Search

The external surface microornament of the glass scallops Catillopecten natalyae and malyutinae is made by calcitic spiny projections consisting of a stem that later divides into three equally spaced and inclined branches (here called aerials). C. natalyae contains larger and smaller aerials, whereas C. malyutinae only secreted aerials of the second type. A remarkable feature is that aerials within each type are fairly similar in size and shape and highly co-oriented, thus constituting a most sophisticated microornament. We demonstrate that aerials are single crystals whose morphology is strongly controlled by the crystallography, with the stem being parallel to the c-axis of calcite, and the branches extending along the edges of the {104} calcite rhombohedron. They grow epitaxially onto the foliated prisms of the outer shell layer. The co-orientation of the prisms explains that of the aerials. We have developed a model in which every aerial grows within a periostracal pouch. When this pouch reaches the growth margin, the mantle initiates the production of the aerial. Nevertheless, later growth of the aerial is remote, i.e. far from the contact with the mantle. We show how such an extremely sophisticated microornament has a morphology and co-orientation which are determined by crystal growth., Instituto de Salud Carlos III Spanish Government CGL2017-85118-P PID2020-116660GB-I00, Unidad Cientifica de Excelencia of the University of Granada UCE-PP2016-05, Junta de Andalucia RNM363, Ministry of Science, ICT & Future Planning, Republic of Korea 13.1902.21.001 075-15-2020-796

Accurate biological models are critical to predict biotic responses to climate change and human‐caused disturbances. Current understanding of organismal responses to change stems from studies over relatively short timescales. However, most projections lack long‐term observations incorporating the potential for transgenerational phenotypic plasticity and genetic adaption, the keys to resistance. Here, we describe unexpected temporal compensatory responses in biomineralization as a mechanism for resistance to altered environmental conditions and predation impacts in a calcifying foundation species. We evaluated exceptional archival specimens of the blue mussel Mytilus edulis collected regularly between 1904 and 2016 along 15 km of Belgian coastline, along with records of key environmental descriptors and predators. Contrary to global‐scale predictions, shell production increased over the last century, highlighting a protective capacity of mussels for qualitative and quantitative trade‐offs in biomineralization as compensatory responses to altered environments. We also demonstrated the role of changes in predator communities in stimulating unanticipated biological trends that run contrary to experimental predictive models under future climate scenarios. Analysis of archival records has a key role for anticipating emergent impacts of climate change., Current understanding of organismal responses to change stems from relatively short‐term studies. Most projections lack long‐term observations incorporating the potential for transgenerational phenotypic plasticity and genetic adaption. We describe unexpected temporal responses in biomineralization as a mechanism for resistance to altered environments in the blue mussel Mytilus edulis. We evaluated archival mussel specimens collected regularly between 1904 and 2016, along with records of environmental descriptors and predators. Contrary to global‐scale predictions, shell production increased over the last century, highlighting a protective capacity of mussels for qualitative and quantitative trade‐offs in biomineralization as compensatory responses to altered environments and predator communities.

The carbonate shell of rhynchonelliformean brachiopods is a relevant archive of seawater temperature / δ18O for the Phanerozoic. However, its application to investigate seasonality is under-exploited despite its wide employment to reconstruct the chemical conditions of past oceans. Indeed studies of brachiopod sclerogeochemistry are still rare compared to some other biogenic archives. In order to better understand if ontogenetic time-series faithfully record the seasonal variability of the surrounding environment, we investigated the incorporation of δ18O in the shell (δ18Oshell) of a modern temperate brachiopod species, Calloria inconspicua, living in the intertidal zone of Otago Harbor (New Zealand). After reconstructing the life history and ontogenetic age using morphological features of the valves, carbonate samples were collected from the inner fibrous layer along the maximum growth axis with a sub-millimetric resolution. Databases provided environmental parameters of seawater for comparison, and time-series of δ18Ofor equilibrium calcite (δ18Oeq) were estimated using salinity and temperature data. Results reveal that maxima and minima of δ18Oshell are well related to morphological markers highlighting yearly growth steps. The count of yearly cycles allowed an estimate of the timing of shell secretion and comparison with the estimated δ18Oeq of the surrounding environment.We found that C. inconspicua can be a robust archive for seasonal variation since it records with high fidelity the local condition of seawater in certain phases of its life. Although the application of the brachiopod shells in sclerogeochemistry and studies of past seasonality is promising, future researches should consider a few potential drawbacks which need to be taken into consideration. These include the fact that growth rate changes over the life-span and that shell is not deposited throughout the entire year (leading to under-estimation of the full seasonal range). Additionally, it is important to ensure that sampling does not sample different thickness within the shell, this returning time-averaged values. Knowledge of likely general biology, population composition, life cycle and shell structure are important prerequisites for the interetation of δ18O time-series measured on fossil brachiopod shells.

Increases in Arctic temperatures have accelerated melting of the Greenland icesheet, exposing intertidal organisms, such as the blue mussel Mytilus edulis, to high air temperatures and low salinities in summer. However, the interaction of these combined stressors is poorly described at the transcriptional level. Comparing expression profiles of M. edulis from experimentally warmed (30 °C and 33 °C) animals kept at control (23‰) and low salinities (15‰) revealed a significant lack of enrichment for Gene Ontology terms (GO), indicating that similar processes were active under all conditions. However, there was a progressive increase in the abundance of upregulated genes as each stressor was applied, with synergistic increases at 33 °C and 15‰, suggesting combined stressors push the animal towards their tolerance thresholds. Further analyses comparing the effects of salinity alone (23‰, 15‰ and 5‰) showed high expression of stress and osmoregulatory marker genes at the lowest salinity, implying that the cell is carrying out intracellular osmoregulation to maintain the cytosol as hyperosmotic. Identification of aquaporins and vacuolar-type ATPase transcripts suggested the cell may use fluid-filled cavities to excrete excess intracellular water, as previously identified in embryonic freshwater mussels. These results indicate that M. edulis has considerable resilience to heat stress and highly efficient mechanisms to acclimatise to lowered salinity in a changing world.

