Epidemic and pathogens

In the early autumn of 2016, a previously unknown parasite, Haplosporidium pinnae, was identified along the Spanish coastline, resulting in the mass mortality of noble pen shells. This situation gave rise to considerable concern among experts and the general public, given the significant risk of species extinction. This resulted in a change in the IUCN status of the pen shell from Vulnerable (VU) to Critically Endangered (CR). By 2020, the parasite had disseminated throughout the Mediterranean, with research indicating that surface currents acted as a vector. H. pinnae exhibits a high degree of specificity towards this species (Pinna nobilis), exhibiting no observable affinity towards related pen shell species within the same family, namely Pinna rudis and Atrina fragilis. At the present time, there are only a few remaining noble pen shells in open waters, with somewhat larger populations remaining only in certain lagoons in France, Spain, and Italy.

It is known that parasites from the genus Haplosporidium have been identified in various species of crabs, mollusks, and tunicates since the nineteenth century. However, they had never previously been recorded in the Pinnidae family. It is hypothesised that climate change has facilitated the arrival and subsequent spread of this parasite.

Haplosporidium pinnae is a microscopic parasite that infects the noble pen shell, rapidly reproducing within its tissue. The parasite is most abundant in the digestive gland, impairing its normal function and leading to the starvation of the individual. The parasite's reproduction begins at temperatures above 13.5°C and salinity levels ranging from 36.5 to 39.7 parts per thousand.