Search

Chitons possess the most elaborate system of shell pores found in any hard-shelled invertebrate. Although chitons possess some anteriorly located sense organs, they lack true cephalization, as their major sensory systems are not concentrated in a distinct head region. Instead, the aesthete system within their shells forms a dense sensory network that overcomes the barrier of their hard dorsal armour. The basic arrangement of neural structures embedded within a solid, opaque matrix, has confounded understanding of the overall network. In this study, we use synchrotron X-ray μCT to visualise the aesthete canal networks inside chiton valves. We selected representatives from all three major chiton clades: Lepidopleurida, the basal branching clade, and Callochitonida and Chitonida, which both have more complex shell morphology, to compare internal structure. Lepidopleurida aesthete canals are oriented vertically and pass directly through the shell to connect with the body. By contrast, aesthetes canals in Callochitonida and Chitonida have complex internal structures with extended horizontal passages, coalescing at the shell diagonal that corresponds to the valve insertion slits. This represents a stepwise evolution of chiton shell form, where thicker and more complex valves require a diverting and rewiring of the entire sensory network. Aspects of the aesthete system, such as the microscopic arrangement of surface pores, have long been used in chiton taxonomy for species diagnoses; insertion slits should also be understood as a secondary feature of the aesthete system. Chiton shell structures that are used for morphological systematics are driven by sensory adaptations., (© 2024 The Author(s). Journal of Morphology published by Wiley Periodicals LLC.)

The turtle shell is a remarkable structure that has intrigued not only evolutionary biologists but also engineering and material scientists because of its multi-scale complexity and various functions. Although protection is its most apparent role, the carapace and plastron are also related to many physiological functions and their shape influences hydrodynamics and self-righting ability. As such, analysing the functional morphology of the shell could help understanding the ecology of Triassic stem-turtles, which will contribute to the century-long debate on the evolutionary origins of turtles. Here, we used 3D imaging techniques to digitize the shells of two of the earliest stem-turtle taxa, Proganochelys and Proterochersis, and submitted their models to biomechanical and shape analyses. We analysed the strength performance under five predation scenarios and tested the function of two morphological traits found in stem-turtles, the epiplastral processes and an attached pelvic girdle. The latter, also present in the crown-lineage of side-necked turtles, has been suggested to increase load-bearing capacity of the shell or to improve swimming in pleurodires. Our results do not confirm the shell-strengthening hypothesis and, together with the results of our shape analyses, suggest that at least one of the first stem-turtles (Proterochersis) was an aquatic animal., (© 2024. The Author(s).)

The problem of the origin of the bony shell in turtles has a two-century history and still has not lost its relevance. First, this concerns the issues of the homology, the sources of formation and the ratio of bones of different nature, that is, thecal and epithecal, in particular. This article analyzes various views on the nature of the shell elements, and proposes their typification, based on modern data on developmental biology. It is proposed that the defining characteristic of the types of shell ossifications is not the level of their anlage in the dermis (thecality or epithecality), but, first of all, the primary sources of their formation: (1) neural crest (nuchal and plastral plates); (2) vertebral and rib periosteum (neural and costal plates); and (3) dermal mesenchyme (peripheral, suprapygal and pygal plates, as well as epithecal elements). In addition, there is complete correspondence between these types of ossifications and the sequence of their appearance in the turtle ontogenesis. The data show fundamental coincidence of the modifications of the ontogenetic development and evolutionary formation of the shell ossifications and are in agreement with a stepwise model for the origin of the turtle body plan. Particular attention is paid to the origin of the epithecal elements of the turtle shell, which correspond to the additional or supernumerary ossifications and seem to have wider distribution among turtles, than previously thought., (© 2024 Wiley Periodicals LLC.)

While the shell of the land snail Cepaea nemoralis is typically classed as yellow, pink, or brown, the reality is that colour variation is continuously distributed. To further understand the origin of the continuous variation, we used crosses of C. nemoralis to compare quantitative measures of the colour with the inferred genotype of the underlying supergene locus. We also used a recently developed linkage map to find quantitative trait loci that may influence colour. The results show that the colour locus of the supergene-at around 31.385 cM on linkage group 11-is involved in determining the quantitative chromatic differences that are perceptible to human vision. We also found some evidence that variation within colour classes may be due to allelic variation at or around the supergene. There are likely other unlinked loci involved in determining colour within classes, but confirmation will require greater statistical power. Although not investigated here, environmental factors, including diet, may also impact upon variation within colour types., (© The Author(s) 2024. Published by Oxford University Press on behalf of the European Society of Evolutionary Biology. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Mollusks encompass enormous disparity, including familiar clams and snails alongside less familiar aculiferans (chitons and vermiform aplacophorans) with complex multicomponent skeletons. Paleozoic fossils trace crown mollusks to forms exhibiting a combination of biomineralized shells and sclerites (e.g., scales, spines, and spicules). We describe a shell-less, Cambrian stem mollusk, Shishania aculeata gen. et sp. nov., with conical, hollow chitinous sclerites and a smooth girdle, broad foot, and mantle cavity. The sclerites have a microstructure of narrow canals consistent with the impressions of chaetal microvilli found in annelids and brachiopods. Shishania sclerites provide a morphological stepping stone between typical chaetae (chitinous bristles) and the external organic part of aculiferan sclerites that encloses a mineralized body. This discovery reinforces a common origin of lophotrochozoan chaetae and the biomineralized aculiferan sclerites, suggesting that the mollusk ancestor was densely covered with hollow chitinous chaetae.

The Ediacaran-Cambrian transition documents a critical stage in the diversification of animals. The global fossil record documents the appearance of cloudinomorphs and other shelled tubular organisms followed by non-biomineralized small carbonaceous fossils and by the highly diversified small shelly fossils between ~ 550 and 530 Ma. Here, we report diverse microfossils in thin sections and hand samples from the Ediacaran Bocaina Formation, Brazil, separated into five descriptive categories: elongate solid structures (ES); elongate filled structures (EF); two types of equidimensional structures (EQ 1 and 2) and elongate hollow structures with coiled ends (CE). These specimens, interpreted as diversified candidate metazoans, predate the latest Ediacaran biomineralized index macrofossils of the Cloudina-Corumbella-Namacalathus biozone in the overlying Tamengo Formation. Our new carbonate U-Pb ages for the Bocaina Formation, position this novel fossil record at 571 ± 9 Ma (weighted mean age). Thus, our data point to diversification of metazoans, including biomineralized specimens reminiscent of sections of cloudinids, protoconodonts, anabaritids, and hyolithids, in addition to organo-phosphatic surficial coverings of animals, demonstrably earlier than the record of the earliest known skeletonized metazoan fossils., (© 2024. The Author(s).)

