ELEMENTAL CHEMISTRY OF ENDOLYMPH AND OTOLITH: PASSIVE RECORDER OR ACTIVE WRITER?

Over the past two decades the chemistry of fish ear stones (otoliths) has been increasingly employed to study the environmental life histories of fish. Under the assumption that the otolith, composed of calcium carbonate in the form of aragonite, passively records the chemistry of the water masses encountered by the fish. Researchers have used the relative abundance of divalent cations compared to calcium ascribing significance to these otolith ratios related to water chemistry. Little is known about the mechanisms of otolith formation nor the potential elemental fractionation that might occur during otolith formation. This study presents data that compares the otolith chemistry to the chemistry of the fluid surrounding the precipitating otolith, the endolymph. In this study we explored the relative abundance of metals in both phases with specific focus on those elements used in environmental life history studies. If, as hypothesized, the otolith is a passive recorder there should be no elemental fractionation between endolymph and otolith. If fractionation between the phases is observed this may point to biological mechanisms leading to fractionation of elements during aragonite growth. Endolymph was extracted from the membranous section of the inner ear in the golden redhorse (Moxostoma). For each sample, sagittal otoliths and endolymphatic material were separated and elemental chemistry measured by DRC-ICP-MS. The endolymph is a supersaturated liquid with trace element concentrations an order of magnitude larger than those in the corresponding otolith. Elemental fractionation between the two phases was not observed suggesting that the otolith may indeed faithfully record the relative abundances of divalent cations in water with little to no biological overprint.