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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Stress is thought to cause increased disease outbreaks and mortality in a number of invertebrates but currently very little information is available on mechanisms linking physiological states of stress and reduced disease resistance in these organisms. In the present study, we examined the possibility that stress alters immune functions, the principal line of defense against pathogens, in a molluscan model, the abalone Haliotis turbeculata. Immune parameters were investigated in abalones subjected to a 15 min mechanical disturbance which, as indicated by noradrenaline and dopamine hemolymphatic levels, resulted in a transient state of physiological stress. During the application of the stressor, immune parameters such as the number of circulating hemocytes, the migratory activity, the phagocytic capacity and the respiratory burst responses of hemocytes, decreased significantly. All parameters returned to initial values within 15-30 min after the end of the disturbance and a transient period of immunostimulation occurred between 100 and 480 min after the stress for all immune parameters except intracellular superoxide anion production. These results indicate that in the abalone H. tuberculata, as in vertebrates, a link exists between stress and the immune system. This may begin to answer why stress and disease outbreaks are linked in shellfish.

Catecholamines and pro-opiomelanocortin (POMC)-derived peptides, some of the central regulators of the stress-response systems of vertebrates, are also present in invertebrates. However, studies are needed to determine how these hormones participate in the organisation of neuroendocrine stress-response axes in invertebrates. Our present work provides evidence for the presence of an adrenergic stress-response system in the oyster Crassostrea gigas. Noradrenaline and dopamine are released into the circulation in response to stress. Storage and release of these hormones take place in neurosecretory cells presenting morphological and biochemical similarities with vertebrate chromaffin cells. Both in vivo and in vitro experiments showed that applications of the neurotransmitters acetylcholine or carbachol caused no significant release of noradrenaline or dopamine. Moreover, the nicotinic antagonists hexamethonium and α -bungarotoxin and the muscarinic antagonist atropine caused no significant inhibition of catecholamine release in stressed oysters. Adrenocorticotropic hormone (ACTH) induced a significant release of noradrenaline, but the release of dopamine in response to ACTH was not significant. These results suggest that, unlike that of vertebrates, the adrenergic stress-response system of oysters is not under the control of acetylcholine and that other factors, such as the neuropeptide ACTH, might control this system.