NORDEEN, J.M.*; SHEFFIELD, E.M.; KINSEY, S.T.: Size- and salinity-dependent enzyme activities in gills of the blue crab

In the lower Cape Fear River estuary, populations of juvenile (< 12 mm carapace width) blue crabs, Callinectes sapidus, are found at low salinity (<5 ppt). This distribution pattern may have ecological benefits, such as predator avoidance, but it also leads to increased metabolic costs associated with ATP-dependent osmoregulation. The osmoregulatory costs should be particularly high for the smallest animals, which have the highest surface area-to-volume ratio. We hypothesized that the increased metabolic cost of size-dependent osmoregulation at low salinities would be reflected in the activity of enzymes of energy metabolism in the gills. We measured activities of hexokinase, lactate dehydrogenase, citrate synthase, glutamate dehydrogenase and hydroxyacyl-CoA-dehydrogenase as a function of animal size and acclimation salinity. Measurements were made on posterior gills (6 and 7), which are principally involved with osmoregulation, and in anterior gills (3 and 4), which are principally respiratory. Salinity treatments were 7 days acclimation to 5, 17 or 34 ppt and four size classes were selected from the smallest juveniles to adults. The data were interpreted both in terms of salinity treatment and animal scaling laws.