Involvement of <i>Vibrio coralliilyticus</i> and Other <i>Vibrio</i> Species in Mass Mortality of the Pacific Oyster <i>Crassostrea gigas</i> Larvae in a Japanese Hatchery

The mass mortality episodes of the Pacific oyster Crassostrea gigas larvae, which occurred in a Japanese hatchery between 2013 and 2014, were investigated bacteriologically. A total of 77 bacterial isolates were obtained from dead larvae, seawater from rearing tanks, and other sources. Immersion infection challenge experiments revealed that 34 of the 77 isolates were highly virulent towards C. gigas larvae, with the LD50 of less than 105 CFU/mL. These virulent isolates were identified as Vibrio coralliiyticus (n = 28), V. tubiashii (n = 3), and unknown Vibrio species (n = 3), based on PCR and partial sequence analyses of three genes (16S rRNA, recA, rpoA). This indicates that both V. coralliilyticus and V. tubiashii, which have long been known worldwide as important pathogens of hatchery-reared bivalve mollusk larvae, are involved in mass mortality of C. gigas larvae cultured in the hatchery. Furthermore, some virulent isolates from diseased oyster larvae in the same hatchery in 1997, which were previously identified as V. splendidus biovar II using conventional phenotypic characterization tests, were re-identified as V. coralliilyticus. These results imply that V. coralliilyticus is a major pathogen of C. gigas larvae in the hatchery.