Reconstructing sea turtle ontogenetic habitat shifts through trace element analysis of bone tissue

Trace element analysis has emerged as a powerful tool to elucidate past movement and habitat use in aquatic animals, but has been underutilized in studies of non-fish species. When applied to sequentially deposited tissues (e.g. fish otoliths, sea turtle humerus bone), the technique can be used to infer aspects of an individual's ecology through time. The goal of this study was to evaluate whether trace elements could be used to reconstruct transitions between oceanic and neritic life stages in 2 species of sea turtle. We sampled the annual humerus bone growth layers of loggerhead Caretta caretta and Kemp's ridley Lepidochelys kemph sea turtles for concentrations of 7 elements (Mg, Ca, Mn, Cu, Zn, Sr, Ba) using laser ablation-inductively coupled plasma-mass spectrometry. Previous studies have demonstrated that stable nitrogen isotope (delta N-15) values can be used to reconstruct ontogenetic shifts between oceanic (offshore) and neritic (nearshore) habitats in these species; therefore, bone delta N-15 data were also collected for comparison. Bone strontium to calcium (Sr:Ca) and barium to calcium (Ba:Ca) ratios were significantly higher in oceanic versus neritic life stages for both species. Changes in bone elemental ratios within individuals coincided with known changes in resource use, as indicated by delta N-15 values, and fell within the range of body sizes and ages typical for oceanic-to-neritic ontogenetic shifts in each species. We conclude that bone Sr:Ca and Ba:Ca ratios may identify oceanic versus neritic resource use in sea turtles, but that additional studies are needed to identify the specific mechanisms underpinning these differences.