Late Cenozoic bivalve extinction in the North Atlantic area has been attributed to environmental deterioration. Within scallops and oysters – groups with a high growth rate – certain taxa which grew exceptionally fast became extinct, while others which grew slower survived. Those which grew exceptionally fast would have obtained protection from predators thereby, so their extinction may have been due to the detrimental effect of environmental change on growth rate and ability to avoid predation, rather than environmental change per se. We investigated some glycymeridid and carditid bivalves – groups with a low growth rate – to see whether extinct forms grew faster than extant forms. Extinct Glycymeris subovata grew at about the same rate as the slowest-growing living glycymeridid and much slower than late Cenozoic examples of extant G. americana, which grew at about the same rate as the fastest-growing living glycymeridid. Extinct G. obovata and extinct Cardites squamulosa ampla also grew slower than G. americana. These findings indicate that within bivalve groups with a low growth rate, extinction or survival of taxa through the late Cenozoic was not influenced by whether they were relatively fast or slow growers. By implication, environmental change acted directly to cause extinctions in these groups.

Predator–prey interactions often drive ecological patterns and are governed by factors including predator feeding rates, prey behavioral avoidance, and prey structural defenses. Invasive species can also play a large ecological role by disrupting food webs, driving local extinctions, and influencing evolutionary changes in prey defense mechanisms. This study documents a\ud substantial reduction in the behavioral and morphological responses of multiple gastropod species (Nucella lapillus, N. ostrina, Urosalpinx cinerea) to an invasive predatory crab (green crab Carcinus maenas) under ocean acidification conditions. These results suggest that climate-related\ud changes in ocean chemistry may diminish non-lethal effects of predators on prey responses including behavioral avoidance. While snails with varying shell mineralogies were similarly successful at deterring predation, those with primarily aragonitic shells were more susceptible to dissolution and erosion under high CO2 conditions. The varying susceptibility to predation among species with similar ecological roles could indicate that the impacts of invasive species like green\ud crabs could be modulated by the ability of native and invasive prey to withstand ocean acidification conditions.

Organisms with long generation times require phenotypic plasticity to survive in changing environments until genetic adaptation can be achieved. Marine calcifiers are particularly vulnerable to ocean acidification due to dissolution and a reduction in shell-building carbonate ions. Long-term experiments assess organisms' abilities to acclimatize or even adapt to environmental change. Here we present an unexpected compensatory response to extensive shell dissolution in a highly calcium-carbonate-dependent organism after long-term culture in predicted end-century acidification and warming conditions. Substantial shell dissolution with decreasing pH posed a threat to both a polar ( Liothyrella uva) and a temperate ( Calloria inconspicua) brachiopod after 7 months and 3 months exposure, respectively, with more extensive dissolution in the polar species. This impact was reflected in decreased outer primary layer thickness in the polar brachiopod. A compensatory response of increasing inner secondary layer thickness, and thereby producing a thicker shell, was exhibited by the polar species. Less extensive dissolution in the temperate brachiopod did not affect shell thickness. Increased temperature did not impact shell dissolution or thickness. Brachiopod ability to produce a thicker shell when extensive shell dissolution occurs suggests this marine calcifier has great plasticity in calcification providing insights into how similar species might cope under future environmental change.

Here, we provide the dataset associated with the research article “Orientation patterns of aragonitic crossed-lamellar, fibrous prismatic and myostracal microstructures of modern Glycymeris shells” [1]. Based on several tools (SEM, EBSD, laser confocal microscopy and FE-SEM) we present original data relative to the microstructure and texture of aragonite crystallites in all Glycymeris shell layers (crossed-lamellar, complex crossed-lamellar, fibrous prismatic and pedal retractor and adductor myostraca) and address texture characteristics at the transition from one layer to the other, identifying similarities and differences among the different layers. Shells were cut transversely, obliquely and longitudinally in order to obtain different orientated sections of the outer and inner layer and of the myostraca. The identification of major microstructural elements was provided by detailed SEM and laser confocal microscopy images. Microstructure and texture characterization was based on EBSD measurements presented as band contrast images and as color-coded crystal orientation maps with corresponding pole figures. Crystal co-orientation was measured with the MUD value. Finally, the distribution of the organic matrix occluded within the outer crossed-lamellar layer was revealed using FE-SEM. These data, besides providing a modern unaltered Glycymeris reference to detect diagenetic alteration in fossil analogs used for paleoenvironmental reconstructions, are useful to better comprehend the mechanisms of bivalve shell formation.

The vesicular microstructure is a very distinctive arrangement of calcite, consisting of hollow cavities (vesicles) of diverse sizes and shapes, usually elongated in the direction of shell thickening. It is uniquely found among living bivalves in a single oyster family, Gryphaeidae. The vesicles are distributed in lenses interleaved with compact foliated layers. We have studied the morphology and distribution of vesicles within the lenses using optical and electron microscopy, and micro-computed tomography. At a small scale, vesicles do not follow a classical von Neumann–Mullins route typical of ideal foams. At a larger scale, the initiation and evolution of a vesicular layer statistically proceed like a foam, with vesicles becoming more numerous, larger and more even in size. In summary, the vesicular material follows a foam-like coarsening to reduce the number of energetically costly interfaces. However, a steady state is never reached because the animal permanently introduces energy in the system by creating new vesicles. The fabrication of the vesicular material is mediated by the production of an emulsion between the extrapallial fluid and the precursor PILP of the calcitic walls within the thin extrapallial space. For this mechanism to proceed, the mantle cells must perform highly sophisticated behaviours of contact recognition and secretion. Accordingly, the vesicular material is under mixed physical–biological control.

