The zebra mussel , Dreissena polymorpha , is a bivalve mollusk that inhabits freshwater courses and saltwater environments. Its common name comes from the color of its shell, a light brown furrowed by dark zigzag stripes.
This bivalve is smaller than other mussel species, adult individuals reaching three centimeters. A striking characteristic of this species is that the individuals grow in colonies, covering the bed and later growing some mussels on top of others. This allows for very high population densities, of thousands of individuals per square meter.
History as an invasive species
The zebra mussel has the dubious privilege of appearing on the list of the 100 most harmful invasive alien species in the world, compiled by the International Union for Conservation of Nature ( IUCN ) .
The natural range of this species is the Black, Caspian and Aral Seas. During the 19th century, the species began to spread through the waterways of the European continent, reaching the Great Lakes of the USA in 1985, to later occupy the Mississippi and the Caribbean coast.
The reason why this species becomes invasive is its high reproduction rate, an adult specimen can release between one and a half million and a half million larvae in the course of a year . This, together with the density of the populations and their great resistance to different environmental conditions, causes the species to occupy waterways or lakes in a short time.
The species is problematic for two reasons: the growth of the colonies causes damage to the infrastructures and alters the composition of the phytoplankton. By forming compact cones of individuals, mussels are capable of blocking pipes or water deposits , which requires the physical removal of the specimens, since they are resistant to chemicals such as chlorine and eliminating them using these systems would negatively affect in the middle.
On the other hand, the water filtering capacity of the species is prodigious, up to 8.5 liters of water per individual per day, which adds to the population density, resulting in a large volume of water being filtered continuously. .
This has two consequences: on the one hand, the amount of phytoplankton present in the water course is reduced , which negatively affects the rest of the species; but on the other hand, the waters are clarified by eliminating the excess of suspended particles.
In some areas of northern Europe with pollution problems in rivers, the zebra mussel is considered beneficial for this reason. However, this has negative effects on other aquatic species, as already mentioned, so it is controversial to classify the species as “beneficial” .
The great resistance of the species, its high reproduction rate and the fact that it directly causes economic damage have made the fight against the zebra mussel a priority in those areas where it is present.
The economic losses that this species produced in the US in a period of 10 years exceeded 1,600 million euros . In Spain, the Ministry of the Environment dedicated 300 million to combat the species between 2003 and 2006, giving the figure of 100 million a year to control the invasion of the zebra mussel in our country.
Situation in Spain of the zebra mussel
The species was detected in the Ebro basin in 2001 , when the density of individuals was still low. In subsequent years it expanded to the Júcar and Segura basins and finished tracing the course of the Ebro, reaching the Undurraga Reservoir, in Vizcaya, in 2011. Other points where adult specimens have been detected are the Sobrón dam, in Burgos, and the Puentelarrá hydroelectric jump, in Álava.
For now it has not expanded to other river basins, although it seems only a matter of time until that happens. But how does the zebra mussel get around ? It is an aquatic organism and is not economically useful, so humans do not intentionally introduce it into the rivers it invades.
For years it was thought that adult specimens were transported in the hulls of recreational boats, where they are fixed once their larval stage is over. However, the answer could be another.
It has been theorized that fishermen could be accidentally transporting live zebra mussel larvae in their fishing baits, which sometimes come from one river and are used in a different one. To avoid this type of accidental transport, it is essential that the relevant administrations report the presence of the zebra mussel as well as the activities that cause its accidental transport, in order to avoid it.
Regarding existing control strategies , there is still no clear solution to the problem, but different approaches are being tested. The use of filters prevents the passage of larvae to water transport conduits, such as those of hydroelectric power plants.
A compound of hydrogen peroxide and iron salts that is lethal to zebra mussels in the presence of sunlight has also been tested , but the dose must be carefully calculated to avoid affecting other species. Perhaps the most extravagant system that has shown its usefulness is to use ultrasound to prevent the larvae from attaching to the substrate and thus completing their life cycle.