(M.Vahl) C.Agardh, 1817
Caulerpa taxifolia is a species of seaweed, an alga of the genus Caulerpa native to the Pacific Ocean and Caribbean Sea. It is widely used ornamentally in aquariums, because it is considered attractive and neat in arrangement, and is easy to establish and care for. The alga has a stem (rhizome) just above the seafloor. From this stem grow vertical fern-like pinnae, whose blades are flat like those of the yew (Taxus), hence the species name taxifolia.
Unlike most aquarium macro algae, C. taxifolia (Killer Algae) has the appearance of a vascular plant with "leaves" arranged neatly up stalks, like a fern. Behind this appearance, the plant is a typical macro alga, without the vascular system to transmit nutrients and cells that plants originally evolved on land have. Caulerpa taxifolia is the biggest unicellular organism.
Caulerpa taxifolia has been described as storing in its "leaves" a single chemical, 'caulerpicin', that is noxious to fish and other would-be predators, though not toxic to the water around it. This is in contrast to 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 decreasing biodiversity of fauna have also been allayed, as species counts have shown this remains about the same.
Outside its native range, C. taxifolia is listed as an invasive species. 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.
In 1980, the staff at the Wilhelma Zoo in Stuttgart, Germany found that a specific strain of this alga thrived in cold aquarium environments. Selective breeding under exposure to both chemicals and ultra-violet light produced even hardier Caulerpa strains. When it eventually found its way into the Mediterranean, widespread concern developed that the algae 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 3,000 hectares (7,400 acres), and was preventing native plants from growing. This concern earned the algae the nickname "Killer Algae" after the title of a book written on the subject. Its author, Marine biologist Alexandre Meinesz, first discovered the alga in the 1980s, and requested the help of the Oceanographic Museum of Monaco, which sat right next to the first known C. taxifolia patch. 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.
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 by the European Oceanographic Observatory of Monaco (based within the Museum of Monaco), 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.
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 for 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 has both male and female individuals and additionally reproduces sexually. 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.), 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 algae 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.
Possible natural control method
Researchers at the University of Nice in France have been studying a tiny aquatic slug which is 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. However, this slug cannot survive in the cooler waters of the Mediterranean and, therefore, is unable to control the invasive alga there.
A field of C. taxifolia amongst seagrass.
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|Wikispecies has information related to Caulerpa taxifolia.|
|Wikimedia Commons has media related to Caulerpa taxifolia.|
- on YouTube
- Killer Algae, 2001 BBC Documentary
- 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.