EFFECTS OF SIZE AND SALINITY ON SODIUM AND WATER PERMEABILITY IN Callinectes sapidus

Susan Pate, Jennifer Check, Robert Roer & Constantinos Moustakas - Dept. of Biological Sciences

University of NC at Wilmington

ABSTRACTIn 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.

INTRODUCTION

Adult blue crabs have long been known to be extremely good hyperosmoregulators in low salinities (Tan & Van Engel, 1966). The hemolymph is maintained isosmotic to the medium down to an external salinity of 25 ppt (~725 mOsm) (Findley & Stickle, 1978). As the medium osmotic concentration decreases below this point, hemolymph osmolarity decreases slowly to an asymptotic value of ~600 mOsm. This value is maintained even into fresh water ([Na+] = 1.5 meq•l-1; [Cl-] = 3.0 meq•l-1) (Cameron, 1978).

The adaptation to life in a hypo-osmotic medium entails the production of large amounts of urine to combat osmotic water gain, and the active uptake of Na+ and Cl- by the gills to combat urinary and electrofusive (i.e. due to passive movement down an electrochemical gradient) salt loss. The primary adaptation is that of active salt uptake, since adult blue crabs demonstrate little of the reduction in osmotic water permeability or urinary salt loss that is evident in truly freshwater species (e.g. crayfish) (Cameron, 1978).

Juvenile crabs have a larger surface-to-volume ratio than adults, and should, therefore, have a far more difficult time maintaining blood osmolarities above that of the medium. The relative metabolic rate of small organisms is higher than that of large organism under similar conditions. The metabolic work associated with osmoregulation increases exponentially with the difference between blood and medium osmotic concentrations (Potts, 1954). Together, these considerations reflect a potentially huge metabolic cost of osmoregulation for juvenile crabs.

It is possible that, unlike adults, juvenile crabs could reduce this workload by decreasing their water and/or salt permeability. The aim of this study was to determine if this is the case.

METHODS

The efflux of water from the crab is due to diffusive exchange across the gills and excretion via the antennal gland to compensate for osmotic water gain. At steady state the net flux is zero, so the rate of influx must equal the rate of efflux. Together, these pathways represent the rate of water turnover in the crab and reflect its total water permeability. Reductions in osmotic water permeability would be reflected in a reduction in the rate of water gain and, hence, a reduction in unidirectional water efflux. To assess the rate of water efflux, crabs were weighed and placed into a beaker or small plastic aquarium containing a measured volume of their acclimation medium to which is added 1 µCi 3H2O per ml. Crabs were left in the radioactive medium for a minimum of 24h in order to reach equilibrium. Crabs were then rinsed in non-radioactive medium and placed in a measured volume of non-radioactive medium. Samples of the medium were taken over time. The rate of appearance of isotope was corrected for body weight and hemolymph specific activity to give the passive flux.

The efflux of Na from the crab represents the passive loss of this ion in response to its electrochemical gradient plus Na loss via the antennal gland. At steady state, the efflux is compensated by an equal and opposite influx due to the active uptake mechanisms previously described. Reduction in active uptake can be accomplished by a reduction in passive loss, the latter requiring, in part, a reduction in gill permeability. Any reduction in gill water and/or sodium permeability will be reflected in a reduction in unidirectional Na efflux. The procedure to assess the unidirectional Na efflux was almost identical to that for water efflux described above. Instead of 3H2O, 22Na was added to the incubation medium at 0.1µCi•ml-1.

ACKNOWLEDGEMENTSThis work was supported by grant DBI 99-78613 from the

National Science Foundation

NSF

Figure 1. Measurements of water efflux, as an indicator of waterpermeability from Callinectes sapidus ranging from 0.27 to 26.10gbody weight. Fluxes were measured at the acclimation salinity foreach crab. The data show an expected decrease in flux with in-creasing mass, but show no decrease in flux with decreased salinity.

Figure 4. Measurements of sodium efflux, as an indicator of sodium permeability from Callinectes sapidus ranging from 0.19 to 164.00g body weight. Fluxes were measured at the acclimation salinity for each crab. The data show an expected decrease in flux with increasing mass at 1000 mOsm. However, there is a substantial decrease in Na permeability with decreased salinity.

Figure 2. Mean flux rates (+ s.d.) for juvenile vs. adult and subadult crabs. The flux rates for juveniles were significantly lower than those for larger crabs at 150 mOsm (p<0.02). There were no significant differences between fluxes in 1000 mOsm vs. 150 mOsm for either group. The increased gradient for osmotic water flux at 150 mOsm indicates that water permeability is lower at 150 mOsm than at 1000 mOsm.

Figure 3. Mean (+s.d.) water efflux of juvenile (<1 g) Callinectes sapidus. There are no differences in water flux across salinities. However, this indicates a progressive decrease in water permeability with decrease in salinity, since the gradient for osmotic water gain increases as salinity decreases.

Figure 5. Mean Na flux rates (+S.E.M.) for juvenile, subadult and adult crabs. The flux rates for juveniles (<2 g) were significantly higher than those for larger crabs at 150 mOsm (p<0.05). There were significant decreases between fluxes in 1000 mOsm vs. 150 mOsm for all groups.

Figure 6. Mean (+S.E.M.) Na efflux of juvenile (<2 g) Callinectes sapidus. There are highly significant differences in Na flux between 1000 mOsm and the lower salinities. However, there is no further decrease in flux from the value at 150 mOsm as salinity decreases to 50 mOsm.