The Implication of Vibrio Bacteria in the Winter Mortalities of the Critically Endangered Pinna nobilis

Abstract

Pinna nobilis populations, constituting the largest bivalve mollusk endemic to the Mediterranean, is characterized as critically endangered, threatened by extinction. Among the various factors proposed as etiological agents are the Haplosporidium pinnae and Mycobacterium sp. parasites. Nevertheless, devastation of the fan mussel populations is still far from clear. The current work is undertaken under a broader study aiming to evaluate the health status of Pinna nobilis population in Aegean Sea, after the mass mortalities that occurred in 2019. A significant objective was also (a) the investigation of the etiological agents of small-scale winter mortalities in the remaining populations after the devastating results of Haplosporidium pinnae and Mycobacterium sp. infections, as well as (b) the examination of the susceptibility of the identified bacterial strains in antibiotics for future laboratory experiments. Microbiological assays were used in order to detect the presence of potential bacterial pathogens in moribund animals in combination with molecular tools for their identification. Our results provide evidence that Vibrio bacterial species are directly implicated in the winter mortalities, particularly in cases where the haplosporidian parasite was absent. Additionally, this is the first report of Vibrio mediterranei and V. splendidus hosted by any bivalve on the Greek coastline.

Keywords: Haplosporidium pinnae; Mycobacterium sp.; Pinna nobilis; Vibrio mediterranei; Vibrio splendidus; Vibrio spp.; antibiotics; mass mortality.

Figure 1

Sampling sites of P. nobilis specimens from Kalloni gulf, Lesvos Island (39.095818, 26.149199) and Maliakos gulf, Fthiotis (38.805781, 22.613020), within the Aegean Sea.

Figure 2

Subculture of a single colony on TCBS with 2% NaCl.

Figure 3

Phylogenetic relationship of the newly described Vibrio haplotypes in comparison with the most closely related ones, obtained from the GenBank database. (a) Maximum likelihood dendrogram including all newly described Vibrio haplotypes in comparison with the most closely related ones. (b) Maximum likelihood dendrogram including only V. mediterranei haplotypes. Haplotype names as in Table 1.

Figure 4

Histological display of the digestive gland of two different P. nobilis specimens. (A) Digestive gland of fan mussel infected with Mycobacterium sp., H. pinnae, and V. splendidus. (B) Digestive gland of fan mussel infected only with Mycobacterium sp. and V. splendidus. Both exhibit heavy inflammation responses and heavy lesions in the connective tissue of the digestive gland. A degenerative process was also indicated in the epithelium of the digestive tubules of both specimens, regardless of the presence of H. pinnae.