Challenging traditional methods of age estimation: elemental and isotopic characterisation of speartooth shark Glyphis glyphis vertebrae

Chondrichthyan (sharks, rays, and chimaeras) age estimation has mostly been informed through counting bands in calcified hard parts such as vertebrae. However, this method is not broadly applicable to all species and is prone to misinterpretation biases. There is a need to improve current age estimation methods, and elemental characterisation of vertebrae is a promising prospect. We analysed vertebrae from speartooth shark Glyphis glyphis, a euryhaline species from tropical Australia, by both micro-X-ray fluorescence (μXRF) and laser ablation multi-collector inductively coupled plasma mass spectrometry to discriminate elemental and isotopic variations. Age estimates from μXRF were compared to the conventional ageing method of transmitted light optical microscopy (TLOM). In addition, we investigated ambient environmental changes along the growth axis of vertebrae through in situ characterisation of strontium (Sr) isotopic ratios (87Sr/86Sr) and conventional solution-based 87Sr/86Sr analyses of water samples from the associated habitat. Age estimation through μXRF indicated that Sr exhibits an alternative banding pattern to that of TLOM. This Sr variation is correlated to strong 87Sr/86Sr variations, not seen in stenohaline marine species. The elemental and isotopic Sr variation recorded along the growth axis of G. glyphis vertebrae is related to the influence of seasonal terrestrial runoff. The predictability of the seasonal rainfall pattern anchors 87Sr/86Sr to time and thereby validates Sr bands as annual (between ages 0-11) while it invalidates bands identified via TLOM. This study suggests that detecting elemental and isotopic variations with predictable seasonal fluctuations in a species’ ambient environment can provide an alternative means of age validation.