Evaluating a short vs. long-term progeny test and investigating physiology associated with survival in extreme low salinity for the eastern oyster Crassostrea virginica

Low salinity negatively affects growth and survival in the eastern oyster, which decreases productivity of aquaculture operations along the mid-Atlantic and Gulf Coasts of the United States. With heavy rainfall events predicted to become more frequent, coastal aquaculture operations face increased risk of prolonged exposure to extreme low salinity conditions. While recent experimental work has determined that survival in extreme low salinity (< 3) is a moderately heritable trait in the eastern oyster, further experimental challenges were conducted to investigate the importance of challenge duration and temperature on the survival phenotype and estimation of its genetic parameters to hone the experimental challenge for potential incorporation into a breeding program. Growth (shell height) and algal removal were also assessed to investigate physiological phenotypes associated with differential survival in extreme low salinity. A subset of individuals from 51 half-sibling families were exposed to one of two experimental challenges: a short-term low salinity (2.5) challenge at constant temperature (27 °C) for 2 months, or a long-term low salinity (2.5) challenge where temperature was adjusted daily to match local ambient conditions for 6-months. Differential mortality was observed across families for both the short-term and long-term low salinity challenges, and narrow-sense heritability estimates were similar for both challenges (short-term h2 = 0.35, long-term h2 = 0.4). Strong phenotypic (rS = 0.89) and genetic (rG = 0.81) correlations for family mortality were found between challenges. Algal clearance metrics over a 24-h clearance experiment differed among families (p < 0.001), but were only weakly associated with family survival in the long-term low salinity exposure (range p = 0.08–0.23, R2 = 5–10%). Growth was negligible during the long-term challenge. However, after being returned to ambient salinities (13), there was no difference in growth rate, wet weight gain, or mortality between oysters from families with low or high survival in the long-term exposure (p > 0.05), indicating that low salinity tolerant oysters can recover following low salinity events. This work shows that a short-term (2 month) low salinity (2.5) challenge at a constant temperature captures the same patterns of family mortality as a 6-month, temperature fluctuating challenge. Additionally, measuring individual oyster clearance rate and the parameters derived from the algal removal curves provide additional insight into the physiological status of oysters under extreme low salinity stress.