Sodium and Water Fluxes in Free-Living Crocodylus porosus in Marine and Brackish Conditions

Abstract

Radioactive sodium and water were used to determine total body water (TBW), exchangeable sodium (ExNa) and water and sodium fluxes in free-living Crocodylus porosus in marine (hyperosmotic; salinity = 25‰–35‰) and brackish (hypoosmotic; salinity = 2‰-7.5‰) sections of the Tomkinson River in northern Australia. At capture, size-corrected TBW and ExNa pools in 62 crocodiles (hatchlings, juveniles, and subadults; weight, 0.108–54.4 kg) were independent of salinity history. To determine fluxes, all animals were released at their capture sites and left undisturbed until recapture. Thirty-seven were recaptured after 7-18 days. Fifteen of the 17 hatchlings recaptured from both salinity categories grew and maintained their condition and hydration status. In contrast, all 20 juveniles and subadults lost weight in the same period, and juveniles in hyperosmotic conditions showed significantly lower hydration and condition factors. Water effluxes in hatchlings were ~80 and ~160 ml · kg−0.63 · day⁻¹ in marine and brackish conditions, respectively. Comparable sodium effluxes were 7.5 and 4.4 mmol ·kg−0.63 · day⁻¹. All crocodiles in hyperosmotic conditions had consistently lower water effluxes (≈ XO.5) and higher sodium effluxes (≈ X1.6) than did crocodiles in brackish water. In both salinity categories, hatchlings had greater water turnover (≈ X1.3, X1.6) and sodium turnover (≈ X1.5, X1.25) than did juveniles and subadults. Interpretation of the field data is complicated by integumentary exchange of sodium and water, a size-related aphagia apparently induced by disturbance, and difficulties of adjusting for allometric differences across a wide range of sizes. Nevertheless, it is clear that C. porosus is able to effect considerable economies of water turnover in hyperosmotic salt water and that the secretory capacity of the lingual glands, as measured in the laboratory, is more than enough to account for the highest sodium effluxes that we measured in C. porosus in the field.