Sunday, August 1

The nacra, the ‘giant mussel’ of the Mediterranean in critical danger of extinction


The nacra or ‘sea silk’ for the ancient Egyptians (due to the bright and agile fabrics that were made with its filaments) is a protected species of bivalve mollusk that reaches up to 120 centimeters. This ‘giant mussel’, which has been present in Mediterranean waters for five million years, has been threatened in recent decades by human activity (extractions of specimens, ship crashes and pollution) and, in the last case and more brutally, since 2016 by a parasitic protozoan (Haplosporidium pinnae) that is annihilating this species with a mortality rate of 100%.

This bivalve is in critical danger of extinction, above other more emblematic species such as the Iberian lynx, the seahorse or the golden eagle. In the last five years it has disappeared from the entire Balearic and Andalusian coastline, and the last specimens in Spain survive in the Mar Menor and the Ebro Delta because the salinity conditions of these sites have prevented the parasite from emerging. However, with the entry of water with a lower salinity index after the Gloria storm in 2020, the population has been reduced to 2.3%.

In the fight against the clock to save the second largest mollusk in the world from extinction, the Institute for Agrifood Research and Technology of Catalonia (Irta), the Aquarium of the University of Murcia, the University of Alicante and the Institute for Environmental Research and Marine Science from the Catholic University of Valencia (Imedmar) collaborate with other international research groups to ensure the reproduction of nacra (Pinna nobilis) in captivity.

Emilio Cortés, a research biologist at the University of Murcia, is hopeful about the possibility of preventing the nacra from disappearing permanently. “We have taken giant steps since we began to study them in 2016,” he says. Cortés’s research group began to delve into this species and its situation in the Mar Menor after the first eutrophication crisis (an increase in nutritive substances in the waters that causes an excess of phytoplankton that, in turn, causes a lack of oxygen, and suffocates marine life). “After the green soup that was formed in the lagoon due to the entry of nutrients, all the nacras died below three meters. We went from more than a million and a half copies, to only 1,500,” laments Cortés.

The Mar Menor is a strategic nacra conservation area since that first eutrophication crisis in 2016. It was then that the surviving nacra began to be counted and studied in order to reproduce them in captivity. However, to date it has not been able to complete the breeding process outside the natural environment. “We need these larvae, the result of reproduction in captivity, to reach the juvenile stage and settle. We have not yet achieved it, but we are close to the University of Murcia”, says the researcher.

Achieving reproduction in captivity would mean ensuring that, despite the poor state of the waters of the Mar Menor or the advance of the parasite in the Mediterranean, the species would not become extinct. However, Francisca Giménez, researcher and professor at the University of Alicante, considers it vitally important to ensure that the Mar Menor ecosystem stops receiving input of nutrients, mainly nitrogen and phosphorus, so as not to end up with those specimens that inhabit the lagoon and to be able to incorporate those who managed to cultivate in the Aquarium of the University of Murcia, dependent on the Vice-Rector’s Office for Research and Internationalization of this institution.

Water quality indicators

Nacras are of fundamental importance in the marine environment due to their work in filtering nutrients, thus serving as indicators of the coastal ecosystem and the quality of its waters. The disappearance of this species would worsen the state of the Mar Menor and with it, the ecological crisis that is suffered in the place.

At present, the recovery of this species is slow and faces serious difficulties, since it not only has to deal with the eutrophication crisis in the Mar Menor, but also has to face the appearance of this new parasite capable of to annihilate practically all the specimens with which it comes into contact.

This protozoan of unknown origin shares almost the same natural conditions to subsist as the nacra (except for the salinity indexes that it can withstand) and is causing a massive death of these animals, due to the fact that it reproduces asexually by spores in its digestive gland. , causing them death by starvation.

A similar case raised serious problems on the East Coast of the United States during the 1950s: the Haplosporidium nelsoni almost managed to finish off the American oysterCrassostrea virginica) and continues to wreak havoc in the colonies to this day. However, some specimens resisted this protozoan and reproduced, giving rise to second and third generations of individuals resistant to it.

Reproduction in captivity and survival in the lagoons

Currently there are extremely few nacras surviving to the Haplosporidium pinnae, Therefore, the only solutions proposed are focused on achieving reproduction in captivity, together with guaranteeing the survival of the specimens present in the lagoons. The latter are in constant danger because a storm or the rise in sea level, as a result of climate change, could reduce their salinity and allow the protozoan to enter these waters. In the case of the Mar Menor, this critical state is enhanced by the pollution that it has suffered for decades, initially caused by wastewater discharges and, currently, by agricultural transformations.

Faced with the need to end the eutrophication crisis, Vox proposed the opening of the Mar Menor to the Mediterranean as a solution. However, this measure would mean a decrease in the salinity levels of the lagoon and the possibility of the parasite expanding, already present in the Mar Menor, and which affected the population of nacras of the encañizada in 2016. The professor of The University of Alicante, like the rest of the Spanish scientific community, is completely opposed to this measure: “It has been proposed from Vox and has the support of powerful groups in the Region, but it is a simplistic initiative that it does not take into account the conditions of the Mar Menor ecosystem. What is needed is to stop the entry of nutrients now, the situation of the lagoon is more fragile and critical with each eutrophication crisis and it is increasingly difficult to recover “.

The general director of the Mar Menor, Miriam Pérez, assures that the Government of the Region of Murcia is carrying out “numerous actions aimed at the protection and conservation of the species”, as well as “outreach, communication and awareness activities” .



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