Patterns of metal accumulation in osteoderms of the Australian freshwater crocodile, Crocodylus johnstoni

The concentrations of 15 metals were measured in the osteoderms (dermal bones) of 30 freshwater crocodiles (Crocodylus johnstoni) from a single population in the Lynd River, northeastern Australia (17°50′S, 144°20′E), that were well characterised with respect to site fidelity, reproductive status and age. Multiple linear regression analyses were used to determine the effects of crocodile size (snout-vent length, 25-128 cm), age (0.7-62.7 years), gender (male or female) and reproductive status (sexually mature or immature) on osteoderm metal concentrations. Gender and reproductive status were not significant (P>0.05) co-predictors of the osteoderm concentration of any metal. In contrast, size, age and osteoderm calcium concentration were highly significant (P<0.001) systematic predictors of the osteoderm concentrations of all metals, except Na and K. Osteoderm metal concentrations were inversely related (P<0.001) to both size (r2=0.52−0.92) and age (r2=0.52−0.84), but positively related (P<0.001) to osteoderm calcium concentration (r2=0.67−0.92). Relative to calcium concentration, the rates of metal accumulation in the osteoderms of C. johnstoni were inversely related to the solubility constant (log Ksp) of the metal as a phosphate; however this relationship was not linear. This finding is consistent with that previously established for the flesh of freshwater bivalves, which like the crocodilian osteoderm, have a calcium phosphate repository in the form of extracellular granules. The constancy of this relationship between rate of metal accumulation and relative solubility for calcium phosphate deposits, despite contrasting Ca accumulation regimes and taxonomic dissimilarity, points to a potential underlying principle that warrants investigation in a greater range of biota. The implications for using the osteoderms of C. johnstoni as an indicator of metal levels in freshwater ecosystems are also discussed.