Differential shell strength of Cepaea nemoralis colour morphs--implications for their anti-predator defence

Arboreal snail genus Amphidromus Albers, 1850 of Southeast Asia: Shell polymorphism of Amphidromus cruentatus (Morelet, 1875) revealed by phylogenetic and morphometric analyses

Species of colourful arboreal snails of the genus Amphidromus from Southeast Asia commonly exhibit high intraspecific variation in shell morphology. Although highly polymorphic Amphidromus specimens with different colouration have been collected at the same locality and were revealed to possess similar genital organs, there is yet no morphometric or DNA analyses of these different shell morphs. This study is the first to reveal that both striped and stripeless morphs of A. cruentatus from Laos and Vietnam belong to the same mitochondrial (COI and 16S rRNA) lineage. Although the shell colouration between the striped and stripeless morphs is markedly different, morphometric and shell outline-based analyses indicated an overall similarity in shell shape. We also revised the systematics of A. cruentatus, in which we treated similar related species, namely A. eudeli, A. fuscolabris, A. thakhekensis, A. gerberi bolovenensis, A. goldbergi, A. pengzhuoani, A. eichhorsti and A. pankowskiae as junior synonyms of A. cruentatus. Amphidromus daoae, A. anhdaoorum, A. stungtrengensis, A. yangbayensis and A. yenlinhae, which were formerly regarded as junior synonyms, are considered as species different from A. cruentatus based on shell morphology and morphometric analyses. Preliminary phylogenetic analyses also retrieved some Amphidromus species groups as distinct mitochondrial lineages.

Convergent evolution of

East Asian terrestrial snails of the family Camaenidae Pilsbry, 1895a are diverse in terms of genus and species numbers, shell morphology and mode of living. This family also includes colourful conical arboreal snails that traditionally have been assigned to the genus Amphidromus Albers, 1850. Yet, the present study shows that, despite their deceiving conchological similarity, some of these East Asian arboreal snails do not belong to the genus Amphidromus or the subfamily Camaeninae Pilsbry, 1895a. The presence of a dart complex comprising a mucous gland, a dart sac, an accessory sac and a proximal accessory sac, along with a pronounced penial caecum and molecular phylogenetic analyses revealed that former ‘Amphidromus’ dautzenbergi, ‘A.’ roemeri and ‘Camaena’ mirifica, and one additional new species belong to Aegistohadra Wu, 2004 (subfamily Bradybaeninae Pilsbry, 1934). Aegistohadra dautzenbergi, comb. nov. and Aegistohadra roemeri, comb. nov. are conical with colourful spiral bands, whereas Aegistohadra mirifica, comb. nov. and Aegistohadra zhangdanae, sp. nov. are heliciform to conical with colourful, variegated spiral and transverse banding patterns. DNA sequence analyses also revealed that each variety of Aegistohadra dautzenbergi could not be differentiated by mitochondrial (cytochrome c oxidase subunit I and 16S rRNA) gene fragments. The phylogenetic position of Aegistohadra within the East Asian camaenids revealed that the similar appearance in shell morphology, microhabitat use and diet to arboreal snails in the genus Amphidromus is homoplastic. Moreover, the presence or absence of a dart complex is also homoplastic and is unsuitable for suprageneric classification. By contrast, the presence of a flagellum and a penial caecum is useful for the suprageneric classification.

Quantitative measures and 3D shell models reveal interactions between bands and their position on growing snail shells

The nature of shell growth in gastropods is useful because it preserves the ontogeny of shape, colour, and banding patterns, making them an ideal system for understanding how inherited variation develops, is established and maintained within a population. However, qualitative scoring of inherited shell characters means there is a lack of knowledge regarding the mechanisms that control fine variation. Here, we combine empirical measures of quantitative variation and 3D modeling of shells to understand how bands are placed and interact. By comparing five-banded Cepaea individuals to shells lacking individual bands, we show that individual band absence has minor but significant impacts upon the position of remaining bands, implying that the locus controlling band presence/absence mainly acts after position is established. Then, we show that the shell grows at a similar rate, except for the region below the lowermost band. This demonstrates that wider bands of Cepaea are not an artifact of greater shell growth on the lower shell; they begin wider and grow at the same rate as other bands. Finally, we show that 3D models of shell shape and banding pattern, inferred from 2D photos using ShellShaper software, are congruent with empirical measures. This work therefore establishes a method that may be used for comparative studies of quantitative banding variation in snail shells, extraction of growth parameters, and morphometrics. In the future, studies that link the banding phenotype to the network of shell matrix proteins involved in biomineralization and patterning may ultimately aid in understanding the diversity of shell forms found in molluscs.