Understanding the underlying reasons for phenotypic plasticity and resulting morphological disparity is one of the key topics of evolutionary research. The phenotypic plasticity of extant and fossil melanopsids has been widely documented. Yet millennial-resolution, well-dated records from small aquatic habitats harboring endemics are scarce. The thermal spring-fed Lake Pețea is an ice age refugia harboring a unique endemic warm-water fauna. Subfossil melanopsids display incredible morphological variability from smooth to keeled, elongated to ribbed, shouldered forms. Numerous morphotypes have been considered as individual taxa with a fluent succession from the smooth elongated to the ribbed, shouldered types. This study presents an extensive morphometric analysis of subfossil melanopsids (ca. 3500 specimens) derived from stratified samples with an independent chronology. The aim was to separate morphotypes for investigations of temporal morphological disparity. Our results challenge the widely accepted hypothesis that proposes the evolution of shouldered, compressed, ribbed shells through a two-step process from smooth elongated spindle-shaped shells. Instead, it suggests that the subfossil shells belong to two distinct taxa present throughout the available stratigraphic data. The main components of shape variation, shape globularity, and shell coiling seem allometry-related. Ribs, striation, and keels appear randomly. High-spired spindle-shaped forms were considered to represent specimens of Microcolpia daudebartii hazayi. Bulkier low-spired and shouldered specimens represent phenotypes of Mi. parreyssii parreyssii. The collective and random distribution of morphotypes from the early stages of the lake's history also refutes the idea of a continuous transformation of the elongated forms into compressed, shouldered ones. Rather points to multiple events and environmental stimuli triggering development. Melanopsids appear in Late Glacial horizons, with Theodoxus prevostianus preferring temperatures above 23°C which may indicate the subordinate presence of hot water microhabitats in cooler waters., (© 2024 Wiley Periodicals LLC.)

The diversification of macroscopic pelagic arthropods such as caryocaridid archaeostracans was a crucial aspect of the Great Ordovician Biodiversification Event, and the plankton revolution. A pelagic mode of life has been inferred for caryocaridids from their common presence in black graptolitic shales alongside carapace morphologies that appear streamlined. However, the hydrodynamic performance within the group and comparisons with other archaeostracans were lacking. Here we use a computational fluid dynamics approach to quantify the hydrodynamic performance of caryocaridids, and other early Palaeozoic archaeostracans including Arenosicaris inflata and Ordovician ceratiocaridids. We show that streamlining of the carapace was an important factor facilitating a pelagic mode of life in caryocaridids, in reducing the drag coefficient and facilitating a broader range of lift coefficients at different angles of attack. However, comparable hydrodynamic performance is also recovered for some ceratiocaridids. This suggests that alongside carapace streamlining, adaptations to appendages and thinning of the carapace were also important for a pelagic mode of life in Ordovician caryocaridids., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Pates, Xue. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The exchange of information and social interactions on broad spatial scales between human groups in the past can be studied through the provenance of key indicators of distant origin recorded at archaeological sites. The remains of shells of mollusk species, especially when crafted as elements of personal ornaments, express aspects of the behaviors and valuations for the populations that selected, transformed, and exchanged such items. In the southern cone of South America, past hunter-gatherer groups traveled long distances and interacted with communities distributed throughout the territory to acquire goods for technological use, visual display or considered highly valued materials. When recorded at distant locations, these goods of extra local origin are very informative regarding the differences between commonly used home ranges and the occasional access to remote spaces. We present the results of the analysis of the archaeomalacological assemblage of the Baño Nuevo 1 site, a cave with exceptional preservation conditions in Central West Patagonia. This site has yielded a diverse group of artifacts made of shells with origins from multiple distances, as well as evidence of the use of marine, freshwater, and terrestrial species. Its deposits, which extend over the last 11,000 years, reveal an antiquity of at least the middle Holocene for the acquisition, manufacture, use and transport of goods as personal ornaments from shells in the macroregion., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Hammond et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Ostracod crustaceans originated at least 500 Ma ago. Their tiny bivalved shells represent the most species-abundant fossil arthropods, and ostracods are omnipresent in a wide array of freshwater and marine environments today and in the past. Derima paparme gen. et sp. nov. from the Herefordshire Silurian Lagerstätte (~430 Ma) in the Welsh Borderland, UK, is one of only a handful of exceptionally preserved ostracods (with soft parts as well as the shell) known from the Palaeozoic. A male specimen provides the first evidence of the appendages of Binodicopina, a major group of Palaeozoic ostracods comprising some 135 Ordovician to Permian genera. The appendage morphology of D. paparme , but not its shell, indicates that binodicopes belong to Podocopa. The discovery that the soft-part morphology of binodicopes allies them with podocopes affirms that using the shell alone is an unreliable basis for classifying certain fossil ostracods, and knowledge of soft-part morphology is critical for the task. Current assignment of many fossil ostracods to higher taxa, and therefore the evolutionary history of the group, may require reconsideration.

A sample of phosphatized, originally calcareous, mollusk shells from the Katian age uppermost Mójcza Limestone at its type locality yielded a few hundred polyplacophoran plates. The chelodids are very rare among them. Three septemchitonid species dominate. They represent a gradation from underived steep roof-like plates to almost cylindrical ones, leaving only a narrow ventral slit for the foot. Apparently, this represents the first step toward the extremely derived 'segmented clam' Bauplan of the Silurian Carnicoleus, with plates completely closed at the venter except for the mouth and anal openings. To enable growth, the plates became thinner and more flexible (or perhaps resorbed) along the dorsum. The tendency toward reduction of the ventral gap of the plates in the early Paleozoic septemchitonid polyplacophorans implies their lack of ability to cling to the substrate with a muscular foot. In compensation, their plates changed toward a more efficient protective function, covering the animal body sides more and more completely. This may explain the origin of the ventral furrow of extant solenogasters hiding the rudimentary foot. An opposite route was chosen by the coeval Acaenoplax lineage, in which the plates did not contact each other, exposing much of the soft body on the dorsum. In both cases the animals appeared to be worm-like, perhaps representing different ways of evolution from the Paleozoic chitons to the extant aplacophorans., (© 2024 Wiley Periodicals LLC.)

Purpose: Accurate identification of medically important intermediate host and vector species is crucial for understanding disease transmission and control. Identifying Bulinus snails which act as intermediate host species for the transmission of schistosomiasis is typically undertaken using conchological and genital morphology as well as molecular methods., Methods: Here, a landmark-based morphometric analysis of shell morphology was undertaken to determine its utility to distinguish the closely related and morphologically similar sister species Bulinus senegalensis and Bulinus forskalii. The method was developed to increase the accuracy of conchological morphology methods to identify Bulinus species in the field. Both species are found in West Africa, but only B. senegalensis is implicated in the transmission of urogenital schistosomiasis., Results: We found when scaled down to the same length, 3-whorl and 4-whorl (juvenile) B. senegalensis shells had a longer spire, narrower body whorl and shorter aperture than B. forskalii. In contrast, 5-whorl (adult) B. senegalensis had a shorter spire, but still had a shorter aperture and narrower body whorl than B. forskalii. Canonical Variate Analysis (CVA) showed minimal overlap between B. senegalensis and B. forskalii for 3-whorl and 4-whorl shells, with a clear separation for 5-whorl shells. Overall, B. senegalensis had a consistently shorter aperture size and narrower body whorl than B. forskalii for all development stages. Spire length was variable depending on the stage of development, with 3-whorl and 4-whorl shells having the opposite trends of adult shells., Conclusions: Our study demonstrates the applicability of landmark-based morphometrics in distinguishing the medically important, Bulinus senegalensis from its morphologically similar sister species, Bulinus forskalii. We recommend using measurements based on spire length, penultimate whorl length, body whorl width and aperture size to differentiate B. senegalensis and B. forskalii, when used with the appropriate information for each shell's development stage., (© 2024. The Author(s).)

