FROM DARK TO LIGHT AND BACK AGAIN: IS PINNA NOBILIS, THE LARGEST MEDITERRANEAN BIVALVE, ON THE BRINK OF EXTINCTION?

Abstract

Pinna nobilis is the largest bivalve of the Mediterranean Sea, where it represents a flagship species. As a possible consequence of several human disturbing activities, at the beginning of ‘80s, populations of fan mussel started a severe demographic decline. To reverse this trend, P. nobilis was included in a regime of full protection which led to a significant recovery of the species at the beginning of the millennium. Unfortunately, P. nobilis is presently facing a dramatic epidemic, which is bringing this species to the brink of extinction. This phenomenon started in early autumn 2016, from the Mediterranean coasts of Spain. Since then, the mass mortality spread quickly eastward reaching almost all Mediterranean areas. First epidemiological surveys ascribed the mass mortality of P. nobilis to the protozoan Haplosporidium pinnae, but recent studies indicated some species of bacteria belonging to the genera Mycobacterium and Vibrio as further or alternative etiological agents. Presently, a multifactorial disease mediated by the combined action of several pathogens seems to be the most probable responsible factor which is favouring the phenomenon. Despite the conservational prominence of P. nobilis, a low number of studies investigated the genetic structure of this species before its mass mortality and all were consistent in evidencing a very good health for populations throughout the whole Mediterranean, pointing out high levels of genetic variability and good genetic connectivity among areas. Now it would be useful to provide an extended molecular survey post-epidemic, for a deeper understanding of the causes of mass mortality of fan mussels.