High diversity and regional endemism in land snails of eastern Tanzania

In February/March 1995 we collected land snails (including slugs) at 12 stations in eastern Tanzania. A total of 571 person-hours yielded 9174 snails assigned to 159 morpho-species. The richest two sites each (<4ha of uniform forest) had 50 species (Amboni Cave) and 48 species (near Amani, Usambaras), nearly as great as the most species-rich sites known in the world; sieving of litter and soil would probably yield more species. In lowland (coastal) forests, both diversity and endemism seemed to decrease from north to south. Most snail species were found within only one of four coastal or one montane geographic regions, indicating substantial regional endemism. Only one species (Achatina fulica) appeared in all five regions, and 84% of all other species were found in only one (61%) or two regions (23%). The predatory streptaxids comprised about half the species and a third of the individuals at the Usambara site, an extremely high ratio of carnivores. Small snails (< 5mm greatest adult shell dimension) – many of which are probably undescribed species – comprise a substantial proportion of Tanzanian molluscan diversity; more surveys are needed, especially because of human pressures on the few forest patches remaining.     

Morphology and aestivation behaviour in some Madagascan acavid land snails

Nine species of Madagascan acavid land snails were compared in a phylogenetic context. The two most plesiomorphic, Clauator johnsoni and C. moreleti, differ from the others by their high-spired shells, short tentacles, short tails, long necks, and crawling mode of hitching the shell along the ground. In the seven more apomorphic species, the crawling mode is smooth, with the shell resting on the tail, and the relative lengths of tail and shell correlate significantly. Among these seven species, three pairs of closest relatives (Helicophanta petiti and H. uesicalis, H. farafanga and H. souuerbiana, Ampeltta decaryi and A. julii) show evidence of phylogenetic constraints on ranked shell size. Aestivation site (as tentatively inferred from rare data) does not correlate with shell shape or size: burrowers have H/D = 2.7 to 0.6 and D = 70 to 25 mm; arboreals have H/D = 0.8 to 0.5 and D = 70 to 30 mm; the species with both the highest spire and the smallest diameter (C. moreleti) is neither a burrower or an arboreal, but stays on the ground surface. Inferred aestivation sites are randomly distributed phylogenetically. Climate shows no correlation, except that the arborcals are only from humid to wet regimes. Uniform shell colouration occurs only in burrowers (C. johnsoni, H. petiti, H. uestcalis), but disruptive shell colouration occurs in all others, including burrowers (H. farafanga, A. decaryt), ground-surface aestivators (C. moreleti), arboreals (H. souuerbiana, A. julii), and semi-arboreals (Ampefita subfunebris). Among all nine species, burrowers have significantly thicker shells (than their close relatives of similar size), wider bodies, and longer snouts than non-burrowers (H. souuerbiana is exceptional in being arboreal despite its huge size and in having the broad foot and snout of a burrower). Thus, although there is some evidence for phylogenetic constraints, natural selection for aestivation and crawling behaviours seems to have dominated the evolution of external body morphology and of shell thickness (but not shell size and shape) in these snails.     

Polygyrid land snail phylogeny: external sperm exchange, early North American biogeography, iterative shell evolution

A fresh assessment of reproductive behaviour and anatomy, combined with new allozyme data, results in a phylogenetic hypothesis for the 23 polygyrid genera that differs substantially from previous attempts. Analyses based on this phylogeny suggest that (a) evolutionary transition from internal to external sperm exchange occurred only once in polygyrids, with unusual functional intermediates still extant; (b) polygyrid biogeographic history paralleled that of plethodontid salamanders, with hypothesized vicariance events at about 145, 120, 65 and 40 MA, and, dispersal events at about 55 and 40 MA; and (c) iterative shell evolution occurred despite a phylogenetic constraint on ontogenetic whorl-expansion rate.     

