Cardiac Morphological Abnormalities Associated with a Mass Mortality Event in Farmed European Sea Bass

Mass mortality events (MMEs) represent a major challenge for aquaculture, with important consequences for production, economic sustainability, and fish welfare. While these events are generally considered multifactorial, the potential contribution of cardiac abnormalities remains poorly understood, particularly in European sea bass (Dicentrarchus labrax). This study investigated cardiac morphology and histopathology in farmed European sea bass associated with a mass mortality event. A total of 48 hearts from four groups were examined, including fish collected during a mortality event and surviving conspecifics from the same commercial farm. Gross cardiac morphology was assessed through ventricular height-to-width ratio (H:W), bulbus arteriosus alignment, and relative bulbus width, whereas histological analyses evaluated myocardial architecture, vascular lesions, and the organization of compact and spongy myocardium. Fish collected during the mortality event exhibited a significantly lower ventricular H:W ratio than surviving fish, while no differences were detected in bulbus alignment or relative bulbus width. Histologically, survivor hearts were characterized by a sparse compact myocardium and extensive lacunar spaces within the spongiosa. In contrast, mortality-event fish displayed a denser myocardial architecture with closely packed trabeculae, reduced lacunar spaces, and a thicker compact layer. These findings provide evidence of marked cardiac remodeling associated with a mass mortality.