How do the organic and mineral fractions drive the opacity of fish otoliths? Insights using Raman microspectrometry

Abstract

International audienceWe investigated the relationships between the opacity and the physico-chemical characteristics of fish otoliths and more specifically their aragonite and organic fractions. The analysis of these two fractions on otolith macrostructures was performed using Raman microspectrometry on both translucent and opaque zones of otoliths of pollock (Pollachius virens) and European hake (Merluccius merluccius). The magnitude of the Raman signatures of the aragonite and organic fractions were strongly correlated to otolith opacity with maxima in translucent zones. Opacity models, built from Raman signatures, successfully predicted the observed opacity for both species. A partial decorrelation of different aragonite signatures between translucent and opaque zones was revealed and discussed in terms of organisation (size, orientation) of aragonite crystals. Two categories of organic signatures with opposite effects on the opacity were identified, suggesting differences in organic compounds and (or) variations in their relative quantities. These original contributions provided new insight for understanding otolith biomineralization mechanisms as well as for interpreting and discriminating otolith macrostructures

Investigating marine bio‐calcification mechanisms in a changing ocean with in vivo and high‐resolution ex vivo Raman spectroscopy

“…Like our in vivo profiling results, high‐resolution ex vivo mapping showed that the corals and fish otolith are composed entirely of aragonite, whereas the foraminifer and coralline alga are entirely calcitic, except for minor aragonite contamination or infilling in the coralline alga. The banding of ν 1 peak intensity in the fish otolith may be due to differing proportions of organics and aragonite crystals (Jolivet, Bardeau, Fablet, Paulet, & Pontual, ). Conversely, micro‐banding patterns in the aragonitic corals are likely caused by variations in calcifying fluid Ω Ar , which could also explain the Mg/Ca banding observed here (Figure c,d) and described previously (Meibom et al, ) because aragonite Mg/Ca increases with Ω Ar or crystal growth rate (AlKhatib & Eisenhauer, ; DeCarlo et al, ).…”

Biogenic and diagenetic indicators in archaeological and modern otoliths: Potential and limits of high definition synchrotron micro-XRF elemental mapping

“…Our model assumes a homogeneous dispersion of organic and mineral fractions, which is known to be a simplification (Dauphin and Dufour, 2008). Variation in organic content with circadian rhythm has been long described in otoliths, although quantitative data remains sparse (Jolivet et al, 2013a). The true ultrastructure of otoliths includes nanometric organic envelopes and variation of organic/mineral ratios within growth increments, which are only beginning to be described and are very difficult to accurately model.…”

An analysis of bivalve larval shell pigments using micro‐Raman spectroscopy

“…Raman spectra of fish otoliths have shown higher organic signals in dark bands; however, these were signals from proteinaceous material and not from pigments. [24] It has been speculated that pigments are modified from food, and therefore, different colors and pigments found in the same species of mollusk may be due to different habitats. [12] Although the source of pigments in larval shells is unknown, if larvae incorporate pigments via algal food (metabolic) sources, pigments from the early PDI shell may differ from those in the later shell because the larva does not feed during PDI formation.…”