Functional aspects of shell geometry in some British land snails

Allometric relationships between the area of the shell mouth and live body weight are examined in 19 species of British land snails. Within species, the rate of increase of mouth area on weight is usually less than the isometric expectation, in spite of the logarithmic spiral pattern of growth in most species. It is suggested that this deviation is due mainly to changes in density with size. Two species which conform to isometric expectation alter the geometry of the shell as they pow.
Between species, the rate of increase in mouth area on weight in adults is much greater than isometric expectation, and the range of mouth areas at standard weight is considerable. These deviations are almost entirely accounted for by differences in shell geometry between large and small species, the former having higher rates of whorl expansion and smaller or non–existent umbilicuses.
The range of loading (weight per unit mouth area) on resting adult snails is thus much less than would be expected. It is suggested that large species are limited in the range of possible shell geometries by the need to minimize loading, while in small species other forces such as desiccation and predation may also be important: the range of geometries is generally larger. Observations on ecology and behaviour tend to support these conclusions.     

Polygyrid relations: a phylogenetic analysis of 17 subfamilies of land snails (Mollusca: Gastropoda: Stylommatophora)

The family of polygyrid land snails in North America is significant for its sympatric shell convergences, diversity of mating systems and complex zoogeography; its monophyly and its relation to other families has long been questionable. Cladistic analysis has been performed on one representative each of 17 subfamilies, including all three polygyrid subfamilies and one or more subfamilies each from all ten of the families that have been proposed as the polygyrid sister groups. Eighteen anatomical synapomorphies are used, of which eight are newly discovered, four are differently assessed from previous studies, and six are traditional. The resulting strict consensus tree of alternative maximum-parsimony cladograms is: (Acavidae (Ammonitellidae Corillidae ((Discidae Oreohelicidae) (Helminthoglyptidae Bradybaenidae Polygyridae (Thysanophoridae (Camaenidae Sagdidae)))))).
According to this working hypothesis, the Polygyridae are monophyletic, and their sister group remains unresolved, although the Acavidae, Ammonitellidae, Corillidae, Discidae and Oreohelicidae can be ruled out. Of the five classifications of stylommatophoran families that have been proposed in the past 12 years, the consensus tree is closest to that of Nordsieck. For future morphological work, three regions are recommended as potentially rich in unused phylogenetic information: the fertilization pouch-seminal receptacle complex, the ureter at the pneumostome and the ventral-chain ganglia. Simultaneous dissection, wkh side-by-side comparisons, is recommended over other methods for detecting homologies in land snails. Molecular characters should be exploited, because of the scarcity and the frequent homoplasy of morphological characters.     

Microgeographical variation in shell size of the land snail Chondrina clienta

Microgeographical variation in shell morphology of the rock-dwelling land snail Chondrina clienta , collected from 30 sites within an area of 0.5 km² on the island of Öland, Sweden, was examined in relation to its own population density and that of a potential competitor (the land snail Balea perversa ) and to environmental factors. Dispersal of marked individuals averaged 83 cm per year within a stone pile and 291 cm in an area of exposed bedrock. Local population density of active C. clienta ranged from 5 to 794 individuals per m². Shell characters were highly intercorrelated, both within and between populations. Principal component analysis revealed that most of the interpopulational variation could be expressed by the single character of shell height, which ranged from 5.54 to 6.94 mm. In all populations, snails of a given size had the same whorl number. Shell size was not influenced by habitat type (exposed rock surface, stone pile or stone wall) or proportion of calcareous stones within habitat. It was, however, negatively correlated with local population density, indicating intraspecific competition, and positively correlated with the degree of plant cover within the habitat. Analysis of variance revealed additionally a density effect of B. perversa on shell size in C. clienta , probably as a result of interspecific competition. Breeding experiments using C. clienta from different sites and carried out under unifrom conditions caused most of the phenotypic variation to disappear, demonstrating the high phenotypic plasticity of the species.     

