The zebra mussel is widely recognized as one of the most serious invasive alien species because of its substantial economic toll and its disruptive effects on ecosystems. It has already established itself in eight of Spain’s fifteen river basin districts following the discovery of adult specimens, with particular concern in the Segura basin where experts believe the transfer from the Tagus River system may have facilitated its arrival.
Reports from the Segura Hydrographic Confederation note the mussels appeared in the Talave reservoir (Albacete) on a Monday. Management measures target maintaining a water level that discourages spread since zebra mussels inhabit depths of roughly 2 to 5 meters. The goal is to dry out existing colonies and hinder further expansion. [Citation: Segura Hydrographic Confederation]
According to data from the Ministry of Ecological Transformation, the zebra mussel now borders the Andalusian Mediterranean basins, including the Guadalquivir, Guadalete and Barbat, Tinto, Odiel and Piedras, as well as the Júcar, Ebro, and Catalonia regions. As a result, several basins are being monitored for potential incursions. [Citation: Ministry of Ecological Transformation]
To date, Miño-Sil, the Galician Coast, Eastern Cantabria, Western Cantabria, Duero, Tagus, and Guadiana have been spared from established populations so far. [Citation: Ministry of Ecological Transformation]
Concha Durán, head of the Júcar Hydrographic Confederation Technical Service and a recognized invader expert, explains that the zebra mussel tends to settle more readily in calcareous terrain because it requires calcium carbonate to build its shell. In contrast, regions with abundant silica appear less conducive to expansion for the moment. [Citation: Concha Durán]
The mussel ranks in the top ten most dangerous invasive alien species globally due to its heavy socioeconomic and ecological impacts, though it does not threaten human health. Any spread is perceived as bad news, according to Durán. [Citation: Durán]
“You have to be tough in the beginning”
The Segura boundary has likely been among the few zones with limited boat traffic controls and monitoring to curb the invader, Durán notes, emphasizing a tough stance when the species is first detected. [Citation: Durán]
Durán stresses that, beyond draining water, a comprehensive study is needed to decide whether to halt reservoir operations such as fishing and other aquatic activities, and to gauge how far the mussel population has progressed. “In my experience, the early phase demands decisive action. Only then can a strategy be learned for living with the problem,” he adds. [Citation: Durán]
This species can block water intakes and is known to affect hydroelectric facilities, irrigation systems, and municipal buildings. Ecologically, its impact is brutal: once it adheres to a surface, it displaces other organisms, including crustaceans with vulnerable survival prospects. As a strong filter feeder, it consumes large amounts of phytoplankton, potentially reshaping entire ecosystems. [Citation: Durán]
Following evidence of a settled population, rapid dispersal is expected to spread to nearby basins. Francisco Oliva, a professor at the University of Murcia and coordinator of the Life Invasaqua project, notes the invasion risk in the context of research to counter exotic aquatic species. [Citation: Oliva]
Given the species’ high reproductive capacity—more than a million larvae can be released per year—and the lack of natural predators in many waters, rapid colonization of affected reservoirs is anticipated. Oliva warns that the Segura basin could face severe socioeconomic impacts on agriculture if the expansion continues. [Citation: Oliva]
Some hydrographic confederations have already invested substantial funds in direct management of the species, with figures surpassing 30 million euros. A study by the Ebro Confederation’s hydrographic authority also indicates substantial indirect losses from zebra mussels, exceeding 1,600 million. [Citation: Ebro Confederation]
Experts suggest reducing reservoir water levels and conducting extensive sampling to define the true population extent, alongside temporary prohibitions on activities that could disseminate larvae to other reservoirs or water systems, including navigation and sport fishing. [Citation: Ebro Confederation]
Transfer, possible culprit?
There is strong concern that this species could colonize the Segura basin via the Tajo-Segura transfer. Researchers and technicians have warned about the need to filter the flow reaching the Talave reservoir, which could be serving zebra mussels from infested areas in the Júcar basin. [Citation: Research Team]
The first appearance of the zebra mussel in Spain date back to 2001, with the Ebro and Júcar basins among the early zones affected. It endangers biodiversity and hydraulic infrastructure, and basin agencies caution that boats are the main vectors of spread, prompting limited navigation on affected dams and a campaign to promote preventive protocols to curb further expansion. [Citation: National Invasive Species Catalog]
Periodic plankton sampling is conducted by basin organizations to detect larvae, while adults are monitored and adjacent spreading points are examined. Zebra mussels originate from the Black, Caspian, and Azov seas, and the International Union for Conservation of Nature classifies them among the 100 most dangerous invasive alien species. They are also listed in the National Catalog of Invasive Alien Species. The adult shell is elongated and triangular, reaching up to four centimeters. The valves display alternating light and dark bands in a zigzag pattern, with color and shape variability. They are not edible.
Further notes emphasize ongoing environmental surveillance and ecosystem risk assessment to guide policy and response strategies.