Rickettsiales‐like organisms in bivalves and marine gastropods: a review

Abstract

AbstractRickettsiales‐like organisms (RLOs) are intracellular bacteria that have been reported to be symbionts of over 60 species of bivalves and marine gastropod molluscs of aquaculture importance around the world. Morphologically, some of these prokaryotes are similar to bacteria from the order Rickettsiales, and in most cases, they form intracytoplasmic inclusions similar to those observed in bacteria of the family Anaplasmataceae. Genetically, only Candidatus Xenohaliotis californiensis (CXc), a parasite of several abalone species, has been classified as a member of the order Rickettsiales. The other two characterized RLOs are distantly related; one belongs to the same class (α‐proteobacteria), and the other belongs to the γ‐proteobacteria. Hyperparasitism of RLOs by bacteriophages has been reported in several species of mollusc and appears to have a negative effect on RLOs. The intracellular lifestyle of RLOs has been associated with cell alterations including hypertrophy and compression of the cytoplasmic organelles, and in most cases, no host reaction is observed by histology. In some cases, a high infection intensity of RLOs has been related to dysfunction of the affected organs that can result in disease, and in other cases, high burdens of RLOs have been associated with mortality, but most of these cases have yet to be confirmed. To date, only CXc has been confirmed as the aetiological agent of a chronic and potentially lethal disease of several abalone species known as withering syndrome. This disease is listed by the World Organization of Animal Health. With the ever‐increasing interest in preserving fisheries and expanding the aquaculture production of mollusc species, more efforts should be focused on the characterization and study of RLOs to elucidate the type of symbiosis they maintain with their host and determine when they are acting as true pathogens that should be subjected to control in aquaculture. This review provides an overview of the literature on RLOs that infect bivalves and gastropods of aquaculture importance, and we discuss the most important aspects of these unique prokaryotes.