Shell thickness in the land snail Theba pisana (Pulmonata: Helicidae)

Shell thickness in Theba pisana was measured in samples from Britain, northern France and the Mediterranean region. Mean adult thickness varied between about 0.08 mm and 0.20 mm. These values are much lower than previously published values. The considerable variation within samples was related to shell size, periods of growth, and resorption of shell material during breeding. Variation between samples was not great. This contrasts with previous work which linked shell thickness variation to large-scale climatic effects. Perhaps availability of calcium may be more important than climate in determining shell thickness in Theba Pisana .     

Molecular phylogeny of the genus Helix (Pulmonata: Helicidae)

The vast diversity of land snail forms is insufficiently understood even in seemingly familiar taxa. This holds for Helix Linnaeus, 1758, a genus with several common edible species which comprises the largest Western Palearctic snails. The taxonomy of this genus, which has a centre of diversity in the eastern Mediterranean, has recently undergone significant changes, in both the delimitation of the genus itself and its species‐level systematics. Here, we compare the lineage diversity of Helix, as revealed by two mitochondrial markers, with the conclusions of the recently published morphology‐based taxonomic revision. For the molecular analysis, we assembled a representative data set covering almost all species of the genus as recognized by the mentioned revision. We obtained sequences not only from fresh and preserved soft tissues but also from dried tissue remains (some of them decades old) from shell collections. Our results show that the genus Helix, in the narrow sense proposed by recent studies, is paraphyletic because the genus Tacheopsis was unambiguously revealed as one of the tip branches of Helix. The monophyly of several species, as presently recognized, was not supported; partly, this may be attributed to a lineage diversity overlooked so far. This holds also for the type species of the genus, H. pomatia, which comprises at least one additional lineage. Greece, the Aegean and western Turkey is the core area for the diversity of Helix and its relatives, and the region is probably a major long‐term refuge for large Helicidae. The highest species diversity is found along the Alpide belt from the western Balkans to southern Turkey. The diversity of Helix in Europe, north of Greece and the Apennines, is a result of a single European radiation. Our data also suggest that past human activities likely influenced the present‐day distributions of some species.     

Microhabitat choice and high temperature tolerance in the land snail Theba pisana (Mollusca: Gastropoda)

Adults of the land snail Theba pisana near the species'northern limit (Tenby. South Wales) spend the daytime inactive attached to stems of tall vegetation, avoiding higher temperatures near the ground, while juveniles remain low down. This may be related to the lesser ability of juveniles to withstand desiccation for long periods, and/or to their requiremcnts for an easily accessible source of suitable food during growth. In the field and laboratory, adults (at least) respond to light and wind direction, avoiding the hottest and most exposed sides of stems on which they rest.
The upper lethal temperature of snails from S. Wales lies between 42 and 46 °C, depending on exposure time. Juveniles are more tolerant than adults, probably because of their greater ability to use evaporative cooling for short periods. Spanish T. pisana are more tolerant (upper lethal limit 46–50 °C) than Tenhy snails; this difference may have a genetic basis. Aestivating Spanish snails are more tolerant than active ones.
Climbing and orientation are essential for both adults and juvenilcs in the Mediterranean habitats where Theba pisana evolved, since ground temperatures can exceed the lethal limit. It is less important at the northern edge of the range, but is still retained by the adults.     

Regulation of infection by a nematode (Muellerius capillaris) in the snail Theba pisana

The regulation of infection by Muellerias capillaris was studied in three groups of Theba pisana. The regulation prior to penetration was very important for all the groups as assessed by the use of snail-conditioned water: a lytic activity was observed and the surviving larvae were less active than the control. The mortality rate of larvae after penetration was low in preinfected local group of snails, medium in the naive nonlocal group, and high in the naive local group.     

Relevance of body size and shell colouration for thermal absorption and heat loss in white garden snails,Theba pisana (Helicidae), from Northern France

The internal temperature of land snails depends on environmental factors, such as exposure to electromagnetic radiation and airflow as well as biotic factors including shell size, shell colouration and thickness or the resting position of the snail. In controlled field experiments, we quantified heating by thermal absorption of light and airflow-induced heat loss in the white garden snail, Theba pisana, from Normandy, France. Heating experiments revealed a significant positive relation of the internal body temperature with illumination period, shell temperature and air temperature at different times of day. The size of the snails was negatively related with both of the given illumination times: smaller animals heated up stronger than larger ones. The temperature at the surface of the shell significantly depended on the illumination period and the time of day. An AIC-based quality assessment of multiple linear modelling showed that, for explaining both shell surface and internal temperature of the soft body, several factors, i.e., exposure time, daytime, shell size and colouration contributed to the best models, respectively. Similarly, heat loss of the soft body after and during exposure of the snails to sunlight by a constant airflow depended on the initial body temperature, shell size, colouration and ambient air temperature. Our study revealed also the importance of both shell size and colouration for the loss of body temperature under natural conditions: small and banded animals that had heated up to temperatures above 30 °C cooled down faster than large and un-banded ones.     

