Water and sodium balance in the estuarine diamondback terrapin (Malaclemys)

Summary

Total body water decreased significantly in terrapins exposed to sea water (SW). Although the intracellular fluid decreased somewhat upon SW exposure, the decline in extracellular fluid was almost twice as great. Under conditions of voluntary drinking after salt loading, terrapins substantially increased the volume of the extracellular fluid while maintaining the intracellular fluid near the freshwater (FW) control levels. FW terrapins were consistently heavier than animals of the same plastron length exposed to SW. Thus expression of body fluid volumes as ml/cm plastron length rather than as % body weight is necessary to correct for the loss of total body water with progressive dehydration. Fasted terrapins in SW lost weight at 0.32% weight/day, whereas the rate in FW was 0.21%/day. Water influx and efflux in SW were 0.17 and 0.16 ml/100 g·h respectively. When the efflux was increased by the calculated value for unmeasured respiratory loss, it exceeded the influex by 0.01 ml/100 g·h. Consequently the net water loss determined with radiotracers (equivalent to 0.24% weight/day) was similar to the difference between the weight losses in SW and FW (0.11%/day). Partitioning studies indicated that the majority of water exchange between the terrapin and SW occurs through the integument. Terrapins in SW underwent a concentration of the body fluids, most of which can be attributed to water loss, not electrolyte gain. The rates of Na influx and efflux were quite low (usually ranging from 6–10 μmoles/100 g·h). In two terrapins the injection of NaCl loads resulted in eight- to 19-fold increases in Na efflux. The uptake of Na from SW occurred orally. The skin was virtually impermeable to Na. The salt gland and possibly the cloaca were the major routes of Na efflux. The injection of NaCl loads resulted in an increase in cephalic Na excretion from a mean of 3.2 μmoles/100 g·h to 32.5 μmoles/100 g·h. Terrapins in SW exhibited a significant increase in bladder urine [K] over the FW controls. There was a direct relationship between plasma [Na], urine [K], and lachrymal salt gland Na−K ATPase content. In comparing SW terrapins with FW painted turtles (Chrysemys) exposed to SW radiotracer studies demonstrated a similarity in Na influx, but there was at least a four-fold increase in water exchange in the painted turtle. It seems likely that the skins of many aquatic reptiles (marine, estuarine and FW) are impermeable to Na but differ markedly in water permeability.