Physiological and biochemical changes associated with massive mortality events occurring in larvae of American oyster (Crassostrea virginica)

Abstract

-In this paper, biochemical and physiological analyses were used to characterize changes associated with mortality event occurred during veliger development of American oyster, Crassostrea virginica. Biochemical analyses included the evaluation of lipid classes, fatty acid composition and total protein content. Larval physiology was evaluated by studying feeding activity, enzymes related to energy metabolism, oxidative stress levels and enzymatic antioxidant defenses. These analyses were complemented by bacterial community analyses as well as by measuring larval oyster performance. We observed that mortality events coincided with (1) strong changes in the surrounding bacterial community; (2) a progressive decrease in feeding activity; (3) higher levels of some lipid classes (free fatty acids, diglycerides, and acetone mobile phospholipids); (4) lower levels of phospholipids and protein; (5) higher contents of non-methylene interrupted dienoic fatty acids (22:2 NMI); (6) a decrease in energy metabolism activity (citrate synthase and cytochrome oxidase activities); (7) a higher oxidative stress (lipid peroxidation level); and (8) an activation of antioxidant defences before mortality (glutathione peroxidase and superoxide dismutase). We hypothesized that mortality emergence was related to higher energy consumption coupled with the progressive decline in feeding activity, lowered energy reserves and a decrease in energy metabolism activity. Thus, the low energy availability limited the efficiency of antioxidant defenses, resulting in a higher oxidative stress.

Mass mortalities of Pacific oysters Crassostrea gigas reflect infectious diseases and vary with farming practices in the Mediterranean Thau lagoon, France

“…However, CS activity was similar regardless of the pathological status of oysters (Table 4). This result contrasts with a previous report of decreased CS and cytochrome oxidase activities during a mortality event in Crassostrea virginica larvae (Genard et al 2011). Similarly, in juvenile Manila clams, respiration rate decreased with the development of brown ring disease symptoms (Flye-SainteMarie et al 2007).…”

“…Activity of HK in labial palps of oysters varies with feeding ration (Bacca 2007, p. 123). Therefore, the reduction in HK activity observed in infected oysters may reflect a decrease in feeding activity, which is symptomatic of pathogen infection in bivalves (Flye-Sainte-Marie et al 2007, Genard et al 2011.…”

Effect of the probiotic strainPhaeobacter gallaeciensisafter bacterial challenge on the complete larval development ofPecten maximus

“…Indeed, we measured an activation of antioxidant defenses (CAT and SOD) 48 h after pathogen exposition in X34 treated and untreated larvae with significantly higher levels in untreated larvae, while no changes in lipids peroxidation (MDA) were recorded. After Genard et al (2011Genard et al ( , 2013, the activation of antioxidant defenses after a bacterial challenge could be associated to the immune response activation in order to manage the production of various reactive oxygen species (ROS) during the intracellular destruction of phagocytized bacterial material, thus avoiding the peroxidation of host tissues. In support of this, our results indicate that larvae developed an accurate immune response triggered by the bacterial infection.…”

“…All of these analyses were performed using a microplate reader (Bioteck Instruments Inc., USA) with methods described in Genard et al (2011). Briefly, samples were homogenized on ice in phosphate buffered saline (PBS) containing 0.1% of triton X-100.…”

“…Therefore, in this study, we aimed to evaluate the probiotic potential of the "X34" strain of Phaeobacter gallaeciensis on the entire larval development (from veliger stages to metamorphosis) of P. maximus, using a bacterial challenge experiment in conditions closer to those used in hatchery. Furthermore, in order to validate the efficiency of the larvae's response to the bacterial challenge, we measured the activation of antioxidant processes as an indicator of the physiological response of larvae to bacterial infection, as proposed by Genard et al (2011Genard et al ( , 2013. We test the hypothesis that a probiotics pre-treatment of several days (to allow colonization of larvae by the probiotics) protect P. maximus larvae against Vibrio pathogen and decrease the antioxidant responses of larvae in contact to pathogen.…”