Thirty-four years of climatic selection in the land snail Theba pisana

Few continuous, long-term studies have measured the intensity and variability of natural selection within a framework of clear adaptive hypotheses. In the snail Theba pisana, the proportion of effectively unbanded shells is higher in exposed habitats than in adjacent acacia thickets, which has been explained by microclimatic selection. Comparisons across an ecotone for 34 consecutive years determined the combined effects on morph frequencies of habitat and changes in weather conditions during summer. The long-term average (±s.e.) frequency of effectively unbanded shells was 0.577±0.011 in the open habitat when compared with 0.353±0.005 in the acacia. The persistent association of shell banding with habitat accounted for 34% of the variation in morph frequencies. Differences among years were also large, representing 23% of the variation. Higher proportions of effectively unbanded snails were associated with hotter, sunnier summers. Thus, temporal variation supports the hypothesis of microclimatic selection, consistent with the spatial association with habitat. Based on observed rates of change, the mean annual selection on this polymorphism was about 0.13, but with a large variance: s was as high as 0.5, but ⩽0.05 in about 40% of the years. The large variance and frequent reversals in direction of selection indicate a potential for rapid genetic change, but with little net change in morph frequencies over three decades, highlighting the value of long-term continuous studies of populations facing natural environmental variation.     

Distribution, hybridization and variation in the Israeli landsnail Levantina (Pulmonata: Helicidae)

The rock-dwelling landsnail Leuantinu is represented in Israel by two species, caesareana and hierosolyma . They are both found abundantly in Mediterranean, steppe, semi-arid and extremely arid habitats, and are probably ancient elements in the Israeli fauna. Their common borderline shows no association with altitude, rainfall, temperature, climatic zones, lithology, phytogeography, or the distribution of predators. The possiblity that one species is actively displacing the other, and that the position of the borderline is temporary, is rejected on grounds both of previous research and of fossil finds.
A narrow hybrid zone occurs in most places along the borderline. Laboratory experiments show that these hybrids are inferior to the parent species in viability and fecundity, and that assortative mating of the two parent species occurs in a mixed population. Hybridization may perhaps be maintained because selection favours the individual that will mate with its own species whenever it can tind one, but is capable of mating with other species when it cannot. In these conditions, hybridization is not a waste of reproductive energy.
Certain morphological changes occur in the vicinity of the hybrid zone. Far from the zone, caesareana resembles hierosolyma in shell-height, shell shape and genitalia; approaching the hybrid zone it diverges from hierosolyma in these characteristics and develops a distinct variant, caesareana var. werneri . This divergence may represent an accumulation of isolating mechanisms in the proximity of the hybrid zone, and thus be a case of reproductive character displacement. If indeed it is, then it differs from most described cases of character displacement in that the species are parapatric, not sympatric, and the displaced character is found not only at the common frontier but also from 2–12 km into the allopatric zone.     

Self-fertilization and cross-fertilization in the land snail Arianta arbustorum (Mollusca, Pulmonata: Helicidae)

The simultaneously hermaphroditic land snail Arianta arbustorum (L.) (Pulmonata: Helicidae) was long believed to be self-incompatible. In the present study, however, uniparental reproduction was observed under laboratory conditions. Snails from two natural populations and from laboratory culture reared in isolation from the subadult stage produced fertile eggs. Over a period of three years after maturation, 38.5% of the unmated snails produced fertile eggs, mostly in the second and third years instead of the first year. Egg production and hatching success (the proportion of eggs that hatched) were lower in unmated snails than in mated snails. The number of hatchlings produced by unmated snails was 1–2% of that produced by mated snails. Hatchlings from unmated snails survived as well as those from mated snails under laboratory conditions. The result indicate that A. arbustorum can self-fertilize but with a great fitness reduction.     

Phylogeography and morphological variability in land snails: the Sicilian Marmorana (Pulmonata, Helicidae)

Land snails have long been recognized as suitable organisms for studying phenotypic differentiation and phylogeny in relation to geographical distribution. Morphological data (shell and anatomy biometry on different geographical scales) and partial sequences from mitochondrial genes (cytochrome oxidase subunit I , 16S rDNA) were used to test whether morphological patterns match phylogeny in a diversified group of Sicilian rock-dwelling land snails belonging to the genus Marmorana . The taxonomic implications of the three character sets (shell and anatomical biometry and molecular data) were also considered. The inferred phylogenetic relationships do not match morphological (shell and genitalia) patterns. This result may significantly modify the current taxonomy. Mitochondrial based reconstructions define several supported clades well correlated with geographic distribution and populations were found to be distributed parapatrically. The progressive decline in mitochondrial DNA sequence similarity over a distance of 250 km is consistent with a model of isolation by distance, a pattern previously recognized for other groups of land snails. For one clade of Marmorana , colonization along Mediterranean trade routes appears to be a possibility.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 809–823.     