A land snail's view of a fragmented landscape

Habitat fragmentation may influence the genetic structure of populations, especially of species with low mobility. So far, these effects have been mainly studied by surveying neutral markers, and much less by looking at ecologically relevant characters. Therefore, we aimed to explore eventual patterns of covariation between population structuring in neutral markers and variation in shell morphometrics in the forest‐associated snail Discus rotundatus in relation to habitat fragment characteristics. To this end, we screened shell morphometric variability and sequence variation in a fragment of the mitochondrial 16S rDNA gene in D. rotundatus from the fragmented landscape of the Lower Rhine Embayment, Germany. The 16S rDNA of D. rotundatus was highly variable, with a total of 118 haplotypes (384 individuals) forming four clades and one unresolved group. There was a geographic pattern in the distribution of the clades with the river Rhine apparently separating two groups. Yet, at the geographic scale considered, there was no obvious effect of fragmentation on shell morphometrics and 16S rDNA variation because G_ST often was as high within, as between forests. Instead, the age of the habitat and (re‐)afforestation events appeared to affect shell shape and 16S rDNA in terms of the number of clades per site. The ecologically relevant characters thus supported the presumably neutral mitochondrial DNA markers by indicating that populations of not strictly stenecious species may be (relatively) stable in fragments. However, afforestation after large clearcuts and habitat gain after the amendment of deforestation are accompanied by several, seemingly persistent peculiarities, such as altered genetic composition and shell characters (e.g. aperture size). © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 839–850.     

Herbivory on calcicolous lichens: different food preferences and growth rates in two co-existing land snails

A total of 32 calcicolous lichen species, one alga and one bryophyte were recorded on a limestone wall in the grassland Great Alvar on the Baltic island of Öland, Sweden. Fourteen (41%) of these 34 species and free-living cyanobacteria showed herbivore damage, most probably due to grazing by the land snails Chondrina clienta and Balea perversa which inhabited the limestone wall. Three laboratory experiments were conducted to examine the food preferences of individuals of C. clienta and B. perversa collected at this site and to evaluate any association between their preference and the net food quality of the lichens to the snails. Chondrina clienta and B. perversa exhibited food preferences, which differed significantly between species. Within species, variation in food choice was similar among individuals. This indicates that snail populations may be composed of polyphagous individuals with similar food preferences. Different lichen species were of different net food quality to the snails as indicated by growth rate differences. In both snail species the most preferred lichen species of the choice experiment caused the largest weight increase in juveniles, viz. Caloplaca flavovirescens for C. clienta and Aspicilia calcarea for B. perversa. This suggest that the snail species studied differ in their abilities to deal with secondary compounds and physical characteristics of certain lichens or that they can utilize the energy and nutrients of these lichens to a different extent. It is suggested that differential food preferences might reduce the intensity of interspecific competition for resources (lichens) between C. clienta and B. perversa.     

Annotated type catalogue of land snails collected from Taiwan (Formosa) in the Natural History Museum,London

The present catalogue lists the type specimens of land snail species, collected from Taiwan and deposited in the Natural History Museum, London. Thirty-seven nominal species described by Pfeiffer, Adams, Nevill, Moellendorff, Godwin-Austen and Gude were traced. I present here information on type status, collection data obtained from the registers and labels of each collection, and annotations on the current taxonomic affiliation. Lectotypes of 28 nominal (sub)species were newly designated. One holotype was fixed originally and two holotypes newly fixed by monotypy. Syntypes of two species and paralectotypes of three species were also discovered in the Museum. No specimen of the species Pupina adamsi Sowerby, 1878, which was supposed to be deposited in the NHM, was found. Pictures of the name-bearing types are provided for further research on biodiversity of the island.     

Trophic divergence despite morphological convergence in a continental radiation of snakes

Ecological and phenotypic convergence is a potential outcome of adaptive radiation in response to ecological opportunity. However, a number of factors may limit convergence during evolutionary radiations, including interregional differences in biogeographic history and clade-specific constraints on form and function. Here, we demonstrate that a single clade of terrestrial snakes from Australia—the oxyuranine elapids—exhibits widespread morphological convergence with a phylogenetically diverse and distantly related assemblage of snakes from North America. Australian elapids have evolved nearly the full spectrum of phenotypic modalities that occurs among North American snakes. Much of the convergence appears to involve the recurrent evolution of stereotyped morphologies associated with foraging mode, locomotion and habitat use. By contrast, analysis of snake diets indicates striking divergence in feeding ecology between these faunas, partially reflecting regional differences in ecological allometry between Australia and North America. Widespread phenotypic convergence with the North American snake fauna coupled with divergence in feeding ecology are clear examples of how independent continental radiations may converge along some ecological axes yet differ profoundly along others.     

