Temperature-induced microstructural changes in shells of laboratory-grown Arctica islandica (Bivalvia)

The physiological responses to titanium dioxide nanoparticles exposure in pearl oysters (Pinctada fucata martensii)

To evaluate the physiological responses to titanium dioxide nanoparticles exposure in pearl oysters (Pinctada fucata martensii), pearl oysters were exposed for 14 days to different levels (0.05, 0.5, and 5 mg/L) of nano-TiO2 suspensions, while a control group did not undergo any nano-TiO2 treatment. And then recovery experiments were performed for 7 days without nano-TiO2 exposure. At days 1, 3, 7, 14, 17, and 21, hepatopancreatic tissue samples were collected and used to examine the activities of protease, amylase, lipase, catalase (CAT), glutathione peroxidase (GPx), superoxide dismutase (SOD), lysozyme (LYS), alkaline phosphatase (AKP), and acid phosphatase (ACP). The microstructure of the nacreous layer in shell was also analyzed by scanning electron microscopy. Results showed that pearl oysters exposed to 5 mg/L of TiO2 nanoparticles had significantly lower protease, amylase, and lipase activities and significantly higher CAT, SOD, GPx, LYS, ACP, and AKP activities than control pearl oysters did even after 7-day recovery (P-values <0.05). Pearl oysters exposed to 0.5 mg/L or 0.05 mg/L of TiO2 nanoparticles had lower protease, amylase, and lipase activities and higher CAT, SOD, GPx, LYS, ACP, and AKP activities than control pearl oysters did during the exposure period. After 7-day recovery, no significant differences in protease, lipase, SOD, GPx, CAT, ACP, AKP, or LYS activities were observed between pearl oysters exposed to 0.05 mg/L of TiO2 nanoparticles and control pearl oysters (P-values >0.05). In the period from day 7 to day 14, indistinct and irregular nacreous layer crystal structure in shell was observed. This study demonstrates that TiO2 nanoparticles exposure influences the levels of digestion, immune function, oxidative stress, and biomineralization in pearl oysters, which can be partially and weakly alleviated by short-term recovery. These findings contribute to understanding the mechanisms of action of TiO2 nanoparticles in bivalves. However, studies should evaluate whether a longer recovery period can restore to their normal levels in the future.

Effects of H2O2 pretreatment on the elemental fingerprints of bivalve shells and their implications for the traceability of geographic origin

The fraudulent mislabelling of seafood geographic origin has been growing due to complex supply chains and growing consumer demand. To address this issue, seafood traceability tools, such as those based on elemental fingerprints (EF) of bivalve shells, have been successfully used to confirm their harvesting location. However, despite the usefulness of these methodologies, there is still room for optimization. Therefore, this study evaluated the effects of a routine procedure during bivalve shells preparation for ICP-MS analysis - their pretreatment with H2O2 to remove organic components. More specifically, the present study evaluated the effects of H2O2 on i) the elemental fingerprints of shells of two bivalve species (Ruditapes philippinarum and Cerastoderma edule) from four different locations over the north-western and the western Iberian coast, and ii) their influence on the accuracy of models (based on the EF of shells) used to confirm the geographic origin of these species. Significant differences were observed between untreated and pretreated shells of R. philippinarum (p within location ranging from 0.0001 to 0.0011) and C. edule (p ranging from 0.0001 to 0.0007 for C. edule) for both their elemental fingerprints as a whole and several individual elements. The accuracy of the models employed to determine the origin of the two bivalve species, using i) untreated shells, ii) pretreated shells, and iii) both pretreated and untreated shells grouped per location, was high, with the models accurately predicting the geographic origin of 100, 90 and 95% of R. philippinarum and 95, 100 and 95% of C. edule, respectively. These results show that the shifts in the EF of bivalve shells promoted by treating them with H2O2 prior to ICP-MS analysis did not affect the accuracy of the models used to confirm the geographic origin of both bivalve species. Therefore, the need to pre-treat bivalve shells with H2O2 can be dismissed in future studies addressing the traceability of bivalves when using ICP-MS, thus contributing to reducing environmental impacts and economic costs associated with this procedure, as well as the time required to obtain results.

