Abstract
Due to the rapid decrease of Pinna nobilis populations during the previous decades, this bivalve species, endemic in the Mediterranean Sea, is characterized as ‘critically endangered’. In addition to human pressures, various pathogen infections have resulted in extended reduction, even population extinction. While Haplosporidium pinnae is characterized as one of the major causative agents, mass mortalities have also been attributed to Mycobacterium sp. and Vibrio spp. Due to limited knowledge concerning the physiological response of infected P. nobilis specimens against various pathogens, this study’s aim was to investigate to pathophysiological response of P. nobilis individuals, originating from mortality events in the Thermaikos Gulf and Lesvos and Limnos islands (Greece), and their correlation to different potential pathogens detected in the diseased animals. In isolated tissues, several cellular stress indicators of the heat shock and immune response, apoptosis and autophagy, were examined. Despite the complexity and limitations in the study of P. nobilis mortality events, the present investigation demonstrates the cumulative negative effect of co-infection additionally with H. pinnae in comparison to the non-presence of haplosporidian parasite. In addition, impacts of global climate change affecting physiological performance and immune responses result in more vulnerable populations in infectious diseases, a phenomenon which may intensify in the future.
Highlights
The endemic sea bivalve species Pinna nobilis (Linnaeus, 1758) is one of the largest bivalve species in Mediterranean marine ecosystems, reaching up to 120 cm shell length and inhabiting coastal areas with Posidonia oceanica (Delile, 1813) or Cymodocea nodosa (Ucria) (Ascherson, 1870) meadows at depths of 0.5–60 m [1,2]
Keeping in mind the lack of complete knowledge concerning the physiological response of infected P. nobilis specimens against various pathogens, the objectives of this study are the investigation of the pathophysiological responses of P. nobilis individuals originating from mortality events, as well their correlation to the different potential pathogens detected in the diseased animals
Haplosporidium pinnae was histologically detected in four out of the six samples used for this research (Figure 2(Aa,Ba,Bb)), while Mycobacterium sp. was histologically detected in all samples used in this research (Figure 2(Ab))
Summary
The endemic sea bivalve species Pinna nobilis (Linnaeus, 1758) is one of the largest bivalve species in Mediterranean marine ecosystems, reaching up to 120 cm shell length and inhabiting coastal areas with Posidonia oceanica (Delile, 1813) or Cymodocea nodosa (Ucria) (Ascherson, 1870) meadows at depths of 0.5–60 m [1,2]. Due to the rapid decrease of P.nobilis populations due to its mass mortalities during the previous decades, this species is included in the Annex II Barcelona Convention (1992) and Annex IV of the EU habitats directive (2007), and it has been characterized as ‘critically endangered’ by the IUCN red list for threatened species [4,5]. Significant mortalities, even extinction in some cases, of P. nobilis within its natural habitats are mostly due to various pathogen infections [8]. While records of this phenomenon began in 2016, it continues to devastate the populations of fan mussels to the present day [9]. The newly described Haplosporidium pinnae is the most probable cause of this ecological risk [10]
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