Abstract

The dynamics of the host-pathogen relationship are crucial for the success of marine bivalve aquaculture. Several Vibrio bacteria have been identified as pathogenic for scallops. This study covered eight months of monitoring of Vibrio spp. in the hemolymph of the scallop Argopecten purpuratus under culture conditions. Following the isolation of culturable Vibrio spp. and their taxonomic assignation, their pathogenic potential was estimated through experimental infections and cytotoxicity assays. The main findings were that: (i) scallops exhibit a low load of Vibrio spp. in the hemolymph, though these loads increase between late winter and early spring; (ii) the identified strains belong to the Splendidus, Pectenicida, and Rumoiensis clades, with the Splendidus clade being predominant, and Vibrio lentus being the most common species among them; and (iii) a high proportion of strains (86%) exhibited production of virulence factors, with four strains showing significant cytotoxic activity on scallop primary cultures of hemocytes. Two of the most virulent Vibrio isolates were confirmed as V. crassostreae HO115 and V. toranzoniae HO139 through genome sequencing. These isolates were then used for immune challenges of juvenile and adult scallops. The juvenile scallops were more affected by both vibrios compared to the adults. The juveniles showed exacerbated immune responses, a low capacity to eliminate Vibrio spp. in the hemolymph, and mortality rates of 26–40% after exposure. The most prevalent virulence-related genes in the genomes of V. crassostreae HO115 and V. toranzoniae HO139 included toxins, tissue-degrading enzymes, adhesion structures and immune evasion mechanisms. Notably, the virulence-related protein R5.7 is present in V. toranzoniae HO139 but absent in V. crassostreae HO115. Additionally, V. crassostreae HO115 featured a Type VI secretion system (T6SS) and a putative plasmid similar to the pathogenic plasmid pGV1512 which contains plasmid partitioning genes (parA/B), a conjugative apparatus (tra), transposases, and a heavy metal resistance cluster (cus/cop), strongly suggesting the presence of an extrachromosomal mobilizable element in V. crassostreae HO115. These findings establish a baseline for understanding the potential impact of Vibrio spp. present in scallop hemolymph. Evaluating in future environmental contexts could enhance our ability to predict how these Vibrio species will affect scallop health and propose solutions to mitigate these effects.