Elemental distribution within polymorphic inconnu (Stenodus leucichthys) otoliths is affected by crystal structure

Abstract

The chemistry and crystal structure of sagittal otoliths from inconnu (Stenodus leucichthys) were examined optically, with an electron microprobe, a scanning electron microscope, and with X-ray diffraction techniques. The distributions of strontium (Sr), sodium (Na), potassium (K), and calcium (Ca) were determined with line scans and area maps of thin, transverse otolith sections. Regions depleted in Sr, Na, and K were found to be discordant with optical annuli and were optically distinct from other regions of the otoliths. These patterns of trace element depletion cannot be explained by models of otolith growth that are based on ionic composition of endolymph fluids as the sole control of otolith composition. Electron micrographs showed the depleted regions of the otoliths to be of a different crystal structure than other regions of the otoliths. X-ray diffraction analyses revealed the presence of vaterite in otoliths with depleted regions, while otoliths without depleted regions showed no evidence of vaterite. The depleted areas may be composed of vaterite, and the crystal structure of vaterite may prevent certain trace elements from incorporating in the otolith. Scientists using fish otolith chemistry to infer environmental conditions or life history should be aware that elemental abundance within otoliths may be affected by other processes as well.