Most molluscs possess shells, constructed from a vast array of microstructures and architectures. The fully formed shell is composed of calcite or aragonite. These CaCO3 crystals form complex biocomposites with proteins, which although typically less than 5% of total shell mass, play significant roles in determining shell microstructure. Despite much research effort, large knowledge gaps remain in how molluscs construct and maintain their shells, and how they produce such a great diversity of forms. Here we synthesize results on how shell shape, microstructure, composition and organic content vary among, and within, species in response to numerous biotic and abiotic factors. At the local level, temperature, food supply and predation cues significantly affect shell morphology, whilst salinity has a much stronger influence across latitudes. Moreover, we emphasize how advances in genomic technologies [e.g. restriction site-associated DNA sequencing (RAD-Seq) and epigenetics] allow detailed examinations of whether morphological changes result from phenotypic plasticity or genetic adaptation, or a combination of these. RAD-Seq has already identified single nucleotide polymorphisms associated with temperature and aquaculture practices, whilst epigenetic processes have been shown significantly to modify shell construction to local conditions in, for example, Antarctica and New Zealand. We also synthesize results on the costs of shell construction and explore how these affect energetic trade-offs in animal metabolism. The cellular costs are still debated, with CaCO3 precipitation estimates ranging from 1-2 J/mg to 17-55 J/mg depending on experimental and environmental conditions. However, organic components are more expensive (~29 J/mg) and recent data indicate transmembrane calcium ion transporters can involve considerable costs. This review emphasizes the role that molecular analyses have played in demonstrating multiple evolutionary origins of biomineralization genes. Although these are characterized by lineage-specific proteins and unique combinations of co-opted genes, a small set of protein domains have been identified as a conserved biomineralization tool box. We further highlight the use of sequence data sets in providing candidate genes for in situ localization and protein function studies. The former has elucidated gene expression modularity in mantle tissue, improving understanding of the diversity of shell morphology synthesis. RNA interference (RNAi) and clustered regularly interspersed short palindromic repeats - CRISPR-associated protein 9 (CRISPR-Cas9) experiments have provided proof of concept for use in the functional investigation of mollusc gene sequences, showing for example that Pif (aragonite-binding) protein plays a significant role in structured nacre crystal growth and that the Lsdia1 gene sets shell chirality in Lymnaea stagnalis. Much research has focused on the impacts of ocean acidification on molluscs. Initial studies were predominantly pessimistic for future molluscan biodiversity. However, more sophisticated experiments incorporating selective breeding and multiple generations are identifying subtle effects and that variability within mollusc genomes has potential for adaption to future conditions. Furthermore, we highlight recent historical studies based on museum collections that demonstrate a greater resilience of molluscs to climate change compared with experimental data. The future of mollusc research lies not solely with ecological investigations into biodiversity, and this review synthesizes knowledge across disciplines to understand biomineralization. It spans research ranging from evolution and development, through predictions of biodiversity prospects and future-proofing of aquaculture to identifying new biomimetic opportunities and societal benefits from recycling shell products. FCT: UID/Multi/04326/2019; European Marine Biological Research Infrastructure Cluster-EMBRIC (EU H2020 research and innovation program) 654008; European Union Seventh Framework Programme [FP7] ITN project 'CACHE: Calcium in a Changing Environment' under REA 60505; NERC Natural Environment Research Council NE/J500173/1 info:eu-repo/semantics/publishedVersion

WOS:000582677500029; International audience; Horizon scanning is an increasingly common strategy to identify key research needs and frame future agendas in science. Here, we present the results of the first such exercise for the field of sclerochronology, thereby providing an overview of persistent and emergent research questions that should be addressed by future studies. Through online correspondence following the 5th International Sclerochronology Conference in 2019, participants submitted and rated questions that addressed either knowledge gaps or promising applications of sclerochronology. An initial list of 130 questions was compiled based on contributions of conference attendees and reviewed by expert panels formed during the conference. Herein, we present and discuss the 50 questions rated to be of the highest priority, determined through an online survey distributed to sclerochronology community members post the conference. The final list (1) includes important questions related to mechanisms of biological control over biomineralization, (2) highlights state of the art applications of sclerochronological methods and data for solving long-standing questions in other fields such as climate science and ecology, and (3) emphasizes the need for common standards for data management and analysis. Although research priorities are continually reassessed, our list provides a roadmap that can be used to motivate research efforts and advance sclerochronology toward new, and more powerful, applications.

Although geographical patterns of species' sensitivity to environmental changes are defined by interacting multiple stressors, little is known about compensatory processes shaping regional differences in organismal vulnerability. Here, we examine large-scale spatial variations in biomineralization under heterogeneous environmental gradients of temperature, salinity and food availability across a 30° latitudinal range (3,334 km), to test whether plasticity in calcareous shell production and composition, from juveniles to large adults, mediates geographical patterns of resilience to climate change in critical foundation species, the mussels Mytilus edulis and M. trossulus. We find shell calcification decreased towards high latitude, with mussels producing thinner shells with a higher organic content in polar than temperate regions. Salinity was the best predictor of within-region differences in mussel shell deposition, mineral and organic composition. In polar, subpolar, and Baltic low-salinity environments, mussels produced thin shells with a thicker external organic layer (periostracum), and an increased proportion of calcite (prismatic layer, as opposed to aragonite) and organic matrix, providing potentially higher resistance against dissolution in more corrosive waters. Conversely, in temperate, higher salinity regimes, thicker, more calcified shells with a higher aragonite (nacreous layer) proportion were deposited, which suggests enhanced protection under increased predation pressure. Interacting effects of salinity and food availability on mussel shell composition predict the deposition of a thicker periostracum and organic-enriched prismatic layer under forecasted future environmental conditions, suggesting a capacity for increased protection of high-latitude populations from ocean acidification. These findings support biomineralization plasticity as a potentially advantageous compensatory mechanism conferring Mytilus species a protective capacity for quantitative and qualitative trade-offs in shell deposition as a response to regional alterations of abiotic and biotic conditions in future environments. Our work illustrates that compensatory mechanisms, driving plastic responses to the spatial structure of multiple stressors, can define geographical patterns of unanticipated species resilience to global environmental change.

