PATE, S.E.*; CHECK, J.F.; ROER, R.D.; MOUSTAKAS, C.T.: Effects of size and salinity on sodium and water permeability in Callinectes sapidus
In order to understand the magnitude of the osmoregulatory stress imposed upon juvenile blue crabs in low salinities, the water and sodium permeabilities of these crabs were measured after acclimation to either sea water (1000 mOsm) or dilute sea water (150 mOsm) and compared to adult and sub-adult specimens. Crabs were incubated in their acclimation medium to which either 3H2O or 22NaCl was added. After at least 24 h equilibration, washout of the isotope was monitored into non-radioactive media. The rate of passive efflux was used as an index of relative permeabilities. In all cases, there was an inverse logarithmic relationship between size and permeability, typical of metabolic rate scaling functions. Acclimation to 150 mOsm induced a decrease in Na permeability for crabs of all sizes, but the decrease was greater for juvenile crabs than for adults. Water efflux was unchanged in both adult and juvenile crabs acclimated to150 mOsm relative to the rate in sea water. Because the osmotic gradient is much larger at low salinity, these data reflect a decrease in water permeability. While juvenile crabs are capable of marked reductions of Na and water permeability in low salinities, the data suggest that this is not sufficient to completely offset a higher metabolic cost associated with osmoregulation relative to adults.