Parallelism and convergence in anuran fangs

The anuran lower jaw is composed of three pairs of bones: dentaries, angulosplenials and mentomeckelians. Although the lower jaw is toothless, except in Gastrotheca guentheri , enlarged fangs or odontoids have evolved at least four times independently in some myobatrachids, hylids, ranids and leptodactylids through both parallel and convergent evolutionary events. Fangs seem to represent the single best design solution to enable an anuran to inflict a bite-like wound, but the biological role of biting varies among species. Fangs are projections of the dentaries in ranids, but in the hylid frog Hemiphractus and in ceratophryine leptodactylids, they form a sinosteotic unit with the dentaries and mentomeckelians. Comparisons of morphology, behaviour and diet among frog taxa with enlarged fangs reveal that the fangs may be the result of either sexual or natural selection. Those fangs that evolved in response to sexual selection seem to be relatively larger than those that resulted of natural selection.     

Molecular evolutionary relationships between partulid land snails of the Pacific

Adaptive radiation of partulid land snails in the tropical Pacific has produced an extraordinary array of distinctive morphological, ecological and behavioural types. Here we use part of the nuclear ribosomal RNA gene cluster to investigate the relationships within and between the three partulid genera, Partula, Samoana and Eua. The genera cluster separately, with Samoana and Partula forming monophyletic groups. With one exception, the molecular data generally support the previous generic classification based on genital morphology, even in species that show a number of characteristics otherwise atypical of the genus. Convergent evolution explains morphological similarities between members of different genera. The phylogeny suggests that Samoana has colonized the Pacific from west to east, originating in the area where Eua, believed to be the most ancient partulid genus, is found. An unexplained anomaly is the reported occurrence of a single species of Samoana in the Mariana Islands of the western Pacific. The genus Partula has a disjunct distribution, encompassing islands both to the east and west of the range occupied by Eua. Partula seems to have spread both eastward and westward after the splitting of the Partula lineage.     

Dragonfly predators influence biomass and density of pond snails

Studies in lakes show that fish and crayfish predators play an important role in determining the abundance of freshwater snails. In contrast, there are few studies of snails and their predators in shallow ponds and marshes. Ponds often lack fish and crayfish but have abundant insect populations. Here we present the results of field surveys, laboratory foraging trials, and an outdoor mesocosm experiment, testing the hypothesis that insects are important predators of pulmonate snails. In laboratory foraging trials, conducted with ten species of insects, most insect taxa consumed snails, and larval dragonflies were especially effective predators. The field surveys showed that dragonflies constitute the majority of the insect biomass in fishless ponds. More focused foraging trials evaluated the ability of the dragonflies Anax junius and Pantala hymenaea to prey upon different sizes and species of pulmonate snails (Helisoma trivolvis, Physa acuta, and Stagnicola elodes). Anax junius consumed all three species up to the maximum size tested. Pantala hymenaea consumed snails with a shell height of 3 mm and smaller, but did not kill larger snails. P. acuta were more vulnerable to predators than were H. trivolvis or S. elodes. In the mesocosm experiment, conducted with predator treatments of A. junius, P. hymenaea, and the hemipteran Belostoma flumineum, insect predators had a pronounced negative effect on snail biomass and density. A. junius and B. flumineum reduced biomass and density to a similar degree, and both reduced biomass more than did P. hymenaea. Predators did not have a strong effect on species composition. A model suggested that A. junius and P. hymenaea have the largest effects on snail biomass in the field. Given that both pulmonate snails and dragonfly nymphs are widespread and abundant in marshes and ponds, snail assemblages in these water bodies are likely regulated in large part by odonate predation.     

Freshwater snails of Oman,South Eastern Arabia

A systematic account is given of the extant freshwater snail fauna of Oman, based on recent collections made in Dhofar and in the northern mountainous areas. Also included are certain species found in brackish coastal localities. A total of 8 freshwater species is regarded as belonging to the fauna of normal freshwater; 7 have been found alive (Thiara scabra, Melanoides tuberculata, Lymnaea natalensis, Gyraulus piscinarum, Biomphalaria arabica, Bulinus wrighti and Indoplanorbis exustus) and one is known only as shells (Paludestrina glaucovirens). This report of G. piscinarum is the first for Oman and may be the first reliable identification for Arabia. Three taxa from brackish water, Gangetia miliacea, Iravadia quadrasi and Melanoides sp., are new for the fauna of Oman and of the whole Arabian peninsula. The finding of G. miliacea provides the first instance of any member of the Stenothyridae living west of the Gulf of Oman and identified from whole snails rather than empty shells. I. quadrasi is the first member of its genus to be found alive west of the Gulf of Oman.     