Flat hydrodynamic shells likely represent an evolutionary trade-off between adaptation to an aquatic lifestyle and the instability of more rounded shells, thought beneficial for self-righting. Trade-offs often result in compromises, this is particularly true when freshwater turtles, with flatter shells, must self-right to avoid the negative effects of inverting. These turtles, theoretically, invest more biomechanical effort to achieve successful and timely self-righting when compared to turtles with rounded carapaces. This increase in effort places these hatchlings in a precarious position; prone to inversion and predation and with shells seemingly maladapted to the act of self-righting. Here, we examine hatchling self-righting performance in three morphologically distinct freshwater turtle species (Apalone spinifera, Chelydra serpentina and Trachemys scripta scripta) that inhabit similar environmental niches. We demonstrate that these hatchlings were capable of rapid self-righting and used considerably less biomechanical effort relative to adult turtles. Despite differences in shell morphology the energetic efficiency of self-righting remained remarkably low and uniform between the three species. Our results confound theoretical predictions of self-righting ability based on shell shape metrics and indicate that other morphological characteristics like neck or tail morphology and shell material properties must be considered to better understand the biomechanical nuances of Testudine self-righting., (© 2024. The Author(s).)

Fractal-like, intricate morphologies are known to exhibit beneficial mechanical behavior in various engineering and technological domains. The evolution of fractal-like, internal walls of ammonoid cephalopod shells represent one of the most clear evolutionary trends toward complexity in biology, but the driver behind their iterative evolution has remained unanswered since the first hypotheses introduced in the early 1800s. We show a clear correlation between the fractal-like morphology and structural stability. Using linear and nonlinear computational mechanical simulations, we demonstrate that the increase in the complexity of septal geometry leads to a substantial increase in the mechanical stability of the entire shell. We hypothesize that the observed tendency is a driving force toward the evolution of the higher complexity of ammonoid septa, providing the animals with superior structural support and protection against predation. Resolving the adaptational value of this unique trait is vital to fully comprehend the intricate evolutionary trends between morphology, ecological shifts, and mass extinctions through Earth's history.

Changes in the structural association of skeletal traits are crucial to the evolution of novel forms and functions. In vertebrates, such rearrangements often occur gradually and may precede or coincide with the functional activation of skeletal traits. To illustrate this process, we examined the ontogeny of African hinge-back tortoises (Kinixys spp.). Kinixys species feature a moveable "hinge" on the dorsal shell (carapace) that enables shell closure (kinesis) when the hind limbs are withdrawn. This hinge, however, is absent in juveniles. Herein, we describe how this unusual phenotype arises via alterations in the tissue configuration and shape of the carapace. The ontogenetic repatterning of osseous and keratinous tissue coincided with shifts in morphological integration and the establishment of anterior (static) and posterior (kinetic) carapacial modules. Based on ex vivo skeletal movement and raw anatomy, we propose that Kinixys employs a "sliding hinge" shell-closing system that overcomes thoracic rigidity and enhances the protective capacity of the carapace. Universal properties of the vertebrate skeleton, such as plasticity, modularity, and secondary maturation processes, contributed to adaptive evolutionary change in Kinixys. We discuss a hypothetical model to explain the delayed emergence of skeletal traits and its relevance to the origins of novel form-to-function relationships., (© 2022 The Authors. The Anatomical Record published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Molluscan shells display a high diversity of external sculpture. Sculptural elements may be symmetrical, where both edges of an element are morphologically similar, or asymmetrical, where one edge is steeper than the other. Asymmetrical sculpture can be ratcheted, with the leading edges (those in the direction of locomotion or growth) less steep than the trailing edges, or imbricated (leading edges steeper than trailing edges). While the ratcheted sculpture is better known, the diversity of imbricated sculpture has remained largely unexplored. In a survey of extant benthic shell-bearing molluscs, we document imbricated sculpture primarily in epifaunal bivalves or on the exposed sectors of shells of semi-infaunal bivalves. Imbricated sculpture is particularly widespread in pteriomorphian bivalves, but it is absent in the subclade Mytiloidea as well as in highly mobile Pectinidae. It also occurs in many carditid bivalves (Archiheterodonta) and in phylogenetically scattered euheterodonts. In several infaunal bivalves including species of Cardites (Carditidae), Hecuba (Donacidae), and Chione (Veneridae), comarginal elements on the posterior sector are imbricated whereas anterior comarginal ridges are ratcheted. Imbricated sculpture in bivalves tends to be concentrated on the upper (left) valves of pectinids or on the posterior sector of both valves in archiheterodonts and euheterodonts. Imbricated sculpture is uncommon in gastropods, even in epifaunal species, but does occur in the collabral ridges in some Vasidae and a few other groups. Expression of imbricated sculpture does not depend on shell mineral composition or microstructure. The ecological distribution and within-shell pattern of expression of imbricated sculpture point to the likelihood that this type of asymmetrical sculpture is both widespread and potentially functional. Additionally, we present a potential methodology whereby shell sculpture categories (symmetrical, ratcheted, and imbricated) may be quantified by comparing the lengths of corresponding leading and trailing edges across the shell surface., (© 2023 Wiley Periodicals LLC.)

Organisms with limited dispersal capabilities should show phenotypic plasticity in situ to keep pace with environmental changes. Therefore, to study the influence of environmental variation on the phenotypic diversity, we chose land snails, Trochulus hispidus and T. sericeus, characterized by high population variability. We performed long-term field studies as well as laboratory and common garden experiments, which revealed that temporal environmental changes generate visible variation in shell size and shape of these snails. Many shell measurements of T. hispidus varied significantly with temperature and humidity in individual years. According to this, the first generation of T. hispidus, bred in controlled laboratory conditions, became significantly different in higher spire and narrower umbilicus from its wild parents. Interestingly, offspring produced by this generation and transplanted to wild conditions returned to the 'wild' flat and wide-umbilicated shell shape. Moreover, initially different species T. hispidus and T. sericeus transferred into common environment conditions revealed rapid and convergent shell modifications within one generation. Such morphological flexibility and high genetic variation can be evolutionarily favored, when the environment is heterogeneous in time. The impact of climate change on the shell morphometry can lead to incorrect taxonomic classification or delimitation of artificial taxa in land snails. These findings have also important implications in the context of changing climate and environment., (© 2022. The Author(s).)

