Effect of relative shell size in turtles on water and electrolyte composition

Interspecific differences in water, sodium, and potassium concentrations of entire freshwater turtles are related to the relative size of the shell. Species with proportionately smaller shells have higher water and potassium concentrations but lower sodium concentrations (expressed as mumol/g wet mass). These effects are associated with variation in the amount of shell bone, which is lower in water and potassium concentrations than other tissues. Within a single species there are also differences in composition related to total wet mass and relative shell mass. Smaller turtles have a relatively larger overall water concentration, a lower overall sodium concentration (expressed as mumol/g wet mass), and a larger relative wet mass of shell (expressed as % total wet mass). The shell in small turtles is lower in relative dry mass, higher in water concentration, and lower in sodium concentration (expressed as mumol/g wet mass) than in large turtles. There is also a striking inverse size-related difference in the proportion of overall sodium that is exchangeable. This is due to a decrease in the proportion of exchangeable sodium in the shell of larger turtles, perhaps related to progressive calcification. The magnitude of the effects of size and/or age on water and electrolyte composition of turtles has not been previously recognized but must now be taken into account before any interspecific or intraspecific comparisons in body composition are made.