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Invading Mollusks Come to Stay |
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By María Laura Mazza*
The introduction of exotic species is one of the biggest threats to freshwater and ocean ecosystems alike, say biologists.
MONTEVIDEO - Small mollusks from Asia and Africa have arrived in the South American estuary of the Río de la Plata, displacing local species, multiplying by the millions, blocking potable water networks, complicating navigation and damaging hydroelectric systems. They are impossible to eradicate, warn biologists.
These shellfish travel as larvae in the bilge water that ships take on in ports as ballast in order to improve stability. The water is discharged when the ship reaches shallower waters, like the Río de la Plata, dumping the exotic species into new ecosystems.
But adult mollusks are also brought from port to port, stuck to the ship's hull, chains or keel.
Once introduced, these organisms can displace native species and alter local ecological conditions, warns a study by scientists from the Argentine Museum of Natural Sciences, Museum of La Plata, University of Mar del Plata and the Argentine Institute of Oceanography.
With no natural enemies -- predators or competitors for resources -- and with a great capacity to adapt themselves to new surroundings, they multiply rapidly.
In just a few years, exotic species of shellfish have expanded north to the rivers feeding into the Río de la Plata, like the Uruguay and Paraná rivers, reaching Brazil and Paraguay. They have also headed south along Argentina's Atlantic coast.
Elimination of these intruding species is "absolutely impossible," says Argentine marine biologist Pablo Penchaszadeh, head of the study that is part of the Environmental Protection Project of the Río de la Plata and Its Sea Coast (FREPLATA), a joint Argentina-Uruguay initiative, with international backing.
The most harmful is the golden mussel, or Limnoperna fortunei, a freshwater bivalve originating in China and Southeast Asia. It is not suitable for human consumption and measures just 10 to 25 millimeters long.
"Since it was first detected in Argentina in 1991 it has advanced nearly 250 km per year. It has turned into a veritable plague," Penchaszadeh told Tierramérica.
The golden mussel reproduces at an alarming rate. Each female lays thousands of larvae. Because it can adhere to any hard surface, it forms colonies that can completely block water pipelines, filters and irrigation canals.
In 1996 this species invaded the Paraná and Paraguay rivers, by 1999 it had reached the Guaíba River basin in Brazil, and in 2001 it appeared in the Uruguay River.
As the golden mussel takes over, the local fauna begins to change -- autochthonous species of mollusks are ousted from the habitat.
Before the golden mussel arrived in the Argentine beach town of Bagliardi, on the Río de la Plata, three snail species were common: Heleobia piscium, Chilina fluminea and Gundlachia concentrica.
But since the foreign species established itself there, the latter two snail species are only seen by chance, according to biologist Gustavo Darrigran, who spotted the limnoperna in 1991.
The invasion expanded to the point of affecting the water supplies at Uruguay's state water company's purification plant, OSE, in Aguas Corrientes, in the south.
This bivalve mussel has settled in all of the hydroelectric dams of the Mercosur region (Southern Common Market - Argentina, Brazil, Paraguay and Uruguay), dams like Salto Grande, on the Uruguay River, and Itaipú and Yaciretá, on the Paraná.
As a result, maintenance and cleaning operations have been stepped up in order to prevent a general collapse of the systems.
Corbicula fluminea, a mussel native to the waters of Turkey, Japan, Indonesia, Australia and Africa, lives buried in the sand -- and now obstructs pipelines and irrigation canals, although it does not adhere to surfaces.
The mollusk accumulates toxic substances in its tissues, therefore may prove a threat to human health because they are a food resource for the fish that humans eat.
There are other invaders, like the poliqueto Laeonereis, a worm-like species living inside sharp, narrow tubes that grow in colonies until they form a sort of reef. In Brazil, they reach a meter long and can cause serious injury to divers.
Their fast rate of reproduction and the lack of means to prevent them from expanding or to eliminate them make these exotic species an explosive problem.
FREPLATA proposes creating a system to process the bilge water of boats before it is dumped into the Río de la Plata.
The authorities apply specific poisons and mollusk-repellant paint is used on ships, but these substances are toxic. One harmless method, but not very practical, is to extract entire colonies of mollusks from the estuary.
"We are focusing on preventing invasions by new species and are attempting methods for controlling the organisms already established are not more harmful than the invaders themselves," said Penchaszadeh, a member of Argentina's National Council Scientific and Technical Research.
The Argentine and Uruguayan governments have not conducted studies on the economic losses caused by the exotic mollusk species, and the local population is generally unaware of the phenomenon, but "the costs of maintenance and cleaning the installations is reflected in the service rates" for water and electricity, Hugo Roche, FREPLATA expert in economic and social issues, told Tierramérica.
Economic globalization has made the problem of invading species a global occurrence. Marine biologists say it is one of the four major threats to continental bodies of water and to oceans, alongside contamination, overexploitation of fishing resources and the alteration or destruction of habitat.
The International Maritime Organization has set up specific programs to help developing countries -- like Argentina and Uruguay -- to reduce the transfer of harmful organisms through ship bilge water.
* María Laura Mazza is a Tierramérica contributor.
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