Spatial and Temporal Variation in Vertebral Geochemical Composition: Evaluating the Potential to Distinguish Natal Origin from Natural Elemental Markers in Elasmobranchs

Differences in the chemical composition of calcified structures are used to reconstruct environmental history and reveal natal origins, dispersal patterns, and metapopulation structure of many marine organisms. Because the use of discrete nursery areas is common among elasmobranchs, distinctive chemical markers may be incorporated into the vertebrae of individuals as they occupy these areas for the first months or years of their lives.  We evaluate the assumptions of elemental analysis for the cartilaginous vertebrae of elasmobranchs and compare the variation observed in this study with that reported for the calcified structures of teleosts and mollusks.  Vertebrae were collected from young-of-the-year scalloped hammerhead sharks (Sphyrna lewini) along the Pacific coast of Mexico and Costa Rica in 2007-2009 to assess patterns of spatial and temporal variation in elemental composition.  Elemental composition was measured using Laser Ablation Inductively Coupled Plasma Mass Spectrometry.  There was consistency in elemental composition (Li, Mg, Ca, Ti, V, Cr, Mn, Co, Cu, Zn, Rb, Sr, Zr, Cd, Ba, La, and Pb) among vertebrae within an individual.   Changes in vertebral elemental composition were correlated with a major life transition, differing significantly between the region of vertebrae that was deposited post-partum in comparison to that which developed while in-utero. Multivariate analysis of variance was applied to examine temporal (intra- and inter-annual) differences in vertebral chemistry within and among sample locations.  The ability to successfully classify individuals to sites of natal origin was assessed through discriminant function analysis.