Divergence in the shell morphology of the land snail genus Aegista (Pulmonata: Bradybaenidae) under phylogenetic constraints

The role of natural selection in phenotypic evolution is central to evolutionary biology. Phenotypic evolution is affected by various factors other than adaptation, and recent focus has been placed on the effects of phylogenetic constraints and niche conservatism on phenotypic evolution. Here, we investigate the relationship between the shell morphology and habitat use of bradybaenid land snails of the genus Aegista and clarify the causes of the divergence in shell morphology among phylogenetically related species. The results of ancestral state reconstruction showed that arboreal species have evolved independently from ground‐dwelling species at least four times. A significant association was found between shell shape and habitat use, despite the existence of a certain degree of phylogenetic constraint between these traits. A principal component analysis showed that arboreal species tend to have a relatively high‐spired shell with a narrow umbilicus. By contrast, ground‐dwelling species have a low‐spired shell with a wide umbilicus. Although the latitude and elevation of the sampling locations showed no relationship with shell morphology, the geology of the sampling locations affected the shell size of arboreal species. The development of a well‐balanced shell shape is one effective method for reducing the cost of locomotion under the force of gravity in each life habitat, resulting in the divergence in shell morphology and the independent evolution of morphologically similar species among different lineages. The present study suggests that ecological divergence is probably the cause of shell morphology divergence in land snails. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 229–241.     

The shape of the mandibular corpus in large fissiped carnivores: allometry,function and phylogeny

Mandibular corpus shape variability was investigated in seven families of both extant and extinct mammalian carnivores using two‐dimensional landmarks and geometric morphometric methods. The landmark configuration represents the position of the fourth premolar relative to the canine and the lower carnassial, plus related features of the corpus profile. Between families, the corpus manifests differences in shape and allometry. Additionally, families differ in the relationship between the angle α (a carnassial trait proxy for hypo‐hypercarnivory) and mandibular corpus shape. When phylogenetic relatedness is taken into account using five different phylogenies as covariates, interspecific scaling is still present but the relationship between α and corpus shape is no longer significant. This suggests that the allometric relationship reflects the physical constraints of mastication, resulting in relatively increased corpus depth with higher loads. In contrast, the angle α is related to corpus curvature and this relationship is phylogenetically constrained by the reduction of the molar battery in large feliforms early during the carnivore radiation. In summary, large fissiped carnivores show extensive functional convergence in mandibular corpus shape although the evolutionary routes leading to such functional convergence are different. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154 , 832–845.     

Scale-dependence in geographic variation in a freshwater gastropod across the Palearctic

ABSTRACT

The reality of spatial clinal variation in morphological traits of freshwater pulmonate snails (Gastropoda: Pulmonata) has repeatedly been questioned or totally discounted. There is a lack of sound statistical evidence in the articles hitherto published on this subject supporting these claims. Here, by means of different analytical methods (analysis of spatial autocorrelation, linear regression analysis, canonical correlation analysis and others), we demonstrate that shell variation in the dwarf pond snail, Galba truncatula, is patterned in space throughout the northern and central Palearctic, with latitudinally-oriented clines in body size and in some shell proportions. Shell size in G. truncatula decreases with latitude and temperature, representing a special case of converse Bergmann cline. However, the temperature itself is hardly the main driver of shell size variation. It is argued that the shorter growing seasons at high latitudes may represent a better explanation for the observed trend. Shell proportions in the dwarf pond snails vary weakly at the macrogeographic scale, being spatially patterned at lower (mesogeographic) scales around 1200–1500?km. In general, spatial variation in G. truncatula shell size is decoupled from variation in shell shape, demonstrating clear scale-dependence similar to that found in different species of terrestrial (non-aquatic) pulmonate snails.     

