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
NSF
ABSTRACT
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.
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.
Figure 1. Measurements of water efflux, as an indicator of water
Figure 3. Mean (+s.d.) water efflux of juvenile (<1 g) Callinectes
Figure 5. Mean Na flux rates (+S.E.M.) for juvenile, subadult and
permeability from Callinectes sapidus ranging from 0.27 to 26.10g
body weight. Fluxes were measured at the acclimation salinity for
each crab. The data show an expected decrease in flux with increasing mass, but show no decrease in flux with decreased salinity.
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.
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.
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.
Figure 2. Mean flux rates (+ s.d.) for juvenile vs. adult and
Figure 4. Measurements of sodium efflux, as an indicator of
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 6. Mean (+S.E.M.) Na efflux of juvenile (<2 g) Callinectes
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.
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.
ACKNOWLEDGEMENTS
This work was supported by grant DBI 99-78613 from the
National Science Foundation