Prevalence and intensity of a rickettsiales-like organism in cultured pleasure oyster, crassostrea corteziensis, from nayarit, mexico.

Fastidious endosymbiotic Rickettsiales-like organisms (RLO) have been observed in the digestive diverticula of the cultured pleasure oyster (Crassostrea corteziensis) from Nayarit, Mexico since 2007. In a few mollusk species, these bacteria have been associated with mortality events and production losses. The type of relationship between the RLO and the pleasure oyster is largely unknown and further investigations are needed to determine if these bacteria warrant management concern in C. corteziensis. In this study, the morphological characteristics of the RLO were studied by histology and SEM, and the taxonomic affiliations of the bacteria were evaluated by 16S rRNA amplicon sequencing. In addition, the prevalence and intensity of the RLO was recorded from 2007 to 2017 by histology. The RLO was observed inside circular basophilic cytoplasmic membrane bound vacuoles (MBV) that had an average length and width of 15.70 ± 15.24 µm and 15.42 ± 14.95 µm respectively. Apart from cellular hypertrophy, no tissue alterations were observed in the areas adjacent to the RLO. Individual bacteria within the MBV were coccoid in shape with an average length of 0.65 ± 0.12 µm and an average width of 0.38 ± 0.09 µm. The bacterial microbiota of a selected number of samples (one sample without RLOs and two samples with RLOs) showed the presence of intracellular parasite OTUs corresponding to the families Rickettsiaceae and Anaplasmataceae, suggesting that the RLO from the pleasure oyster is associated with the order Rickettsiales. A mean prevalence of 5 % was observed throughout the study period and the majority of the organisms (89 %) presented low intensity of Grade 1 (30-61 RLO) of the MBV. A higher prevalence of the RLO was observed during warmer months. The lack of tissue alterations, the low prevalence and the low intensity of the MBV suggest that the RLO from C. corteziensis is a commensal endosymbiont that presents little risk for oyster production in Nayarit, México. However, regular monitoring is needed to detect if any variation in this relationship occurs, mainly in a scenario where extreme environmental fluctuations may occur.