The taxonomic composition of the biota of the Coralline Crag Formation (early Pliocene, eastern England) provides conflicting evidence of seawater temperature during deposition, some taxa indicating cool temperate conditions by analogy with modern representatives or relatives, others warm temperate to subtropical/tropical conditions. Previous isotopic (δ18O) evidence of seasonal seafloor temperatures from serial ontogenetic sampling of bivalve mollusk shells indicated cool temperate winter (

A survey of 759 predatory drill holes in Late Cretaceous and Paleogene molluscan and serpulid worm prey from Seymour Island (Antarctica) has allowed exploration of the effects of the K-Pg mass extinction on predator-prey dynamics at this high latitude site. Circular holes (0.62–6.41 mm in diameter), suggesting a large gastropod predator, most probably “Vanikoropsis” arktowskiana, occur throughout the study interval. Analysis suggests that the driller was a catholic predator capable of attacking a range of prey types, showing some degree of size and stereotypic handling behaviour. Although there were changes in prey choice across the extinction event, these reflect changes in available prey, and in particular limited options immediately post extinction. There were no significant changes in failure rate (Prey Effectiveness) over the mass extinction. Our findings suggest that at this site, for this particular predator, aside from menu choice, predation dynamics post-extinction was “business as usual”.

Members of the gastropod superfamily Stromboidea (Littorinimorpha) are characterised by their elaborate shell morphologies, distinctive mode of locomotion, and often large and colourful eyes. This iconic group comprises over 130 species, including many large and charismatic species. The family Strombidae is of particular interest, largely due to its commercial importance and wide distribution in tropical and subtropical waters. Although a few strombid mitochondrial genomes have been sequenced, data for the other four Recent families in Stromboidea are lacking. In this study we report seven new stromboid mitogenomes obtained from transcriptomic and genomic data, with taxonomic representation from each Recent stromboid family, including the first mitogenomes for Aporrhaidae, Rostellariidae, Seraphsidae and Struthiolariidae. We also report a new mitogenome for the family Xenophoridae. We use these data, along with published sequences, to investigate the relationships among these and other caenogastropod groups. All analyses undertaken in this study support monophyly of Stromboidea as redefined here to include Xenophoridae, a finding consistent with morphological and behavioural data. Consistent with previous morphological and molecular analyses, including those based on mitogenomes, monophyly of Hypsogastropoda is confirmed but monophyly of Littorinimorpha is again rejected.

Mitogenomic trees for Bivalvia have proved problematic in the past, but several highly divergent lineages were missing from these analyses and increased representation of these groups may yet improve resolution. Here, we add seven new sequences from the Anomalodesmata and one unidentified semelid species ( Bryopa lata, Euciroa cf. queenslandica, Laternula elliptica, Laternula truncata, Lyonsia norwegica, Myadora brevis, Tropidomya abbreviata, “Abra” sp.). We show that relationships in a mitogenomic tree for the Class are improved by the addition of seven anomalodesmatans from this highly divergent clade, but are still not completely consistent with relationships recovered in studies of nuclear genes. We suggest that some anomalous relationships (for instance the non-monophyly of Bivalvia) may be partially explained by compositional heterogeneity in the mitogenome and suggest that the addition of more taxa may help resolve both this effect and possible instances of long branch attraction. We also identify several curious features about anomalodesmatan mitogenomes. For example, many protein-coding gene boundaries are poorly defined in marine bivalves, but particularly so in anomalodesmatans, primarily due to non-conserved boundary sequences. The use of transcriptomic and genomic data together enabled better definition of gene boundaries, the identification of possible pseudogenes and suggests that most genes are translated monocistronically, which contrasts with many other studies. We also identified a possible case of gene duplication of ND5 in Myadora brevis (Myochamidae) . Mitogenome size in the Anomalodesmata ranges from very small compact molecules, with the smallest for Laternula elliptica (Laternulidae) only 14,622 bp, to Bryopa lata (Clavagellidae) which is at least 31,969 bp long and may be >40,000 bp. Finally, sampled species show a high degree of sequence divergence and variable gene order, although intraspecific variation in Laternula elliptica is very low.

Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. Generating their mineralized hard parts most marine invertebrates thus produce metastable aragonite rather than the stable polymorph of CaCO3, calcite. After death of the organism, the physiological conditions which were present during biomineralisation are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features. In order to understand the diagenetic replacement of biogenic aragonite to non-biogenic calcite, we subjected Arctica islandica mollusc shells to hydrothermal alteration experiments. Experimental conditions were between 100 °C and 175 °C with reaction durations between one and 84 days, and alteration fluids simulating meteoric and burial waters, respectively. Detailed microstructural and geochemical data were collected for samples altered at 100 °C (and at 0.1 MPa pressure) for 28 days and for samples altered at 175 °C (and at 0.9 MPa pressure) for 7 and 84 days, respectively. During hydrothermal alteration at 100 °C for 28 days, most but not all of the biopolymer matrix was destroyed, while shell aragonite and its characteristic microstructure was largely preserved. In all experiments below 175 °C there are no signs of a replacement reaction of shell aragonite to calcite in X-ray diffraction bulk analysis. At 175 °C the replacement reaction started after a dormant time of 4 days, and the original shell microstructure was almost completely overprinted by the aragonite to calcite replacement reaction after 10 days. Newly formed calcite nucleated at locations which were in contact with the fluid, at the shell surface, in the open pore system, and along growth lines. In the experiments with fluids simulating meteoric water, calcite crystals reached sizes up to 200 micrometres, while in the experiments with Mg-containing fluids the calcite crystals reached sizes up to one mm after 7 days of alteration. Aragonite is metastable at all applied conditions. A small bulk thermodynamic driving force exists for the transition to calcite, which is augmented by stresses induced by organic matrix and interface energies related to the nanoparticulate architecture of the biogenic aragonite. We attribute the sluggish replacement reaction to the inhibition of calcite nucleation in the temperature window from ca. 50 °C to ca. 170 °C, or, additionally, to the presence of magnesium. Correspondingly, in Mg2+-bearing solutions the newly formed calcite crystals are larger than in Mg2+-free solutions. Overall, the aragonite-calcite transition occurs via an interface-coupled dissolution-reprecipitation mechanism, which preserves morphologies down to the sub-micrometre scale and induces porosity in the newly formed phase. The absence of aragonite replacement by calcite at temperatures lower than 175 °C contributes to explain why aragonitic or bimineralic shells and skeletons have a good potential of preservation and a complete fossil record.

Ocean acidification threatens many ecologically and economically important marine calcifiers. The increase in shell dissolution under the resulting reduced pH is an important and increasingly recognized threat. The biocomposites that make up calcified hardparts have a range of taxon-specific compositions and microstructures, and it is evident that these may influence susceptibilities to dissolution. Here, we show how dissolution (thickness loss), under both ambient and predicted end-century pH (approx. 7.6), varies between seven different bivalve molluscs and one crustacean biocomposite and investigate how this relates to details of their microstructure and composition. Over 100 days, the dissolution of all microstructures was greater under the lower pH in the end-century conditions. Dissolution of lobster cuticle was greater than that of any bivalve microstructure, despite its calcite mineralogy, showing the importance of other microstructural characteristics besides carbonate polymorph. Organic content had the strongest positive correlation with dissolution when all microstructures were considered, and together with Mg/Ca ratio, explained 80–90% of the variance in dissolution. Organic content, Mg/Ca ratio, crystal density and mineralogy were all required to explain the maximum variance in dissolution within only bivalve microstructures, but still only explained 50–60% of the variation in dissolution.

The shells of the bivalves Glycymeris glycymeris and Glycymeris nummaria are widely used for environmental studies. They consist of aragonite and comprise four different microstructures and textures from outer to inner shell surfaces: crossed-lamellar, myostracal, complex crossed-lamellar and fibrous prismatic. We characterize with SEM, EBSD, laser-confocal microscopy and AFM imaging mineral unit size, morphology and orientation of crystallites in the different microstructural arrangements and at the transition from one microstructure to the other. We also characterize the microstructure and texture of adductor and pedal retractor myostraca and address structural characteristics at the transition from crossed-lamellar to myostracal assemblies. We find that the crossed-lamellar layer has a three-dimensional crystallographic orientational order. Each set of first-order lamellae consists of twinned aragonite; the two sets of first-order lamellae are misoriented to each other by about 30 to 40° while retaining an approximately parallel a-axis; they do not show any particular twin relationship. Myostracal aragonite grows homoepitactically onto the crossed-lamellar aragonite, but is clearly a separate microstructure, with its own crystallite size and morphology. Within adductor and pedal myostraca, prisms increase in size towards inner surfaces. In contrast to the other shell layers, the myostraca form through competitive growth. The complex crossed-lamellar aragonite initially inherits the three-dimensional texture of the crossed-lamellar microstructure, but with growth develops an axial texture, which is transmitted to the underlying fibrous prismatic microstructure. With this work we provide a modern, unaltered, reference for fossil Glycymeris shells to be used for detection of diagenetic overprint in fossil Glycymeris analogs., GC acknowledge financial support of the 2017 Italian Ministry PRIN RX9XXYProject “Biota resilience to global change: biomineralization of planktic and benthic calcifiers in the past, present and future” to E. Erba. AGC is funded by project CGL2017-85118-P of the Spanish Ministerio de Ciencia e Innovación, the Unidad Científica de Excelencia UCE-PP2016-05 of the University of Granada, and the Research Group RNM363 of the Junta de Andalucía (AGC).

Biological interactions between organisms have long been believed to be very important in structuring communities and, when scaled up over geological time, in the evolution of organisms. Investigations of palaeontological evidence for predator–prey interactions have been popular pursuits, and a number of attractive hypotheses have been proposed which link increased predation pressure with a wide range of morphological and ecological changes which are apparent over the course of the Phanerozoic. In particular studies of fossil drill holes and repair scars in shelly prey have been common targets for research. However, the nature of some of our data has been rather anecdotal and restricted in range. Perhaps we should be more concerned that we are not picking up the true range of natural variability. This paper aims to highlight the sources of variability in our data and, going forward, to urge the collection of quantitative data from many more samples and (palaeo)environmental settings in order that we might properly be able to separate the intrinsic natural variability in our data from robust temporal or spatial trends.

