(M. Vahl) C. Agardh, 1817
Caulerpa taxifolia is a species of seaweed, an alga of the genus Caulerpa. Native to the Indian Ocean, it is widely used ornamentally in aquariums, because it is considered attractive and neat in arrangement, unlike some other macro algae, and is easy to establish and care for, though to the point of being invasive. 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).
Unlike most aquarium macro algae, taxifolia appears to be a normal, vascular plant with "leaves" arranged neatly up stalks like a fern. This is just an appearance, the plant is a typical macro alga, without the normal vascular system to transmit nutrients and cells that plants originally evolved on land have.
Caulerpa taxifolia has been described as storing, in its "leaves" a single chemical that is noxious to fish and other would-be predators, though not toxic to the water around it. This is in contrast to other plants which produce a variety of toxins, but in reduced amounts. On the other hand, studies have found that there is reduced pollution and toxicity in waters where it grows invasively, as around port cities in the Mediterranean. Original concerns about it reducing biodiversity of fauna have also been allayed, as species counts have shown it remains about the same. 
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, widespread concern developed as several speculative accounts suggested that the plant 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 Jacques Cousteau’s Oceanographic Museum of Monaco in 1984. Ten years later, the claim was made that Caulerpa had grown to cover 7,400 acres (29.9 km2), and was preventing native plants from growing. This concern earned the plant the dubious nickname "Killer Algae" after the title of an entire book written on the subject. Its author, 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. The parties bickered publicly for years over whether the species was natural or invasive, and whether the museum had released it or not, at the expense of sound scientific research on the species and its ecological significance.
Beds of the algae typically inhabit polluted, nutrient-rich areas such as sewage outfalls, explaining its spread among port cities in the Mediterranean Sea. This actually reduces the pollution in those areas, as the caulerpa consumes it: In an eight year study of Caulerpa beds in the French Bay of Menton, it was found that the alga reduced pollution and aided in the recovery of native Posidonia seagrass.
Despite claims that as many as half of fish species have disappeared from areas where Caulerpa grows, 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 species richness and epiphytic plant diversity is greater in Caulerpa than in pure sea grass. Thus, in contrast to widely publicized reports to the contrary, the species appears to have many beneficial ecological effects on aquatic communities in the Mediterranean Sea.
In fact, aquarist Jean Jaubert, director of the aforementioned Oceanographic Museum of Monaco, has said that the affected areas in the nearby Bay of Menton have been exaggerated 100-fold.
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.
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 colonization 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 filled up with chlorine and killed living organisms inside it, not only the unwanted alga but also fish, invertebrates and other seaweeds. The killing of such other organisms was not desirable but was deemed preferable to letting the plant grow unchecked.
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 eradicate this invasive alga.
- Guiry, M.D. & Guiry, G.M. (2007). "Genus: Caulerpa taxonomy browser". AlgaeBase version 4.2 World-wide electronic publication, National University of Ireland, Galway. Retrieved 2007-09-23.
- Pierre Madl & Maricela Yip (2004). "Literature Review of Caulerpa taxifolia". BUFUS-Info 19 (31). Retrieved 2007-06-10.
- Bright, C. 1998. Life out of bounds: Bioinvasion in a borderless world. W. W. Norton & Company, New York.
- Meinesz, A. 1999. Killer algae. University of Chicago Press, Chicago.
- Frakes, T.A. 2001. Killer algae: Ecological disaster or media hysteria? Submitted during the hearing in California concerning the proposed bill # 1334 available at http://www.aquarium-design.com/reef/caulerpa.html
- Jaubert, J.M., J.R.M. Chisholm, G. Passeron-Seitre, D. Ducrot, H.T. Ripley, and L. Roy. 1999. No deleterious alterations in Posidonia beds in the Bay of Menton (France) eight years after Caulerpa taxifolia colonization. Journal of Phycology 25:1113-1119.
- Relini, G., M Relini, and G. Torchia. (1998) Fish biodiversity in a Caulerpa taxifolia meadow in the Ligurian Sea. Italian Journal of Zoology 65 Supplement:465-470.
- Francour, P., M. Harmelin-Vivien, J. G. Harmelin, and J. Duclerc. 1995. Impact of Caulerpa taxifolia colonization on the littoral ichthyofauna of north-western Mediterranean sea. Hydrobiologia 300-301:345-353.
- Thibaut, T. 2001. "Elysia subornata a potential control agent of the alga Caulerpa taxifolia in the Mediterranean Sea", Journal of the Marine Biological Association of the United Kingdom
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (February 2008)|
- 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
- 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.