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Back in the early 1980’s, the Wilhelmina Zoo in Stuttgart was looking into various types of seaweed for use in their aquarium displays. They settled on a species known as Caulerpa taxifolia, since its bright green, feathery fern-like fronds were quite pretty, and it was both hardy and fast-growing. In addition, it produces chemicals that make it taste awful to marine animals, so it wouldn’t get eaten.

By repeatedly subjecting specimens to harsh aquarium conditions and selecting the ones that survived the best, researchers developed Caulerpa taxifolia (Vahl) C. Agandh, a new-and-improved, genetically distinct strain which was particularly hardy and fast-growing. This variety was ideal for their purposes and it was shared with other museums and aquariums. For a time, all was well and good in the world of marine botany. In 1984, however, a square meter patch of this new variety of Caulerpa was found in the Mediterranean off the shore of Monaco, right outside the Oceanographic Museum.

Evidently a little piece of it was flushed down a drain. But while those organizations involved in dealing with the accidental release exercised their blame-pointing fingers, Caulerpa spread. It was, after all, particularly hardy and fast-growing. By the time anyone got around to doing anything about it, the infestation covered several acres and was beyond anyone’s control. By 2001, there were thousands of acres of this remarkably prolific plant clogging coastal waters around the Mediterranean.

A Caulerpa infestation looks like a vast meadow of leafy, green fronds. And nothing else. The meadows have been compared to fields of wet, overgrown Astroturf. It grows as much as three inches a day, fast enough to crowd out other algae, and since it tastes awful, aquatic herbivores won’t go near it. Because it creates immense fields of desert-like undersea monotony, humans are crowded out as well. The bland landscape becomes uninteresting to divers, diminishing the tourism industry; and seaside fishermen lose interest as their favorite fish move on to more accommodating waters.

Its appearances as far away as Spain, Croatia, and Tunisia are believed to be the result of Caulerpa being picked up by fishing nets and anchors. Since it’s particularly hardy, it isn’t bothered by typical harbor pollution and it happily lies waiting to hitch a ride. A tiny fragment no larger than a fingernail is all that is needed to spawn a new plant, which means that the usual mechanical seaweed-removal methods can actually spread an infestation rather than contain it.

A Caulerpa meadow

Controlling the plant with natural predators is also problematic. While there are a few mollusks that will munch happily on the other strains of Caulerpa, they cannot tolerate the temperatures of the Mediterranean, and they would be unable to eat it fast enough to be useful anyway.

Meanwhile, Caulerpa was working its way around the globe via the aquarium trade. In 2000, two small patches were found off the shore of San Diego, CA. Additional patches were spotted off southeast Australia. It is believed that these were the result of people dumping the contents of their salt-water aquaria down the drain.

Aware of the danger, agencies in California sprang into action. The state passed a law banning the possession and sale of nine species of Caulerpa. The City of San Diego topped them by banning all Caulerpa species. The infestations were dealt with through drastic measures. Marine biologists led an all-out assault on the invader. Armed with the latest weapons in botanical warfare, they completely covered and sealed the patches with black plastic tarps to cut off sunlight. Next, they pumped deadly chlorine under the tarps, killing every last trace of the enemy – along with anything else that had the misfortune to be trapped along with it. Six years and $7 million later, California can boast the world’s first successful victory over a Caulerpa invasion. Mediterranean countries are doing what they can with mechanical removal, and Australia is trying copper sulfate, a potent herbicide. In both places, the infestations are too large for the California treatment.

Meanwhile the United States has declared a war on mutant seaweed, exercising the federal Noxious Weed Act (1999) and the federal Plant Protection Act (2000) to ban the importation, interstate sale, and transport of the menacing Caulerpa. A public education campaign in California is also underway. Nonetheless, a recent survey showed that there are still stores selling the banned species. Not only do home hobbyists frequently lack the expertise to identify the illegal immigrants, but dealers, distributors, and inspectors may also lack this knowledge. Bans have also been enacted in Spain, France, and Australia.

A "Wanted" poster from the California Environmental Protection Agency

Invasive species are certainly nothing new. The current infestation of rabbits in Australia is perhaps the most well-known example of a foreign species causing the decline and extinction of various indigenous species. This invasion was launched by a landowner who immigrated from England and wanted to continue his rabbit-hunting hobby. Additionally, in 19th Century America, “acclimation societies” were trendy among those well-to-do who wanted to better mankind through scientific dabbling. Members encouraged the introduction and spread of non-native species for various beneficial uses. Kudzu, presently infesting the southeast US, was brought over from Japan for use as animal fodder and ground cover. House sparrows were imported from Britain to control insects, only to quickly become as annoying a pest as the insects they were meant to control. Even after it became apparent that the introduced species were becoming problems, they still continued bringing in new ones.

What separates Caulerpa from other invasive varieties is that it does not occur in nature. It is the product of selective-breeding genetic manipulation, pre-packaged with a man-made advantage which allows it to out-compete natural species. Although Genetically Modified (GM) crops are invaluable in feeding millions of poor⁠— usually by producing more nutrients, growing in harsher environments, producing larger yields, and/or resisting predators⁠— Caulerpa serves as a sobering illustration of the risks involved in developing alternatives which are hardier than their natural cousins. Today this mutant seaweed serves as a test case for the control of an accidentally-released unnatural strain, demonstrating the importance of quick and comprehensive action.

The ease at which it spreads and grows has earned Caulerpa a spot on the World Conservation Union’s “100 World’s Worst Invasive Alien Species” List. It remains to be seen whether or not the spread of Caulerpa can be contained, or whether it will wind up being the equivalent of aquatic kudzu. Perhaps over time we’ll find some Caulerpa-consuming sea creatures we can introduce to feast upon the unwanted plants; and hopefully we won’t need to find something to control the Caulerpa eaters. Failing that, some genetic tinkering might yield some kind of particularly hardy and fast-growing Caulerpa predator, and we can pit one mutant against another. With enough time, tarps, and successive creature-eaters, our victory over un-nature is inevitable.