## 1 Introduction  
The fan mussel _Pinna nobilis_ is endemic to the Mediterranean Sea and is one of the largest bivalves that reach a size of up to 86 cm [1]. However, according to data from the Adriatic Sea, the maximum length is up to 120 cm [2]. It is a long-living bivalve, up to 20 years according to [3] [PERSON] et al. (1993). On the other hand, an age of 27 years has been reported in the Thermaikos Gulf (Greece) [4]. Recruitment [3] is highly variable, occurring at depths between 0.5 and 60 m, mostly on soft bottom areas overgrown by meadows of the seagrass _Posidonia oceanica_, _Cymodeca nodosa_, _Zostera marina_ or _Zostera noliti_[2], but also on bare sandy bottoms [5, 6]. The state of fan mussel populations can be discussed by referring to two periods, one before 2016 and the other after. Up until 2016, the population of _Pinna nobilis_ was endangered as a result of recreational and commercial fishing for food, using its shell for decorative purposes, and incidental killing by trawling and anchoring. In the European Union, it has been listed as an endangered species and was placed under strict protection according to the European Council Directive 92/43/EEC. In some areas, dense populations of 15-16 ind. per ha\({}^{-1}\) were reduced to 1 ind. per ha\({}^{-1}\) in a single year [7]. Destruction of eggs, larvae and adults by chemical pollutants and regression of its common habitat, the seagrass beds of _Posidonia oceanica_ due to anthropogenic activities, have also contributed to the acceleration of the decline of this formerly abundant species [8]. This situation persisted until 2016, when all the aforementioned problems faded into the background, as this largest shellfish of our sea almost no longer graced the seabed. After 2016, and over the following 3-4 years, fan mussel populations in the Mediterranean almost disappeared due to a massive die-off caused by the protozoan _Haplosporidium pinnae_[9]. The die-off reached such proportions in subsequent years that the European authoritiesdecided to classify the fan mussel as a critically endangered species. on the IUCN global red list of threatened species [10]. In 2019, the first cases of mortality were recorded in our southern Adriatic aquatorium. In the Bay of Mali Ston (where we conducted research and maintained fan mussel breeding stocks), during the summer months of the same year, the natural population of pen shells perished, as well as all specimens we had in the breeding stock cultivated since 2005 [11]. Prior to 2016, knowledge of the biology and ecology of the species was fragmentary, and several aspects needed further investigation [3; 12]. Investigations focused on the age and growth rate of the _Pinna nobilis_ in natural conditions [6; 13; 14; 15; 16]. Biofouling was also investigated [17], and its distribution and habitat [18], organisms that are symbiotically related, such as the shrimp, _Pontonia pinmophylax_ and the crab, _Nepinnotheres pinmotheres_ (Decapoda) associated with this bivalve [18]. In the Western and Central Mediterranean, research involved sexual development and maturation [19], the influence of hydrodynamic forces on population structure [12], and monitoring of the populations [20]. Research was also conducted on growth under natural conditions, mainly on the seafloor [5; 7; 12; 13; 14; 15; 21; 22; 23]. Gametogenesis was also investigated, with findings indicating it is generally similar to other shellfish species [24]. The potential for suspension farming was explored in the eastern Mediterranean [25] alongside growth possibilities in commercial production conditions for oyster and mussel parks in the Adriatic [11]. Mass mortality of populations and critical endangerment now require more intensive research on potential laboratory cultivation and natural repopulation in order to save the remaining specimens and stimulate repopulation.