Caulerpa taxifolia is a green seaweed that is widely distributed in the world’s tropical seas. In Africa it occurs naturally in the Gulf of Guinea in the west, and Tanzania, Kenya, Somalia and the Red Sea in the east, as well as Madagascar, the Maldives and Seychelles. However, a robust, cold-tolerant strain has become an aggressive invader outside the natural range of the species. It has spread throughout the northern Mediterranean, where it has had a devastating impact on local biodiversity. It has also been found in the coastal waters of Tunisia in North Africa, as well as the United States and Australia. Now there are concerns that it might be introduced to other parts of Africa, and threaten marine communities there too
Rapid and serious impact
In 1984, a small patch of Caulerpa taxifolia was discovered growing in the sea outside the Monaco Aquarium, from which it probably escaped. It began spreading rapidly, colonising the seafloor from the shore down to a depth of 100 metres. Before long the species had invaded France, Spain, Italy and Croati a, and now carpets more than 11 000 acres of the northern Mediterranean seafloor. It forms dense monocultures, excluding indigenous seaweeds and seagrasses as well as corals, seafans and sponges.
Caulerpa has caused a reduction in the diversity and abundance of invertebrates and fish, both by destroying natural habitat and by producing toxic secondary metabolites that provide chemical defence against herbivores. The seagrass beds originally served as important nursery areas for many fish species, so the invasion has likely affected the
overall production, biomass and distribution of fish in the Mediterranean. Furthermore, the caulerpa beds are an impediment to net fisheries and are of little interest to recreational divers because of their limited biodiversity. As a result, they have been accused of negatively impacting commercial fisheries and coastal tourism.
It was long suspected that the seaweed is a hardier clone of the original tropical species, developed under artificial aquarium conditions. Genetic studies have lent support to this theory, and confirmed that the Mediterranean populations and collections in several European aquaria represent a single strain. The Mediterranean strain only reproduces asexually, allowing tiny pieces of the seaweed to grow into whole plants. This has facilitated its spread, as fragments dispersed on fishing nets and anchors are able to start new colonies. The species’ chemical defences undoubtedly contribute to its invasiveness, by increasing its competitive success over indigenous seaweeds.
Introductions to the United States and Australia have probably been via releases from home aquaria. Caulerpa taxifolia is a popular aquarium plant and the Mediterranean strain has been widely traded.
Numerous eradication methods have been used against caulerpa infestations, with varying effect.
Mechanical methods have included hand-harvesting, suction-dredging and covering with sand or tarps to exclude sunlight.
Some success has been achieved with chemical control methods involving the application of chlorine, copper sulphate, or even rock salt where caulerpa occurs in shallow areas on hard substrata.
Biological control options are also being investigated. Sea slugs belonging to the Sacoglossan group of molluscs feed only on seaweeds of the order Caulerpales,
sucking out the cell sap and accumulating the toxins to make themselves less palatable to predators. Tests are therefore being conducted to assess their potential as biocontrol agents.
Reference: Matthews S. & Brandt K. Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004