Nacra is a substantial bivalve, native to the Mediterranean. Among the largest mollusks on the planet, it can reach roughly a meter in length over a span of about fifteen years. Its value goes beyond size: it helps filter the sea and provides shelter for a surprising array of other species. Sponges, sea anemones, algae and many little creatures live inside and around its shell, creating a busy, living microcosm beneath the water’s surface.
Since 2016, nacra has faced a severe threat. Haplosporidium pinnae, a lethal parasite, has decimated most of the population, wiping out about 99.9 percent and shrinking numbers from millions to only several thousand. This mass die-off led to its listing as critically endangered on the IUCN Red List in 2019, signaling a pressing conservation crisis along the coasts where it thrives.
Conservationists describe the situation as dire and emphasize the need for wide-scale coordination and action. The loss of this keystone species would ripple through the marine ecosystem. Patricia Prado, a researcher at the Sant Carles de la Ràpita Institute of Technology and Agrifood in the Ebro Delta, has helped depict the stark reality of the crisis. Prado is part of eight Mediterranean research centers involved in the Pinnarca project, a European Life program aimed at safeguarding mollusks from extinction.
Examples of nacra on the coast of the Region of Murcia CARM Mar Menor Canal
Haplosporidium pinnae appears unusually deadly. Prado suspects it may work in concert with bacteria carried by mussels. The parasite’s origin remains unknown, though strong links point to human activity such as global shipping and agricultural runoff. Spores can persist not only in shells but also in surrounding water, complicating efforts to limit the spread.
It’s not just a parasite
Other factors contribute to the mussel decline. In the Mar Menor, a saline lagoon in southeast Spain, populations have dropped from about 1.7 million to roughly 1,000. Excess nitrogen from agricultural fertilizers fuels eutrophication, which robs marine life of oxygen and hits shellfish particularly hard. Young nacra rely on stable salinity and clean water to grow, making them especially vulnerable during early life stages.
Nearby in the Ebro delta, an invasive blue crab, likely hitchhiked on ships from the United States, has emerged as a voracious competitor, threatening native crab species and upsetting local ecological balance.
The largest nacra population in the delta is in the shallow, protected waters of Alfacs Bay, where as many as 90,000 individuals remain. The parasite seems to favor certain salinity levels, and Prado and her team hope to relocate some individuals to areas with lower salinity to support recovery. This effort is part of ongoing work to sustain the remaining nacra and maintain genetic diversity by exchanging individuals between groups to prevent inbreeding and its deadly consequences.
Examples of nacra in the Spanish Mediterranean CAR
Efforts continue to raise healthy individuals. Mussel farming remains an important activity in the delta, with strong expertise in hatcheries and cultivation. Yet nacra reproduction is slow and irregular, making captive breeding challenging. Prado notes that nacra exhibit a population regulation mechanism that can suppress reproduction at certain times, which can complicate restoration timelines.
Reasons for hope?
Some scientists stay cautiously optimistic. A marine ecologist at AZTI, Ángel Borja, emphasizes that science plays a clear role in identifying problems, communicating them to the public, and developing affordable solutions. In recent years, there have been encouraging examples of conservation success, suggesting that deliberate action can yield positive results even amid significant threats to marine life.
Mike Elliott, a Professor of Marine Sciences at the University of Hull, notes that many EU member states have programs to halt biodiversity decline and restore habitat, species numbers, and overall health. Recreating habitat benefits biodiversity and can strengthen communities against climate-change impacts like rising sea levels. In most cases, habitat restoration allows a species to recover if nearby populations can help repopulate it.
Elliott also cautions that nacra is long-lived and slow to grow, which means sustained protection is essential. Habitats once lost can be difficult to restore fully, underscoring the need for ongoing management and monitoring.
Prado stresses the urgency of saving nacra due to its foundational role in the ecosystem. It provides a habitat in itself; losing nacra would reduce biodiversity and alter coastal food webs. The intervention is crucial because waiting for nature to heal amid human-driven ocean pressures is not a viable option.
Some argue that extinction can be a natural part of evolution, but Prado reminds readers that a species already known and relied upon should not vanish without concerted effort. A broad segment of researchers agrees that immediate, informed action can preserve nacra and protect the broader reef and benthic community that depends on it.
Citation: Guardian, 2022 report on mollusk mass mortality and recovery prospects in the Spanish Mediterranean. …
Environment department contact address removed for privacy.