Multiple scale patterns of shell and anatomy variability in land snails: the case of the Sicilian Marmorana (Gastropoda: Pulmonata, Helicidae)

Certain major aspects of phenotypic diversity are still largely unexplained. When phenotypic patterns do not relate to habitat variables, fine analysis of morphological patterns and their distribution sheds light on the origin of diversity. Among invertebrates, snails are an ideal model for studying the roles of the neutral processes and selection involved in creating diversity. To understand patterns and processes of variability on different scales (regional: areas; local: sites), morphological variability of two sets of characters (shell and genitalia) was quantified in a group of rock-dwelling land snails of the genus Marmorana (Pulmonata, Helicidae). To analyse shell variability, partitioning of the overall variation into size and shape components was analysed by a principal component-based approach. Shell shape and size variability is not significantly influenced by any environmental pressure. Variability at site scale is mainly attributed to shell size, which is a trait demonstrated to have a high degree of phenotypic plasticity. No sharp changes were observed for genitalia. Moreover, allometries between shell size and genitalia measurements involve a few populations. The observed multiple scale patterns are in line with the hypothesis that genital variance may be selectively controlled to maintain function.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 359–370.     

Spatial variation of shell morphometrics in the subantarctic land snail <Emphasis Type="Italic">Notodiscus hookeri</Emphasis> from Crozet and Kerguelen Islands

The land snail Notodiscus hookeri, widely distributed in subantarctic islands, shows a large intraspecific variation in shell morphology. In the present work, shell size and form of individuals from populations located in Crozet and Kerguelen archipelagos were investigated by means of multivariate statistics. Variation in shell morphometrics was analysed after the partitioning of the overall variation into size and shape components by means of a principal component-based approach. Shell size shows a significant spatial heterogeneity, which seems essentially related to environmental pressures. Previous works pointed to a greater conchological variation between populations from Kerguelen but present observations show that intra-island variances are not significantly different in the two islands studied. Variation in shell shape splits the populations into two main entities because of different allometric relationships between two shell height components and all other measurements. However, using geographical affinities of populations as instrumental variables shows that more complex environmental features interfered in population clustering.     

Genome‐wide association analyses reveal polygenic genomic architecture underlying divergent shell morphology in Spanish Littorina saxatilis ecotypes

Gene flow between diverging populations experiencing dissimilar ecological conditions can theoretically constrain adaptive evolution. To minimize the effect of gene flow, alleles underlying traits essential for local adaptation are predicted to be located in linked genome regions with reduced recombination. Local reduction in gene flow caused by selection is expected to produce elevated divergence in these regions. The highly divergent crab‐adapted and wave‐adapted ecotypes of the marine snail Littorina saxatilis present a model system to test these predictions. We used genome‐wide association (GWA) analysis of geometric morphometric shell traits associated with microgeographic divergence between the two L. saxatilis ecotypes within three separate sampling sites. A total of 477 snails that had individual geometric morphometric data and individual genotypes at 4,066 single nucleotide polymorphisms (SNPs) were analyzed using GWA methods that corrected for population structure among the three sites. This approach allowed dissection of the genomic architecture of shell shape divergence between ecotypes across a wide geographic range, spanning two glacial lineages. GWA revealed 216 quantitative trait loci (QTL) with shell size or shape differences between ecotypes, with most loci explaining a small proportion of phenotypic variation. We found that QTL were evenly distributed across 17 linkage groups, and exhibited elevated interchromosomal linkage, suggesting a genome‐wide response to divergent selection on shell shape between the two ecotypes. Shell shape trait‐associated loci showed partial overlap with previously identified outlier loci under divergent selection between the two ecotypes, supporting the hypothesis of diversifying selection on these genomic regions. These results suggest that divergence in shell shape between the crab‐adapted and wave‐adapted ecotypes is produced predominantly by a polygenic genomic architecture with positive linkage disequilibrium among loci of small effect.     

