RESUMEN
Mass Mortality Events (MMEs) affecting the noble pen shell Pinna nobilis have been reported since 2016. In this work, we used an in vitro flow cytometric assay to evaluate phagocytosis, coupled with cytology and Electron Microscopy (TEM), to define animal immunocompetence following infection by P. nobilis Picornavirus (PnPV). The study was performed on 27 animals in July 2021 and May 2022 on two natural population from the Ebro Delta (Catalonia, Spain) and animals maintained in captivity at facilities in Valencia and Murcia Aquarium. Hemolymph was collected in the field and in captivity as a non-destructive sampling method. Based on dimension and internal complexity, flow cytometry identified three haemocyte types, distinguished in granulocytes, hyalinocytes and a third type, biggest in size and with high internal complexity and granularity. Those cells corresponded at ultrastructure to hemocytes with advanced phases of PnPV infection and related to cytopathic effect of the replicating virus displaying numerous Double Membrane Vesicles (DMVs) and cells corpse fusion. The results showed that pen shell in captivity had significantly lower Total Hemocyte Count (THC) compared with natural population of Alfacs Bay (mean number of 7-9 x 104 vs 2-5 x 105 cells/mL, respectively). FACS (Fluorescence-activated cell sorting) based phagocytosis analysis demonstrate that animals in captivity at IMEDMAR-UCV and Murcia Aquarium, had scarce or absent ability to phagocyte the two stimuli (Staphylococcus aureus and Zymosan A) (10,2 % ± 1,7 of positives) if compared with the natural population in Alfacs Bay (28,5 % ± 5,6 of positive). Ultrastructure images showed that PnPV itself can lead to an alteration of the hemocyte cytoskeleton, impairing the capabilities to perform an active phagocytosis and an efficient phagolysosome fusion.