Clearance of Vibrio campbellii injected into the hemolymph of Callinectes sapidus, the Atlantic blue crab: The effects of prior exposure to bacteria and environmental hypoxia

The Atlantic blue crab, Callinectes sapidus   (Rathbun), lives in a bacteria-rich environment that experiences daily fluctuations in water quality. In the present study, we tested the hypothesis that crustaceans with prior or ongoing exposure to bacteria in their hemolymph have an increased susceptibility to subsequent infections, and that acute exposure to low dissolved oxygen (hypoxia) and elevated carbon dioxide levels (hypercapnia) may further confound the ability of blue crabs to counter a subsequent infection. Adult male blue crabs held in well-aerated (normoxic; PO2=20.7kPa; CO2 < 0.06 kPa; pH 7.8–8.0) or hypercapnic hypoxic (HH; PO2=4kPa; CO2 = 1.8 kPa; pH 6.9–7.2) seawater received an injection (pre-challenge dose) of 1 × 105Vibrio campbellii g−1 crab. Control animals were injected with an equivalent dose of HEPES-buffered saline (1 μl g−1 crab). At 2 h or 24 h after the pre-challenge injection, both Vibrio and saline-pre-challenged animals were injected with a dose of live V. campbellii (1 × 105 g−1 crab). This second injection will be referred to as a second injection or challenge injection. Degradation in or physical removal of intact bacteria from hemolymph was quantified using real-time PCR; bacteriostasis was quantified as the percentage of intact bacteria that could not be recovered by selective plating. We demonstrated that bacteriostasis occurs in the hemolymph of blue crabs. Furthermore, blue crabs that received a challenge injection 2 h after a pre-challenge dose of V. campbellii cleared culturable bacteria from their hemolymph more rapidly when compared to animals that received a pre-challenge dose of saline. This enhanced clearance of culturable bacteria was associated with an increase in antibacterial activity in the cell-free hemolymph. However, the enhanced clearance of culturable bacteria disappeared when the time interval between the pre-challenge and challenge dose was extended to 24 h and when crabs were held in HH seawater throughout the experiment. Neither the time interval between the pre-challenge and the challenge dose nor exposure to HH altered the pattern of intact bacterial clearance in blue crabs. These results demonstrate that prior exposure to bacteria does not increase the susceptibility of C. sapidus to a second, sublethal dose of V. campbellii. In fact, a recent exposure to V. campbellii enhances the ability of blue crabs to render bacteria non-culturable and the immune mechanisms/effectors responsible for this are short lived and appear to be sensitive to low dissolved oxygen and high carbon dioxide concentrations in the environment.