IKERD, J.L.**; BURNETT, K.G.; BURNETT, L.E.; College of Charleston, South Carolina; College of Charleston, South Carolina; College of Charleston, South Carolina: Effects of salinity on the accumulation of the bacterium Vibrio campbellii in the gills of the blue crab, Callinectes sapidus (Rathbun)

When hemocytes, the major cellular component of the crustacean immune system, come in contact with bacteria, they aggregate to form nodules. We have shown that hemocyte aggregates accumulate in the gills of Vibrio-injected crabs, blocking the flow of hemolymph and decreasing oxygen uptake. This blockage may also affect ionic regulation at low salinities. At low salinity, gill epithelia thicken, especially in the posterior gills. We suggest that hemocyte aggregates more easily block the smaller hemolymph spaces in the gills of crabs in low salinity compared to high salinity. Greater blockage should result in fewer Vibrio entering the gill. Thus, we would expect to see smaller numbers of culturable Vibrio in the gills of low salinity crabs and especially the posterior gills. In the present study crabs were acclimated to either 30 ppt or 10 ppt, and injected with V. campbellii (2.5 x 104/g crab) which expresses green fluorescent protein and resistance to the antibiotics kanamycin and chloramphenicol. Thirty minutes after injection, individual gills were removed, homogenized, and the number of Vibrio colony forming units (CFU) were measured. Mean Vibrio CFU/g gill were significantly higher in crabs held at 30 ppt (66,956 3,146 SEM; n=7) than in crabs held at 10 ppt (30,084 1,467 SEM; n=7) (P<0.05). At both salinities, there are significantly smaller numbers of Vibrio in anterior gills, which are 3 times smaller than posterior gills, indicating that hemolymph spaces are smaller. These results suggest that gills in which epithelia have thickened in response to low salinity are more easily blocked when a crab is challenged with bacteria. (NSF IBN-0212921)