Shellfish die-off event: can Spanish giant mussels recover? | Environment

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In the shallow waters and seagrass of Spain’s Ebro Delta, there are almost no solid surfaces for the creatures to cling to. This is where the fan mold (Pinna nobilis) Between. Sponges, sea anemones, algae, and life forms not found on any other surface in the region live on its shell, while octopuses, oysters, and some fish live inside.

The bivalve, also known as the noble feather shell, is unique to the Mediterranean and is the second largest in the world, with some specimens reaching one meter in length over a lifespan of 15 years. It plays a crucial role in water filtration and is home to an array of underwater creatures.

But the fan mussel fell victim to the parasite Haplosporidium pennes, which since 2016 has wiped out 99.9% of the population, reducing its number from millions to thousands. This mass mortality event led to the mollusc being added in 2019 to the International Union for Conservation of Nature (IUCN) red list as critically endangered. ” The situation is critical. We need large-scale coordination because we are witnessing the extinction of a very important species,” says Patricia Prado, a researcher at the Institute of Agribusiness and Technology of Sant Carles de la Ràpita in the delta of the Ebro. The institute is one of the eight Mediterranean research centers that make up the Pinnarca Projectan EU Life program aimed at saving the mollusc from extinction.

The fan mussel, seen in the Ebro delta in Spain. The species can live 15 years and reach one meter in length. Photo: Courtesy of Instituto de Investigación y Tecnología Agroalimentaria

H. penna is exceptionally deadly. Prado thinks this may be because it works in conjunction with bacteria that mussels can already carry. She says the origin of the parasite is unknown but suspects it is linked to human activity, be it global warming, shipping or agriculture. The spores of the parasite live in water, not just in fan mussels.

Protozoa aren’t the only cause of mussel decline. In the Mar Menor, a saline lagoon in southeastern Spain, the population has fallen from 1.7 million to just 1,000 due to excess nitrogen in the water caused by fertilizer runoff used in agriculture. The process, known as eutrophication, deprives marine life of oxygen, and fan mussels are among the victims.

In the Ebro Delta, young mussels are also victims of the blue crab, an invasive species that is said to have arrived from the United States by sea and whose voracious appetite has practically wiped out the native species of crab.

Researcher Patricia Prado at work in the Ebro Delta.
Researcher Patricia Prado at work in the Ebro Delta. Photo: Courtesy of Instituto de Investigación y Tecnología Agroalimentaria

The largest population of fan mussels in the delta is found in the sheltered, shallow waters of the Bahia del Alfacs, where up to 90,000 can survive. It appears the parasite needs an optimal level of salinity to thrive, and Prado and his team hope that moving individuals to parts of the delta with lower salt levels can help the mussels thrive.

“We try to maintain the surviving population and exchange individuals between groups to avoid inbreeding, otherwise they will become extinct due to lack of genetic diversity,” she says.

They also try to breed healthy individuals. Mussel farming being a major industry in the delta, there is a lot of experience in this area, but so far they have been unlucky. The mussel is slow to reproduce and does not do so every year. Plus, says Prado, “it has a mechanism to regulate the population, which makes it difficult for us to reproduce them if it’s programmed not to reproduce at certain times.”

However, Dr. Ángel Borja, a marine ecologist at the AZTI institute, which is part of the Basque Alliance for Research and Technology, says there is cause for optimism. “Science plays a leading role in identifying problems and communicating them to society, but especially in developing affordable solutions,” he says. “Despite the threats to the ocean, in recent decades we have seen many examples of positive conservation outcomes, so there is reason to speak of ‘ocean optimism’.

Mike Elliott, Professor of Marine Science at the University of Hull, adds: “All countries, and especially those in the EU, have plans and projects to halt the decline in biodiversity and restore habitats, numbers of species and the size of their populations, such as Pinna.

The invasive Atlantic blue crab, pictured in a river in Spain, is also a threat to young fan mussels.
The invasive Atlantic blue crab, pictured in a river in Spain, is also a threat to young fan mussels. Photography: Wirestock/Alamy

“Recreating habitat can not only be good for biodiversity, but it could also protect people from the effects of climate change, such as sea level rise. In many cases, recreating habitat will allow a species to recover, as long as there are populations nearby to repopulate it.

But, Elliott adds, “Long-lived, slow-growing species with patchy populations, such as Pinna, will need greater protection and may also be more difficult to restore once habitat is lost.

Prado insists that trying to save the fan mussel is vital because of its crucial role in the ecosystem. “It’s a habitat in itself, so losing the fan mussel means losing biodiversity,” says Prado. “We have to intervene because there is no point in waiting for things to improve on their own, especially since we are the source of a lot of problems.

“People argue that it’s part of a process, that some animals will go extinct and others will replace them, but we as a species depend on the ones that already exist.”

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