THE EFFECT OF pH ON THE KILLING ACTIVITY OF HEMOCYTES IN THE PACIFIC OYSTER, CRASSOSTREA GIGAS     Steven M. Allen and Louis Burnett Grice Marine Laboratory, University of Charleston South Carolina, 205 Fort Johnson Rd., Charleston, SC     In recent years there has been an increase in the occurrence of summer mortalities of the commercially important Pacific oyster, Crassostrea gigas.  These mortalities occur during the late summer when water and air temperatures are at their highest.  C. gigas are grown intertidally and are, therefore, air exposed for hours at a time.  An oyster closed during air exposure depletes the oxygen stores within the shell and builds up CO2 acidifying the tissues.  The  average pH of hemolymph from an oyster which is submerged in well aerated water (18°C) and ventilating is 7.52 (0.04 SEM; N=35).  The average pH of of hemolymph from an oyster which is aerially exposed for 4h in 30°C air is 6.83 (0.02 SEM; N=26).  We hypothesize that stresses associated with air exposure inhibit the immune system of the oyster and contribute to the summer mortalities.  The focus of the present study was to determine if the innate immunity provided by hemocytes was decreased by low pH.  The ability of hemocytes to kill the bacterium Vibrio parahaemolyticus was assessed using an in vitro killing assay.  Hemocytes were  treated with low pH and challenged in vitro with V. parahaemolyticus.  A tetrazolium dye reduction assay was used to quantify the number of viable bacteria, from which a killing index was calculated.  No significant difference was found between the two treatments pH 7.6 and pH 6.6 (p < 0.01; N=14).   ODRP Grant No. NA96RG0488