To understand the molecular responses of Pinctada fucata with different shell colors to salinity stress, we used transcriptome sequencing on the mantle of P. fucata with a black shell and red shell color under the salinity of 20, 35, and 50. The 414 and 2371 differentially expressed genes (DEGs) in P. fucata with a black shell under low- or high-salt stress, while there were 588 and 3009 DEGs in P. fucata with a red shell. KEGG pathway enrichment analysis showed that, under low salt stress, the DEGs of P. fucata with the black shell were significantly enriched in pathways MAPK signaling pathway, protein processing in endoplasmic reticulum, vitamin B6 metabolism, longevity regulating pathway-multiple species, estrogen signaling pathway and antigen processing and presentation, the DEGs of P. fucata with a red shell were significantly enriched in pathways vitamin B6 metabolism. Under high salt stress, the DEGs of P. fucata with a red shell were significantly enriched in pathways arginine biosynthesis. 11 DEGs were randomly selected for quantitative real-time PCR, and the results were consistent with the RNA-seq. In addition, under high salt stress, DEGs were enriched into some pathways related to osmotic regulation and immune defense of P. fucata with black shell and red shell, such as Glycolysis / Gluconeogenesis, AMPK signaling pathway, Beta-Alanine metabolism, Glycine, serine and threonine metabolism, MAPK signaling pathway and Phagosome. The study showed that high salt stress had a greater influence on P. fucata with two shell colors, and P. fucata with a black shell made a positive immune defense response. Our results will improve to further understand the salt tolerance mechanism of P. fucata with different shell colors., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Even though mollusks' capacity to repair shell damage is usually studied in response to a single event, their shells have to defend them against predatory and environmental threats throughout their potentially multi-decadal life. We measured whether and how mollusks respond to chronic mechanical stress. Once a week for 7 months, we compressed whole live California mussels (Mytilus californianus) for 15 cycles at ∼55% of their predicted one-time breaking force, a treatment known to cause fatigue damage in shells. We found mussels repaired their shells. Shells of experimentally stressed mussels were just as strong at the end of the experiment as those of control mussels that had not been experimentally loaded, and they were more heavily patched internally. Additionally, stressed shells differed in morphology; they were heavier and thicker at the end of the experiment than control shells but they had increased less in width, resulting in a flatter, less domed shape. Finally, the chronic mechanical stress and repair came at a cost, with stressed mussels having higher mortality and less soft tissue than the control group. Although associated with significant cost, mussels' ability to maintain repair in response to ongoing mechanical stress may be vital to their survival in harsh and predator-filled environments., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

Mollusks have developed a broad diversity of shelled structures to protect against challenges imposed by biological interactions(e.g., predation) and constraints (e.g., [Formula: see text]-induced ocean acidification and wave-forces). Although the study of shell biomechanical properties with nacreous microstructure has provided understanding about the role of shell integrity and functionality on mollusk performance and survival, there are no studies, to our knowledge, that delve into the variability of these properties during the mollusk ontogeny, between both shells of bivalves or across the shell length. In this study, using as a model the intertidal mussel Perumytilus purpuratus to obtain, for the first time, the mechanical properties of its shells with nacreous microstructure; we perform uniaxial compression tests oriented in three orthogonal axes corresponding to the orthotropic directions of the shell material behavior (thickness, longitudinal, and transversal). Thus, we evaluated whether the shell material's stress and strain strength and elastic modulus showed differences in mechanical behavior in mussels of different sizes, between valves, and across the shell length. Our results showed that the biomechanical properties of the material building the P. purpuratus shells are symmetrical in both valves and homogeneous across the shell length. However, uniaxial compression tests performed across the shell thickness showed that biomechanical performance depends on the shell size (aging); and that mechanical properties such as the elastic modulus, maximum stress, and strain become degraded during ontogeny. SEM observations evidenced that compression induced a tortuous fracture with a delamination effect on the aragonite mineralogical structure of the shell. Findings suggest that P. purpuratus may become vulnerable to durophagous predators and wave forces in older stages, with implications in mussel beds ecology and biodiversity of intertidal habitats., (© 2022. The Author(s).)

The function-optimized properties of biominerals arise from the hierarchical organization of primary building blocks. Alteration of properties in response to environmental stresses generally involves time-intensive processes of resorption and reprecipitation of mineral in the underlying organic scaffold. Here, we report that the load-bearing shells of the brachiopod Discinisca tenuis are an exception to this process. These shells can dynamically modulate their mechanical properties in response to a change in environment, switching from hard and stiff when dry to malleable when hydrated within minutes. Using ptychographic X-ray tomography, electron microscopy and spectroscopy, we describe their hierarchical structure and composition as a function of hydration to understand the structural motifs that generate this adaptability. Key is a complementary set of structural modifications, starting with the swelling of an organic matrix on the micron level via nanocrystal reorganization and ending in an intercalation process on the molecular level in response to hydration., (© 2021. The Author(s).)

Assessing the taxonomic importance of the suture line in shelled cephalopods is a key to better understanding the diversity of this group in Earth history. Because fossils are subject to taphonomic artifacts, an in-depth knowledge of well-preserved modern organisms is needed as an important reference. Here, we examine the suture line morphology of all known species of the modern cephalopods Nautilus and Allonautilus. We applied computed tomography and geometric morphometrics to quantify the suture line morphology as well as the conch geometry and septal spacing. Results reveal that the suture line and conch geometry are useful in distinguishing species, while septal spacing is less useful. We also constructed cluster trees to illustrate the similarity among species. The tree based on conch geometry in middle ontogeny is nearly congruent with those previously reconstructed based on molecular data. In addition, different geographical populations of the same species of Nautilus separate out in this tree. This suggests that genetically distinct (i.e., geographically isolated) populations of Nautilus can also be distinguished using conch geometry. Our results are applicable to closely related fossil cephalopods (nautilids), but may not apply to more distantly related forms (ammonoids)., (© 2021. The Author(s).)

Recent technical innovations are revealing surprising patterns in mollusc shell pigmentation, such as an unexpectedly modest role for melanins and rapid divergences in the mix of pigments used to achieve similar colour patterns. The elucidation of the molecular genetic basis of shell pigmentation has been slow, probably because of the high genome complexity of gastropods and bivalves. Recent work within the old field of evolutionary ecology of shell pigmentation allows a greater role for the analysis of large-geographic-scale patterns (sometimes employing citizen-science data), as well as experimental field studies. However, the field remains dominated by land snails as model organisms, while colour pattern evolution in marine gastropods and bivalves, particularly those not exposed to visual predators, remains mysterious., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Biological systems have a remarkable capability of synthesizing multifunctional materials that are adapted for specific physiological and ecological needs. When exploring structure-function relationships related to multifunctionality in nature, it can be a challenging task to address performance synergies, trade-offs, and the relative importance of different functions in biological materials, which, in turn, can hinder our ability to successfully develop their synthetic bioinspired counterparts. Here, we investigate such relationships between the mechanical and optical properties in a multifunctional biological material found in the highly protective yet conspicuously colored exoskeleton of the flower beetle, Torynorrhina flammea Combining experimental, computational, and theoretical approaches, we demonstrate that a micropillar-reinforced photonic multilayer in the beetle's exoskeleton simultaneously enhances mechanical robustness and optical appearance, giving rise to optical damage tolerance. Compared with plain multilayer structures, stiffer vertical micropillars increase stiffness and elastic recovery, restrain the formation of shear bands, and enhance delamination resistance. The micropillars also scatter the reflected light at larger polar angles, enhancing the first optical diffraction order, which makes the reflected color visible from a wider range of viewing angles. The synergistic effect of the improved angular reflectivity and damage localization capability contributes to the optical damage tolerance. Our systematic structural analysis of T. flammea 's different color polymorphs and parametric optical and mechanical modeling further suggest that the beetle's microarchitecture is optimized toward maximizing the first-order optical diffraction rather than its mechanical stiffness. These findings shed light on material-level design strategies utilized in biological systems for achieving multifunctionality and could thus inform bioinspired material innovations., Competing Interests: The authors declare no competing interest.