The shell material of marine benthic bivalves provides a sensitive archive of water chemistry immediately above the sediment–water interface, which in turn is affected by sedimentary geochemistry and redox reactions. Sulfate has a major controlling effect on sedimentary carbon cycling, particularly the processes of methane production and oxidation, with lower concentrations of sulfate likely resulting in an increase in sedimentary methane production. Whilst it is accepted that ocean sulfate varied markedly across the Phanerozoic, evidence of changes in methane production in sediments has so far been lacking. There is potential for the oxidation products of sedimentary methane to be preserved and detected in marine fossils. Here we present the results of high resolution carbonate isotope records from two taxa of well-preserved shallow-infaunal bivalve (Lahillia and Cucullaea) collected from the marine shelf succession across the Cretaceous–Paleogene (K–Pg) boundary in Seymour Island, Antarctica. The succession has pre-existing subtle indications of more abundant methane, and the time period is characterized by much lower marine sulfate concentrations than modern. These shell carbonate–carbon isotope records vary widely: at one extreme, shells have typical average values and small ranges compatible with a contemporaneous marine dissolved inorganic carbon (DIC) source and modern-style sedimentary carbon cycling. At the other, the shells have large-amplitude annual cycles of carbon isotopic variability of up to 23.8‰ within a single year of growth and shell carbonate δ 13 C compositions as negative as −34‰. Shells with these increased ranges and unusually negative values are found at discrete intervals and across both bivalve taxa. The contribution of methane required to explain the most negative carbonate–carbon isotopic values in the bivalve shells is extremely high (between 30 to 85% of bottom-water DIC based on mass balance calculations). Records of organic-carbon isotopes from the same succession remained between −26.1 and −21.7‰ throughout, suggesting that methane influence was restricted to bottom-waters. A lack of authigenic carbonate in the section indicates that methane oxidation progressed aerobically and may have provided a significant driver for transient bottom water de-oxygenation. Where methane addition is indicated, the seasonal sensitivity precludes control by methane hydrates. We argue that these data represent the increased importance and sensitivity of methanogenesis in the sediments, enabled by lower ocean sulfate concentrations during the Late Cretaceous. The tendency towards a more dynamic role for marine methane production and oxidation is likely to apply to other times of low marine sulfate in Earth's history.

Although geographic patterns of species’ sensitivity to global environmental changes are defined by interacting multiple stressors, little is known about the biological mechanisms shaping regional differences in organismal vulnerability. Here, we examine large-scale spatial variations in biomineralisation under heterogeneous environmental gradients of temperature, salinity and food availability across a 30° latitudinal range (3,334 km), to test whether plasticity in calcareous shell production and composition, from juveniles to large adults, mediates geographic patterns of resilience to climate change in critical foundation species, the mussels Mytilus edulis and M. trossulus. We find mussels produced thinner shells with a higher organic content in polar than temperature regions, indicating decreasing shell calcification towards high latitudes. Salinity was the major driver of regional differences in mussel shell deposition, and in shell mineral and organic composition. In low-salinity environments, the production of calcite and organic shell layers was increased, providing higher resistance against dissolution in more corrosive waters. Conversely, under higher-salinity regimes, increased aragonite deposition suggests enhanced mechanical protection from predators. Interacting strong effects of decreasing salinity and increasing food availability on the compositional shell plasticity in polar and subpolar mussels during growth predict the deposition of a thicker external organic layer (periostracum) under forecasted future environmental conditions. This marked response potential of Mytilus species suggests a capacity for increased protection of high-latitude mussel populations from ocean acidification. Our work illustrates that mechanisms driving plastic responses to the spatial structure of multiple stressors can define geographic patterns of unforeseen species resilience to global environmental change.

The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. A correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies (Casella et al., 2017) have shown that replacement of biogenic carbonate by inorganic calcite occurs via an interface coupled dissolution–reprecipitation mechanism. Furthermore, for a comprehensive assessment of alteration, structural changes have to be assessed on the nanoscale as well, which documents the replacement of pristine nanoparticulate calcite by diagenetic nanorhombohedral calcite (Casella et al., 2018a, b). In the present contribution we investigated six different modern biogenic carbonate microstructures for their behaviour under hydrothermal alteration in order to assess their potential to withstand diagenetic overprint and to test the integrity of their preservation in the fossil record. For each microstructure (a) the evolution of biogenic aragonite and calcite replacement by inorganic calcite was examined, (b) distinct carbonate mineral formation steps on the micrometre scale were highlighted, (c) microstructural changes at different stages of alteration were explored, and (d) statistical analysis of differences in basic mineral unit dimensions in pristine and altered skeletons was performed. The latter analysis enables an unequivocal determination of the degree of diagenetic overprint and discloses information especially about low degrees of hydrothermal alteration.

Shape variability represents an important direct response of organisms to selective environments. Here, we use a combination of geometric morphometrics and generalised additive mixed models (GAMMs) to identify spatial patterns of natural shell shape variation in the North Atlantic and Arctic blue mussels, Mytilus edulis and M. trossulus, with environmental gradients of temperature, salinity and food availability across 3980 km of coastlines. New statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the developmental and genetic contributions to shell shape and made it possible to identify general relationships between blue mussel shape variation and environment that are independent of age and species influences. We find salinity had the strongest effect on the latitudinal patterns of Mytilus shape, producing shells that were more elongated, narrower and with more parallel dorsoventral margins at lower salinities. Temperature and food supply, however, were the main drivers of mussel shape heterogeneity. Our findings revealed similar shell shape responses in Mytilus to less favourable environmental conditions across the different geographical scales analysed. Our results show how shell shape plasticity represents a powerful indicator to understand the alterations of blue mussel communities in rapidly changing environments.

The large, burrowing bivalve Laternula elliptica is an abundant component of shallow-water soft-substrate communities around Antarctica but its congeners are temperate and tropical in distribution and their phylogenetic relationships are obscure. A new molecular analysis of Laternulidae species shows that there are two distinct clades, one of Exolaternula species, E. spengleri and E. liautaudi, possessing a ligamental lithodesma and a larger clade of species lacking the lithodesma. Of the latter, Laternula elliptica is a sister taxon to temperate and tropical species, including those that live around the coasts of Australia from Tasmania to Darwin. It is suggested that L. elliptica was left isolated around Antarctica following the opening of the Tasman Gateway and initiation of the Circum-Antarctic Current as Australia drifted northwards following the final breakup of Gondwana. A further scenario is that as Australia moved closer to Asia, species spread into tropical habitats and more widely to the Red Sea and Japan. Exolaternula species have a likely Tethyan origin and the present-day range is from the Arabian Gulf, around southern Asia and as far north as southern Russia.