Strong Coupling between Biomineral Morphology and Sr/Ca of Arctica islandica (Bivalvia)—Implications for Shell Sr/Ca-Based Temperature Estimates

Bivalve shells serve as powerful high-resolution paleoclimate archives. However, the number of reliable temperature proxies is limited. It has remained particularly difficult to extract temperature signals from shell Sr/Ca, although Sr is routinely employed in other biogenic aragonites. In bivalves, Sr/Ca is linked to the prevailing microstructure and is sometimes affected by kinetics. Here, the hypothesis is tested that temperature can be reconstructed from shell Sr/Ca once microstructure and/or growth-rate-related bias has been mathematically eliminated. Therefore, the relationship between Sr/Ca and increment width, as well as biomineral unit size, has been studied in three different shell portions of field-grown Arctica islandica specimens. Subsequently, microstructure and/or growth-rate-related variation was removed from Sr/Ca data and residuals compared to temperature. As demonstrated, the hypothesis could not be verified. Even after detrending, Sr/Ca remained positively correlated to water temperature, which contradicts thermodynamic expectations and findings from inorganic aragonite. Any temperature signal potentially recorded by shell Sr/Ca is overprinted by other environmental forcings. Unless these variables are identified, it will remain impossible to infer temperature from Sr/Ca. Given the coupling with the biomineral unit size, a detailed characterization of the microstructure should remain an integral part of subsequent attempts to reconstruct temperature from Sr/Ca.

Microstructural and Genetic Insights Into the Formation of the “Winter Diffusion Layer” in Japanese Pearl Oyster Pinctada fucata and Its Relation to Environmental Temperature Changes

Phenotypic plasticity in molluscan shell microstructures may be related to environmental changes. The “winter diffusion layer,” a shell microstructure of the Japanese pearl oyster Pinctada fucata, is an example of this phenomenon. In this study, we used P. fucata specimens with shared genetic background to evaluate the seasonal plasticity of shell microstructures, at molecular level. To detect the seasonal changes in shell microstructure and mineral composition, shells of multiple individuals were periodically collected and analyzed using scanning electron microscopy and Raman spectrophotometry. Our observations of the winter diffusion layer revealed that this irregular shell layer, located between the outer and middle shell layers, had a sphenoid shape in radial section. This distinct shape might be caused by the internal extension of the outer shell layer resulting from growth halts. The winter diffusion layer could be distinguished from the calcitic outer shell layer by its aragonitic components and microstructures. Moreover, the components of the winter diffusion layer were irregular simple prismatic (the outer and inner sublayers) and homogeneous structures (the middle sublayer). This irregular formation occurred until April, when the animals resumed their “normal” shell formation after hibernation. To check for a correlation between gene expression and the changes in microstructures, we conducted qPCR of seven major biomineralization-related shell matrix protein-coding genes (aspein, prismalin-14, msi7, msi60, nacrein, n16, and n19) in the shell-forming mantle tissue. Tissue samples were collected from the mantle edge (tissue secreting the outer shell layer) and mantle pallium (where the middle shell layer is constructed) of the same individuals used for microstructural observation and mineral identification that were collected in January (winter growth break period), April (irregular shell formation period), and August (normal shell formation period). Statistically significant differences in gene expression levels were observed between mantle edge and mantle pallium, but no seasonal differences were detected in the seasonal expression patterns of these genes. These results suggest that the formation of the irregular shell layer in P. fucata is caused by a currently unknown genetic mechanism unrelated to the genes targeted in the present study. Further studies using big data (transcriptomics and manipulation of gene expression) are required to answer the questions herein raised. Nevertheless, the results herein presented are essential to unravel the intriguing mystery of the formation of the winter diffusion layer, which may allow us to understand how marine mollusks adapt or acclimate to climate changes.