International Association
for Open Knowledge
on Invasive Alien Species
A parasite causes the collapse of the protected fan mussel (Pinna nobilis) populations in the Aegean Sea
7 February 2019

Researchers at the University of the Aegean, the Hellenic Center for Marine Research and the University of Thessaly (Greece) have provided the first evidence that a highly invasive parasite has recently invaded the Aegean Sea and caused the mass mortality of fan mussel populations.

Fan mussel (Pinna nobilis) is an emblematic Mediterranean bivalve that can exceed 1 m in length and 45 years of life (pictured). Its populations in the western Mediterranean (Spain, Italy, France) have suffered substantial declines since 2016, when the parasite Haplosporidium pinnae apparently arrived in the Mediterranean. Its origin is unknown but it is likely an introduced species that arrived very recently.

The researchers surveyed thirteen sites in the coastal waters of Lesvos Island (Aegean Sea) and reported a mass mortality event of fan mussels, with mortalities exceeding 93% of the populations on average. Using genetic and histological evidence, it was confirmed that this mass mortality event was due to the parasite H. pinnae that managed to invade the Aegean Sea, two years after its arrival in the western Mediterranean.

Maria Sini, post-doctoral researcher in the University of the Aegean, explains the importance of the findings: “Pinna nobilis is an endemic species that is only found in the Mediterranean Sea. Hence, the recent large-scale declines of the fan mussel populations can potentially lead to a complete extinction of this animal. A close collaboration between local authorities, scientists, stakeholders, and recreational divers acting as citizen scientists, is needed to address this critical threat, through the detection and effective protection of natural parasite-resistant populations.”

Professor Vasileios Bakopoulos, among the co-authors of this study, said: “The destruction of the digestive gland of fan mussels by the parasite and the absence of host tissue reactions were eminent, and alarming, features of histopathology of the disease. Fan mussel populations with natural resistance to the parasite should be identified and the factors affecting disease development elucidated in order to provide tools for the species’ protection and conservation.”

The existence of one healthy population in the study area gives reason for some hope for the survival of the species. The lead author of the study, professor Stelios Katsanevakis from the University of the Aegean said: “We need to further investigate the factors affecting the capability of the parasite to cause mortality of fan mussels, and thus predict the spatial distribution of healthy populations. Urgent measures to strictly protect the remaining healthy populations are needed to secure the conservation of the species.”

The findings of this study have been published in Aquatic Invasions, an open access journal of the International Association for Open Knowledge on Invasive Alien Species (INVASIVESNET) and can be found here.

For further information about this study contact Prof. Stelios Katsanevakis at or call +30-6948080681.

For more information about INVASIVESNET, please contact our Media and Communications Officer at

© 2021 International Association for Open Knowledge on Invasive Alien Species | Registered Association 221.451
Registered Office: PL 6, 00981 Helsinki, Finland