Element composition of shark vertebrae shows promise as a natural tag

Abstract

Reconstructing movements and environmental histories of sharks may be possible by using the element composition of vertebrae, but unlocking such possibilities requires an understanding of the effects of extrinsic and intrinsic factors on element composition. We assessed water temperature and pH effects (independently and in combination) on vertebral chemistry of Port Jackson sharks while accounting for intrinsic factors (condition and sex) using indoor aquaria and outdoor mesocosm environments, where the latter may better reflect natural field conditions. We analysed eight element:Ca ratios (7Li, 8B, 24Mg, 55Mn, 65Cu, 88Sr, 138Ba and 238U) by laser ablation inductively coupled plasma mass spectrometry and found positive temperature-dependant responses for multiple elements, including B:Ca, Mn:Ca, Sr:Ca and Ba:Ca (r2=0.43, 0.22, 0.60 and 0.35 respectively), whereas pH had a minor effect on vertebral Mg:Ca and Li:Ca (r2=0.10 and 0.31 respectively). As shown for teleost otoliths, condition affected element composition (Mn:Ca), suggesting potential physiological influences on element uptake. The suitability of vertebral chemistry as a natural tag appears to be element specific, and likely governed by a suite of potentially codependent extrinsic and intrinsic factors. Overall, variations in vertebrae chemistry show promise to reconstruct movements and habitat use of cartilaginous fishes. Yet, further research is required to understand the ubiquitous nature of the findings presented here.