Breaking down shell strength: inferences from experimental compression and future directions enabled by 3D printing

Mollusc and brachiopod shells have served as biological armour for hundreds of millions of years. Studying shell strength in compression experiments can provide insights into macroevolution, predator-prey dynamics, and anthropogenic impacts on aquatic ecosystems. These studies have been conducted across fields including palaeontology, ecology, conservation biology and engineering using a range of techniques for a variety of purposes. Using this approach, studies have demonstrated that predators can cause changes in prey shell morphology in the laboratory over both short timescales and over longer evolutionary timescales. Similarly, environmental factors such as nutrient concentration and ocean acidification have been shown to influence shell strength. Experimental compression tests have been used to study the functional morphology of shell-crushing predators and to test how the taphonomic state of shells (e.g. presence of drill holes, degree of shell degradation) may influence their likelihood of being preserved in the fossil record. This review covers the basic principles and experimental design of compression tests used to infer shell strength. Although many investigations have used this methodology, few provide a detailed explanation of how meaningfully to interpret data generated using compression experiments for those unfamiliar with this method. Furthermore, this review provides a compilation of the findings of studies that have employed these experimental methods to address specific themes: taphonomy, morphology, predation, environmental variables, and climate change. Many authors have used experimental compression tests, however, disparities among methodologies (e.g. in experimental design, taxa, specimen preservation, etc.) limit the applicability of findings from taxon-specific studies to broader eco-evolutionary questions. The review highlights confounding factors, such as shell thickness, size, damage, microstructure, and taphonomic state, and address how they can be mitigated using three-dimensional (3D)-printed model shells. 3D prints have been demonstrated as valuable proxies for understanding aspects of shell morphology that cannot otherwise be experimentally isolated. Using 3D printed models allows simplification of complex biological systems for idealized experimental studies. Such studies can isolate specific aspects of shell morphology to establish fundamental relationships between form and function. Establishing standardized methods of testing shell strength in this way will not only permit comparison across studies but also will enable investigators systematically to add complexity to their models.

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.

Structure-behaviour correlations between two genetically closely related snail species

Species, through their structure and composition, have evolved to respond to environmental constraints. Predator-prey interactions are among environmental pressures that can lead to speciation, but it remains unclear how this pressure can be related to the material structure and performance. Recently, two land snails, Karaftohelix editha and Karaftohelix gainesi, were found to exhibit divergent phenotypes and responses to predation despite sharing the same habitat and most of their genome. Indeed, under attack from a beetle, K. editha snails retract into their shell whereas K. gainesi snails swing their shell. In this paper, we looked at the microstructure, composition, morphology and mechanics of the shells of those two species and discuss potential relationships between material structure and the snail defence behaviour. The results of this study provide additional arguments for the role of predator-prey interactions on speciation, as well as an unusual approach for the design of biomimetic structures adapted to a particular function.

Thrush anvils are calcium source hotspots for many bird species

Calcium is one of the most important elements determining reproductive success in birds, and snail shells are a well-known source of calcium for egg-laying females. In Europe, song thrushes, Turdus philomelos, break snails open at locations called anvils, eat the soft parts and leave the broken shells. Based on observational studies in 2011–2017 in western Poland, we showed that thrush anvils were visited by 54 other bird species (mainly smaller species) that collected the broken shell fragments. The frequency of visits to anvils differed among species and changed over the course of the breeding season, but anvils were especially used by females during the pre-laying period. Our study is the first description of such widespread exploitation of thrush anvils by other bird species, and we think that this phenomenon has been overlooked owing to the difficulties in recording anvils located, for example, on single small stones, stumps of felled trees and man-made objects such as glass bottles. We conclude that thrush anvils can be an important source of calcium for many bird species that adapt behaviourally to use these places. Hence, anvils should be recognized as localized hotspots for bird diversity.

Snail shell colour evolution in urban heat islands detected via citizen science

The extreme environmental conditions that prevail in cities are known to cause selection pressures leading to adaptive changes in wild, city-dwelling, organisms (urban evolution). The urban heat island, elevated temperatures in the city centre due to a combination of generation, reflection, and trapping of heat, is one of the best recognised and most widespread urban environmental factors. Here, we use a citizen-science approach to study the effects of urban heat on genetically-determined shell colour in the land snail Cepaea nemoralis in the Netherlands. We use smartphone applications to obtain colour data on almost 8000 snails throughout the country. Our analysis shows that snails in urban centres are more likely to be yellow than pink, an effect predicted on the basis of thermal selection. Urban yellow snails are also more likely to carry dark bands at the underside of the shell; these bands might affect thermoregulation in yet underexplored ways.

