Respiration rate and intestinal microbiota as promising indicators for assessing starvation intensity and health status in Pacific oyster (Crassostrea gigas)

Abstract

Implementing early disease warning is vital to prevent large-scale mortality of mollusks cultured in open waters. Starvation-induced emaciation syndrome is an undeniable prerequisite for mollusk mortality. Nevertheless, the indicators of mollusk response to starvation remain poorly understood. In this study, the effects of a 44-day starvation period on the physiological performance and symbiotic microbiota of oysters were investigated. Starvation resulted in reduced growth and survival and notable damage to intestinal structure, including shrinking lumens and shortened intestinal walls and villi. Analyses of the respiration rate and intestinal microbiota, including OTUs, taxonomy, and predicted functions, revealed a stepped change pattern with a tipping point on day 10, indicating that oysters employed distinct strategies to adapt to prolonged starvation. Biomarkers associated with nutrient digestion and uptake, such as Firmicutes and Staphylococcus abundance, as well as the Firmicutes/Bacteroidetes ratio, progressively declined with increasing starvation. Conversely, Proteobacteria, an indicator of energy disequilibrium and unstable gut microbiota, and pathogenic Vibrio, showed significant enrichment. As starvation progressed, bacterial competition intensified, as evidenced by the reduced network connectivity and the increasing percentage of negative correlations. Functional predictions demonstrated an early-stage enrichment of degradation functions followed by later-stage suppression during prolonged starvation. In contrast, biosynthesis functions exhibited the opposite trend, suggesting that intestinal microbiota adjusted their specific functions to cope with food deprivation. Collectively, a conceptual model of starvation stress tolerance limits (SSTL) was introduced to define critical thresholds and candidate markers for evaluating starvation intensity and oyster health. This study offers valuable insights into non-feeding mollusks' adaptation to prolonged starvation and provides potential indicators for mollusk mortality events.