Elizabeth M. Harper

Calcifying bivalves account for > 80% of the total macrozoobenthic biomass in the southern Baltic Sea. Salinity declines from around 30 in the western Kattegat to 7 in the central Baltic Proper providing a natural gradient along which to investigate the impact of lowering salinity on shelled organisms. Low salinity conditions constrain biomineralisation with limited calcification substrates and periods of aragonite undersaturation. For species with wholly aragonitic shells like the cockle, Cerastoderma glaucum , shell maintenance and production can be challenging. While studies have investigated the corrosive effects of ocean acidification on calcifying organisms, few have considered the long-term consequences of low salinity. This study investigated the shell characteristics of C. glaucum sampled in 2016 and 2024–25 along the southern Baltic salinity gradient to examine variations in shell thickness, shape and condition. Comparisons were made with populations sampled from coastal lagoons in 2024–25 (UK and France) where seasonal salinity fluctuations are significant (< 20 to > 40). This comparative study revealed Baltic populations of C. glaucum exposed to salinities of < 10 to produce shells that are up to 85.2% thicker than populations in more variable environments. However, their size is limited to lengths of 20 mm, around 58% of the usual length achieved by individuals in salinities of > 10. Integral to survival, the shell offers protection from predation and adverse ambient conditions. Ocean freshening poses significant threats to biomineralisation and understanding the response of calcifying species to low salinity is fundamental for assessing their adaptive capacity to future projections of declining salinity.

Ocean freshening due to increased precipitation and ice melting in a warming world poses a significant threat to marine calcifiers. The reduced availability of calcification substrates and an undersaturated calcium carbonate state challenge shell construction and maintenance. The corrosive potential of ocean acidification on biomineralized skeletons is well understood, but few studies have investigated the corrosive potential of significant freshwater input into marine habitats. To examine the susceptibility of invertebrate biocomposites to low salinity, we exposed blocks containing polished surfaces of pristine shell material of six bivalve taxa, representing different mineralogies and microstructures, to a salinity gradient (0–45‰) for 180 days. By measuring the loss of shell thickness, we revealed a significant correlation between dissolution and decreasing salinity. Significantly different amounts of shell thickness loss were observed across microstructures, revealing mineralogy and organic content as important predictors for dissolution. Aragonite layers lost significantly more shell thickness than calcite, and higher organic content retarded dissolution across microstructures. Overall, shell dissolution at low salinities is significant and comparable to dissolution predicted under end-of-century ocean acidification scenarios. The compromised integrity of marine invertebrate biomineralized skeletons because of ocean freshening is of critical concern with future predicted increases in precipitation and sea ice melt.

The shells of bivalved molluscs comprise, in general, few microstructures and very few textures. In the case of ostreoid oysters, a high diversity has been observed. The shells consist of columnar-prismatic, foliated, granular calcite and myostracal-prismatic aragonite. Furthermore, voids are incorporated into the ostreoid shell: the pores of the vesicular shell segments and the blades/laths of the chalk lenses. These initiate formation of additional microstructures and textures. We investigated the shells of Magallana gigas, Ostrea stentina, Ostrea edulis (Ostreidae), Hyotissa hyotis, Hyotissa mcgintyi and Neopycnodonte cochlear (Gryphaeidae) with high-resolution, low-kV, electron backscatter diffraction (EBSD) measurements and scanning electron microscopy (FE-SEM) imaging and review the diversity of ostreoid Ca-carbonate microstructures and textures. From a crystallographic perspective, we (i) characterized the sub-micrometer crystal assembly pattern of ostreoid microstructures and textures, (ii) investigated crystal organization at the changeover from one microstructure into the other and (iii) examined how curved crystal surfaces are generated at inner shell surface as well as within the shell, in and at aggregations of folia and foliated units. We show that Ostreoidea are capable of secreting single crystalline, graded and dendritic calcite within the same shell and, hence, are able to vary strongly the degree of crystal co-alignment. We demonstrate that Ostreoidea myostracal aragonite is twinned, while shell calcite is not twinned, neither within different microstructures nor at the changeover between adjacent microstructures. We highlight the very specific microstructure of the foliated shell and demonstrate the strongly regulated gradedness of both the c- and a*-axes orientation of the foliated calcite crystallites.

The microstructure of bivalve foliated calcite is extraordinary. It consists of units formed of stacks of folia with individual folia consisting of arrowhead-ended crystal laths. We investigated the texture of the foliated microstructure, the texture of individual and arrays of folia and the texture of assemblies of foliated units of the gryphaeid oyster Hyotissa hyotis with low kV, high-resolution, electron backscatter diffraction (EBSD). We base our understanding of the foliated texture on the combined interpretation of crystallographic aspects of individual and stacks of folia with the nature of crystal organization in a folium, a foliated unit and in foliated unit aggregations. Calcite c- and a*-axes arrangement in a folium is single-crystal-like. Due to the parallel organization of adjacent laths in a folium and the stacked arrangement of folia in a foliated unit, the assembly of calcite c- and a*-axes in foliated units is graded. The result is a ring-like distribution of c- and a*-axes orientations in the pole figures; nonetheless, the orientation rings are substructured by c- and a*-axes orientation clusters. The direction of the arrowhead endings of the laths is coincident with the growth direction of the shell. The morphology of arrowheaded laths initiates the formation of planes with {105}, {106} directions and a parallel orientation to the inner shell surface. H. hyotis’s foliated microstructure has a specific texture that is not fully understood. We discuss axial, spherulitic, turbostratic-like textures the foliated microstructure and suggest that the foliated texture of H. hyotis can, to some degree, be described with a turbostratic pattern.

