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Ocean acidification (OA), a consequence of the increase in anthropogenic emissions of carbon dioxide, causes major changes in the chemistry of carbonates in the ocean with deleterious effects on calcifying organisms. The pH/PCO2 range to which species are exposed in nature is important to consider when interpreting the response of coastal organisms to OA. In this context, emerging approaches, which assess the reaction norms of organisms to a wide pH gradient, are improving our understanding of tolerance thresholds and acclimation potential to OA. In this study, we deciphered the reaction norms of two oyster species living in contrasting habitats: the intertidal oyster Crassostrea gigas and the subtidal flat oyster Ostrea edulis, which are two economically and ecologically valuable species in temperate ecosystems. Six-month-old oysters of each species were exposed in common garden tanks for 48 days to a pH gradient ranging from 7.7 to 6.4 (total scale). Both species were tolerant down to a pH of 6.6 with high plasticity in fitness-related traits such as survival and growth. However, oysters underwent remodelling of membrane fatty acids to cope with decreasing pH along with shell bleaching impairing shell integrity and consequently animal fitness. Finally, our work revealed species-specific physiological responses and highlights that intertidal C. gigas seem to have a better acclimation potential to rapid and extreme OA changes than O. edulis. Overall, our study provides important data about the phenotypic plasticity and its limits in two oyster species, which is essential for assessing the challenges posed to marine organisms by OA.

The purpose of this study was to examine the lipid class content of larval stages of the sea scallop Placopecten magellanicus during development and to examine the potential effects of varying feeding regimes on larval lipid content, growth, survival, settlement behavior, and survival of postlarval stage. The potential of lipid class ratios to forecast larval growth, survival, settlement behavior, and success was examined. At the start of exogenous feeding (day 4) three diets, which differed in triacylglycerol (TAG) content, were applied. Diet A consisted of Isochrysis sp. and Pavlova lutheri, diet B was a mix of Isochrysis sp. and Chaetoceros muelleri, and diet C consisted of the same two species, but C. muelleri was grown under silicate deprivation to enhance TAG accumulation. Larvae were periodically sampled for lipid class analysis, growth measurement and survival assessment. Behavior of pediveliger larvae for each diet was recorded using an endoscopic camera during settlement. Experiments were replicated twice and repeated 1 mo later. Our study shows that TAG level in larval food was positively correlated with growth rate, larval TAG content and, as a consequence, larval "quality," as measured by TAG-sterol (ST) or TAG-phospholipid (PL) ratios, prior to settlement. A positive relation between number of competent larvae produced and larval quality at day 8 was found, suggesting that survival at competency was partly explained by the recovery efficiency of energetic reserves as TAG after embryogenesis. Higher growth rates obtained with the diet enriched with TAG reflect its high caloric content and the presence of sufficient essential fatty acids. TAG-ST ratio of competent larvae was negatively correlated with settlement success (day 40). High... [ABSTRACT FROM AUTHOR]

The aim of this study was to test the validity of a lipid based indicator of larval quality of sea scallop Placopecten magellanicus. Objectives were 2-fold: (1) to determine the link between lipid class content and reproductive state of adults in the field and in the laboratory and (2) to follow lipid class content, growth, and survival during embryonic and early larval development. Adult scallops were periodically sampled during gametogenesis for lipid class and histological analysis of the gonads in the field at two locations and in the laboratory after feeding three different diets. Females were induced to spawn and lipid class content, larval growth, and survival of five batches of eggs were followed for 8 days after fertilization. Site, diet, and time had significant effects on lipid class composition of male and female gonads and gametogenesis of females. Triacyglycerol accumulation during vitellogenesis was characteristic of female gonads and explained respectively 56.4% and 71.3% of the variability in maturity and egg size. When spawning was induced, no major effect of location or diet on lipid composition of gonad and subsequent eggs was detected. Nevertheless, the mean number of eggs produced by females increased with atresia level in gonad, suggesting that egg quantity was incompatible with egg quality. Lipid class composition during embryogenesis and young larval development showed a high demand for triacyglycerol. [ABSTRACT FROM AUTHOR]

Of all environmental factors, seawater temperature plays a decisive role in triggering marine diseases. Like fever in vertebrates, high seawater temperature could modulate the host response to pathogens in ectothermic animals. In France, massive mortality of Pacific oysters, Crassostrea gigas, caused by the ostreid herpesvirus 1 (OsHV-1) is markedly reduced when temperatures exceed 24°C in the field. In the present study we assess how high temperature influences the host response to the pathogen by comparing transcriptomes (RNA sequencing) during the course of experimental infection at 21°C (reference) and 29°C. We show that high temperature induced host physiological processes that are unfavorable to the viral infection. Temperature influenced the expression of transcripts related to the immune process and increased the transcription of genes related to the apoptotic process, synaptic signaling and protein processes at 29°C. Concomitantly, the expression of genes associated with catabolism, metabolite transport, macromolecule synthesis and cell growth remained low from the first stage of infection at 29°C. Moreover, viral entry into the host might have been limited at 29°C by changes in extracellular matrix composition and protein abundance. Overall, these results provide new insights into how environmental factors modulate host–pathogen interactions.