HATCHERY MORTALITIES OF LARVAL OYSTERS CAUSED BY Vibrio tubiashii AND Vibrio coralliilyticus

Hatchery production of bivalve shellfish has been hampered by the occasional presence of opportunistic pathogens, particularly Vibrio coralliilyticus and Vibrio tubiashii.  The present study reports the results of several avenues of research to better define these pathogens and the roles they play in eliciting larval oyster mortalities.  Many of the outbreaks originally attributed to V. tubiashii were actually determined by DNA sequencing to be from V. coralliilyticus, better known as a coral pathogen.  We sequenced a V. tubiashii (ATCC 19109) and showed that it consists of over 5.5 million base pairs on two chromosomes and four plasmids.  We also sequenced a highly pathogenic V. coralliilyticus (strain RE98) which has two chromosomes and two plasmids totaling over 6 million base pairs.  Some significant differences can be noted between these two pathogens, including the presence of a zinc-metalloprotease gene in V. coralliilyticus which is missing in V. tubiashii.  By means of this gene, the presence of V. coralliilyticus can be confirmed by PCR or a novel dipstick assay that is currently under development.  

The relative lethal doses of V. coralliilyticus and V. tubiashii in larval Eastern oysters (Crassostrea virginica) and Pacific oysters (C. gigas) were identified.  The LD₅₀ values, calculated at 6 days post challenge with V. coralliilyticus, ranged from 1.1 × 10⁴ to 4.0 × 10⁴ cfu/ml seawater.  In contrast, the V. tubiashii strains were highly infectious toward Eastern oyster larvae (LD₅₀ = 3.8 × 10³ to 1.2 × 10⁴ cfu/ml) but were noninfectious toward healthy Pacific oyster larvae.  Vibrio coralliilyticus has been repeatedly identified as a pathogen of larval Pacific oysters, but the presence of V. coralliilyticus in East Coast hatcheries has not been reported.  We identified the first V. coralliilyticus-associated outbreak in an East Coast hatchery and showed high levels of V. coralliilyticus in sick larvae, low levels in seemingly healthy larvae, and low to moderate levels in algal cultures.  The number of spawns required, millions of larvae produced through metamorphosis, and percent production losses by month are shown in Table 1.  Overall, a 34% loss was observed in 2015 and was associated with the presence of V. coralliilyticus.