The shell of Ostreoidea is specific among that of bivalves, as it comprises many crystal microstructures and crystal textures. This stems from the fact that into the basic shell structure, consisting of columnar, foliated, granular calcite and myostracal aragonite, voids are incorporated: the pores of the vesicular segments and the blades/laths of the chalk lenses. Furthermore, cementation to uneven-surfaced substrate requires the formation of corrugated shell morphologies and adjustments of crystal organization for the generation of convex/concave shell surfaces. We investigated the shell of Magallana gigas, Ostrea stentina, Ostrea edulis (Ostreidae) and Hyotissa hyotis, Hyotissa mcgintyi, Neopycnodonte cochlear (Gryphaeidae) with high-resolution, low-kV, electron backscatter diffraction (EBSD) measurements and scanning electron microscopy (FE-SEM) imaging. From a crystallographic perspective we, (i) characterized the sub-micrometer crystal assembly of ostreoid microstructures and textures, (ii) investigated crystal organization at the changeover from one microstructure into the other, and (iii) examined how curved crystal surfaces are generated at inner shell surface and within the shell, in and at aggregations of folia. We show that Ostreoidea are capable to secrete, within the same shell, single crystalline, graded and polycrystalline calcite, the latter with almost random orientation. We demonstrate that Ostreoidea myostracal aragonite is twinned, while Ostreoidea calcite is not twinned, neither the calcite within the different microstructures, nor the calcite at the changeover from one microstructure into the other. We highlight the very specific microstructure of the foliated shell and demonstrate the strongly regulated gradedness of both, the c- and a*-axes orientation of the foliated calcite crystallites.

Eyes within the marine gastropod superfamily Stromboidea range widely in size, from 0.2 to 2.3 mm - the largest eyes known in any gastropod. Despite this interesting variation, the underlying evolutionary pressures remain unknown. Here, we use the wealth of material available in museum collections to explore the evolution of stromboid eye size and structure. Our results suggest that depth is a key light-limiting factor in stromboid eye evolution; here, increasing water depth is correlated with increasing aperture width relative to lens diameter, and therefore an increasing investment in sensitivity in dim light environments. In the major clade containing all large-eyed stromboid families, species observed active during the day and the night had wider eye apertures relative to lens sizes than species observed active during the day only, thereby prioritising sensitivity over resolution. Species with no consistent diel activity pattern also had smaller body sizes than exclusively day-active species, which may suggest that smaller animals are more vulnerable to shell-crushing predators, and avoid the higher predation pressure experienced by animals active during the day. Within the same major clade, ancestral state reconstruction suggests that absolute eye size increased above 1 mm twice. The unresolved position of Varicospira, however, weakens this hypothesis and further work with additional markers is needed to confirm this result.

Oxygen stable isotope (δ¹⁸O) analysis of marine biogenic carbonates is widely used to study the temperature at which the hard parts precipitated. This technique enables investigation of the environmental conditions experienced by carbonate-shelled organisms throughout their life. We used this approach on four bivalve species from the Coralline Crag Formation (Early Pliocene; UK), where warm- and cold-associated species co-occur in the Ramsholt Member. Our results reveal species-specific differences in reconstructed temperature. Two distinct climatic settings were identified: a cold setting with seafloor temperatures of 7.3 °C in winter and 12.4 °C in summer, and a warm one with seafloor temperatures of 10.4 °C in winter to above 14 °C in summer. These absolute temperatures were calculated using a modelled value of seawater δ¹⁸O (+0.1‰) that is open to question, but the disparity in temperatures exists whatever value is used. Interestingly, the reconstructed temperatures for some of the studied species align with those of their modern relatives: Arctica islandica records the coldest temperatures in this dataset, while Centrocardita squamulosa ampla indicates warmer conditions. Aequipecten opercularis, a eurythermal pectinid, exhibited the broadest range of temperatures. The interval during which the Coralline Crag was deposited (c. 4.4‒3.8 Ma) is generally regarded as relatively warm. However, the presence of A. islandica and the absolute seawater temperatures derived from shell δ¹⁸O suggest the occurrence of sufficiently long cool periods for the establishment and survival of this long-lived species. Data from specimens of C. squamulosa ampla and A. islandica collected from a single bed point to glacial/interglacial-type climate fluctuation. These results confirm that the presence of A. islandica is a marker for cold temperate environments. These results also confirm that assemblage information is not sufficient to determine palaeoclimatic conditions, and that reconstructed temperatures from shell δ¹⁸O can reveal time-averaging in fossil shell beds.

Climate change is causing increased coastal freshening in Antarctica, leading to reduced salinity. For Antarctica’s endemic echinoderms, adapted to the stable polar environment, the impact of rapid reductions in coastal salinity on physiology and behaviour is currently unknown. Six common Antarctic echinoderms (the sea urchin Sterechinus neumayeri ; the sea star Odontaster validus ; the brittle star Ophionotus victoriae ; and three sea cucumbers Cucumaria georgiana , Echinopsolus charcoti and Heterocucumis steineni ), were directly transferred from ambient salinity (34.5‰) to a range of salinity dilutions (29–9‰) for 24 h. All species showed reduced activity and the establishment of a temporary osmotic gradient between coelomic fluid and external seawater. Most species exhibited a depression in oxygen consumption across tolerated salinities; however, at very low salinities that later resulted in mortality, oxygen consumption increased to levels comparable to those at ambient. Low salinity tolerance varied substantially between species, with O. victoriae being the least tolerant (24 h LC ₅₀ (lethal for 50% of animals) = 19.9‰) while E. charcoti and C. georgiana demonstrated the greatest tolerance (24 h LC ₅₀ = 11.5‰). These findings demonstrate the species-specific response of Antarctica’s endemic echinoderms to short-term hypoosmotic salinity events, providing valuable insight into this phylum’s ability to respond to an underreported impact of climate change.