More than four years after the “killer seaweed” Caulerpa taxifolia was discovered in two lagoons in Southern California, biologists are cautiously claiming victory over the highly invasive seaweed, listed as one of the top 100 invasive species on earth. A hearty tropical seaweed, Caulerpa is a popular decorative aquarium plant that has smothered vast tracts of the Mediterranean Sea in a desert of green. It probably infested the Mediterranean when fragments escaped from an aquarium in Monaco. Caulerpa taxifolia grows as a dense smothering blanket, covering and killing all native aquatic vegetation in its path when introduced in a susceptible non-native marine habitat. Fish, invertebrates, marine mammals, and sea birds that are dependent on native marine vegetation are displaced or die off from the areas where they once thrived.

California waters were likely infested by someone releasing the contents of a home tank into a storm drain or waterway. Diver surveys have revealed no new strands of the plant in either the Agua Hedionda Lagoon in Carlsbad or Huntington Harbour in Orange County, the only two sites in North America where Caulerpa taxifolia has been found.

A California Sea Grant-funded study led by Susan Williams, director of the Bodega Marine Laboratory, found that concentrations of chlorine bleach, which plays a key role in eradication, should be maintained at 125 parts per million for at least 30 minutes in both the water column and sediments to kill the plant and rhizoids. However, “the use of chlorine as a rapid method of eradication needs to be weighed against the value of the native community, because chlorine kills indiscriminately,” the researchers stated in the May issue of the journal Marine Ecology Progress Series.

Other ways California Sea Grant has supported eradication efforts include co-hosting the first “International Caulerpa Taxifolia Conference” in 2002 and funding research projects documenting aspects of the plant's basic biology and looking into the prevalence of Caulerpa species in retail aquarium stores. The state legislature has banned the intrastate sale and possession of nine Caulerpa species, including Caulerpa taxifolia.

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