L. E. Taplin

Lingual salt glands inCrocodylus acutus andC. johnstoni and their absence fromAlligator mississipiensis andCaiman crocodilus

Summary1.Lingual salt glands, secreting hyperosmotic Na/K solutions in response to methacholine, are present inCrocodylus acutus andC. johnstoni but apparently absent from the alligatorids,Alligator mississipiensis andCaiman crocodilus.2.Both secretory rates (6–20 μmol/100 g·h) and concentrations (450–600 mM Na) of glandular secretions are essentially identical in the marine/estuarineC. acutus andC. porosus and significantly higher than in the freshwaterC. johnstoni (1–2 μmol/100 g·h; 320–420 mM Na).3.Lingual glands inAlligator secrete isosmotic Na/K at low rates (1–2 μmol/100 g·h) while those ofCaiman show no response to methacholine.4.The physiological contrast between alligatorids and crocodylids is reflected in distinct differences in the superficial appearance of the tongue and lingual pores.5.It is postulated that the alligatorid condition of low secretory capacity and isosmotic secretion reflects the primitive salivary function of lingual glands from which the salt-secreting capability in crocodylids was derived.

Survival and Growth of Hatchling Crocodylus porosus in Saltwater Without Access to Fresh Drinking Water

SummaryIt has been suggested that C. porosus select nest sites which provide a source of freshwater for hatchlings during the dry season. From a mark-recapture study, we conclude that hatchling C. porosus can survive and grow in hyperosmotic saltwater without drinking freshwater. Hence, the siting of nests is unlikely to be the consequence of a requirement by hatchlings for freshwater. Considered along with other information, our observations imply that hatchling C. porosus have functional salt glands.

Sodium and water budgets of the fasted estuarine crocodile,Crocodylus porosus, in sea water

Summary1.Quantitative sodium and water budgets have been constructed for the estuarine crocodile,Crocodylus porosus, unfed in sea water in the laboratory.C. porosus is homeostatic for exchangeable Na in sea water but is unable to achieve homeostasis of plasma Na concentration or total body water when fasted.2.The lingual salt glands account for 55% of the total Na efflux compared with only 2% excreted by the renal/cloacal complex. The urinary system appears to be the principal route for excretion of K.3.The integument is essentially impermeable to Na but permeable to water, as in other estuarine/marine reptiles. Much of the Na and water exchange across the skin appears to take place across cephalic epithelia, particularly the buccal mocosa.4.The permeable buccal mucosa is costly in terms of direct water loss and indirect losses tied to excretion of excess Na. Lowering of buccal permeability may be limited by the necessity to retain some functional glandular tissue and sensory organs in the mouth.5.There is no evidence to suggest thatC. porosus feeds selectively on invertebrate prey of low Na or high water content despite the apparent selective advantage of minimizing water losses associated with extra-renal Na excretion.

Osmoregulation by the Broad-Snouted Caiman, "Caiman latirostris", in Estuarine Habitat in Southern Brazil

Abstract The broad-snouted caiman Caiman latirostris, of South America mostly frequents freshwater but occurs also in estuaries. Nothing of substance is known of its osmoregulatory physiology but, in the light of accumulating evidence that alligatorids lack specialised adaptations for life in hyperosmotic waters, we anticipated its physiology would be more similar to that of Alligator mississippiensis than the euryhaline Crocodylus porosus, which has both lingual salt glands and a more complex renal:cloacal system. This proved to be the case. Caiman captured in estuaries of the Ilha do Cardoso in southern Brazil were effective hypo-osmotic osmoregulators in salinities of 0–24 ppt (seawater = 35 ppt). Plasma osmolarity, sodium and chloride were similar to those in other crocodilians and not influenced by salinity. Plasma urea was low and did not vary with salinity. We found no evidence of lingual or other salt glands. Urinary electrolyte concentrations varied considerably with salinity and in ways reminiscent of A. mississippiensis but very different from C. porosus. Ca. latirostris dehydrated in seawater more rapidly than C. porosus and had substantially higher integumental permeability to water. Caiman did not drink seawater but rehydrated rapidly when returned to freshwater (FW). We found small caiman (<500 g) only in very low salinities (<3 ppt) and larger caiman closer to the sea. We postulate that medium to large Ca. latirostris can take advantage of the feeding opportunities presented by the estuarine mangal despite lacking the physiological specialisations of crocodylids. Two individuals which we re-sighted by chance had travelled at least 600 m in 2–3 days, showing that every caiman we captured or saw was within easy reach of FW. Most likely their habitation of the estuary and its mangal is achieved through a combination of low surface area:volume ratio, relatively impermeable skin, and periodic access to FW.

