How a creepy alga is strangling
the Med
By Gemma Gouldby
Winner of the 16-19 group
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 2ยท5 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 a 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 20C 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.
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