LIFE 10 NAT/ES/000582
Lucha contra las especies invasoras en las cuencas hidrográficas de los ríos Tajo y Guadiana en la Península Ibérica
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Zebra Mussel (Dreissena polymorpha)

Species description

Bivalve mollusc originally native to the Pontocaspian region (Black Sea, Caspian Sea, Aral Sea). This species takes its name from the black and white zig-zag shape pattern on its shell, which is simlar to a zebra skin. It is a small mollusc not longer than 3 cm long which attaches itself to the substratum through the byssus, forming a long dense cluster. This species lives in freshwaters, and it could survive in salty waters as well; although it mainly stays in stagnant or still waters. It  might be exposed to the air for periods of more than 5-6 days without being damaged, and gets nourishment from phytoplnakton.

Zebra mussels are unisexual (although hermaphroditism cases have taken place), and the ratio of male to female individuals is by and large equal. Mussel females reproduce during their second year of life, they having an external fertilization. It lays around 40,000 eggs, and planktonic larvae are born some days later. These grow and spread quickly, shifting into the juvenile stage in one month. Zebra mussels might produce up to one million descendants in one year time.
Pathways of Introduction and Expansion

Sailing is considered to be one of the main vectors for the introduction of this species in most areas or countries in which it is an alien invasive species. From the 19th century on, zebra mussel spread all over Europe through inter-river navigation channels built in those areas. It was in the 80s when it started to invade North America through sea transport. Nowadays, it has colonized many continental waters (rivers, lakes, reservoirs, etc.) in North America, and Central and Western Europe. In order to spread, mussels stick to ship's hulls, or larvae are carried in ballast water, fishing gear, boots, vessels, etc.
Impact

This species feeds mainly on phytoplankton; this way, it directly competes with native species displacing them, and also increasing organic matter levels, which affects the quality of continental waters. This species causes great ecologycal imbalance, since it covers the substratum of the whole area it invades: riverbeds, pebbles and rocks, riverside vegetation, native bivalves shells, any hydraulic structures, turbines, drainage systems, water storage, ship hulls, engines, and also anchors, jetties, water treatment plants, reservoirs, dams, irrigation ditches, intake pipes, etc. It may also obstruct pipes, irrigation channels, or hydraulic lines in general. The accumulation of thousands of valves from dead specimens drastically alters substratum features in riverbeds, beaches on river edges, and river sediments. Its presence makes oxygen levels decrease, which may produce cases of anoxia. It has also been observed an increase in the transparency of the water column, which favors the development of benthic plants in affected areas, and also the accumulation of organic matter in the riverbed.

The economical impact this species causes is directly related to the obstruction of any type of water systems (drinkable water, industries, nuclear and hydroelectric plants, etc.), as well as huge amounts of money necessary to repair any damage it causes.
Ficha del CATÁLOGO ESPAÑOL DE ESPECIES EXÓTICAS INVASORA

ESTRATEGIA NACIONAL PARA EL CONTROL DEL MEJILLÓN CEBRA (Dreissena polymorpha) EN ESPAÑA
This species grows fast, and reproduces every month. This way, it quickly creates huge mussel groups made up of hundreds or thousands of individuals per square metre, their empty shells accumulating in the river banks and bed. In Spain, the first zebra mussel specimens were found in the basin of the river Llobregat (Barcelona). Its origin is unknown, but they disappeared after floodings in October, 1982. In August 2001, a group of Catalan malacologists, and naturalists from the Grupo de Natura Freixe de Flix (Tarragona) found them again in the lower Ebro (Catalonia), from Xerta to Ribarroja reservoir (Aragón). Many theories about the introduction of the zebra mussel in the Iberian Peninsula have been considered. It has not been discovered yet, but it is most probable that a vessel containing huge quantities of zebra mussel or its ballast water full of larvae might be the cause. Nowadays, it is not only present in the river Ebro basin, but also in the rivers Júcar, Segura, and Guadalquivir.
Dreissena polymorpha - © Corey Raymond
Dreissena polymorpha - © Corey Raymond
Dreissena polymorpha - © Creative Commons
Dreissena polymorpha - © Creat. Commons