Multiple Non-Species-Specific Pathogens Possibly Triggered the Mass Mortality in Pinna nobilis

Abstract

The fan mussel, Pinna nobilis, represents the largest bivalve endemic to the Mediterranean Sea. Since 2016, dramatic mass mortality of this species has been observed in several areas. The first surveys suggested that Haplosporidium pinnae (currently considered species-specific) was the main etiological agent, but recent studies have indicated that a multifactorial disease may be responsible for this phenomenon. In this study, we performed molecular diagnostic analyses on P. nobilis, P. rudis, and bivalve heterologous host species from the island of Sardinia to shed further light on the pathogens involved in the mass mortality. The results support the occurrence of a multifactorial disease and that Mycobacterium spp. and H. pinnae are not necessarily associated with the illness. Indeed, our analyses revealed that H. pinnae is not species-specific for P. nobilis, as it was present in other bivalves at least three years before the mass mortality began, and species of Mycobacterium were also found in healthy individuals of P. nobilis and P. rudis. We also detected the species Rhodococcus erythropolis, representing the first report in fan mussels of a bacterium other than Mycobacterium spp. and Vibrio spp. These results depict a complicated scenario, further demonstrating how the P. nobilis mass mortality event is far from being fully understood.

Keywords: Haplosporidium pinnae; Mycobacterium sp.; Pinna rudis; Rhodococcus erythropolis; Sardinia; fan mussel; heterologous host species; multifactorial disease; sentinel species.

Figure 1

Individuals of Pinna nobilis found dead with shell completely detached from the bottom.

Figure 2

Sampling sites of Pinna nobilis, Pinna rudis, and heterologous host species (HHS) throughout the coast of Sardinia Island. Sampling site codes are reported in Table 1. In the site IPI, we collected Pinna nobilis and Pinna rudis. HHS1: Mytilus galloprovincialis from Olbia; HHS2 Ruditapes decussatus from Porto Pozzo; HHS3: Ruditapes decussatus from Tortolì. The maps used are available at the site: https://d-maps.com/index.php?.

Figure 3

Whole body of the individual of Pinna nobilis that was collected immediately after dying (PN19 in Table 1).

Figure 4

Bayesian phylogenetic tree showing the relationships among bacteria taxa. The column on the right (nucleotide divergence threshold (NDT T.E.) indicates the taxonomic entities found by the NDT method. The sequences obtained in the present study are reported in red font. The asterisk indicates the sequences of bacteria isolated from individuals of Pinna rudis.