Calls,colours, shape,and genes: a multi‐trait approach to the study of geographic variation in the Amazonian frog Allobates femoralis

Evolutionary divergence in behavioural traits related to mating may represent the initial stage of speciation. Direct selective forces are usually invoked to explain divergence in mate‐recognition traits, often neglecting a role for neutral processes or concomitant differentiation in ecological traits. We adopted a multi‐trait approach to obtain a deeper understanding of the mechanisms behind allopatric divergence in the Amazonian frog, Allobates femoralis. We tested the null hypothesis that geographic distance between populations correlates with genetic and phenotypic divergence, and compared divergence between mate‐recognition (acoustic) and ecological (coloration, body‐shape) traits. We quantified geographic variation in 39 phenotypic traits and a mitochondrial DNA marker among 125 individuals representing eight populations. Geographic variation in acoustic traits was pronounced and tracked the spatial genetic variation, which appeared to be neutral. Thus, the evolution of acoustic traits tracked the shared history of the populations, which is unexpected for pan‐Amazonian taxa or for mate‐recognition traits. Divergence in coloration appeared uncorrelated with genetic distance, and might be partly attributed to local selective pressures, and perhaps to Batesian mimicry. Divergence in body‐shape traits was low. The results obtained depict a complex evolutionary scenario and emphasize the importance of considering multiple traits when disentangling the forces behind allopatric divergence. ©2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 826–838.     

Tuning in to multiple predators: conflicting demands for shell morphology in a freshwater snail

1. We examined the response to chemical cues from fish and crayfish, two predators with contrasting feeding modes, and their single and combined effect on shell morphology in the freshwater snail Radix balthica. 2. Snails were subjected to four treatments: tench (Tinca tinca), signal crayfish (Pacifastacus leniusculus), a combination of tench and signal crayfish and no predators (control). Shell shape, crushing resistance and shell thickness were quantified. We also analysed whether shape or shell thickness contributes most to crushing resistance. 3. Chemical cues from the fish induced a rounder shell shape in R. balthica, a thicker shell and a higher crushing resistance, whereas crayfish chemical cues had no effect on shell morphology, shell thickness or crushing resistance. Shell shape contributed more to crushing resistance than shell thickness. 4. The combined predator treatment showed an intermediate response between the fish and crayfish treatments. Shell roundness was reduced compared with the fish treatment, but the reduced crushing resistance that comes with a less rounded shell was compensated by an increased investment in extra shell material, exceeding that of the fish treatment. 5. Our study extends previous studies of multipredator effects on phenotypically plastic freshwater snails by showing that the snails are able to fine‐tune different elements of morphology to counter predator‐specific foraging modes.     

Colonization,stability, and adaptation in a transplant experiment of the polymorphic land snail Cepaea nemoralis (Gastropoda: Pulmonata) at the edge of its geographical range

At the eastern margins of the geographical distribution in Europe, populations of Cepaea nemoralis are sparse and limited to urban environments to which they are possibly confined by relatively warmer climates. In 1999 we introduced 1101 C. nemoralis individuals originating from nine urban populations to a rural location in the area. The snails established a viable population, which suggests that confinement to urban settings is dispersal‐ rather than climate‐limited. The snails filled available habitats at a rate of approximately 400–600 m² year^?1. On the whole, morph frequencies remained remarkably stable; changes that occurred are attributable to segregation of alleles or chromosomes. However, snails responded to habitat heterogeneity: consistent and predictable divergence occurred between habitat types, such that light‐shelled snails were repeatedly more frequent in the open than in adjoining shaded habitats. This suggests the operation of climatic and/or visual selection. As the whole area encompassing seven distinct habitat patches was only 0.3 ha, and the maximum duration of population divergence was only 11 years (fewer than four snail generations), these results indicate extremely small temporal and spatial scales of adaptation during initial phases of population establishment and spread. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 462–470.     

