The northern quahog dominates shellfish aquaculture in the state of Florida and is ranked 3rd in the nation for total production. The clam industry in Florida faces two environmental challenges, extended low salinity caused by hurricanes and heavy storms. In this study, the feasibility of improving low salinity tolerance in northern quahogs was evaluated using a challenge experiment on ~1-year-old clams from 4 distinct groups. These groups consist of two commercially produced stocks (Com and BF) and two research-produced stocks (HS and Ctl), one of which is the F1 offspring of heat-tolerant broodstock. Tolerance to acute low salinity stress was conducted at 5 ppt for 18 days by decreasing salinity from 22 ppt to 5 ppt over 6 days. Any mortalities observed during the challenge period were recorded and tissue was taken for genotyping and parentage analysis. The clams were brought back to 22 ppt over 4 days. All survivors at LD 50 were remeasured and weighed to compare the amount of stress applied throughout the challenge. Siphon clips and hemolymph samples were taken of survivors for later genotyping and the biopsied survivors were deployed in a fluctuating-salinity lease site for continued culture.
During the low-salinity challenge, peak mortality occurred on day 18 at 5 ppt, which coincided with LD 50 for the challenge. Of the 4 stocks of clams used for this challenge, the HS stock, which was the F1 heat shock-resistant clams, showed the most susceptible with a significantly lower survival rate. After 8-month of cultivating in the field, the survival and biometrics of these clams were recorded. Both research stocks had a high survival rate (HS-FC = 71.43% and Ctl-FC = 65.95%) compared to the commercially produced stocks (Com-FC = 39.47% and BF-FC = 47.42%).
These final survivors will be used as broodstock for F1 offspring production and will be genotyped for heritability of tolerance and genetic correlations analysis. This study is an initiation of low-salinity clam breeding. It is expected that the results will lay the foundation for further clam breeding for resistance to low salinity and summer heat stress to support the sustainability of the Florida clam industry.