Cold-hardiness in several species of land snails

1. 1.|Mean supercooling points (SCP) of Anguispira alternata (Say) gradually declined in autumn and increased in spring. SCP also declined in autumn in Discus conrkhitei (Newcomb) and Gastrocopta armifera (Say).

2. 2.|Supercooling ability varied among the species, being the greatest in G. armifera.

3. 3.|Prolonged exposure of winter animals to high temperatures resulted in only a modest increase in SCP.

4. 4.|Low temperature survival of D. cronkhitei and G. armifera was slightly poorer than predicted by the mean SCP.

Ecological and morphological diversification within single species and character displacement in Mandarina,endemic land snails of the Bonin Islands

The endemic land snail species Mandarina hahajimana has undergone extensive habitat and morphological diversification within the Hahajima islands in the Bonin archipelago. This species has diversified into populations with ground, arboreal and semi-arboreal life histories. In addition, arboreal populations and semi-arboreal populations show diversification in preferences of species and positions of the tree on which they are found. Shell morphologies of M. hahajimana exhibit remarkable geographical variation, and they have a clear relationship with their life histories. The morphological variation of M. hahajimana results from adaptation to different lifestyles. The habitats of these populations influence the relationships with other species of Mandarina coexisting with M. hahajimana. This suggests that the morphological and ecological divergence within M. hahajimana has been induced by competitive interaction with other species of Mandarina. Character displacement may have played an important role in promoting adaptive radiation of Mandarina in the Bonin Islands.     

The probability of genetic parallelism and convergence in natural populations

Genomic and genetic methods allow investigation of how frequently the same genes are used by different populations during adaptive evolution, yielding insights into the predictability of evolution at the genetic level. We estimated the probability of gene reuse in parallel and convergent phenotypic evolution in nature using data from published studies. The estimates are surprisingly high, with mean probabilities of 0.32 for genetic mapping studies and 0.55 for candidate gene studies. The probability declines with increasing age of the common ancestor of compared taxa, from about 0.8 for young nodes to 0.1–0.4 for the oldest nodes in our study. Probability of gene reuse is higher when populations begin from the same ancestor (genetic parallelism) than when they begin from divergent ancestors (genetic convergence). Our estimates are broadly consistent with genomic estimates of gene reuse during repeated adaptation to similar environments, but most genomic studies lack data on phenotypic traits affected. Frequent reuse of the same genes during repeated phenotypic evolution suggests that strong biases and constraints affect adaptive evolution, resulting in changes at a relatively small subset of available genes. Declines in the probability of gene reuse with increasing age suggest that these biases diverge with time.     

The age of Alu subfamilies

Using Kimura's distance measure we have calculated the average age of all major Alu subfamilies based on the most recent available data. We conclude that AluJ sequences are some 26 Myr older than previously thought. Furthermore, the origin of the FLA (Free Left Arm) Alu family can be traced back to the very beginning of the mammalian radiation.One new minor subfamily is reported and discussed in the context of sequence diversity in major Alu subfamilies. Correspondence to: J. Jurka     

Unraveling the thread of nature's tapestry: the genetics of diversity and convergence in animal pigmentation

Animals display incredibly diverse color patterns yet little is known about the underlying genetic basis of these phenotypes. However, emerging results are reshaping our view of how the process of phenotypic evolution occurs. Here, we outline recent research from three particularly active areas of investigation: melanin pigmentation in Drosophila, wing patterning in butterflies, and pigment variation in lizards. For each system, we highlight (i) the function and evolution of color variation, (ii) various approaches that have been used to explore the genetic basis of pigment variation, and (iii) conclusions regarding the genetic basis of convergent evolution which have emerged from comparative analyses. Results from these studies indicate that natural variation in pigmentation is a particularly powerful tool to examine the molecular basis of evolution, especially with regard to convergent or parallel evolution. Comparison of these systems also reveals that the molecular basis of convergent evolution is heterogeneous, sometimes involving conserved mechanisms and sometimes not. In the near future, additional work in other emerging systems will substantially expand the scope of available comparisons.