Mitochondrial phylogeny and biogeographic history of the Greek endemic land-snail genus Codringtonia Kobelt 1898 (Gastropoda, Pulmonata, Helicidae)

The aim of this work was to infer the phylogeny of the Greek endemic land-snail genus Codringtonia Kobelt 1898, estimate the time frame of the radiation of the genus, and propose a biogeographic scenario that could explain the contemporary distribution of Codringtonia lineages. The study took place in the districts of Peloponnese, Central Greece and Epirus of mainland Greece. Sequence data originating from three mtDNA genes (COI, COII, and 16S rDNA) were used to infer the phylogeny of the eight nominal Codringtonia species. Furthermore, the radiation time-frame of extant Codringtonia species was estimated using a relaxed molecular clock analysis and mtDNA substitution rates of land snails. The phylogenetic analysis supported the existence of six Codringtonia lineages in Greece and indicated that one nominal species (Codringtonia neocrassa) might belong to a separate genus distantly related to Codringtonia. The time frame of differentiation of Codringtonia species was placed in the Late Miocene-Pleistocene epoch. The dispersal-vicariance analysis performed indicated that most probably Codringtonia exhibited a north-to-south spread with the ancestral area being that of central Greek mainland, accompanied with duplication (speciation) and vicariance events.     

The Pleistocene glaciations and the evolutionary history of the polytypic snail species Arianta arbustorum (Gastropoda, Pulmonata, Helicidae)

The evolutionary history of the snail Arianta arbustorum is controversial. This diverse, polytypic species has two distinct forms: one, with a globular shell and closed umbilicus, is found from lowland to high altitudes; the other, with a depressed shell and open umbilicus, is found at a few scattered, high altitude localities. What is the origin of these two forms? Some believe that the depressed shell is a recent, local, ecotypic adaptation to alpine environments. Others believe that this form is a relic of an ancestral condition that may have survived the Pleistocene glaciations on nunatak-like montane refugia, while the globular shell is a derived condition and its presence at high altitudes follows post-Pleistocene recolonisation. We analysed a portion of the mitochondrial gene cytochrome oxidase I for 100 snails of the species A. arbustorum, three additional Arianta species, and nine outgroup taxa from five genera, in order to understand the phylogeographic history of the species. Despite some confounding artefacts that are likely due to introgression among the morphological forms, the resulting phylogeny shows that the depressed shell is plesiomorphic, while the globular shell is derived. Moreover, their disparate histories suggest that the depressed shell variety survived the glaciations in pockets of alpine refugia, while the globular shell variety recolonised the alpine environment post-glacially.     

Snails in the desert: Species diversification of Theba (Gastropoda: Helicidae) along the Atlantic coast of NW Africa

The spatial subdivision of species often plays a pivotal role in speciation. Across their entire range, species are rarely panmictic and crucial consequences of spatial subdivision are (1) random genetic drift including historical factors, (2) uniform selection, and (3) divergent selection. Each of these consequences may result in geographic variation and eventually reproductive isolation, but their relative importance in speciation is still unclear. In this study, we used a combination of genetic, morphological, and climatic data to obtain a comprehensive picture of differentiation among three closely related, parapatrically distributed taxa of the land snail genus Theba occurring along the Atlantic coasts of South Morocco and Western Sahara. We conducted Mantel and partial Mantel tests to relate phenotypic and genotypic variation of these species to geography and/or climate. As null hypothesis for an evolutionary scenario, we assumed nonadaptive speciation and expected a pattern of isolation by distance among taxa. Rejection of the null hypothesis would indicate isolation by environment due to adaptation. Generally, genetic drift plays an important role but is rarely considered as sole driver of speciation. It is the combination of drift and selection that predominantly drives speciation. This study, however, provides a potential example, in which nonadaptive speciation, that is, genetic drift, is apparently the main driver of shaping the diversity of Theba in NW Africa. Restriction of gene flow between populations caused by geographic isolation probably has played an important role. Climate oscillations during the Plio‐ and Pleistocene may have led to repeated ecological changes in NW Africa and disruptions of habitats promoting differentiation by geographic isolation. The inferred evolutionary scenario, however, did not fully explain the incongruence between the AFLP‐ and mtDNA‐tree topologies. This incongruence might indicate past hybridization among the studied Theba forms.     