Homeostasis of plasma electrolytes, water and sodium pools in the estuarine, crocodile, Crocodylus porosus, from fresh, saline and hypersaline waters

Summary1.Total body water and exchangeable Na pools have been measured in hatchling and juvenile Estuarine Crocodiles captured from a wide range of salinity (0–64‰). Plasma electrolyte concentrations are presented for hatchlings over the same range of salinity.2.Plasma electrolyte concentrations in hatchlings are constant across the entire salinity range studied. Hatchlings and juveniles, up to 5 kg body weight, maintain constant weight-corrected total body water pools but show a decline in exchangeable sodium pools with increasing salinity, suggesting a shift in the distribution of electrolytes or water between extra- and intra-cellular fluid compartments.3.Both water and Na pools scale allometrically with body weight (allometric coefficients of 0.984 and 0.944 respectively). Expression of weight-specific pool sizes in units of ml/100 g or mmol/kg is, therefore, potentially misleading. Demonstration of homeostasis with respect to pool size depends upon the expression of pool size in units of ml or mmol per unit length and upon detailed consideration of weight/length and volume/length scaling relationships. The implications of these findings for future studies of the ecology of C. porosus in saline habitats are discussed.

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

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.

Drinking of fresh water but not seawater by the estuarine crocodile (Crocodylus porosus)

Abstract 1. 1. The roles of drinking and integumental exchange in water balance of unfed estuarine crocodiles, Crocodylus porosus, have been examined in both seawater and fresh water. 2. 2. In seawater, C. porosus avoids drinking despite the loss of substantial amounts of water across the integument. Water loss across the buccal mucosa accounts for 60% of the total. 3. 3. In fresh water, C. porosus experiences a small net uptake across the integument which is insufficient to compensate for urinary water losses. Water balance is achieved through drinking. 4. 4. Together with data from other field and laboratory studies, the evidence supports the view that C. porosus in seawater may drink fresh water facultatively but does not depend on drinking for maintenance of hydromineral balance. 5. 5. Review of the available information on drinking in other aquatic reptiles suggests that the results for C. porosus should not be extrapolated to the Crocodylia as a whole. It appears that further study of crocodilians and other aquatic reptiles will reveal a wider range of drinking strategies than has been recognized previously.

Nile crocodiles, Crocodylus niloticus, and estuarine crocodiles, Crocodylus porosus, show similar osmoregulatory responses on exposure to seawater

1. Nile crocodiles, reared in fresh water and exposed acutely to seawater, suffer marked dehydration and hypernatraemia. Cloacal urine osmolarity and potassium concentration increased markedly but urine sodium remains low. 2. Hypernatraemia is increased when secretion from the lingual salt glands is prevented. 3. C. niloticus appears not to drink seawater. 4. Similarities in osmoregulatory response between estuarine and Nile crocodiles suggest that the lingual salt glands of C. niloticus are functional in salt water, playing an important role in sodium balance. 5. Significant differences in the function of the renal/cloacal complex of Alligator and Crocodylus emphasize further the differences between these two groups of crocodilian and provide support for the postulated marine ancestry of many or all of the Crocodylidae.

Osmoregulation of the Australian freshwater crocodile, Crocodylus johnstoni, in fresh and saline waters

An unusual saltwater population of the “freshwater” crocodilian, Crocodylus johnstoni, was studied in the estuary of the Limmen Bight River in Australias Northern Territory and compared with populations in permanently freshwater habitats. Crocodiles in the river were found across a large salinity gradient, from fresh water to a salinity of 24 mg · ml-1, more than twice the body fluid concentration. Plasma osmolarity, concentrations of plasma Na+, Cl-, and K+, and exchangeable Na+ pools were all remarkably constant across the salinity spectrum and were not substantially higher or more variable than those in crocodiles from permanently freshwater habitats. Body fluid volume did not vary; condition factor and hydration status of crocodiles were not correlated with salinity and were not different from those of crocodiles from permanently fresh water. C. johnstoni clearly has considerable powers of osmoregulation in waters of low to medium salinity. Whether this osmoregulatory competence extends to continuously hyperosmotic environments is not known, but distributional data suggest that C. johnstoni in hyperosmotic conditions may require periodic access to hypoosmotic water. The study demonstrates a physiological capacity for colonisation of at least some estuarine waters by this normally stenohaline freshwater crocodilian.