Juvenile oysters Crassostrea gigas cultured in the Bay of Morlaix (France) have suffered unexplained summer mortalities for over a decade. In the present study, we tested the hypothesis that a bacterial pathogen could be responsible for this phenomenon. A first attempt failed to isolate a bacterial pathogen from moribund or weak oysters. Only non-pathogenic, probably opportunistic, bacteria were isolated. As an alternative approach, we focused on oysters presenting reduced stressresponse capacities (determined by circulating noradrenaline measurements), a characteristic of juvenile oysters entering an early phase of the disease. Cultures of bacterial isolates on TCBS plates revealed that a Vibrio strain was present in diseased oysters and scarce or absent in healthy oysters. Experimental infections indicated that this Vibrio can cause mortalities of juvenile oysters when injected at concentrations ranging from 10 4 to 10 8 CFU oyster -1. Similarly to the summer mortality disease, the Vibrio isolate caused higher mortalities at higher temperatures; apparently, it could not be transmitted horizontally, it did not affect adult oysters and it induced stress-response dysfunctions in juvenile oysters. Phenotypic and genotypic characterizations identified the pathogen as Vibrio splendidus. Taken together, the present results satisfy Koch's postulate and suggest that this bacterial strain is probably responsible for the juvenile oyster summer mortalities in the Bay of Morlaix. KEY WORDS: Crassostrea gigas · Summer mortality · Juveniles · Vibrio splendidus · Stress · Noradrenaline Resale or republication not permitted without written consent of the publisherDis Aquat Org 46: [139][140][141][142][143][144][145] 2001 tality rates decreased within 24 to 48 h . A similar approach gave similar results when applied to Crassostrea virginica to elucidate the etiology of the JOD (Boettcher et al. 1999, Elston 1999b). In addition, previous studies indicated that 2 to 3 wk before mortalities occur, juvenile oysters showed signs of neuroendocrine system dysfunction. Indeed, the stress-induced noradrenaline (NA) responses were reduced in these oysters . In the present study, juvenile oysters presenting this early sign were selected to test the hypothesis that a bacterial pathogen was responsible for the juvenile oyster summer mortalities observed in the Bay of Morlaix. MATERIALS AND METHODSOysters. Twenty batches (n ≥ 500 organisms per batch) of juvenile Crassostrea gigas oysters originating from different hatchery or oyster farm stocks were placed on an experimental field site in the Bay of Morlaix between May and September 1999. They consisted of 2 reference batches, 1 wild-caught batch (named Batch B) and 1 hatchery produced batch (named Batch V), which experienced low mortality (< 5%), and of 18 other wild-caught or hatchery produced batches which experienced 10 to 65% mortality (including Batch RRB, which suffered 63.75% mortality). Juvenile oysters belonging to batches exhibiting > 45% mortality were termed 'natura...

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New experiments comparing the effects of 3 species of phytoplankton Prorocentrum minimum Schiller 1933 (PM), Thalassiosira rotula Meunier 1910 strains (TR1) and (TR2), and Skeletonema pseudocostatum (SPC) on the fecundity and egg-hatching rates of Calanus helgolandicus females are described. To further determine the chemical factors causing the inhibitory process related to diatoms, the biological findings were linked with an analysis of aldehydic oxylipins (metabolites derived from the oxidative transformation of fatty acids). Members of this compound class have previously been reported to inhibit copepod egg development. Comparison of the inhibitory properties with results from the chemical analysis, as well as bioassays with synthetic samples using sea urchin eggs as a model system, showed that the observed effect is not restricted to the previously identified volatile aldehydes decadienal and decatrienal but, in fact, depends on a reactive structural element (α,β,γ,δ-unsaturated aldehyde). In addition, the fatty acid content of the algae was verified with special emphasis on eicosapentaenoic acid (EPA), which is currently under discussion for its metabolic value and defensive potential. Our results demonstrate that only certain diatoms inhibit hatching in copepods and that this effect is independent of the EPA content of the diet. Moreover, saturated aldehydes, like the widely distributed tridecanal, did not affect our assay with sea urchin eggs. Since the observed hatching inhibition, as well as the capability for the formation of reactive aldehydes, is highly species and even isolate dependent, it would appear that no prediction of food quality of certain species can be given without a detailed analysis.KEY WORDS: Phytoplankton · Oxylipin · Diatom · Copepod · Hatching · Inhibition · Activated defence Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 245: [33][34][35][36][37][38][39][40][41][42][43][44][45] 2002 nature of the factors that have anti-proliferative properties in marine pelagic algae and their ecological relevance. Miralto et al. (1999) identified reactive volatile aldehydes from the diatom Thalassiosira rotula Meunier 1910 (TR1) with the ability to inhibit egg hatching in copepods. As shown in Fig. 1, these aldehydes, namely 2E,4Z-decadienal (hereafter decadienal) and 2E,4Z,7Z-decatrienal (hereafter decatrienal) are derived from arachidonic acid (C20:4 n-6) and eicosapentaenoic acid (EPA, C20:5 n-3), respectively (Pohnert 2000). These metabolites thus belong to the oxylipins, a class of compounds that is by definition derived from the oxidative transformation of fatty acids. Their formation is initialised by the action of lipoxygenases, an enzyme class that has been previously detected to be active in numerous diatoms ( Fig. 1) (for a review see Pohnert & Boland 2002). For example, the freshwater diatom Gomphonema parvulum uses EPA as a precursor for the formation of hormosirene, a volatile C11-hydrocarbon released together with a seco...