Molecular characterization of an Endozoicomonas-like species associated with mass mortalities of king scallop Pecten maximus L. in United Kingdom

Abstract

The king scallop Pecten maximus L. is the fastest growing fishery in the UK and currently the second most valuable. Mass mortality events in scallops have been reported associated with rickettsia-like organisms (RLOs) world-wide, however the causative agents were uncharacterised. In May 2013 and 2014 two mass mortality events affecting king scallops were recorded in the Lyme Bay Marine Protected Area (MPA) South West England. Histopathological examination showed gill epithelial tissue infected with RLO at varying intensity. Ultrastructural examination confirmed the intracellular location in affected epithelial cells. Partial 16S rDNA sequences of a putative RLO obtained from infected king scallop gill samples collected from both mortality events were identical and had 99.4% identity to 16S rDNA sequences obtained from Candidatus endonucleobacter bathymodioli and 95% with Endozoicomonas spp. In situ hybridisation assays using 16S rDNA probes confirmed the presence of the sequenced RLO gene in the gill tissues. Additional DNA sequences of the bacterium were obtained using high-throughput (Illumina) sequencing and bioinformatic analysis identified over 1000 genes with high similarity, ranging from 77-87% identity, to genes from Endozoicomonas spp. Specific PCR assays were developed and applied to screen for the presence of 16S rDNA RLO sequences in king scallop gill tissues collected in the Lyme Bay MPA during 2015. There was 100% prevalence of the RLO in these gill tissues and the 16S rDNA sequences identified were identical to the sequence found during the previous mortality event.