Stefania Milano

Transgenerational acclimation to seawater acidification in the Manila clam Ruditapes philippinarum: Preferential uptake of metabolic carbon

Abstract Ocean acidification may interfere with the calcifying physiology of marine bivalves. Therefore, understanding their capacity for acclimation and adaption to low pH over multiple generations is crucial to make predictions about the fate of this economically and ecologically important fauna in an acidifying ocean. Transgenerational exposure to an acidification scenario projected by the end of the century (i.e., pH 7.7) has been shown to confer resilience to juvenile offspring of the Manila clam, Ruditapes philippinarum. However, whether, and to what extent, this resilience can persist into adulthood are unknown and the mechanisms driving transgenerational acclimation remain poorly understood. The present study takes observations of Manila clam juveniles further into the adult stage and observes similar transgenerational responses. Under acidified conditions, clams originating from parents reproductively exposed to the same level of low pH show a significantly faster shell growth rate, a higher condition index and a lower standard metabolic rate than those without prior history of transgenerational acclimation. Further analyses of stable carbon isotopic signatures in dissolved inorganic carbon of seawater, individual soft tissues and shells reveal that up to 61% of shell carbonate comes from metabolic carbon, suggesting that transgenerationally acclimated clams may preferentially extract internal metabolic carbon rather than transport external seawater inorganic carbon to build shells, the latter known to be energetically expensive. While a large metabolic carbon contribution (45%) is seen in non-acclimated clams, a significant reduction in the rate of shell growth indicates it might occur at the expense of other calcification-relevant processes. It therefore seems plausible that, following transgenerational acclimation, R. philippinarum can implement a less costly and more efficient energy-utilizing strategy to mitigate the impact of seawater acidification. Collectively, our findings indicate that marine bivalves are more resilient to ocean acidification projected for the end of the century than previously thought.

Drivers of shell growth of the bivalve, Callista chione (L. 1758) – Combined environmental and biological factors

Seasonal shell growth patterns were analyzed using the stable oxygen and carbon isotope values of live-collected specimens of the bivalve Callista chione from two sites in the Adriatic Sea (Pag and Cetina, Croatia). Micromilling was performed on the shell surface of three shells per site and shell oxygen isotopes of the powder samples were measured. The timing and rate of seasonal shell growth was determined by aligning the δ18Oshell-derived temperatures so that the best fit was achieved with the instrumental temperature curve. According to the data, shells grew only at very low rates or not at all during the winter months, i.e., between January and March. Shell growth slowdown/shutdown temperatures varied among sites, i.e., 13.6 °C at Pag and 16.6 °C at Cetina, indicating that temperature was not the only driver of shell growth. Likely, seasonal differences in seawater temperature and food supply were the major component explaining contrasting growth rates of C. chione at two study sites. Decreasing shell growth rates were also associated with the onset of gametogenesis suggesting a major energy reallocation toward reproduction rather than growth. These results highlight the need to combine sclerochronological analyses with ecological studies to understand life history traits of bivalves as archives of environmental variables.

Environmental and biological factors influencing trace elemental and microstructural properties of Arctica islandica shells

Long-term and high-resolution environmental proxy data are crucial to contextualize current climate change. The extremely long-lived bivalve, Arctica islandica, is one of the most widely used paleoclimate archives of the northern Atlantic because of its fine temporal resolution. However, the interpretation of environmental histories from microstructures and elemental impurities of A. islandica shells is still a challenge. Vital effects (metabolic rate, ontogenetic age, and growth rate) can modify the way in which physiochemical changes of the ambient environment are recorded by the shells. To quantify the degree to which microstructural properties and element incorporation into A. islandica shells is vitally or/and environmentally affected, A. islandica specimens were reared for three months under different water temperatures (3, 8 and 13 °C) and food concentrations (low, medium and high). Concentrations of Mg, Sr, Na, and Ba were measured in the newly formed shell portions by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The microstructures of the shells were analyzed by Scanning Electron Microscopy (SEM). Shell growth and condition index of each specimen were calculated at the end of the experimental period. Findings indicate that no significant variation in the morphometric characteristics of the microstructures were formed at different water temperatures or different food concentrations. Shell carbonate that formed at lowest food concentration usually incorporated the highest amounts of Mg, Sr and Ba relative to Ca+2 (except for Na) and was consistent with the slowest shell growth and lowest condition index at the end of the experiment. These results seem to indicate that, under food limitation, the ability of A. islandica to discriminate element impurities during shell formation decreases. Moreover, all trace element-to‑calcium ratios were significantly affected by shell growth rate. Therefore, physiological processes seem to dominate the control on element incorporation into A. islandica shells.

Microstructures in relation to temperature-induced aragonite-to-calcite transformation in the marine gastropod Phorcus turbinatus

Mollusk shells represent important archives for paleoclimatic studies aiming to reconstruct environmental conditions at high temporal resolution. However, the shells, made of calcium carbonate in the form of aragonite and /or calcite, can be altered through time which may undermine the suitability for any reconstruction based on geochemical proxies (i.e., stable isotopes, radiocarbon). At present, the diagenetic processes involved in this chemical and physical deterioration are still poorly understood. The present study aims to shed light on the onset and development of diagenetic alteration in the aragonitic shell of Phorcus turbinatus. To artificially mimic diagenesis, shells of P. turbinatus were exposed to elevated temperatures. The transformation of the mineral phase was monitored by means of Confocal Raman Microscopy whereas the structural changes were investigated using Scanning Electron Microscopy and Atomic Force Microscopy. The results indicate that the two distinct shell layers (prismatic layer and nacre) respond differently to the elevated temperatures, suggesting that the different microstructural organization and organic content may drive the onset and spread of the aragonite-to-calcite transformation. Furthermore, changes in the microstructural arrangement became visible prior to the mineralogical transition. Our results demonstrate that the specific physico-chemical characteristics of structurally different areas within the biogenic carbonates have to be taken into account when studying the phase transformation occurring during diagenesis.

Oxygen Isotope Composition of Arctica islandica Aragonite in the Context of Shell Architectural Organization: Implications for Paleoclimate Reconstructions: SHELL ARCHITECTURE AND OXYGEN ISOTOPES

The analysis of the stable isotopic composition of bivalve shells provides the data needed to construct climate records at high temporal resolution. Yet, the reproducibility of the results and the effect of microstructural organization on the isotopic signature and measurements have not been extensively studied. Here, we examine the architectural changes within Arctica islandica shells, specifically if samples from microstructurally different shell layers differ in respect to stable oxygen isotope values. The oxygen isotope profiles of two microstructurally different shell layers, each sampled at different temporal resolution, were compared to each other. Our results show that aragonite, collected from the layer that is dominated by cross-acicular/lamellar structures, tends to be enriched in heavier oxygen isotopes compared to samples from portions of the outer shell layer dominated by homogeneous microstructure. In some cases, this difference exceeded 0.3 &, which can significantly affect the interpretation of a recorded environmental signal. Observed differences in stable oxygen isotope data may be associated with the physiology of the mollusk and the physical and chemical composition of studied shell layers.