Spermatocyte chromosome analysis of Helicella virgata (Pulmonata: Helicidae): silver-stained and C-banded chromosomes.

Chromosome numbers of the snail Helicella virgata from the fields of Castellammare del Golfo (Sicily) are n = 26 and 2n = 52. Silver-staining analyses of testicular cells suggest that both mitotic and meiotic chromosomes are involved in nucleolus organization. A within-individual variability in NOR-banding pattern is present in each of the 20 specimens analyzed.     

Yeasts and fungi in excrements of the sand hill snail,Theba pisana

Eight microbial species were isolated and identified from excrements of the sand hill snail,Theba pisana, viz. 3 filamentous fungi, 3 basidiomycetous yeasts and 2 yeast-like ascomycetous fungi.     

Shell polymorphism in a southern peripheral population of Cepaea nemoralis (L.) (Pulmonata: Helicidae) in Spain

Populations of the land snail Cepaea nemoralis have been investigated at the southern limits of its range in Spain, the Upper Tajo region, in order to analyse the distribution of the species and the variation of polymorphism in relation to environmental conditions. For this purpose two natural subareas, which differ in topography, soil nature, climate and vegetation have been considered. Cepaea nemoralis is confined to the river banks of the area; it is found in open and shaded biotopes of subarea 1 where it is relatively common, and it is almost restricted to shaded biotopes of subarea 2 where it becomes progressively rarer towards the south. Relative abundance of the species may be related to the amount of calcareous soils and climate, the restriction in its southern distribution being probably the result of extreme solar radiation, high temperature and low humidity. The two subareas differ in the frequency of the main polymorphic characters. These differences may be related to the kind of biotope. Subarea 1 richer in more open and exposed biotopes have higher frequencies of yellow, 00000, 00300, and unfused bands, whereas the restriction to poplar groves because of the lack of suitable exposed biotopes in subarea 2 may account for the higher proportion of pink five-bandeds with the bands fused together. This kind of variation with biotope is consistent with an explanation in terms of differences in the temperature absorbance of different morphs and a metabolic response to differences in temperature amplitude of the biotopes. Other characters (lip colour, punctate bands, hyalozonate bands, etc.) are not polymorphic in this region, thus, suggesting that at the periphery environmental conditions are important not only in determining the distribution of the species but exerting a marked effect on the genetic structure of the populations.     

Molecular phylogeny, taxonomy and evolution of the land snail genus Iberus (Pulmonata: Helicidae)

Partial DNA sequences of two mitochondrial genes [cytochrome oxidase subunit I (COI) and 16S rRNA] from 59 specimens of Iberus were used to test the validity of the described morphospecies of this genus, and examine genetic divergences within and between main phylogenetic groups. Both gene fragments showed phylogenetic concordance. The COI gene was found to be faster evolving than the 16S gene and was fully protein-coding with no insertions or deletions. 16S rRNA was more informative than COI for resolving basal nodes. Both individual and combined analyses of the two gene fragments revealed five main phylogroups. These five groups are genetically unique lineages that are allopatrically distributed and considered to have full species status. Further subdivisions were also considered. Shell morphology was suitable for delimiting species boundaries, but several incongruences between morphology and mtDNA phylogeny were observed. These incongruences were considered consequence of hybridization between Iberus cobosi and Iberus marmoratus , and the result of shell shape polymorphism in Iberus rositai . According to spatial patterns of sequence divergence, life habits and shell morphology may be concluded that the keeled-flat shelled snails independently originated several times within Iberus and they could represent cases of similar shell adaptation to a karstic arid environment.     

Morphological and genetic differentiation of Eremina desertorum (Gastropoda,Pulmonata, Helicidae) in Egypt

To understand the processes that result in morphological and genetic diversity, we studied the differentiation of the land snails Eremina d. desertorum and Eremina desertorum irregularis in the deserts of northern Egypt. These two taxa are differentiated with regard to shell size and shape and are separated by a narrow hybrid zone west of Alexandria. The lack of differences in the genitalia and the lack of reciprocal monophyly of the mitochondrial haplotypes of E. d. desertorum and E. desertorum irregularis support their classification as subspecies rather than distinct species. Low genetic distances indicate that the differentiation is probably less than half a million years old. The genetic data indicate a population expansion in agreement with other evidence that the Nile region in northern Egypt was more humid well into historical times than today. Shell size and shape are correlated with a climatic gradient from cooler and more humid conditions along the Mediterranean coast to arid and hot conditions in the interior. The decrease of body size with decreasing precipitation and increasing temperature might be explained by limited time for food intake in the more arid regions. The shape differences between the taxa are partly an indirect consequence of selection for body size, but are also directly affected by selection for reduction of aperture size.