Stress and stress-induced neuroendocrine changes increase the susceptibility of juvenile oysters (Crassostrea gigas) to Vibrio splendidus

Oysters are permanently exposed to various microbes, and their defense syst em is continuously solicited to prevent accumulation of invading and pathog enic organisms. Therefore, impairment of the animal's defense system usuall y results in mass mortalities in cultured oyster stocks or increased bacter ial loads in food products intended for human consumption. In the present s tudy, experiments were conducted to examine the effects of stress on the ju venile oyster's resistance to the oyster pathogen Vibrio splendidus. Oyster s (Crassostrea gigas) were challenged with a low dose of a pathogenic V. sp lendidus strain and subjected Co a mechanical stress V days later. Both mor tality and V. splendidus loads increased in stressed oysters, whereas they remained low in unstressed animals, Injection of noradrenaline or adrenocor ticotropic hormone, two key components of the oyster neuroendocrine stress response system, also caused higher mortality and increased accumulation of I;. splendidus in challenged oysters. These results suggest that the physi ological changes imposed by stress, or stress hormones, influenced host-pat hogen interactions in oysters and increased juvenile C. gigas vulnerability to Vibrio splendidus.