Soil calcium availability influences shell ecophenotype formation in the sub-antarctic land snail, Notodiscus hookeri

Phylogenomic analyses reveal incongruences between divergence times and fossil records of freshwater snails in East Asia

Fossils provide important insight into our understanding of phylogenetic history by serving as calibration points for divergence time estimation. However, uncertainties in the fossil record due to parallel evolution and convergent evolution can critically affect estimates of node ages. Here, we compare and contrast estimates of phylogenetic divergence with geologic and fossil history for two freshwater snail genera of the family Viviparidae in East Asia (Cipangopaludina and Margarya). Cipangopaludina species are commonly widely distributed species in East Asia, but extant Margarya species are endemic to the ancient lakes in Yunnan, China. According to some previous studies, parallel evolution or convergent evolution of shell morphology has occurred in the family several times which may affect divergence time estimation using fossil records. In this study, we used SNP data derived from ddRAD-seq loci to investigate population demographic history of both genera. Our results show a common pattern of lake endemic lineages diversifying from widely distributed lineages in the Miocene, and multiple colonization to a single ancient lake occurred in the Pleistocene. Our results indicate substantial incongruence among estimated phylogenomic divergence times, some fossil records, and formation ages of ancient lakes. These findings suggest some fossil records may be misidentified in these groups and highlight the need to carefully evaluate geological evidence and fossil records when using these for divergence time estimation.

Mass Spectrometry Imaging of Specialized Metabolites for Predicting Lichen Fitness and Snail Foraging

Lichens are slow-growing organisms supposed to synthetize specialized metabolites to protect themselves against diverse grazers. As predicted by the optimal defense theory (ODT), lichens are expected to invest specialized metabolites in higher levels in reproductive tissues compared to thallus. We investigated whether Laser Desorption Ionization coupled to Mass Spectrometry Imaging (LDI-MSI) could be a relevant tool for chemical ecology issues such as ODT. In the present study, this method was applied to cross-sections of thalli and reproductive tissues of the lichen Pseudocyphellaria crocata. Spatial mapping revealed phenolic families of metabolites. A quantification of these metabolites was carried out in addition to spatial imaging. By this method, accumulation of specialized metabolites was observed in both reproductive parts (apothecia and soralia) of P. crocata, but their nature depended on the lichen organs: apothecia concentrated norstictic acid, tenuiorin, and pulvinic acid derivatives, whereas soralia mainly contained tenuiorin and pulvinic acid. Stictic acid, tenuiorin and calycin, tested in no-choices feeding experiments, were deterrent for N. hookeri while entire thalli were consumed by the snail. To improve better knowledge in relationships between grazed and grazing organisms, LDI-MSI appears to be a complementary tool in ecological studies.

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.

Overcoming deterrent metabolites by gaining essential nutrients: A lichen/snail case study

Specialised metabolites in lichens are generally considered repellent compounds by consumers. Nevertheless, if the only food available is lichens rich in specialised metabolites, lichenophages must implement strategies to overcome the toxicity of these metabolites. Thus, the balance between phagostimulant nutrients and deterrent metabolites could play a key role in feeding preferences. To further understand lichen-gastropod interactions, we studied the feeding behaviour and consumption in Notodiscus hookeri, the land snail native to sub-Antarctic islands. The lichen Usnea taylorii was used because of its simple chemistry, its richness in usnic acid (specialised metabolite) and arabitol (primary metabolite) and its presence in snail habitats. Choice tests in arenas with intact lichens versus acetone-rinsed lichens were carried out to study the influence of specialised metabolites on snail behaviour and feeding preference. Simultaneously, usnic acid and arabitol were quantified and located within the lichen thallus using HPLC-DAD-MS and in situ imaging by mass spectrometry to assess whether their spatial distribution explained preferential snail grazing. No-choice feeding experiments, with the pure metabolites embedded in an artificial diet, defined a gradual gustatory response, from strong repellence (usnic acid) to high appetence (D-arabitol). This case study demonstrates that the nutritional activity of N. hookeri is governed by the chemical quality of the food and primarily by nutrient availability (arabitol), despite the presence of deterrent metabolite (usnic acid).