Continued sampling of the latest Eocene to earliest Oligocene Gries Ranch Formation in Lewis County, Washington State, has yielded new heterobranch microgastropod species. Orbitestella kieli sp. nov., is the third fossil species of this genus and family Orbitestellidae from western North America. Two new species of Ammonicera, A. rolani sp. nov. and A. danieli sp. nov., are together only the second fossil record of this genus and the family Omalogyridae from the northeastern Pacific Ocean. New specimens of two previously recorded species, O. palaiopacifica Squires Goedert and A. benhami Squires Goedert, from early Eocene rocks of the Crescent Formation provide new data regarding shell morphology. The fossil record of both Ammonicera and Orbitestella in western North America is restricted to early Eocene to earliest Oligocene age rocks in Washington State.

The New Zealand green-lipped mussel aquaculture industry is largely dependent on the supply of young mussels that wash up on Ninety Mile Beach (so-called Kaitaia spat), which are collected and trucked to aquaculture farms. The locations of source populations of Kaitaia spat are unknown and this lack of knowledge represents a major problem because spat supply may be irregular. We combined genotypic (microsatellite) and phenotypic (shell geochemistry) data in a geospatial framework to determine if this new approach can help identify source populations of mussels collected from two spat-collecting and four non-spat-collecting sites further south. Genetic analyses resolved differentiated clusters (mostly three clusters), but no obvious source populations. Shell geochemistry analyses resolved six differentiated clusters, as did the combined genotypic and phenotypic data. Analyses revealed high levels of spatial and temporal variability in the geochemistry signal. Whilst we have not been able to identify the source site(s) of Kaitaia spat our analyses indicate that geospatial testing using combined genotypic and phenotypic data is a powerful approach. Next steps should employ analyses of single nucleotide polymorphism markers with shell geochemistry and in conjunction with high resolution physical oceanographic modelling to resolve the longstanding question of the origin of Kaitaia spat.

Sympatric coexistence of recently diverged species raises the question of barriers restricting the gene flow between them. Reproductive isolation may be implemented at several levels, and the weakening of some, e.g. premating, barriers may require the strengthening of the others, e.g. postcopulatory ones. We analysed mating patterns and shell size of mates in recently diverged closely related species of the subgenus Littorina Neritrema (Littorinidae, Caenogastropoda) in order to assess the role of premating reproductive barriers between them. We compared mating frequencies observed in the wild with those expected based on relative densities using partial canonical correspondence analysis. We introduced the fidelity index (FI) to estimate the relative accuracy of mating with conspecific females and precopulatory isolation index (I PC ) to characterize the strength of premating barriers. The species under study, with the exception of L. arcana, clearly demonstrated preferential mating with conspecifics. According to FI and I PC , L. fabalis and L. compressa appeared reliably isolated from their closest relatives within Neritrema. Individuals of these two species tend to be smaller than those of the others, highlighting the importance of shell size changes in gastropod species divergence. L. arcana males were often found in pairs with L. saxatilis females, and no interspecific size differences were revealed in this sibling species pair. We discuss the lack of discriminative mate choice in the sympatric populations of L. arcana and L. saxatilis, and possible additional mechanisms restricting gene flow between them.

Snapping shrimp (Alpheidae) are decapod crustaceans named for the snapping claws with which they produce cavitation bubbles. Snapping shrimp use the shock waves released by collapsing cavitation bubbles as weapons. Along with their distinctive claws, snapping shrimp have orbital hoods, extensions of their carapace that cover their heads and eyes. Snapping shrimp view the world through their orbital hoods, so we asked if the surfaces of the orbital hoods of the snapping shrimp Alpheus heterochaelis have features that minimize the scattering of light. Using SEM, we found that surface features, primarily microbial epibionts, covered less space on the surfaces of the orbital hoods of A. heterochaelis (∼18%) than they do elsewhere on the carapace (∼50%). Next, we asked if these surface features influence aerophobicity. By measuring the contact angles of air bubbles, we found the orbital hoods of A. heterochaelis are less aerophobic than other regions of the carapace. Surfaces that are less aerophobic are more likely to have cavitation bubbles adhere to them and are more likely to have shock waves cause new cavitation bubbles to nucleate upon them. Computational modeling indicates the orbital hoods of A. heterochaelis face a functional trade-off: fewer surface features, such as less extensive communities of microbial epibionts, may minimize the scattering of light at the cost of making the adhesion and nucleation of cavitation bubbles more likely., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

A fossil oyster bed (Ostrea edulis) was recently encountered offshore Helgoland (German Bight). Oysters are important filter feeders in marine environments and their habitat structure supports a large associated biodiversity. The European flat oyster Ostrea edulis has historically occurred in vast populations in the North Sea, but declined massively in the early 20th century. The ecological restoration of Ostrea habitats is a current focal point in the North Sea. To better understand the mechanisms that caused the local collapse of the oyster population, this study investigated the size structure, weight, and age of the shells, along with the spatial dimensions, seafloor properties, and environmental context of the oyster bed. The results show that the demise of the population occurred around 700 CE, ruling out excessive harvest as a driver of decline. Synchronicity of increased geomorphological activity of rivers and concurrent major land use changes in early medieval Europe suggest that increased sedimentation was a viable stressor that reduced the performance of the oysters. The shells provided no indication of a demographically poor state of the oyster bed prior to its demise, but manifested evidence of the wide-spread occurrence of the boring sponge Cliona sp. Our study challenges the assumption of a stable preindustrial state of the European flat oyster in the North Sea, and we conclude that the long-term variability of environmental conditions needs to be addressed to benchmark success criteria for the restoration of O. edulis., Competing Interests: The authors have declared that no competing interests exist.

Variations in the number and arrangement of scutes often are used for species identification in hard-shelled sea turtles. Despite the conserved nature of scute arrangements, anomalous arrangements have been noted in the literature for over a century, with anomalies linked to sub-optimal environmental conditions in the nest during development. Long-held assumptions suggest that anomalous scute arrangements are indicative of underlying physiological or morphological anomalies, with presumed long-term survival costs to the individual. Here, we examined a 25-year photo database of two species of sea turtle (Caretta caretta and Chelonia mydas) captured incidentally and non-selectively on the eastern coast of Florida. Our results suggest that C. mydas is substantially more variable with respect to the arrangement of carapacial scutes, while C. caretta had a relatively higher proportion of individuals with anomalous plastron scute arrangements. We also show evidence that (a) the forms and patterns of anomalous scutes are stable throughout growth; (b) there is limited evidence for selection against non-modal arrangements in the size classes that were examined; and (c) that their frequency has remained stable in juvenile cohorts from 1994 until present. These findings indicate that there may not be a survival cost associated with anomalous scute arrangements once the turtles reach juvenile size classes, and that variation in scute arrangements within populations is relatively common., (© 2020 Wiley Periodicals LLC.)

