet the seafloor to a depth of over 100 feet. It also secretes toxins poisonous to fishes and invertebrates that deter these potential herbivores.

Containing and eradicating the weed is challenging, but possible. Unlike most algae, which release gametes dispersed by ocean currents, this strain spreads through vegetative reproduction-fragments that break off from the alga regrow into new individuals.

Biologists from Merkel & Associates who found the weed in the lagoon are applying chlorine to the plant to kill it, and an extensive monitoring system is in place to devise further eradication strategies.