Changes to calcium carbonate (CaCO3) biomineralization in aquatic organisms is among the many predicted effects of climate change. Because otolith (hearing/orientation structures in fish) CaCO3 precipitation and polymorph composition are controlled by genetic and environmental factors, climate change may be predicted to affect the phenotypic plasticity of otoliths. We examined precipitation of otolith polymorphs (aragonite, vaterite, calcite) during early life history in two species of sturgeon, Lake Sturgeon, (Acipenser fulvescens) and White Sturgeon (A. transmontanus), using quantitative X-ray microdiffraction. Both species showed similar fluctuations in otolith polymorphs with a significant shift in the proportions of vaterite and aragonite in sagittal otoliths coinciding with the transition to fully exogenous feeding. We also examined the effect of the environment on otolith morphology and polymorph composition during early life history in Lake Sturgeon larvae reared in varying temperature (16/22 °C) and pCO2 (1000/2500 µatm) environments for 5 months. Fish raised in elevated temperature had significantly increased otolith size and precipitation of large single calcite crystals. Interestingly, pCO2 had no statistically significant effect on size or polymorph composition of otoliths despite blood pH exhibiting a mild alkalosis, which is contrary to what has been observed in several studies on marine fishes. These results suggest climate change may influence otolith polymorph composition during early life history in Lake Sturgeon.

In this study we quantified the percent CaCO3 polymorph composition in otoliths of larval and juvenile Lake Sturgeon Acipenser fulvescens via X‐ray microdiffraction. Sagittal otoliths of sub‐adults were primarily composed of aragonite (> 90%) while the lapilli otoliths were 100% vaterite. This is the first time the presence of aragonite in otoliths has been reported in an acipenseriform and is surprising given that the ability to form aragonite otoliths was not thought to have evolved until the separation of teleost and holostean species from other Actinopterygian fishes (e.g., sturgeon, paddlefish, gar).

Non-essential elements like Sr2+ are incorporated via Ca2+ transport proteins due to their similar chemical properties and are substituted for Ca2+ in hard structures of fishes. Few studies have investigated the uptake kinetics of non-essential elements or the effect the ambient environment has on uptake rates. We tested the hypothesis that temperature and environmental Ca2+ activity would influence uptake and subsequent deposition rates of Sr2+ in the fin rays of Lake Sturgeon, Acipenser fulvescens. Michaelis-Menten substrate inhibition models were used to measure the kinetics of Sr2+ uptake on Lake Sturgeon larvae that were exposed to varying temperature and Ca2+ activities. Sr2+ influx increased at higher temperatures (maximum JmaxSr=56.5 pmol.g-1.h-1) and decreased when larvae were exposed to increasing activities of Ca2+ (minimum JmaxSr=6.4 pmol.g-1.h-1) indicating Ca2+ has an inhibitory effect on Sr2+ influx. Furthermore, Sr2+ was preferentially accumulated in fin rays and partitioning was significantly affected by temperature and Ca2+ activity providing, for the first time, an understanding of the underlying physiological mechanisms involved in elemental uptake and deposition of non-essential metals in sturgeons.

Fish otoliths, or ear bones, are comprised of the CaCO 3 polymorphs (aragonite, calcite and vaterite), which can occur either alone or in combination. The polymorph phase abundance in an otolith depends on, as yet, unexplained genetic and environmental factors. Most fish otoliths are comprised of the densest CaCO 3 polymorph, aragonite. Sturgeon otoliths, on the other hand, contain significant amounts of the rare and the structurally enigmatic polymorph, vaterite. Sturgeon otoliths are frequently comprised of agglomerations of small microcrystalline vaterite spherulites (<300 μ m in diameter), that range in shape from nearly perfect spheres to oblate spheroids. These spherulites are similar to the synthetic vaterite microspheres employed in laser trapping applications. Vaterite spherulites from both hatchery-reared (juvenile) and wild (adult) Lake Sturgeon exhibit extreme crystallographic texture as evidenced by X-ray diffraction patterns and their reconstructed pole-figures determined here. The vaterite crystallites making up the spherulites have excellent registry in both the axial and equatorial directions. Whether synthesized or natural, the texture manifested in these spherulites suggests that vaterite nucleates and grows similarly in vivo otolith formation as well as from laboratory synthesis. The uniaxial optical character of the vaterite spherulites, confirmed by these diffraction experiments and combined with their large birefringence, makes them well suited for laser trapping applications.

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