The dissolution of the delicate shells of sea butterflies, or pteropods, has epitomised discussions regarding ecosystem vulnerability to ocean acidification over the last decade. However, a recent demonstration that the organic coating of the shell, the periostracum, is effective in inhibiting dissolution suggests that pteropod shells may not be as susceptible to ocean acidification as previously thought. Here we use micro-CT technology to show how, despite losing the entire thickness of the original shell in localised areas, specimens of polar species Limacina helicina maintain shell integrity by thickening the inner shell wall. One specimen collected within Fram Strait with a history of mechanical and dissolution damage generated four times the thickness of the original shell in repair material. The ability of pteropods to repair and maintain their shells, despite progressive loss, demonstrates a further resilience of these organisms to ocean acidification but at a likely metabolic cost., Sea butterflies, or pteropods, are often presented as being at threat from ocean acidification on account of their fragile shells being susceptible to dissolution. Here the authors show that pteropods are able to perform extensive repair to damaged shells, suggesting they may not be as vulnerable as previously thought.

Oyster shells are mainly composed of layers of foliated microstructure and lenses of chalk, a highly porous, apparently poorly organized and mechanically weak material. We performed a structural and crystallographic study of both materials, paying attention to the transitions between them. The morphology and crystallography of the laths comprising both microstructures are similar. The main differences were, in general, crystallographic orientation and texture. Whereas the foliated microstructure has a moderate sheet texture, with a defined 001 maximum, the chalk has a much weaker sheet texture, with a defined 011 maximum. This is striking because of the much more disorganized aspect of the chalk. We hypothesize that part of the unanticipated order is inherited from the foliated microstructure by means of, possibly, {0118} twinning. Growth line distribution suggests that during chalk formation, the mantle separates from the previous shell several times faster than for the foliated material. A shortage of structural material causes the chalk to become highly porous and allows crystals to reorient at a high angle to the mantle surface, with which they continue to keep contact. In conclusion, both materials are structurally similar and the differences in orientation and aspect simply result from differences in growth conditions., Funding was provided by projects CGL2013-48247-P and CGL2017-85118-P (to A.G.C., E.M.H. and A.G.-S.) of the Spanish Ministerio de Economía, Industria y Competitividad. A.G.C. also acknowledges the Research Group RNM363 (Junta de Andalucía).

The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale. In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175°C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons. We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product. At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase., We thank the German Research Council (DFG) for financial support in the context of the collaborative research initiative CHARON (DFG Forschergruppe 1644, grant agreement no. SCHM 930/11-1).

Surface seawaters are becoming more acidic due to the absorption of rising anthropogenic CO2. Marine calcifiers are considered to be the most vulnerable organisms to ocean acidification due to the reduction in the availability of carbonate ions for shell or skeletal production. Rhychonelliform brachiopods are potentially one of the most calcium carbonate-dependent groups of marine organisms because of their large skeletal content. Little is known, however, about the effects of lowered pH on these taxa. A CO2 perturbation experiment was performed on the New Zealand terebratulide brachiopod Calloria inconspicua to investigate the effects of pH conditions predicted for 2050 and 2100 on the growth rate and ability to repair shell. Three treatments were used: an ambient pH control (pH 8.16), a mid-century scenario (pH 7.79), and an end-century scenario (pH 7.62). The ability to repair shell was not affected by acidified conditions with >80% of all damaged individuals at the start of the experiment completing shell repair after 12 weeks. Growth rates in undamaged individuals >3 mm in length were also not affected by lowered pH conditions, whereas undamaged individuals

Shells of brachiopods are excellent archives for environmental reconstructions in the recent and distant past as their microstructure and geochemistry respond to climate and environmental forcings. We studied the morphology and size of the basic structural unit, the secondary layer fibre, of the shells of several extant brachiopod taxa to derive a model correlating microstructural patterns to environmental conditions. Twenty-one adult specimens of six recent brachiopod species adapted to different environmental conditions, from Antarctica, to New Zealand, to the Mediterranean Sea, were chosen for microstructural analysis using SEM, TEM and EBSD. We conclude that: 1) there is no significant difference in the shape and size of the fibres between ventral and dorsal valves, 2) there is an ontogenetic trend in the shape and size of the fibres, as they become larger, wider, and flatter with increasing age. This indicates that the fibrous layer produced in the later stages of growth, which is recommended by the literature to be the best material for geochemical analyses, has a different morphostructure and probably a lower organic content than that produced earlier in life. In two species of the same genus living in seawater with different temperature and carbonate saturation state, a relationship emerged between the microstructure and environmental conditions. Fibres of the polar Liothyrella uva tend to be smaller, rounder and less convex than those of the temperate Liothyrella neozelanica, suggesting a relationship between microstructural size, shell organic matter content, ambient seawater temperature and calcite saturation state.