Under pressure: force resistance measurements in box mites (Actinotrichida, Oribatida)

Background

Mechanical defenses are very common and diverse in prey species, for example in oribatid mites. Here, the probably most complex form of morphological defense is known as ptychoidy, that enables the animals to completely retract the appendages into a secondary cavity and encapsulate themselves. The two groups of ptychoid mites constituting the Ptyctima, i.e. Euphthiracaroidea and Phthiracaroidea, have a hardened cuticle and are well protected against similar sized predators. Euphthiracaroidea additionally feature predator-repelling secretions. Since both taxa evolved within the glandulate group of Oribatida, the question remains why Phthiracaroidea lost this additional protection. In earlier predation bioassays, chemically disarmed specimens of Euphthiracaroidea were cracked by the staphylinid beetle Othius punctulatus, whereas equally sized specimens of Phthiracaroidea survived. We thus hypothesized that Phthiracaroidea can withstand significantly more force than Euphthiracaroidea and that the specific body form in each group is key in understanding the loss of chemical defense in Phthiracaroidea. To measure force resistance, we adapted the principle of machines applying compressive forces for very small animals and tested the two ptyctimous taxa as well as the soft-bodied mite Archegozetes longisetosus.

Results

Some Phthiracaroidea individuals sustained about 560,000 times their body weight. Their mean resistance was about three times higher, and their mean breaking point in relation to body weight nearly two times higher than Euphthiracaroidea individuals. The breaking point increased with body weight and differed significantly between the two taxa. Across taxa, the absolute force resistance increased sublinearly (with a 0.781 power term) with the animal's body weight. Force resistance of A. longisetosus was inferior in all tests (about half that of Euphthiracaroidea after accounting for body weight). As an important determinant of mechanical resistance in ptychoid mites, the individuals' cuticle thickness increased sublinearly with body diameter and body mass as well and did not differ significantly between the taxa.

Conclusion

We showed the feasibility of the force resistance measurement method, and our results were consistent with the hypothesis that Phthiracaroidea compensated its lack of chemical secretions by a heavier mechanical resistance based on a different body form and associated build-up of hemolymph pressure (defensive trade-off).

Shell colour, temperature, (micro)habitat structure and predator pressure affect the behaviour of Cepaea nemoralis

Although shell colour polymorphism of the land snail Cepaea nemoralis is a well-known phenomenon, proximate and ultimate factors driving its evolution remain uncertain. Polymorphic species show variation in behavioural responses to selective forces. Therefore, we estimated effects of various environmental factors (temperature, humidity, food availability, (micro)habitat structure and predatory pressure) on behavioural response (frequency of locomotion, climbing and hiding) of C. nemoralis morphs, in experimental and natural conditions. In the experimental part of study, the frequency of locomotion was negatively affected by temperature and the presence of food and positively influenced by the presence of light. Morphs significantly differed in behavioural responses to environmental variability. Pink mid-banded and yellow five-banded morphs climbed less often and hide in shelter more often than yellow and pink unbanded individuals when temperature was low and food was absent. Snails fed most often at moderate temperature compared to low and high temperatures. Field investigations partially confirmed differences among morphs in frequency of climbing, but not in terms of probability of hiding in sheltered sites. In natural colonies, temperature and (micro)habitat structure significantly affected frequency of climbing as well as hiding in shelter. Snails more often hid in sheltered sites where thrushes preyed on Cepaea. Tendency of unbanded morphs to climb trees may have evolved under avian predatory pressure as thrushes forage on a ground. Tendency of banded morphs to hide in sheltered sites may reflect prey preferences for cryptic background. The results implicate that differential behaviour of C. nemoralis morphs compensate for their morphological and physiological limitations of adaptation to habitat.

Candidate genes for shell colour polymorphism in Cepaea nemoralis

The characteristic ground colour and banding patterns on shells of the land snail Cepaea nemoralis form a classic study system for genetics and adaptation as it varies widely between individuals. We use RNAseq analysis to identify candidate genes underlying this polymorphism. We sequenced cDNA from the foot and the mantle (the shell-producing tissue) of four individuals of two phenotypes and produced a de novo transcriptome of 147,397 contigs. Differential expression analysis identified a set of 1,961 transcripts that were upregulated in mantle tissue. Sequence variant analysis resulted in a set of 2,592 transcripts with single nucleotide polymorphisms (SNPs) that differed consistently between the phenotypes. Inspection of the overlap between the differential expression analysis and SNP analysis yielded a set of 197 candidate transcripts, of which 38 were annotated. Four of these transcripts are thought to be involved in production of the shell’s nacreous layer. Comparison with morph-associated Restriction-site Associated DNA (RAD)-tags from a published study yielded eight transcripts that were annotated as metallothionein, a protein that is thought to inhibit the production of melanin in melanocytes. These results thus provide an excellent starting point for the elucidation of the genetic regulation of the Cepaea nemoralis shell colour polymorphism.

