Effects of salt loading on salt gland function in the euryhaline turtle,Malaclemys terrapin

Abstract

Summary. The estuarine turtle, Malaclemys terrapinis able to ionregulate when acclimated to fresh water, 55% sea water or 100% (full strength) sea water, but when in 100% sea water it does not volume regulate successfully. Orbital gland secretions collected by a new eye cup method are very low in animals from all three salinities without salt load. After salt loading the animals from all three groups produce an orbital gland secretion with a sodium concentration greater than sea water. The concentration of ions and kinetics of the response are similar in all three groups. Orbital gland secretion returns to control preload levels well before the injected load is excreted. There is no correlation between the plasma sodium concentration and any of the parameters of the orbital gland response. There is also no correlation between the concentration of sodium in the tear fluid or the rate of sodium excretion and the level of K+-stimulated p-nitrophenylphosphatase activity in the gland. Some of these unexpected results may relate to the estuarine habitat occupied by Malaclemys. there is more knowledge about osmoregulation (see Discussion). One pertinent fact is that Malaclernys progressively lose weight when in sea water for short term (Bentley et al. 1967) or for long terms (Cowan 1970). This might indicate that the animal is not osmoregulating perfectly, and that salt gland secretions are not hypertonic to sea water.To study the function of the salt gland and its role in osmoregulation, an eyecup method was devised which avoids some of the problems of the earlier methods (see Discussion) and which allows measurement of ion concentrations in the secretions.

Total body water and water turnover rates in the estuarine diamondback terrapin (Malaclemys terrapin) during the transition from dormancy to activity

“…Terrapins regulate body fluid composition through a combination of active salt excretion via the cephalic salt glands (Schmidt-Nielsen and Fange, 1958;Bentley et al, 1967;Cowan, 1981), low integument permeability (Bentley et al, 1967), opportunistic drinking of fresh or brackish water (Davenport and Macedo, 1990), and modifications in behavior (Dunson and Seidel, 1986). Seasonal shifts between aquatic and terrestrial habitats may also play a role in osmoregulation (Spivey, 1998;Butler, 2002;Haramis et al, 2011;Southwood Williard and Harden, 2011;Harden and Williard, 2012;Akins et al, 2014).…”

The blood biochemistry of overwintering diamondback terrapins (Malaclemys terrapin)

“…Taken together, these results indicate relatively low water exchange during winter dormancy (e.g., fresh rain water uptake, salt water ingestion via feeding, and excessive urine excretion). Because salt glands are energetically expensive to maintain (Bentley et al, 1967;Borut and Schmidt-Nielsen, 1963;Whittam, 1963) and may function primarily as a means to excrete excess salts ingested while foraging (i.e., in response to salt loading, Cowan, 1981), it is unlikely to be triggered during dormancy, when we have evidence to suggest they are principally hypophagic. Finally, there is little evidence to suggest that terrapins were maintaining osmotic balance during winter via extracellular storage of water (see Davenport and Macedo, 1990;Robinson and Dunson, 1976) because mean total body water (TBW%) did not change between pre-and post-spring emergence in terrapins (Harden et al, 2014).…”

“…Evasive strategies that reduce evaporative water loss and salt and water exchange with environment include impermeable integument, hypophagy, winter mud burial, aestivation, basking, and water retention via urine reduction (Bentley et al, 1967;Brennessel, 2006;Gilles-Baillien, 1973), which all ultimately reduce metabolic costs associated with maintaining osmotic balance with the environment (Bentley et al, 1967). Compensatory strategies that involve active (i.e., energy-requiring) uptake or extrusion of water or salts include the active exchange of organic osmolytes across cell membranes, the secretion of salts from body to environment via lachrymal salt gland in order to adjust osmotic pressure (Cowan, 1981;Dunson, 1970), and the detection, active uptake, and extracellular storage of fresh rain water when readily available (Bels et al, 1995;Davenport and Macedo, 1990;Robinson and Dunson, 1976).…”

A stereological analysis of the effect of buffer osmolarity on a salt secreting epithelium, under variable physiological conditions

“…Table 1 gives the results for an experiment in which hatchling Malaclemys were acclimatized to fresh water or to salt water for 14 days. Under these conditions the animals from fresh water have plasma osmotic pressures of approximately 100 mosmol less than animals kept in sea water and the salt glands are activated (Cowan, 1981). The basic ultrastructure of the salt glands have been described adequately (Cowan, 1969(Cowan, , 1970 and will not be repeated.…”