2002 WINNER

How a creepy alga is strangling the Med

By Gemma Gouldby

Some call it the killer alga. Despite the sensational nickname, and unlike John Wyndham's man-eating Triffids, no one has been killed by these seemingly innocent marine weeds. However, this menacing invader threatens to devastate Mediterranean marine life.

The weed is a giant single-celled saltwater plant, growing up to 205 metres long, called Caulerpa taxifolia. ''Caulerpa'' is derived from Latin and translates to ''creeping stem''. It grows like a strawberry plant putting out runners - except with one vital difference: it propagates by vegetative (as opposed to sexual) reproduction.

A minute piece of the plant, caught in an anchor chain or floating in ballast water, can rapidly clone itself to generate an entire new colony wherever it settles. It suffocates the native plant life, destroying the breeding grounds and home of thousands of marine creatures. In addition, the resulting dense carpet of lush vegetation is virtually inedible to local fauna due to a potent toxin it contains; fish and other marine creatures starve while the creeping alga grows on, unrestrained.

Caulerpa taxifolia is native to the oceans surrounding the Caribbean and the Indo-Pacific, where it is relatively rare and poses no threat. Indeed, it is even a secondary food source for the tropical fish. Due to its vibrant green colour and rapid acclimatisation to artificial conditions, it was distributed to several European aquariums since the 1960s for use in tropical fish displays. In 1984, a square metre of a mystery green alga was discovered in the Mediterranean, next to the Oceanographic Museum of Monaco, by Professor Alexandre Meinesz of the University of Nice-Sophia Antipolis. He identified it as Caulerpa taxifolia. It seemed that small piece of the alga was inadvertently unleashed into the sea when the museum washed out its tanks. Directors of the museum claimed instead that it had been carried from the tropics by ocean currents; this was later disproved by DNA evidence.

Scientists from Monaco forecast that the small amount of the tropical plant, unused to temperatures below 20 deg C would not survive the Mediterranean winter. Yet they could not have been more wrong; by 1989 an entire hectare was densely covered. Over the ensuing years the cloned alga spread along the Mediterranean coast, often being carried unknowingly by vessels from harbour to harbour. Areas off the coasts of Croatia, Spain, France, Italy, Tunisia, California and southern Australia have since been colonised.

The alien taxifolia is different from its tropical cousin; it grows more vigorously, is resistant to low temperatures and shows a significant increase in size (threefold, according to Prof Meinesz). These properties have resulted in ecological dominance and are attributed to genetic differences between the two strains. Therefore, a genetic mutation - possibly caused by exposure to ultraviolet radiation or chemicals in the aquariums - could have been responsible for the metamorphosis from harmless tropical seaweed to ecological tyrant.

Numerous attempts have been made to reclaim the increasingly impoverished Mediterranean seabed. Manual uprooting, suction, ultrasound and high-pressure water jets have all proved futile. Re-growth is common, and even with the fine nets surrounding the area, loose fragments of taxifolia escape to form new colonies, exacerbating the problem.

The use of predatory sea-slugs, however, remains a feasible solution. These pierce the algal cell wall and suck out the cell contents, storing the toxin (to which they are immune) for their own defence against predators. Although these fascinating creatures successfully kill the plants and prevent re-growth, they are slow eaters; hundreds would be required per square metre. They do, however, reproduce extremely rapidly.

Even so, the deeper regions of the Mediterranean may still prove too cold for the sea-slugs, thus creating a safe haven in which the alien alga could still flourish. Unsurprisingly, there is conflict between scientists; some are understandably wary of introducing another foreign species into the Mediterranean. Each day it spreads, the likelihood of success for any potential control method decreases.

With the growth of the pharmaceutical and biotechnology industries, modern generations of biologists have, like a moth attracted to a bright light, become increasingly drawn towards molecular research. Although such work is tremendously important, the study of the ''whole-organism'' facets of this wide-ranging science has to continue.

Until now, taxifolia has often failed to capture the interest of all but the most dedicated marine ecologists and enthusiasts --- invasive species are rarely treated with equal urgency as those that are endangered. Yet without the diligent work of such people, the Mediterranean seabed could become uniform and monotonous, devoid of its colourful marine life. The so-called ''old-fashioned'' biological sciences must not be neglected - as Caulerpa taxifolia has highlighted.