Wettability and dynamics of water droplet on a snail shell

HYPOTHESIS

There are many natural surfaces with special wettabilities. Snail shells have unique rough structures, which indicates a specific wettability. In this study, the surface of a snail shell was simulated using epoxy resins, and water droplet dynamics on original and simulated snail shells were investigated to understand its special wettability.

EXPERIMENTS

The shell of the Euhadra sandai species of snails was used. The surface structure of the snail shell was simulated using epoxy resins. The surface of this EP resin was treated with UV-O₃ for different periods of time. Wettabilities and dynamics of water droplet on the samples were characterized.

FINDINGS

The surface of the snail shell with a water contact angle of approximately 85° caused the droplet to spread, which is the first report of water droplet dynamics on the shell surface. The behavior of a water droplet on the shell transformed from the Cassie state into the Wenzel state. Changes in the contact angle and diameter of the droplet base on the snail shell were larger than those on the epoxy resins. The surface roughness and chemical heterogeneity of the snail shell led to distortion of the three-phase contact line and enhancement of the spreading of the water droplet.

Litter decomposition driven by soil fauna, plant diversity and soil management in urban gardens

In the face of growing urban densification, green spaces in cities, such as gardens, are increasingly important for biodiversity and ecosystem services. However, the influences of urban green space management on biodiversity and ecosystem functioning (BEF) relationships is poorly understood. We investigated the relationship between soil fauna and litter decomposition in 170 urban garden sites along a gradient of urbanisation intensity in the city of Zurich, CH. We used litter bags of 1 and 4 mm mesh size to evaluate the contribution of soil meso- and macrofauna on litter decomposition. By using multilevel structural equation models (SEM), we investigated direct and indirect environmental effects and management practices on litter decomposition and litter residue quality. We evaluated the role of taxonomic, functional and phylogenetic diversity of soil fauna species on litter decomposition, based on a sample of 120 species (81,007 individuals; 39 collembola, 18 earthworm, 16 isopod, 47 gastropod species). We found highest litter decomposition rates using 4 mm mesh size litter bags, highlighting the importance of soil macrofauna. Urban warming, a proxy for urbanisation intensity, covaried positively, whereas soil disturbances, such as intensive soil and crop management, were negatively correlated with decomposition rates. Interestingly, soil fauna species richness decreased, with the exception of gastropods, and soil fauna abundance increased with urban warming. Our data also show that plant species richness positively affected litter decomposition by increasing soil fauna species richness and microbial activity. A multivariate analysis of organic compounds in litter residues confirmed the importance of soil fauna species richness and garden management on litter decomposition processes. Overall, we showed, that also in intensively managed urban green spaces, such as gardens, biodiversity of plants and soil fauna drives key ecosystem processes. Urban planning strategies that integrate soil protecting management practices may help to maintain important ecosystem services in this heavily used urban environment.

The influence of habitat preferences on shell morphology in ecophenotypes of Trochulus hispidus complex

Trochulus hispidus and T. sericeus are hairy snails widely distributed in Europe. They differ in shell morphology and are usually found in various land habitats. However, their morphology does not match genetic distance as they do not form distinct clades. Therefore, it is interesting to determine to what extent environmental factors can control their phenotypes. We analysed the morphological traits and many environmental features of their habitats to find relationships between these parameters and explain ecological reasons for this plasticity. We found many statistically significant correlations between morphological traits and environmental variables. Illumination, forestation, precipitation and temperature occurred the most important features discriminating habitats of these snails. It turned out that T. sericeus prefers forests and moist shaded places, while T. hispidus chooses more dry habitats and open areas exposed to the sun. T. sericeus is also probably more tolerant to low and variable temperatures. The hair durability is also correlated with their habitats: the shell of T. hispidus is mostly hairless but hairs almost always cover the shell of T. sericeus. These results support the hypothesis that the lack of hairs is associated with the loss of a potential adaptive function due to the change from wet to dry habitats. The hairs facilitate the adherence of snails to herbaceous plants during feeding when the humidity levels are high. The morphological divergence of T. hispidus and T. sericeus is the result of phenotypic plasticity and selection associated with the habitat, which affect both the shell shape and the hair durability. Since T. hispidus and T. sericeus do to not represent separate biological species and their variability has no genetic basis, they should be considered as ecophenotypes. This and our previous studies suggest that phenotypic plasticity in widely distributed Trochulus species is quite common and may have been of ancestral origin.