A coiled shell is the most evident feature of the typical Bauplan of a gastropod mollusc. However, at least 54 families independently evolved an apparently simplified shell morphology: the limpet. Species with this largely uncoiled, depressed shell morphology occur in almost every aquatic habitat and are associated to a number of different lifestyles and diets. The marine gastropod family Capulidae includes 18 recognised genera, the large majority of which are coiled, but with a number of limpet-like species. Capulid shell plasticity is also associated to a broad range of feeding ecologies, from obligate suspension feeders to kleptoparasites. To investigate the evolution of the limpet-like shell in the family Capulidae we performed an ancestral state reconstruction analysis on a time-calibrated phylogenetic tree (COI, 16S, and ITS2) including 16 species representing a good deal of its morphological diversity. Our results identified at least three capulid lineages that independently evolved limpet-like shells, suggesting that a recurrent limpetization process characterizes this family. One of the limpet-like genera was undescribed and was here named Cryocapulus n. gen. We suggest that capulids evolved from a coiled suspension feeder lineage and that the shift to kleptoparasitism, which occurred in the family ancestor, may have represented a strategy to save energy through the exploitation of the water current produced by the host. Probably the major drivers of shell evolution in capulids are related to their ecology, most of them being kleptoparasites, include the shape and the kind of host substrate, and lead to the repeated acquisition of a limpet-like shape., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Many parasites and hosts are embroiled in an on-going arms race that affects the evolution of each participant. One such battle is between parasitic nematodes and terrestrial gastropods which have co-evolved for 90-130 MY. Recently, snails have been shown to encase and kill invading nematodes using their shell as a defence mechanism. However, there is remarkably little known about this process in terms of understanding where, when and how nematodes are fixed within the shell. Also there has never been any attempt to observe this process using methods other than light microscopy. Therefore, we used micro CT scanning of a Cepaea nemoralis shell (a common host for nematodes) to 3D visualise encased nematode parasites and quantify morphological parameters. By taking this approach future studies could use micro CT scanning of fossil shells in conchology collections to understand nematode/snail co-evolution.

One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

The beetle horn primordium is a complex and compactly folded epithelial sheet located beneath the larval cuticle. Only by unfolding the primordium can the complete 3D shape of the horn appear, suggesting that the morphology of beetle horns is encoded in the primordial folding pattern. To decipher the folding pattern, we developed a method to manipulate the primordial local folding on a computer and clarified the contribution of the folding of each primordium region to transformation. We found that the three major morphological changes (branching of distal tips, proximodistal elongation, and angular change) were caused by the folding of different regions, and that the folding mechanism also differs according to the region. The computational methods we used are applicable to the morphological study of other exoskeletal animals.

Littorina saxatilis is becoming a model system for understanding the genomic basis of ecological speciation. The parallel formation of crab-adapted ecotypes that exhibit partial reproductive isolation from wave-adapted ecotypes has enabled genomic investigation of conspicuous shell traits. Recent genomic studies suggest that chromosomal rearrangements may enable ecotype divergence by reducing gene flow. However, the genomic architecture of traits that are divergent between ecotypes remains poorly understood. Here, we use 11,504 single nucleotide polymorphism (SNP) markers called using the recently released L. saxatilis genome to genotype 462 crab ecotype, wave ecotype and phenotypically intermediate Spanish L. saxatilis individuals with scored phenotypes. We used redundancy analysis to study the genetic architecture of loci associated with shell shape, shape corrected for size, shell size and shell ornamentation, and to compare levels of co-association among different traits. We discovered 341 SNPs associated with shell traits. Loci associated with trait divergence between ecotypes were often located inside putative chromosomal rearrangements recently characterized in Swedish L. saxatilis. In contrast, we found that shell shape corrected for size varied primarily by geographic site rather than by ecotype and showed little association with these putative rearrangements. We conclude that genomic regions of elevated divergence inside putative rearrangements were associated with divergence of L. saxatilis ecotypes along steep environmental axes-consistent with models of adaptation with gene flow-but were not associated with divergence among the three geographical sites. Our findings support predictions from models indicating the importance of genomic regions of reduced recombination allowing co-association of loci during ecological speciation with ongoing gene flow., (© 2020 European Society For Evolutionary Biology. Journal of Evolutionary Biology © 2020 European Society For Evolutionary Biology.)

Pteropods, a group of holoplanktonic gastropods, are regarded as bioindicators of the effects of ocean acidification on open ocean ecosystems, because their thin aragonitic shells are susceptible to dissolution. While there have been recent efforts to address their capacity for physiological acclimation, it is also important to gain predictive understanding of their ability to adapt to future ocean conditions. However, little is known about the levels of genetic variation and large-scale population structuring of pteropods, key characteristics enabling local adaptation. We examined the spatial distribution of genetic diversity in the mitochondrial cytochrome c oxidase I (COI) and nuclear 28S gene fragments, as well as shell shape variation, across a latitudinal transect in the Atlantic Ocean (35°N-36°S) for the pteropod Limacina bulimoides. We observed high levels of genetic variability (COI π = 0.034, 28S π = 0.0021) and strong spatial structuring (COI Φ ST  = 0.230, 28S Φ ST  = 0.255) across this transect. Based on the congruence of mitochondrial and nuclear differentiation, as well as differences in shell shape, we identified a primary dispersal barrier in the southern Atlantic subtropical gyre (15-18°S). This barrier is maintained despite the presence of expatriates, a gyral current system, and in the absence of any distinct oceanographic gradients in this region, suggesting that reproductive isolation between these populations must be strong. A secondary dispersal barrier supported only by 28S pairwise Φ ST comparisons was identified in the equatorial upwelling region (between 15°N and 4°S), which is concordant with barriers observed in other zooplankton species. Both oceanic dispersal barriers were congruent with regions of low abundance reported for a similar basin-scale transect that was sampled 2 years later. Our finding supports the hypothesis that low abundance indicates areas of suboptimal habitat that result in barriers to gene flow in widely distributed zooplankton species. Such species may in fact consist of several populations or (sub)species that are adapted to local environmental conditions, limiting their potential for adaptive responses to ocean changes. Future analyses of genome-wide diversity in pteropods could provide further insight into the strength, formation and maintenance of oceanic dispersal barriers., (© 2020 The Authors. Journal of Evolutionary Biology published by John Wiley & Sons Ltd on behalf of European Society for Evolutionary Biology.)

MicroRNAs (miRNAs) regulate post-transcription gene expression by targeting genes and play crucial roles in diverse biological processes involving body color formation. However, miRNAs and miRNA-targets underlying shell color polymorphism remain largely unknown in mollusca. Using four shell colors full-sib families of the Pacific oyster Crassostrea gigas, we systematically identified miRNAs and miRNA-targets in the mantles, which organ could produce white, golden, black or partially pigmented shell. RNA sequencing and analysis identified a total of 53 known miRNA and 91 novel miRNAs, 47 of which were detected to differentially express among six pairwise groups. By integrating miRNA and mRNA expression profiles, a total of 870 genes were predicted as targets of differentially expressed miRNAs, mainly involving in biomineralization and pigmentation through functional enrichment. Furthermore, a total of four miRNAs and their target mRNAs were predicted to involve in synthesis of melanin, carotenoid or tetrapyrrole. Of them, lgi-miR-317 and its targets peroxidase and lncRNA TCONS_00951105 are implicated in acting as the competing endogenous RNA to regulate melanogenesis. Our studies revealed the systematic characterization of miRNAs profiles expressed in oyster mantle, which might facilitate understanding the intricate molecular regulation of shell color polymorphism and provide new insights into breeding research in oyster.