Environmental Heterogeneity and Morphological Variability in Pisidium subtruncatum (Sphaeriidae,Bivalvia)

On the example of P. subtruncatum, a widely distributed bivalve, we studied the relationship between environmental and morphological variability. The main questions addressed were: (1) are there differences in shell morphology between different populations, (2) are they dependent on sediment composition, hydrology or water chemistry, (3) are there differences in morphological variability between stable and unstable environments. Measurements of specimen from nine sampling sites showed high variability in shell shape and size. Length‐height relation, symmetry, width and dry weight of the shell all varied significantly. Shell shape was significantly correlated with sediment composition and dependent on hydrological connectivity, with higher, asymmetric shells occurring in sandy sediments and lower, rounded shells in muddy sediments. Analysis of intrapopulation morphometric variation showed that variability is higher in habitats with high temporal and spatial environmental variability and lower in habitats with low or reduced environmental heterogeneity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of genotype and geography on shell shape and morphometric trait variation among North Atlantic blue mussel (Mytilus spp.) populations

The influence of geography and genotype on shell shape (outline) and trait (morphometric) variation among North Atlantic blue mussels and their hybrids has been examined. Shape differences among reference taxa (Mytilus trossulus, Mytilus edulis and Mytilus galloprovincialis) were consistent with an association between taxon‐specific genes and shape genes. Newfoundland M. edulis × M. trossulus populations and northern Quebec M. trossulus populations exhibited an uncoupling of taxon‐specific genes from shape genes, whereas Nova Scotia M. trossulus populations and SW England M. edulis × M. galloprovincialis populations exhibited an association between taxon‐specific genes and shape genes. We found no evidence of a geographic effect (NE versus NW Atlantic) for shape variation, indicating that the genotype effect is stronger than any geographic effect at macrogeographic scales. Pronounced differences were observed in trait variability consistent with an association between taxon‐specific genes and trait genes in European populations, and trait divergence of New York M. edulis from all European mussels. Trait variability in mussels from Newfoundland, Nova Scotia and northern Quebec indicated an uncoupling of taxon genes from trait genes, whereas trait variability in SW England M. edulis × M. galloprovincialis populations was consistent with background genotype, indicating a strong association between taxon genes and trait genes. A pronounced macrogeographic split (NE versus NW Atlantic) regardless of taxonomy was observed, indicating that geography exerts a greater influence than genotype on trait variation at the macrogeographic scale. This is consistent with pronounced within‐taxon genetic divergence, indicative of different selection regimes or more likely of different evolutionary histories of mussels on either side of the North Atlantic. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96 , 875–897.     

Evidence for ecological speciation in the sister species Candidula unifasciata (Poiret, 1801) and C. rugosiuscula (Michaud, 1831) (Helicellinae, Gastropoda)

This paper studies the speciation of two land snail species in south-east France. By using two mitochondrial and one nuclear gene, the species are shown to be sister species within the larger clade of western European Candidula species. The species never occurred syntopically, but a narrow contact zone population was identified. Recent range expansions inferred from phylogeographical methods indicated that the present day distributions are not limited by the colonization capacity of the species. Analysis of environmental variables suggested distribution along an ecotone. The gastropod community is also correlated to this gradient. Morphometric shell analysis showed that the divergence between the sister species is a result of desiccation-resistant shell characters. We therefore conclude that the cause for the ecological speciation is most probably the establishment of the Mediterranean climate ≈ 3.2 mya. A model of ecological speciation with a moving ecotone is put forward.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79, 611–628.     

Tracing geographical patterns of population differentiation in a widespread mangrove gastropod: genetic and geometric morphometrics surveys along the eastern African coast

In the present study, we assessed the inter‐ and intrapopulation genetic and morphological variation of Cerithidea decollata along the eastern coast of Africa. The population structure of C. decollata along the latitudinal gradient was examined by sequencing 420 bp of the mitochondrial cytochrome c oxidase I (COI) gene in 172 snails from 29 sites, in a combined analysis with geometric morphometrics in 1799 snails from 32 sites. Analysis of molecular variance and spatial analysis of molecular variance showed a moderate spatial population differentiation from Kenya to the Republic of South Africa, suggesting genetic divergence between the northern, central, and southern regions. This structure appears to be the result of life‐history traits combined with oceanographic features. Haplotype network and mismatch analysis suggest a recent population expansion during the Holsteinian interglacial period in the northern region and several colonization events in the central and southern regions. The morphometric approach suggests that morphological variation in shell shape is somewhat independent of the genetic divergence, revealing an overlap of shape across the latitudinal gradient but significant differences among‐population at a local level. This may indicate that similar ecological pressures are acting along the coast, leading to the occurrence of similar morphological characters. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107 , 647–663.     

