Shell shape does not accurately predict self-righting ability in hatchling freshwater turtles.

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.

Fractal-like geometry as an evolutionary response to predation?

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.

Imbricated shell sculpture in benthic bivalves.

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.

Skeletal repatterning enhances the protective capacity of the shell in African hinge-back tortoises (Kinixys).

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.

Temporal variation in climatic factors influences phenotypic diversity of Trochulus land snails.

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.

Bivalves maintain repair when faced with chronically repeated mechanical stress.

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.

Transcriptome analysis of the mantle tissue of Pinctada fucata with red and black shells under salinity stress.

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.

Evaluation of remodeling and geometry on the biomechanical properties of nacreous bivalve shells.

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.

Mechanical adaptation of brachiopod shells via hydration-induced structural changes.

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.

Significance of the suture line in cephalopod taxonomy revealed by 3D morphometrics in the modern nautilids Nautilus and Allonautilus.

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).

Microstructural design for mechanical-optical multifunctionality in the exoskeleton of the flower beetle Torynorrhina flammea.

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.

New species of latest Eocene/earliest Oligocene microgastropods (Heterobranchia: Orbitestellidae and Omalogyridae) from the Gries Ranch Formation, Lewis County, Washington State, USA.

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.

Combining genotypic and phenotypic variation in a geospatial framework to identify sources of mussels in northern New Zealand.

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.

Premating barriers in young sympatric snail species.

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 (IPC) 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 IPC, 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.

The late Holocene demise of a sublittoral oyster bed in the North Sea.

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.

3D morphology of nematode encapsulation in snail shells, revealed by micro-CT imaging.

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.

The orbital hoods of snapping shrimp have surface features that may represent tradeoffs between vision and protection.

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.

Computational analyses decipher the primordial folding coding the 3D structure of the beetle horn.

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.

Evo-devo of shell colour in gastropods and bivalves.

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.

A modern scleractinian coral with a two-component calcite-aragonite skeleton.

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.