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...

Very few studies have analysed the niches of pelagic protist in details. This is because for most protists, both an accurate species definition and methods for routine detection and quantification of cells are lacking. The morphospecies Micromonas pusilla, a marine unicellular green alga, is the most ubiquitous and cosmopolitan picoeukaryote described to date. This species comprises several independent genetic lineages or clades, which are not currently distinguishable based on comparison of their morphology or biogeographical distribution. Molecular probes were used to detect and quantify the genetic clades of M. pusilla in samples from temperate, polar and tropical environments in order to assess potential ecological niche partitioning. The three clades were detected in all biogeographical regions studied and were commonly found in sympatry. Cell abundances recorded for clades A and B were high, especially at coastal stations. Clade C, when detected, was always at low abundances and is suggested to be a low-light clade. Shifts in the contribution of clades to total M. pusilla abundance were observed along environmental gradients, both at local and basin-wide scales. This suggests that the phylogenetic clades occupy specific niches and confirms the existence of cryptic species within the morphospecies M. pusilla. Parameters which can precisely explain the distribution of these cryptic species remain to be elucidated.

Oysters are permanently exposed to various microbes, and their defense system is continuously solicited to prevent accumulation of invading and pathogenic organisms. Therefore, impairment of the animal's defense system usually results in mass mortalities in cultured oyster stocks or increased bacterial loads in food products intended for human consumption. In the present study, experiments were conducted to examine the effects of stress on the juvenile oyster's resistance to the oyster pathogen Vibrio splendidus. Oysters (Crassostrea gigas) were challenged with a low dose of a pathogenic V. splendidus strain and subjected to a mechanical stress 3 days later. Both mortality and V. splendidus loads increased in stressed oysters, whereas they remained low in unstressed animals. Injection of noradrenaline or adrenocorticotropic hormone, two key components of the oyster neuroendocrine stress response system, also caused higher mortality and increased accumulation of V. splendidus in challenged oysters. These results suggest that the physiological changes imposed by stress, or stress hormones, influenced host-pathogen interactions in oysters and increased juvenile C. gigas vulnerability to Vibrio splendidus.