Sodium regulation in the tidepool copepod Tigriopus californicus

1. 1. Body fluid water and sodium regulation in the marine copepod Tigriopus californicus were studied over a range of external sodium concentrations extending from 0.26 to 1.45 M. 2. 2. Body water content was constant at 87.5% of body weight. 3. 3. Body sodium content increased from 0.2 to 4.0/miol/mg dry weight with salinity, however, the increase was not as rapid as would be predicted if the internal Na were in equilibrium with external Na. 4. 4. Calculated mean internal Na concentration as well as upper and lower limits of extracellular Na concentrations were below the line of conformity with external medium. 5. 5. Silver staining techniques revealed possible ventral sites of Na transport which did not stain in ouabain, 2,4-dinitrophenol nor Na-free medium. 6. 6. The results are consistent with hypoionic regulation via active sodium transport over the salinity range observed.