Bivalves protect themselves from predators using both mechanical and behavioral defenses. While their shells serve as mechanical armor, bivalve shells also enable evasive behaviors such as swimming and burrowing. Therefore, bivalve shell shape is a critical determinant of how successfully an organism can defend against attack. Shape is believed to be related to shell strength with bivalve shell shapes converging on a select few morphologies that correlate with life mode and motility. In this study, mathematical modeling and 3D printing were used to analyze the protective function of different shell shapes against vertebrate shell-crushing predators. Considering what life modes different shapes permit and analyzing the strength of these shapes in compression provides insight to evolutionary and ecological tradeoffs with respect to mechanical and behavioral defenses. These empirical tests are the first of their kind to isolate the influence of bivalve shell shape on strength and quantitatively demonstrate that shell strength is derived from multiple shape parameters. The findings of this theoretical study are consistent with examples of shell shapes that allow escape behaviors being mechanically weaker than those which do not. Additionally, shell elongation from the umbo, a metric often overlooked, is shown to have significant effects on shell strength.

A substantial body of research has been accumulated around ammonoids over several decades. A core aspect of this research has been attempting to infer their life mode from analysis of the morphology of their shells and the drag they incur as that shell is pushed through the water. Tools such as Westermann Morphospace have been developed to investigate and scaffold hypotheses about the results of these investigations. We use computational fluid dynamics to simulate fluid flow around a suite of 24 theoretical ammonoid morphologies to interrogate systematic variations within this space. Our findings uphold some of the long-standing expectations of drag behavior; conch inflation has the greatest influence over ammonoid drag. However, we also find that other long-standing assumptions, such as oxyconic ammonoids being the best swimmers, are subject to substantial variation and nuance resulting from their morphology that is not accounted for through simple drag assessment., (Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology 2020. This work is written by US Government employees and is in the public domain in the US.)

In order to study evolutionary pattern and process, we need to be able to accurately identify species and the evolutionary lineages from which they are derived. Determining the concordance between genetic and morphological variation of living populations, and then directly comparing extant and fossil morphological data, provides a robust approach for improving our identification of lineages through time. We investigate genetic and shell morphological variation in extant species of Penion marine snails from New Zealand, and extend this analysis into deep time using fossils. We find that genetic and morphological variation identify similar patterns and support most currently recognized extant species. However, some taxonomic over-splitting is detected due to shell size being a poor trait for species delimitation, and we identify incorrect assignment of some fossil specimens. We infer that a single evolutionary lineage (Penion sulcatus) has existed for 22 myr, with most aspects of shell shape and shell size evolving under a random walk. However, by removing samples previously classified as the extinct species P. marwicki, we instead detect morphological stasis for one axis of shell shape variation. This result demonstrates how lineage identification can change our perception of evolutionary pattern and process. [Genotyping by sequencing; geometric morphometrics; morphological evolution; Neogastropoda; phenotype; speciation; stasis.]., (© The Author(s) 2020. Published by Oxford University Press, on behalf of the Society of Systematic Biologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The shells of ammonoid cephalopods are among the most recognizable fossils, whose fractally folded, internal walls (septa) have inspired many hypotheses on their adaptive value. The enduring explanation for their iterative evolution is that they strengthen the shell against pressure at increasing water depths. The fossil record does not definitively support this idea and much of the theoretical mechanical work behind it has suffered from inaccurate testing geometries and conflicting results. By using a different set of mathematical methods compared with previous studies, I generate a system of finite-element models that explore how different parameters affect the shell's response to water pressure. Increasing the number of initial folds of the septa ultimately has little to no effect on the resulting stress in the shell wall or the septum itself. The introduction of higher-order folds does reduce the tensile stress in the shell wall; however, this is coupled with a higher rate of increase of tensile stress in the septum itself. These results reveal that the increase in complexity should not be expected to have a significant effect on the shell's strength and suggests that the evolution of ammonitic septa does not reflect a persistent trend towards deeper-water habitats.

Climate changes in the Arctic may weaken the currently tight pelagic-benthic coupling. In response to decreasing sea ice cover, arctic marine systems are expected to shift from a 'sea-ice algae-benthos' to a 'phytoplankton-zooplankton' dominance. We used mollusc shells as bioarchives and fatty acid trophic markers to estimate the effects of the reduction of sea ice cover on the food exported to the seafloor. Bathyal bivalve Astarte moerchi living at 600 m depth in northern Baffin Bay reveals a clear shift in growth variations and Ba/Ca ratios since the late 1970s, which we relate to a change in food availability. Tissue fatty acid compositions show that this species feeds mainly on microalgae exported from the euphotic zone to the seabed. We, therefore, suggest that changes in pelagic-benthic coupling are likely due either to local changes in sea ice dynamics, mediated through bottom-up regulation exerted by sea ice on phytoplankton production, or to a mismatch between phytoplankton bloom and zooplankton grazing due to phenological change. Both possibilities allow a more regular and increased transfer of food to the seabed. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Epigenetic variation might play an important role in generating adaptive phenotypes by underpinning within-generation developmental plasticity, persistent parental effects of the environment (e.g., transgenerational plasticity), or heritable epigenetically based polymorphism. These adaptive mechanisms should be most critical in organisms where genetic sources of variation are limited. Using a clonally reproducing freshwater snail (Potamopyrgus antipodarum), we examined the stability of an adaptive phenotype (shell shape) and of DNA methylation between generations. First, we raised three generations of snails adapted to river currents in the lab without current. We showed that habitat-specific adaptive shell shape was relatively stable across three generations but shifted slightly over generations two and three toward a no-current lake phenotype. We also showed that DNA methylation specific to high-current environments was stable across one generation. This study provides the first evidence of stability of DNA methylation patterns across one generation in an asexual animal. Together, our observations are consistent with the hypothesis that adaptive shell shape variation is at least in part determined by transgenerational plasticity, and that DNA methylation provides a potential mechanism for stability of shell shape across one generation., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.)

