Caulerpa taxifolia

Caulerpa taxifolia is a species of seaweed (a type of algae), native to the Indian Ocean, that has been commonly used as ornamentation in aquarium installations around the world. The algae has a stem which spreads horizontally just above the seafloor, and out of this stem there grow vertical fern-like pinnae, whose blades are flat like yew, hence the species name "taxifolia" (the genus of yew is "taxus"). The algae produces a large amount of a single chemical that is toxic to fish and other would-be predators. This is in contrast to other plants which produce a variety of toxins, but in reduced amounts.

A specific strain of this algae was found to thrive in cold aquarium environments by the staff at the Wilhelmina Zoo in Stuttgart, Germany in 1980. At the zoo aquarium, selective breeding under exposure to chemicals and ultra-violet light made the Caulpera even hardier^[1], and when it eventually found its way into the Mediterranean, it created an invasive species panic because of fears that it threatened to alter the entire ecosystem by crowding out other seaweed while being inedible to animals.

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Initial infection

It appears that, in 1984, this seaweed was accidentally released into coastal waters of the Mediterranean Sea just below the Oceanographic Museum of Monaco. It was claimed that caulerpa had grown to cover 7400 acres (Bright 1998). A belief arose that it was preventing other plants from growing, leading to the nickname "Killer Algae". A study in 2001 concluded that claims of huge areas smothered by this algae have been exaggerated 100-fold (Frakes 2001). The plant began to spread between ports along the Mediterranean coast. C. taxifolia has been discovered off the coasts of Australia and the United States, though none of those encroachments have been anywhere near the scale of what is happening in the Mediterranean.

For several years, the origin of the species was unclear. Marine biologist Alexander Meinez first discovered the plant in the 1980s, and requested the help of the Monaco Oceanographic Museum, which sat right next to the first known taxifolia patch. However, the director of the museum argued that this invasion probably happened naturally, the result of ocean currents carrying a tropical species into the area. Bickering over whether the species was natural or invasive, and whether the museum had released it or not, contributed to a delay that allowed the plant to spread beyond control.^{[citation needed]} The museum continued to deny releasing the plant, although former director Jacques-Yves Cousteau eventually expressed the belief that it was the only reasonable explanation.

It is often reported that as many as half of fish species have disappeared from areas where caulerpa grows. But scientific studies have shown that fish diversity and biomass are equal or greater in caulerpa meadows than in seagrass beds (Relini et al. 1998), that caulerpa had no effect on composition or richness of fish species (Francour et al. 1995), and that epiphytic plant richness is greater in caulerpa than in sea grass (Frakes 2001).

Reproduction mechanism

The aquarium strain reproduces asexually, that is, vegetatively: the viscous, elastic white fluid inside the stem was found under the microscope to contain only male gametes. Rate of growth can be as fast as a centimeter per day. If any small part is severed from the rest of the algae, this small part will regrow into another algae. Anchors of ships and fishing nets can serve as carriers of Caulerpa. Thus the algae has been found to jump from the coast of one port city to the coast of another port city. The natural strain reproduces sexually and has both male and female individuals. Gametes are expelled from each gender and meet to form a zygote which then goes through two larval stages before becoming an adult.

Other infections

In 2000 the strain was found in the coast of California (U.S.A.), near San Diego, and also in the coast of New South Wales, Australia. The California invasion was small enough to be considered controllable: it was covered with tarpaulin which was held down with sandbags at the edges of the infestation. Then chlorine was poured in through tubes which fed into certain openings in the tarpaulin: the interior of the tarpaulin fills up with chlorine and kills living organisms inside it, including the killer algae but also fish and plants. The killing of such other organisms was not desirable but was deemed preferable to letting the weed grow unchecked, which would in the end yield greater havoc on these same creatures.

The appearance off the Californian coast was most probably caused by an aquarium owner improperly dumping the contents, allowing C. taxifolia to flow through a storm sewer into the lagoon where the invasion was discovered. California has since passed a law forbidding the possession, sale or transport of Caulerpa taxifolia within the state. There is also a federal law under the Noxious Weed Act forbidding interstate sale and transport of the aquarium strain Caulerpa.

In July, 2006, the algae had been declared eradicated from the two Southern Califor nia locations (Agua Hedionda Lagoon in Carlsbad and Seagate Lagoon in Huntington).

Possible natural control method

Researchers at the University of Nice in France have been studying a tiny aquatic slug which is known to be a natural predator of C. taxifolia. Called Elysia subornata, it was found off the coast of Florida, in waters warmer than those in the Mediterranean. This slug is believed to feed exclusively on Caulerpa taxifolia, by sticking its proboscis into the stem and sucking out the white viscous liquid inside the stem: this causes the algae to become limp, discolored, and dead. As the slug does so, it absorbs the algae's poison. The slug has an enzyme which neutralizes the noxious effect of the poison, and at the same time the poison protects the slug from being eaten by fish. This slug cannot survive in the cooler waters of the Mediterranean, however, and so is unable to check the growth of the killer weed.

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