Natural variability and effects of cleaning and storage procedures on vertebral chemistry of the blacktip shark Carcharhinus limbatus

Abstract

Key methodological assumptions regarding the degree of natural variability and influence of sample handling and storage of elasmobranch vertebral chemistry were assessed using laser-ablation inductively coupled plasma mass spectrometry. Vertebral chemistry of juvenile blacktip sharks Carcharhinus limbatus was examined to identify whether differences existed among different regions of the vertebral column, between thoracic vertebrae of individual fish or within individual vertebrae. Additionally, the effects of bleach exposure and storage in ethanol on vertebral chemistry were compared. No significant variation in vertebral chemistry was found among different regions of the vertebral column or between thoracic vertebrae, but significant differences among transect locations within individual vertebrae were observed. The variation at all three levels appears comparable with published data on sagittal otoliths of bilaterally symmetrical teleost fishes. The experimental assessment of potential treatment effects indicated vertebral chemistry was not significantly affected by bleach or exposure to ethanol. Taken together, these results support the idea that vertebrae taken from the same region of the vertebral column can be treated as equivalent and at least certain elements remain robust to exposure to bleach and ethanol.

Effects of sample cleaning and storage on the elemental composition of shark vertebrae

“…However, Lewis did not quantify [Na:Ca], and sonicated the experimental vertebrae in ultrapure water for 60 min before analysis, which may have equalized the concentrations of the more labile elements ([Na:Ca] and [Mg:Ca]), or perhaps the short bleach treatment of 5 min was not long enough to induce elemental changes as we found. In addition, Lewis utilized laser transects and not spot analyses, so averaging values over the entire transect may have masked potential bleach effects near the edge of the vertebrae. Tillett et al found no elemental variations between vertebrae pairs exposed and unexposed to bleach, but only four specimens were considered and bleach exposure procedures were not documented.…”

“…Na and Mg concentrations in Terubok, tropical shad Tenualosa toli , otoliths were higher in samples immediately removed and stored dry, than in samples stored frozen or in 70% ethanol . McMillan et al (see their Supplemental Materials, Table S4) found that vertebrae [Na:Ca] significantly increased with bleach exposure (40 vs 120 min), whereas [Mg:Ca] decreased, and [Mn:Ca] increased only after 2 h. Lewis investigated how exposure to bleach for 5 min and then ethanol storage for 3 months affected the chemistry of blacktip shark vertebrae and found a significant difference in [Mn:Ca] ( p = 0.04) due to the handling and storage treatment, but no change in [Li:Ca], [Mg:Ca], [Sr:Ca], [Ba:Ca] or [Pb:Ca]. However, Lewis did not quantify [Na:Ca], and sonicated the experimental vertebrae in ultrapure water for 60 min before analysis, which may have equalized the concentrations of the more labile elements ([Na:Ca] and [Mg:Ca]), or perhaps the short bleach treatment of 5 min was not long enough to induce elemental changes as we found.…”

“…The trace elemental composition of vertebrae is increasingly used to reconstruct the environments previously encountered by elasmobranchs and has been used to validate growth band deposition rate . Despite the surge in recent studies that have utilized vertebral elemental chemistry, few have systematically investigated how cleaning and storage procedures may alter the primary elemental composition of hydroxyapatite structures. Such knowledge is crucial for the accurate interpretation of elasmobranch life history patterns established from historical collections …”

Element composition of shark vertebrae shows promise as a natural tag

“…Several studies have examined sources of intra-individual natural variation in element composition of shark vertebrae and demonstrated no variation among regions of the vertebral column (e.g. cervical, thoracic, precaudal;Lewis et al 2017), among vertebrae within similar regions of the vertebral column (e.g. Lewis et al 2017;McMillan et al 2017a), with the exception of Zn (Raoult et al 2018), and little to no variation within similar regions of a vertebra (e.g.…”

“…cervical, thoracic, precaudal;Lewis et al 2017), among vertebrae within similar regions of the vertebral column (e.g. Lewis et al 2017;McMillan et al 2017a), with the exception of Zn (Raoult et al 2018), and little to no variation within similar regions of a vertebra (e.g. Tillett et al 2011;Smith et al 2016;Lewis et al 2017).…”