An experimental study of ²²⁶Ra and ⁴⁵Ca accumulation from the aquatic medium by freshwater turtles (fam. Chelidae) under varying Ca and Mg water concentrations

Abstract

Snapping turtles Elseya dentata (Gray) from Magela Creek, Northern Territory, were exposed under laboratory conditions for up to 30 days to waters resembling the inorganic composition of Magela Creek water during the Wet season, with background and elevated Ca and Mg concentrations, that were labelled with ²²⁶Ra and ⁴⁵Ca. The resulting concentrations of ⁴⁵Ca in muscle, skin, gut, liver, shell bone and leg bone of E. dentata equilibrated or approached equilibrium by 12–18 days. Among the experiments, the concentrations of ⁴⁵Ca in all six tissues were inversely related to turtle mass. An increase in the Ca water concentration by a factor of 15 increased the ⁴⁵Ca concentration in all six tissues. The arithmetic factors of increase in the concentration in each tissue were proportional or more than proportional to the factor of increase in Ca water concentration; this factor was highest for muscle tissue (26.6). An increase in the Mg water concentration by a factor of 48 reduced the ⁴⁵Ca concentration in all tissues except skin where it increased. The concentration of ²²⁶Ra in each tissue (except the gut) was positively related to its ⁴⁵Ca concentration and inversely related to turtle mass in muscle, skin and liver. With the exception of the skin, the increased Ca water concentration did not reduce the ²²⁶Ra in the tissues but increased the ²²⁶Ra concentration in bone and muscle. The increased Mg water concentration had an inverse effect on the ²²⁶Ra concentrations in all tissues except shell. With the exception of the skin the effects of increased Ca and Mg water concentrations and turtle size on ²²⁶Ra concentrations in the tissues of E. dentata were similar to their effects on ⁴⁵Ca tissue concentrations, indicating the similar metabolic behaviour of ²²⁶Ra and ⁴⁵Ca in E. dentata.

Exposures of the species Elseya latisternum (Gray), Emydura signata (Ahl) and Chelodina longicollis (Shaw), which are the same or closely related to species reported to occur in Magela Creek, to ⁴⁵Ca-labelled Sydney tap water for 7 days demonstrated their ability to also accumulate ⁴⁵Ca from their aquatic medium. The patterns of ⁴⁵Ca concentrations in the tissues of these species indicated that they were inversely related to turtle mass, as demonstrated in E. dentata. The concentrations of ⁴⁵Ca accumulated in the tissues were also comparable to those found in single specimens of E. dentata and E. victoriae (Gray) that were exposed for 7 days to simulated Magela Creek water. The data also indicated the larger long-necked C. longicollis accumulated less ⁴⁵Ca per gram of tissue than similar-sized, short-necked species E. signata and E. latisternum, suggesting that long-necked turtles from Magela Creek would accumulate less ²²⁶Ra from their aquatic medium than similar-sized short-necked species.

The capacity of E. dentata to accumulate ²²⁶Ra from the aquatic medium is about two orders of magnitude less than that of the tissue of the freshwater mussel Velesunio angasi (Sowerby) exposed under similar experimental conditions.