The combined effects of low salinity and Dermo on oysters from the St. Lucie Estuary

The viability of the eastern oyster, Crassostrea virginica, is designated as an ecological performance measure for the management of freshwater inflows to the St. Lucie Estuary, Florida. Thus, oysters derived from the St. Lucie Estuary were tested for their physiological response to stress, measured as altered condition index and RNA/DNA ratios, resulting from changes in salinity and infection by the protozoan parasite Perkinsus marinus, the agent of Dermo, a common oyster disease. Pilot studies consisted of the development of a real-time PCR-based assay for P. marinus infection, procedures to infect oysters with the pathogen by injection method and procedures for the measurement of RNA/DNA ratios. The general experimental design was to assess the response of non-injected and injected C. virginica to low salinity challenges. Two scenarios for salinity stress were tested: one in which oysters were subjected to a single reduction in salinity and one in which an initial reduction in salinity was followed by a recovery phase and then subjected to a second challenge of reduced salinity. Condition index was more responsive to changes in salinity regimes than to P. marinus infection. Changes in the RNA/DNA ratio were responsive to the infection status, but not changes in salinity; the pattern of change in the RNA/DNA ratio generally followed changes in the measured levels of infection. The lack of mortalities showed that these oysters were able to tolerate short periods of reduced salinity.