The inability of organisms to cope in changing environments poses a major threat to their survival. Rising carbon dioxide concentrations, recently exceeding 400 μatm, are rapidly warming and acidifying our oceans. Current understanding of organism responses to this environmental phenomenon is based mainly on relatively short‐ to medium‐term laboratory and field experiments, which cannot evaluate the potential for long‐term acclimation and adaptation, the processes identified as most important to confer resistance. Here, we present data from a novel approach that assesses responses over a centennial timescale showing remarkable resilience to change in a species predicted to be vulnerable. Utilising museum collections allows the assessment of how organisms have coped with past environmental change. It also provides a historical reference for future climate change responses. We evaluated a unique specimen collection of a single species of brachiopod (Calloria inconspicua) collected every decade from 1900 to 2014 from one sampling site. The majority of brachiopod shell characteristics remained unchanged over the past century. One response, however, appears to reinforce their shell by constructing narrower punctae (shell perforations) and laying down more shell. This study indicates one of the most calcium‐carbonate‐dependent species globally to be highly resilient to environmental change over the last 120 years and provides a new insight for how similar species might react and possibly adapt to future change., Current knowledge of how organisms will respond to ocean warming and acidification is based on relatively short‐ to medium‐term laboratory and field experiments, which cannot evaluate the potential for adaptation. We present a novel approach of utilising museum collections of a brachiopod species collected every decade at one site from 1900 to 2014 to assess how organisms have coped with past environmental change. This study indicates the resilience of one of the most vulnerable species globally to environmental change over the past 120 years and provides an insight for how similar species might react to future change.

Multiple groups of bivalve molluscs produce calcitic shell layers, many of these broadly classified as "prismatic." Various pteriomorphian bivalves (such as oysters, pterioids, and mussels) secrete prismatic microstructures with high organic content and clear, strong biological control. However, we present the results of a detailed analysis by scanning electron microscopy (SEM), thermogravimetric analysis, and electron backscatter diffraction to characterize the calcitic prisms in two different clades within the euheterodont bivalves: the extant Chama arcana and the extinct rudists. These results show that the form of prisms constructed is both closely similar between the two taxa and significantly different from those of the pteriomorph bivalves. Most notably, C. arcana and the extinct rudists lack the clear organic outer envelopes and uniform polygonal, cross-sectional appearance. Instead, they form interdigitating crystals of very varied diameters, with some crystals encapsulating others. We advocate retaining the term "fibrillar prisms" to classify these euheterodont microstructures. These fibrillar prisms are more closely similar to abiotic speleothem deposits than to the calcitic prisms of pteriomorph bivalves. We argue that calcite prism growth in euheterodonts is dominated by abiotic constraints whereas, in pteriomorphs (such as oysters, pterioids, and mussels), it is under strong biological control.

The occurrence of pearls in the true' oysters, the Ostreioidea, is poorly documented despite being the most produced mollusc species in the world. Oysters of the Crassostrea genus were collected in two different sites in southern Portugal where both Crassostrea angulata and C. gigas are present, namely in: (1) the Ria Formosa lagoon where pearls were not observed (N = 446); and (2) the Guadiana estuary where pearls were found in 12 out of the 798 oysters analysed. The pearls were located mainly at the edge of the right mantle lobe in the inhalant chamber and their maximum length ranged from 0.9 to 5.5 mm. Almost all the pearls had a white-cream colouration with the exception of two pearls that had a black-brown colour. X-ray diffraction analysis of one pearl showed that it was entirely calcitic with no traces of either aragonite or vaterite. The pearls observed were therefore non-nacreous pearls. Scanning electron microscopy (SEM) revealed a diversity of microstructures including prismatic, foliae-like sheets and blocky textures, i.e. highly reminiscent of the host oyster shell microstructures. Parasites (e.g. parasitic copepods, Haplosporidium-like plasmodia) and signs of diseases (e.g. foot disease) were observed in some of the oysters analysed, but they were not associated with the occurrence of pearls. The present work is one of the few studies on the occurrence of natural pearls in true' oysters and to our knowledge the first description of their microstructure by SEM. PROSANI [31-03-01-FEP-002] European Union Atlantic Area Transnational Programme [2009-1/123] info:eu-repo/semantics/publishedVersion

The hypothesized collapse in primary productivity associated with the Cretaceous-Paleogene (K-Pg) mass extinction would have been particularly severe on suspension feeders dependent on phytoplankton. Previous research on the ecological dynamics of erect bryozoans in the Danish Basin supported the phytoplankton crash hypothesis in showing a major postextinction increase in the skeletal biomass of cyclostome bryozoans relative to the more nutrient-demanding cheilostome bryozoans. New data on the ecological dynamics between these two bryozoan clades across the K-Pg boundary are provided to determine whether the postextinction cyclostome spike is also evident in encrusting bryozoans and over a greater geographical area. Changes across the K-Pg were quantified in encrusting bryozoans using three metrics: (1) abundance of colonies; (2) area of substrate covered; and (3) colony form. Data from the southeastern United States were added to those from Denmark to control for regional factors. Our analyses indicate a globally uniform change among encrusting bryozoans but without the strong postextinction cyclostome spike seen previously in Danish erect bryozoans. An increased proportion of sheet over runner cyclostome colonies was found in the lower Danian of Denmark and the southeastern United States, despite the expectation that runners with widely dispersed zooids should fare better in low nutrient conditions. Furthermore, weighed samples of erect bryozoans from the basal Danian of Maastricht (Netherlands) and the southeastern United States failed to reveal a strong cyclostome spike. These findings question the phytoplankton crash hypothesis, or at least the extent to which this kill mechanism influenced suspension feeders during the K-Pg mass extinction.

We investigated the spikes on the outer shell surface of the endolithic gastrochaenid bivalve genus Spengleria with a view to understand the mechanism by which they form and evaluate their homology with spikes in other heterodont and palaeoheterodont bivalves. We discovered that spike formation varied in mechanism between different parts of the valve. In the posterior region, spikes form within the translucent layer of the periostracum but separated from the calcareous part of the shell. By contrast those spikes in the anterior and ventral region, despite also forming within the translucent periostracal layer, become incorporated into the outer shell layer. Spikes in the posterior area of Spengleria mytiloides form only on the outer surface of the periostracum and are therefore, not encased in periostracal material. Despite differences in construction between these gastrochaenid spikes and those of other heterodont and palaeoheterodont bivalves, all involve calcification of the inner translucent periostracal layer which may indicate a deeper homology.