A comparative field study of Crassostrea ariakensis (Fujita 1913) and Crassostrea virginica (Gmelin 1791) in relation to salinity in Virginia

We examined survival, growth, and disease susceptibility of triploid Crassostrea ariakensis (= rivularis) and compared results with that of diploid Crassostrea virginica. Two hundred and fifty oysters (age = 2 yr, mean shell height = 60-64 mm) of each species were deployed at duplicate sites, (Chesapeake Bay, and the Atlantic Coast of Virginia) within low, medium, and high salinity regimes respectively (< 15%, 15-25%, > 25%). Over the course of the study, from June 1998 to September 1999, C. virginica exhibited low survival, modest growth and high disease susceptibility. In contrast, C. ariakensis exhibited high survival, high growth rate, and low disease susceptibility. At low salinity sites, final mean cumulative mortality of C. virginica (81%) was significantly higher than that of C. ariakensis (14%). At medium and high salinity sites, all C. virginica died before the end of the study whereas final mean cumulative mortality in C. ariakensis was 13 to 16%. After 1 year of deployment, mean shell height of C. virginica at low, moderate, and high salinity sites was respectively 70, 80 and 73 mm. In comparison, mean shell height of C. ariakensis was respectively 93, 121 and 137 mm. At low salinity sites, mean growth rate of C. virginica was not significantly different from that of C. ariakensis. At medium and high salinity sites, mean growth rate of C. virginica was significantly lower than that of C. ariakensis. Prevalence and intensity of Perkinsus marinus infections were significantly higher in C. virginica than in C. ariakensis. During the second summer of disease exposure, prevalence in C. virginica was 100% at all sites whereas in C. ariakensis it ranged from 0 to 28%. Heavy intensity of infections were prevalent in C. virginica whereas infections in C. ariakensis were limited to light intensity. Haplosporidium nelsoni (MSX) was present in C. virginica, but absent in C. ariakensis. Mud worms (Polydora spp.) were present in both oyster species, but infestations were low and did not appear to affect condition or growth. In summary, wide salinity tolerance and low disease susceptibility were associated with high survival and growth of C. ariakensis in Chesapeake Bay and the Atlantic Coast of Virginia.