Where do elements bind within the otoliths of fish?

Abstract

Otolith element analyses are used extensively to reconstruct environmental histories of fish based on the assumption that elements substitute for calcium within the CaCO3 otolith structure. However, elements may also be incorporated within the protein component of the otolith in addition to the direct substitution for calcium in the mineral component, and this could introduce errors in environmental reconstructions. The aim of the present study was to determine whether elements were incorporated into the protein or mineral components of otoliths and the relative proportion of each element in each component. Element concentrations from whole ground otoliths and the isolated protein component were quantified using solution inductively coupled plasma mass spectrometry (ICP-MS). Of the 12 elements investigated, most were found in both the proteinaceous and mineral components, but always in greater concentrations in the latter. Elements considered ‘non-essential’ to fish physiology with Ca-like properties (i.e. alkaline metals) were present in the mineral component in relatively high concentrations. Elements essential to fish physiology with smaller atomic radii than Ca (i.e. transition metals) were distributed throughout the protein and mineral components of the otolith. These findings enhance our understanding of element incorporation in the otolith and, ultimately, improve interpretations of otolith-based environmental reconstructions.