The Viviparidae, commonly known as River Snails, is a dominant group of freshwater snails with a nearly worldwide distribution that reaches its highest taxonomic and morphological diversity in Southeast Asia. The rich fossil record is indicative of a probable Middle Jurassic origin on the Laurasian supercontinent where the group started to diversify during the Cretaceous. However, it remains uncertain when and how the biodiversity hotspot in Southeast Asia was formed. Here, we used a comprehensive genetic data set containing both mitochondrial and nuclear markers and comprising species representing 24 out of 28 genera from throughout the range of the family. To reconstruct the spatiotemporal evolution of viviparids on a global scale, we reconstructed a fossil-calibrated phylogeny. We further assessed the roles of cladogenetic and anagenetic events in range evolution. Finally, we reconstructed the evolution of shell features by estimating ancestral character states to assess whether the appearance of sculptured shell morphologies was driven by major habitat shifts. The molecular phylogeny supports the monophyly of the three subfamilies, the Bellamyinae, Lioplacinae, and Viviparinae, but challenges the currently accepted genus-level classification in several cases. The almost global distribution of River Snails has been influenced both by comparatively ancient vicariance and more recent founder events. In Southeast Asia, Miocene dispersal was a main factor in shaping the modern species distributions. A recurrent theme across different viviparid taxa is that many species living in lentic waters exhibit sculptured shells, whereas only one strongly sculptured species is known from lotic environments. We show that such shell sculpture is habitat-dependent and indeed evolved several times independently in lentic River Snails. Considerably high transition rates between shell types in lentic habitats probably caused the co-occurrence of morphologically distinct shell types in several lakes. In contrast, directional evolution toward smooth shells in lotic habitats, as identified in the present analyses, explains why sculptured shells are rarely found in these habitats. However, the specific factors that promoted changes in shell morphology require further work. [biogeographical analyses; fossil-calibrated phylogeny; fossil-constrained analyses; Southeast Asia; stochastic character mapping.]., (© The Author(s) 2020. Published by Oxford University Press, on behalf of the Society of Systematic Biologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Nipponites is a heteromorph ammonoid with a complex and unique morphology that obscures its mode of life and ethology. The seemingly aberrant shell of this Late Cretaceous nostoceratid seems deleterious. However, hydrostatic simulations suggest that this morphology confers several advantages for exploiting a quasi-planktic mode of life. Virtual, 3D models of Nipponites mirabilis were used to compute various hydrostatic properties through 14 ontogenetic stages. At each stage, Nipponites had the capacity for neutral buoyancy and was not restricted to the seafloor. Throughout ontogeny, horizontally facing to upwardly facing soft body orientations were preferred at rest. These orientations were aided by the obliquity of the shell's ribs, which denote former positions of the aperture that were tilted from the growth direction of the shell. Static orientations were somewhat fixed, inferred by stability values that are slightly higher than extant Nautilus. The initial open-whorled, planispiral phase is well suited to horizontal backwards movement with little rocking. Nipponites then deviated from this bilaterally symmetric coiling pattern with a series of alternating U-shaped bends in the shell. This modification allows for proficient rotation about the vertical axis, while possibly maintaining the option for horizontal backwards movement by redirecting its hyponome. These particular hydrostatic properties likely result in a tradeoff between hydrodynamic streamlining, suggesting that Nipponites assumed a low energy lifestyle of slowly pirouetting in search for planktic prey. Each computed hydrostatic property influences the others in some way, suggesting that Nipponites maintained a delicate hydrostatic balancing act throughout its ontogeny in order to facilitate this mode of life., Competing Interests: The authors have declared that no competing interests exist.

The pattern of shell shape variation in populations of the top shell, Phorcus sauciatus from Madeira Island (NE Atlantic) was analysed as a function of substrate type and wave exposure, using geometric morphometrics. We hypothesized that P. sauciatus shows morphological variations to inhabit contrasting environments. Highly significant differences in shell shape were found depending on both substrate type and coastal exposure. The most marked differences were found between exposed and sheltered environments. Rounded shells in exposed environments may be explained by physiological reasons, since larger muscles are needed to ensure attachment to substrate in areas subject to harsh conditions. On the other side, conically-shaped shells may accommodate a larger body but with a smaller foot, an adaptation to sheltered environments. Slight shape variations were also observed among substrates, mostly in the degree of differentiation of some whorls. Differences could be related to a particular use of habitat and/or to the degree of exposure to water current. These results suggest that P. sauciatus is locally adapted to varying coastal hydrodynamics and may be considered as a good model in studies on adaptations of fauna to certain climate change effects., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier GmbH. All rights reserved.)

The colour of oyster shells is a very diverse characteristic morphotype, forming intriguing vivid patterns both on the inside and outside of the shell. In the present study, we have identified for the first time, the presence of several porphyrins as constituents of the shell pigmentation of the Crassostrea gigas oyster consumed worldwide. The precise molecular structures of halochromic, fluorescent and acid-soluble porphyrins, such as uroporphyrin and turacin, are unambiguously determined by reverse phase liquid chromatography combined with high resolution mass spectrometry. Their presence account for the purple colouration of shells but also for the dark colouration of adductor muscle scars. We have also defined the endogenous origin of these porphyrins, specifically secreted or accumulated by the shell forming tissue. These findings are pioneering analytical proofs of the existence of the haem pathway in the edible oyster Crassostrea gigas, evidenced by the chemical identification of haem side-products and supported by the recent publication of the corresponding oyster genome.

Echinoderms are an exceptional group of bilaterians that develop pentameral adult symmetry from a bilaterally symmetric larva. However, the genetic basis in evolution and development of this unique transformation remains to be clarified. Here we report newly sequenced genomes, developmental transcriptomes, and proteomes of diverse echinoderms including the green sea urchin (L. variegatus), a sea cucumber (A. japonicus), and with particular emphasis on a sister group of the earliest-diverged echinoderms, the feather star (A. japonica). We learned that the last common ancestor of echinoderms retained a well-organized Hox cluster reminiscent of the hemichordate, and had gene sets involved in endoskeleton development. Further, unlike in other animal groups, the most conserved developmental stages were not at the body plan establishing phase, and genes normally involved in bilaterality appear to function in pentameric axis development. These results enhance our understanding of the divergence of protostomes and deuterostomes almost 500 Mya.

The Indian Ocean hydrothermal vent is a region where a new oceanic crust is formed by magma at the interface of the deep-sea bed over 2000 m in depth. Here we examined for the first time the exoskeleton structure and mechanical properties of the bythograeid crab Austinograea Rodriguezensis living in hydrothermal vents. Scanning electron microscope and energy dispersive x-ray were used for structural analysis, and a nanoindentation system was used for mechanical analysis. The exoskeleton was divided into four layers: epicuticle, exocuticle, endocuticle, and membrane. The thickness of each layer was different from that of other crustaceans previously reported. Additionally, the number of constituent elements, composition ratio, and hardness of each layer were unique among previously studied crabs. This observation indicates that those characteristics might have evolved for creatures with a hard exoskeleton living in the deep-sea hydrothermal vent., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

Gastropod shells may present large spines and sharp shapes that vary according to environmental, taxonomic, and evolutionary factors. In these cases, classic morphometric methods used to study shell contour might not provide a clear representation of morphological shell based on angular decomposition of contour. The present study analyzed and compared for the first time the robustness of the contour analysis using wavelet transformed and Elliptic Fourier descriptors for gastropod shells including enlarged spines. For that, we analyzed two geographical and ecological separated populations of Bolinus brandaris from the NW Mediterranean Sea. Results showed that contour analysis of gastropod shells with enlarged spines can be analyzed using both methodologies, but the wavelet analysis provided a better local discrimination. From an ecological perspective, shells with spines of different sizes were observed in both localities suggesting a wide plasticity of the species., (Copyright © 2020 Elsevier GmbH. All rights reserved.)