Caulerpa taxifolia

Caulerpa taxifolia
Scientific classification
Kingdom:	Plantae
Division:	Chlorophyta
Class:	Bryopsidophyceae
Order:	Bryopsidales
Family:	Caulerpaceae
Genus:	Caulerpa
Species:	C. taxifolia
Binomial name
Caulerpa taxifolia (M. Vahl) C. Agardh, 1817

Caulerpa taxifolia is a species of seaweed, an alga of the genus Caulerpa. Native to the Indian Ocean, it has been widely used ornamentally in aquariums. The alga has a stem which spreads horizontally just above the seafloor, and from this stem grow vertical fern-like pinnae, whose blades are flat like yew, hence the species name "taxifolia" (the genus of yew is "Taxus"). The alga 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.

It is one of two algae on the list of the world's 100 worst invasive species compiled by the IUCN Invasive Species Specialist Group.

Invasive species

A specific strain of this alga was found to thrive in cold aquarium environments by the staff at the Wilhelma Zoo in Stuttgart, Germany in 1980. Selective breeding under exposure to chemicals and ultra-violet light made the Caulerpa even hardier, and when it eventually found its way into the Mediterranean, it created widespread concern as it threatened to alter the entire ecosystem by crowding out native seaweed while being inedible to animals.

It is thought that the seaweed was accidentally released into coastal waters of the Mediterranean Sea just below the Oceanographic Museum of Monaco in 1984. Ten years later, Caulerpa had grown to cover 7,400 acres (30 km²), preventing native plants from growing and leading to the nickname "Killer Algae". Caulerpa overgrows the seabed and disrupts the food cycle dependent on the native plant life. 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 these 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 Alexandre Meinesz first discovered the plant in the 1980s, and requested the help of the Monaco Oceanographic Museum, which sat right next to the first known C. 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. 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 reported that as many as half of fish species have disappeared from areas where Caulerpa grows. However, scientific studies have shown that fish diversity and biomass are equal or greater in Caulerpa meadows than in seagrass beds, that Caulerpa had no effect on composition or richness of fish species, and that epiphytic plant richness is greater in Caulerpa than in sea grass. A study in 2001 concluded that claims of huge areas smothered by this alga have been exaggerated 100-fold.

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 alga, this small part will regrow into another alga. Anchors of ships and fishing nets can serve as carriers of Caulerpa. Thus this alga 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 sex and meet to form a zygote which then goes through two larval stages before becoming an adult.

Other infestations

In 2000, the strain was found on the coast of California (U.S.A.), near San Diego, and also on 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 alga 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 California 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 alga had been declared eradicated from the two Southern California 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 C. taxifolia, by sticking its proboscis into the stem and sucking out the white viscous liquid inside the stem: this causes the alga to become limp, discolored, and dead. As the slug does so, it absorbs the alga'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.

References

• Peplow, M. 2005. "Algae create glue to repair cell damage", Nature
• "Start-up drills for oil in algae" by Martin LaMonica, CNET News.com, May 20, 2005, retrieved July 13, 2006
• Theodoropoulos, David. 2003. Invasion Biology: Critique of a Pseudoscience. pages 42,159. Avvar Books, Blythe, CA. 237 p. ISBN 0-9708504-1-7

External links

• In-depth article on invasions of Caulerpa taxifolia, source as escaped aquarium plant, etc.
• Caulerpa Taxifolia fact sheet
• An excerpt from Killer Algae by Alexandre Meinesz
• Caulerpa taxifolia at the Center for Invasive Species Research
• "Deep Sea Invasion" Nova (TV series) broadcast April 1, 2003
• Species Profile- Caulerpa, Mediterranean Clone (Caulerpa taxifolia), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Caulerpa, Mediterranean Clone.