USING PROBIOTIC BACTERIA TO REDUCE LARVAL MORTALITY OF Crassostrea gigas DUE TO THE BACTERIAL PATHOGEN Vibrio coralliilyticus AND IMPROVE METAMORPHOSIS

High mortalities of Pacific oyster (Crassostrea gigas) larvae and spat associated with an increase in concentrations of the pathogenic bacterium Vibrio coralliilyticus (Vcor), are commonly reported. This study examined the ability of probiotic bacteria, which inhibited the growth of Vcor cultured on agar plates, to reduce mortality of Pacific oyster larvae in the presence of Vcor. We have identified three high-performing probiotic strains that significantly reduced mortality of larvae exposed to lethal concentrations of Vcor when added 24 hours post egg fertilization. Larval mortality was minimized to background levels when all three probiotics were added in combination (Figure 1).

A thirty-day experiment showed that a single addition of the probiotic mix to larval cultures resulted in a significant increase in larval shell length 12 days post fertilization (Larvae only mean=153.0 ± SE 1.5µm, Probiotics mean=164.5 ± SE 2.6µm;(T test: t(8) = 3.86, P=0.0048), and greatly increased the proportion of larvae that successfully metamorphosed and grew for at least six days as spat (Figure 2). The ability of these probiotics to reduce larval oyster mortality in the presence of Vcor pathogens and to improve metamorphosis success independent of disease challenge could greatly improve the consistency and production of seed oysters in hatcheries.