From May to October 2019, multiple mass mortality events (MMEs) of Pinna nobilis were observed along Croatian coastline starting from the south-east and rapidly progressing in north-western direction. Time dynamics of the MMEs closely followed general speed and direction patterns of surface sea-currents, advancing approximately 350 km in less than 3 months. Surveillance, clinical evaluation, and sample collection were performed on multiple sites with various degrees of mortality rates. Moribund P. nobilis individuals were collected and subjected to pathological, molecular, and microscopical investigation. Affected animals were positive for Mycobacterium in 70% of the individuals, and Haplosporidium pinnae was present in 58% of the cases. Observed pathological lesions were most severe where concurrent presence of both pathogens was confirmed (in 45.8% of moribund individuals). Moderate to strong lesions were observed in animals positive for Mycobacterium only (25% of cases), and lesions were absent or minor to moderate when only H. pinnae was confirmed (16% of cases). Considering the rapid and severe spread of the MMEs, the areas less exposed to major sea currents appeared to be at lower risk of pathogen transmission. Surveillance activities along the Croatian coastline identified several P. nobilis populations in such “lower risk” areas without apparent mortality or clinical symptoms. Such areas are of particular interest as source of potentially healthy individuals to support active recovery actions.

Disease outbreaks in several ecologically or commercially important invertebrate marine species have been reported in recent years all over the world. Mass mortality events (MMEs) have affected the noble pen shell ( Pinna nobilis), causing its near extinction. Our knowledge of the dynamics of diseases affecting this species is still unclear. Early studies investigating the causative etiological agent focused on a novel protozoan parasite, Haplosporidium pinnae, although further investigations suggested that concurrent polymicrobial infections could have been pivotal in some MMEs, even in the absence of H. pinnae. Indeed, moribund specimens collected during MMEs in Italy, Greece, and Spain demonstrated the presence of a bacteria from within the Mycobacterium simiae complex and, in some cases, species similar to Vibrio mediterranei. The diagnostic processes used for investigation of MMEs are still not standardized and require the expertise of veterinary and para-veterinary pathologists, who could simultaneously evaluate a variety of factors, from clinical signs to environmental conditions. Here, we review the available literature on mortality events in P. nobilis and discuss approaches to define MMEs in P. nobilis. The proposed consensus approach should form the basis for establishing a foundation for future studies aimed at preserving populations in the wild.

The Kornati National Park (Croatia) is considered an environment with minimal anthropogenic input. The purpose of this study was to determine the isotopic characteristics of the sediment and muscle tissues of the banded dye-murex Hexaplex trunculus. We selected locations in the park according to their estimated risk of anthropogenic pollution (large, lower, and minimal). Isotopic analyses of the sedimentary organic carbon (δ¹³C_org values) showed that the sedimentary organic matter in locations with P. oceanica meadows (Piškera, Vrulje) was enriched in ¹³C compared to that of locations with the influx of terrestrial organic matter. The δ¹³C and δ¹⁵N values of the muscle tissues of H. trunuclus were the highest in the two locations with the highest possible anthropogenic impact (−14.47‰ and −15.66‰ for δ¹³C_org, +8.87‰ and +10.4‰ for δ¹⁵N). The high δ values may indicate the presence of the pigment indirubin (C₁₆H₁₀O₂N₂) and other derivatives that cause the purple coloration but are also elevated because of the discharge of untreated sewage from a nearby marina and village.