Author: IUCN Mediterranean

Mass mortality events of the endemic bivalve Pinna nobilis (pen shell) have been recorded in the Western Mediterranean populations over the last year and a half. They first occurred in the south-east of the Iberian Peninsula and Balearic Islands in late 2016 and have been rapidly spreading ever since, causing the mortality of around 99% of individuals in all infected populations. To date, the mortality has been confirmed along the entire Spanish coast and in several locations in Corsica and Italy (Naples and Sicily).

Known areas affected by the mortality outbreak of Pinna nobilis in March 2018.

Experts from Spain have confirmed that the outbreak is caused by a new species of haplosporidian parasite, which is found in digestive glands of infected individuals. When infected in their natural environment, pen shells display mantle retraction, no reaction to stimuli and open valves, as they are no longer able to close their shells. The death is attributed to direct blockage of the digestive gland by the parasite and subsequent starvation of the bivalve. Once a population is infected, the likelihood of survival of any individual is very low, with no possibility of creating buffer zones.

First observations seem to indicate that the cycle of the parasite is at least partially influenced by the temperature, and therefore the seasonal increase in sea water temperatures is likely to intensify the problem and favour the spread of the disease to new regions.

In a recent meeting organised by the IUCN Centre for Mediterranean Cooperation and the Spanish Ministry of Agriculture and Fisheries, Food and Environment, experts from affected countries have gathered to discuss and explore possible coping measures and develop recommendations for this serious threat to the survival of the species. Given the elevated and rapid rate of expansion of the parasite thus far, the viability of Pinna nobilis populations throughout the Mediterranean could be seriously compromised in the following years. There is an urgent need to assess the current status of the populations elsewhere and start the coordination of a rescue programme in the countries from the Western and Central Mediterranean region where the parasite has not arrived yet.

Based on the outcomes of the meeting, the IUCN Centre for Mediterranean Cooperation is proposing a series of recommendations for Marine Protected Areas in the Mediterranean, as they represent key areas for early action and rapid implementation of conservation measures:

  1. INCREASE MONITORING FREQUENCY for the MPAs located in Western and Central Mediterranean. The monitoring should be conducted particularly every month during summer time but also in winter. This should include:
  • Monitor (by visual means) the status of all the population of Pinna nobilis and Pinna rudis in your MPA. You might would like to explore the possibility of assistance with local organisations, dive centers and research groups.
  • The monitoring should also include to take tissue biopsies of the populations in your MPA to find out the presence or absence of the parasite. Even if individuals look healthy, is IMPORTANT to be sure the parasite has not arrived to your MPA. For information regarding the tissue analysis, we recommend you contact a local/national lab with expertise on molecular and histological analysis.
  • Report any unusual changes in the populations. If a mass mortality of the population is observed, contact as soon as possible your national administration. Please also let us know to be able to report to other areas and regions.
  1. COLLABORATE IN THE IDENTIFICATION OF PINNA NOBILIS HOTSPOTS in the entire region- populations with high densities in areas that still haven’t been affected by the pathogen, as priority sites for the early implementation of management measures. Please contact us if your MPA presents a significant (high density with more than 10 or 20 individuals per 100m2) population of P.nobilis.

  2. RAISE THE ISSUE AT NATIONAL LEVEL AND ADVOCATE FOR THE DEVELOPMENT OF A RESCUE PROGRAMME that would target both adult and juvenile specimens. First rescue programmes are already underway in Spain and France, with the aim of rearing the Pinna in captivity and exploring the possibility of transplantation of the resilient juveniles in the future.


Further information:


Contact: Maria del Mar Otero, IUCN Centre for Mediterranean Cooperation at