Mitochondrial DNA phylogeny and morphological diversity in the genus Mastus (Beck, 1837): a study in a recent (Holocene) island group (Koufonisi, south-east Crete)

Three endemic Cretan land snail species of the genus Mastus (Beck, 1837) from the island group of Koufonisi (south-east Crete) and the eastern part of Crete, were studied by multivariate analysis of shell morphology and analysis of mtDNA sequences. The phylogeny of the populations studied and the processes effecting the genetic and morphological diversity of the species were investigated. Extremely high mtDNA sequence divergence was observed both within and between populations. The Cretan populations could not be distinguished morphologically, while the populations of the islets were more distinct. We argue that the active geological past of the area (including sea level changes) and the long-term presence of humans has produced a mixing up of Mastus populations leading to the accumulation of high divergence of mtDNA sequences on a small spatial scale. The limited morphological diversity and the distinct shell 'identity' of the islets' populations can be attributed to the selective pressures of the island group.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78, 383–399.     

Species limits in molecular phylogenies: a cautionary tale from Australian land snails (Camaenidae: Amplirhagada Iredale, 1933)

By complementing two independent systematic studies published recently on the Western Australian land snail Amplirhagada, we compare levels of morphological variation in shells and genitalia with those in the mitochondrial markers cytochrome c oxidase (COI) and 16S to evaluate the utility of mtDNA markers for delimiting species. We found that penial morphology and mitochondrial divergence are generally highly consistent in delimiting species, while shells have little overall taxonomic utility in these snails. In addition to this qualitative correspondence, there is almost no overlap between intraspecific and interspecific genetic distances in COI, with the highest intraspecific and lowest interspecific distance being 6%. This value is twice the general level suggested as a DNA barcode threshold by some authors and higher than the best average found in stylommatophoran land snails. Although in Amplirhagada land snails DNA barcoding may provide meaningful information as a first‐pass approach towards species delimitation, we argue that this is due only to specific evolutionary circumstances that facilitated a long‐termed separate evolution of mitochondrial lineages along spatial patterns. However, because in general the amounts of morphological and mitochondrial differentiation of species depend on their evolutionary history and age, the mode of speciation, distributional patterns and ecological adaptations, and absence or presence of mechanisms that prevent gene flow across species limits, the applicability of DNA barcoding has to be confirmed by morphological studies for each single group anew. Based on evidence from both molecular and morphological markers, we describe six new species from the Bonaparte Archipelago and revise the taxonomy of a further two. © 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 165 , 337–362.     

Selection on hybrids of ecologically divergent ecotypes of a marine snail: the relative importance of exogenous and endogenous barriers

Unravelling the form of selection acting on hybrids of ecotypes undergoing ecological speciation is essential to understand the mechanisms behind the evolution of reproductive isolation in the face of gene flow. Shell phenotype is known to be affected by natural selection and is involved in the fitness of the marine snail Littorina saxatilis. Here, we studied the association between shell traits and fitness in hybrids in order to determine the relative role of exogenous and endogenous selection in this hybrid zone of L. saxatilis. We show that directional selection is the predominant mode of selection among hybrids. We also show its heterogeneity, affecting different shell traits, within populations at the level of the microhabitat. Therefore, endogenous selection mechanisms are most probably lacking in this hybrid zone and exogenous barriers (pre‐ and post‐zygotic) are possibly one of the main forces behind the evolution of barriers to gene flow between these ecologically divergent ecotypes. This study shows how this barrier might represent an important type of reproductive isolation within ecological speciation, and this should be taken into account in future studies of speciation in hybrid zones. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 391–400.     

PHYLOGENETIC CONSTRAINTS IN KEY FUNCTIONAL TRAITS BEHIND SPECIES’ CLIMATE NICHES: PATTERNS OF DESICCATION AND COLD RESISTANCE ACROSS 95 DROSOPHILA SPECIES

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92–95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate‐to‐strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids’ high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate‐to‐high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change.     

