Respiration and energy metabolism during exercise in Callinectes sapidus, the Atlantic blue crab: Effects of the bacterial pathogen Vibrio campbellii
Thibodeaux, Lindy Kay
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The Atlantic blue crab, Callinectes sapidus, lives in estuarine and coastal environments where exposure to disease-causing microorganisms occurs. Bacteria from the genus Vibrio are naturally abundant in marine environments and are commonly associated with the exoskeleton and within the hemolymph of blue crabs and have the potential to cause mortality in these organisms. First, the impact of the marine bacterium Vibrio campbellii on survival of blue crabs was assessed. Three replicate trials of an LD50 bacteria challenge assay were performed. We determined that V. campbellii can cause mortality in blue crabs with an average 48 h LD50 value of 6.2 x 10^5 CFU g^-1 crab. Secondly, the effect of bacterial exposure on metabolism was determined following injection of V. campbellii or saline (control). Injection of V. campbellii caused a 30% reduction in resting oxygen uptake 4 h after injection, which decreased further to 42% at 24 h, accompanied only by a small increase in circulating hemolymph lactate. These data support the hypothesis that hemocyte aggregates forming around bacteria in the gills interfere with normal respiratory and circulatory function of the gills. Since blue crabs depend on the ability to effectively move around in their environment to survive, we also investigated how exposure to bacteria affects the energetic costs of 30 min of walking at 8 m min ^-1. Blue crabs more than doubled their aerobic and anaerobic metabolism in response to moderate walking in well-oxygenated conditions. However, exposure to V. campbellii caused a significant depression in aerobic metabolism during and after walking supplemented only by a small increase in anaerobic metabolism. Patterns of phosphagen and adenylate consumption within an active muscle were not affected. The ability of blue crabs to supply the necessary energy requirements of walking is remarkably and largely unaffected by treatment with Vibrio; however, Vibrio-injected crabs are less aerobic while doing so. This depressed metabolic condition in response to bacteria, present even during moderate activity, could be caused by a gill blockage by hemocyte aggregations in response to bacteria or may represent an internally regulated depression of metabolism.