A systematic review of animal predation creating pierced shells: implications for the archaeological record of the Old World

Background

The shells of molluscs survive well in many sedimentary contexts and yield information about the diet of prehistoric humans. They also yield evidence of symbolic behaviours through their use as beads for body adornments. Researchers often analyse the location of perforations in shells to make judgements about their use as symbolic objects (e.g., beads), the assumption being that holes attributable to deliberate human behaviour are more likely to exhibit low variability in their anatomical locations, while holes attributable to natural processes yield more random perforations. However, there are non-anthropogenic factors that can cause perforations in shells and these may not be random. The aim of the study is compare the variation in holes in shells from archaeological sites from the Old World with the variation of holes in shells pierced by mollusc predators.

Methods

Three hundred and sixteen scientific papers were retrieved from online databases by using keywords, (e.g., ‘shell beads’; ‘pierced shells’; ‘drilling predators’); 79 of these publications enabled us to conduct a systematic review to qualitatively assess the location of the holes in the shells described in the published articles. In turn, 54 publications were used to assess the location of the holes in the shells made by non-human predators.

Results

Almost all archaeological sites described shells with holes in a variety of anatomical locations. High variation of hole-placement was found within the same species from the same site, as well as among sites. These results contrast with research on predatory molluscs, which tend to be more specific in where they attacked their prey. Gastropod and bivalve predators choose similar hole locations to humans.

Discussion

Based on figures in the analysed articles, variation in hole-location on pierced shells from archaeological sites was similar to variation in the placement of holes created by non-human animals. Importantly, we found that some predators choose similar hole locations to humans. We discuss these findings and identify factors researchers might want to consider when interpreting shells recovered from archaeological contexts.

What explains rare and conspicuous colours in a snail? A test of time-series data against models of drift, migration or selection

It is intriguing that conspicuous colour morphs of a prey species may be maintained at low frequencies alongside cryptic morphs. Negative frequency-dependent selection by predators using search images ('apostatic selection') is often suggested without rejecting alternative explanations. Using a maximum likelihood approach we fitted predictions from models of genetic drift, migration, constant selection, heterozygote advantage or negative frequency-dependent selection to time-series data of colour frequencies in isolated populations of a marine snail (Littorina saxatilis), re-established with perturbed colour morph frequencies and followed for >20 generations. Snails of conspicuous colours (white, red, banded) are naturally rare in the study area (usually <10%) but frequencies were manipulated to levels of ~50% (one colour per population) in 8 populations at the start of the experiment in 1992. In 2013, frequencies had declined to ~15-45%. Drift alone could not explain these changes. Migration could not be rejected in any population, but required rates much higher than those recorded. Directional selection was rejected in three populations in favour of balancing selection. Heterozygote advantage and negative frequency-dependent selection could not be distinguished statistically, although overall the results favoured the latter. Populations varied idiosyncratically as mild or variable colour selection (3-11%) interacted with demographic stochasticity, and the overall conclusion was that multiple mechanisms may contribute to maintaining the polymorphisms.

The danger within: the role of genetic, behavioural and ecological factors in population persistence of colour polymorphic species

Polymorphic species have been the focus of important work in evolutionary biology. It has been suggested that colour polymorphic species have specific evolutionary and population dynamics that enable them to persist through environmental changes better than less variable species. We suggest that recent empirical and theoretical work indicates that polymorphic species may be more vulnerable to extinction than previously thought. This vulnerability arises because these species often have a number of correlated sexual, behavioural, life history and ecological traits, which can have a simple genetic underpinning. When exacerbated by environmental change, these alternate strategies can lead to conflict between morphs at the genomic and population levels, which can directly or indirectly affect population and evolutionary dynamics. In this perspective, we identify a number of ways in which the nature of the correlated traits, their underpinning genetic architecture, and the inevitable interactions between colour morphs can result in a reduction in population fitness. The principles illustrated here apply to all kinds of discrete polymorphism (e.g. behavioural syndromes), but we focus primarily on colour polymorphism because they are well studied. We urge further empirical investigation of the genetic architecture and interactions in polymorphic species to elucidate the impact on population fitness.