Save your host,save yourself? Caste‐ratio adjustment in a parasite with division of labor and snail host survival following shell damage

Shell damage and parasitic infections are frequent in gastropods, influencing key snail host life‐history traits such as survival, growth, and reproduction. However, their interactions and potential effects on hosts and parasites have never been tested. Host–parasite interactions are particularly interesting in the context of the recently discovered division of labor in trematodes infecting marine snails. Some species have colonies consisting of two different castes present at varying ratios; reproductive members and nonreproductive soldiers specialized in defending the colony. We assessed snail host survival, growth, and shell regeneration in interaction with infections by two trematode species, Philophthalmus sp. and Maritrema novaezealandense, following damage to the shell in the New Zealand mud snail Zeacumantus subcarinatus. We concomitantly assessed caste‐ratio adjustment between nonreproductive soldiers and reproductive members in colonies of the trematode Philophthalmus sp. in response to interspecific competition and shell damage to its snail host. Shell damage, but not parasitic infection, significantly increased snail mortality, likely due to secondary infections by pathogens. However, trematode infection and shell damage did not negatively affect shell regeneration or growth in Z. subcarinatus; infected snails actually produced more new shell than their uninfected counterparts. Both interspecific competition and shell damage to the snail host induced caste‐ratio adjustment in Philophthalmus sp. colonies. The proportion of nonreproductive soldiers increased in response to interspecific competition and host shell damage, likely to defend the parasite colony and potentially the snail host against increasing threats. These results indicate that secondary infections by pathogens following shell damage to snails both significantly increased snail mortality and induced caste‐ratio adjustments in parasites. This is the first evidence that parasites with a division of labor may be able to produce nonreproductive soldiers according to environmental factors other than interspecific competition with other parasites.     

Molecular phylogenetics and morphological variation reveal recent speciation in freshwater mussels of the genera Arcidens and Arkansia (Bivalvia: Unionidae)

Intra‐ and interspecific morphological variation due to both phenotypic plasticity and evolutionary convergence hinder the work of taxonomists and lead to over‐ and underestimates of species richness. Nevertheless, most species on Earth are recognized solely based on morphological characters. We used molecular phylogenetic and morphometric techniques to examine two freshwater mussel species. One is common and widespread, while the other is imperiled and endemic to the Interior Highlands of the USA. Phylogenetic and molecular clock analyses revealed that divergence of Arcidens confragosus and Arkansia wheeleri is small and relatively recent. Divergence in these and other taxa is probably due to isolation of streams in the Interior Highlands. Morphometric analyses showed distinct shell shapes using traditional morphometrics, but not through geometric morphometrics. Outlined shell shapes are indistinguishable; geometric morphometrics could not capture a three‐dimensional component. Our analyses support the validity of these two species as congeners, with the nomen Arcidens (Simpson 1900) having priority. Because shell morphologies are both heritable and environmentally determined, our study emphasizes the importance of considering both molecular and morphometric analyses for identification of freshwater molluscs of conservation concern. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 535–545.     

Changes in the selection differential exerted on a marine snail during the ontogeny of a predatory shore crab

Empirical estimates of selection gradients caused by predators are common, yet no one has quantified how these estimates vary with predator ontogeny. We used logistic regression to investigate how selection on gastropod shell thickness changed with predator size. Only small and medium purple shore crabs (Hemigrapsus nudus) exerted a linear selection gradient for increased shell‐thickness within a single population of the intertidal snail (Littorina subrotundata). The shape of the fitness function for shell thickness was confirmed to be linear for small and medium crabs but was humped for large male crabs, suggesting no directional selection. A second experiment using two prey species to amplify shell thickness differences established that the selection differential on adult snails decreased linearly as crab size increased. We observed differences in size distribution and sex ratios among three natural shore crab populations that may cause spatial and temporal variation in predator‐mediated selection on local snail populations.