Intrathalline Metabolite Profiles in the Lichen Argopsis friesiana Shape Gastropod Grazing Patterns

Lichen-gastropod interactions generally focus on the potential deterrent or toxic role of secondary metabolites. To better understand lichen-gastropod interactions, a controlled feeding experiment was designed to identify the parts of the lichen Argopsis friesiana consumed by the Subantarctic land snail Notodiscus hookeri. Besides profiling secondary metabolites in various lichen parts (apothecia, cephalodia, phyllocladia and fungal axis of the pseudopodetium), we investigated potentially beneficial resources that snails can utilize from the lichen (carbohydrates, amino acids, fatty acids, polysaccharides and total nitrogen). Notodiscus hookeri preferred cephalodia and algal layers, which had high contents of carbohydrates, nitrogen, or both. Apothecia were avoided, perhaps due to their low contents of sugars and polyols. Although pseudopodetia were characterized by high content of arabitol, they were also rich in medullary secondary compounds, which may explain why they were not consumed. Thus, the balance between nutrients (particularly nitrogen and polyols) and secondary metabolites appears to play a key role in the feeding preferences of this snail.

Which Specialized Metabolites Does the Native Subantarctic Gastropod Notodiscus hookeri Extract from the Consumption of the Lichens Usnea taylorii and Pseudocyphellaria crocata?

Notodiscus hookeri is the only representative of terrestrial gastropods on Possession Island and exclusively feeds on lichens. The known toxicity of various lichen metabolites to plant-eating invertebrates led us to propose that N. hookeri evolved means to protect itself from their adverse effects. To validate this assumption, the current study focused on the consumption of two lichen species: Usnea taylorii and Pseudocyphellaria crocata. A controlled feeding experiment was designed to understand how the snail copes with the unpalatable and/or toxic compounds produced by these lichen species. The occurrence of two snail ecophenotypes, represented by a mineral shell and an organic shell, led to address the question of a metabolic response specific to the phenotype. Snails were fed for two months with one of these lichens and the chemical profiles of biological samples of N. hookeri (i.e., crop, digestive gland, intestine, and feces) were established by HPLC-DAD-MS and compared to that of the lichens. N. hookeri appears as a generalist lichen feeder able to consume toxic metabolite-containing lichens, independently of the ecophenotype. The digestive gland did not sequester lichen metabolites. The snail metabolism might be based on four non-exclusive processes according to the concerned metabolites (avoidance, passive transport, hydrolysis, and excretion).

Insights from the Shell Proteome: Biomineralization to Adaptation

Bivalves have evolved a range of complex shell forming mechanisms that are reflected by their incredible diversity in shell mineralogy and microstructures. A suite of proteins exported to the shell matrix space plays a significant role in controlling these features, in addition to underpinning some of the physical properties of the shell itself. Although, there is a general consensus that a minimum basic protein tool kit is required for shell construction, to date, this remains undefined. In this study, the shell matrix proteins (SMPs) of four highly divergent bivalves (The Pacific oyster, Crassostrea gigas; the blue mussel, Mytilus edulis; the clam, Mya truncata, and the king scallop, Pecten maximus) were analyzed in an identical fashion using proteomics pipeline. This enabled us to identify the critical elements of a "basic tool kit" for calcification processes, which were conserved across the taxa irrespective of the shell morphology and arrangement of the crystal surfaces. In addition, protein domains controlling the crystal layers specific to aragonite and calcite were also identified. Intriguingly, a significant number of the identified SMPs contained domains related to immune functions. These were often are unique to each species implying their involvement not only in immunity, but also environmental adaptation. This suggests that the SMPs are selectively exported in a complex mix to endow the shell with both mechanical protection and biochemical defense.