Some aspects of the breeding biology of the equatorial land snail Limicolaria martensiana (Achatinidae: Pulmonata)

A study of the breeding season of the polymorphic equatorial land snail Limicolaria martensiana Smith (Achatinidae: Pulmonata) lasting six years and six months in populations in the Kampala District (lat. 0 20') and the Equator line shows that the snail breeds continuously throughout the year but has bimodal peaks in its breeding which occurs during the dry seasons, while during the rainy seasons breeding is low. It is the decrease in the amount of rainfall which induces peak breeding and as a result there is an inverse relationship between the amount of rainfall and the breeding intensity. The evolution of ovoviviparity with the ability to retain eggs and/or young in utero during aestivation is the phenomenon that enables L. martensiana to have peak breeding during the dry seasons. Moisture which enables young to feed and thus grow is an ultimate factor in determining breeding in L. martensiana . The gestation period is 10 to 14 days during the rainy seasons but the snail retains its eggs and/or young in utero during the dry seasons and only deposits them when rain returns. The eggs take 18.6±5.43 days ( N= 130) to hatch after they are laid. Hatching in utero and outside occurs by the young eating the egg shells. Oogenesis in the snails occurs at all times. L. martensiana stores viable allosperms which it can use for further reproduction.     

Ecological aspects of polymorphism in an African land snail, Limicolaria martensiana

In Uganda the herbivorous land snail, Limicolaria martensiana occurs in well-defined populations. Samples of snails from 49 populations reveal the existence of five distinct colour forms, the frequency of which varies markedly from population to population. There is a common streaked form that occurs in all populations and a series of pallid forms that may be present or absent. Late Pleistocene fossils from the Western Rift indicate that the polymorphism is at least 8000–10,000 years old. The polymorphism is not correlated with broad geographical components of the environment or with habitat. It is however correlated with population density, and in one population a change in density resulted in a change in the relative frequency of the colour forms. Human activities have probably contributed to the dispersal of the snails to new areas, and because of these the founder effect cannot be ruled out as contributing to the polymorphism.     

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 .     

Systematics and distribution of the land snail Pene (Pulmonata: Enidae) in Israel

Formerly considered a subgenus of Buliminus, Pene is here, for reasons given, considered as a separate genus. Four species ( syriacus, sidoniensis, galilaea and auriculata ) inhabit Israel. A new subspecies, syriacus ramimensis , is described from north-east Galilee.
Northwards, towards the borderline with s. syriacus , the ratio mouth height/mouth diameter of s. ramimensis becomes first gradually larger, then reverses abruptly. The northernmost s. ramimensis sample is in a peculiar intermediate position suggesting that it represents a syriacus-ramimensis hybrid population which has reached stabilization.
Distribution borders are usually correlated with rain or substratum. The sidoniensis/galilaea borderline is unusual in that it crosses an area of nearly uniform climate, substratum and vegetation. Along it sidoneinsis and galilaea are found within centimetres of each other, suggesting that the borderline is here determined by interspecific competition rather than by physical factors.     

Phylogeny of the three brown banded land snail genus Humboldtiana (Pulmonata: Humboldtianidae)

The three brown banded land snail genus Humboldtiana (Pulmonata: Humboldtianidae) comprises 50 species of insular distribution from southern Texas to central México; its low vagility and dispersal potential are reflected in high levels of endemism and highly restricted distributions. A current scheme of classification recognizes six subgenera: Polyomphala, Oreades, Gymnopallax, Clydonacme, Aglotrochus, and Humboldtiana (the latter containing three species groups). In the present work, mitochondrial 16S rDNA and partial ITS region and ribosomal large subunit (5.8S+ITS-2+28S) sequences from 26 recognized species and 2 populations of unknown identity were studied to assess phylogenetic relationships of Humboldtiana species using parsimony and Bayesian analysis, and AU test was used to compare the different phylogenetic hypotheses. The same four major clades were recovered by both methods (buffoniana, queretaroana, montezumae and fortis), although none of them support the current scheme of classification, with the only exception being the subgenus Gymnopallax included in the clade buffoniana. The results suggest that speciation patterns observed in the genus Humboldtiana could be explained as a morphostatic radiation.     

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.     

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.     

A comparative study of the ultrastructure and mineralogy of calcified land snail eggs (Pulmonata: Stylommatophora)

This study indicates that eggs containing calcium carbonate crystals occur in at least 36 of the 65 known families of the land snails (class Gastropoda: order Stylommatophora). Eggs from 22 of these families were available for examination. The x-ray diffraction data, available for the first time for 21 of these families, shows that these egg shells are all made of calcite only, or of a combination of calcite with smaller amounts of aragonite. All of the snail (body) shells examined were made of aragonite only. This is the first ultrastructural investigation of these egg shells, and it indicates that the eggs exhibit enough structural diversity to allow identification of parental animals to genus, and often to species level solely on the basis of egg shell ultrastructure. All of the calcified eggs may be divided into two groups: (1) partly calcified, with discrete crystals of CaCo₃ dispersed in the jelly layer, and (2) heavily calcified, with a hard, brittle egg shell made of fused crystals of CaCO₃ much like an avian egg. Both types of calcified eggs occur in oviparous as well as in ovoviviparous snails. Because of the wide distribution of calcified eggs in the Stylommatophora, and because of the occurrence of heavily calcified eggs in ancient families such as Partulidae, Endodontidae, and Zonitidae, the calcified egg is viewed as a primitive land snail trait associated with terrestrial adaptation. The function of the calcified egg shell, in addition to mechanical support of egg contents, is to supply the developing embryo with enough calcium to form the embryonic shell by the time of hatching.     

Polymorphic microsatellites in the African freshwater snail,Bulinus forskalii (Gastropoda,Pulmonata)

Eleven microsatellites were isolated in the freshwater snail Bulinus forskalii, intermediate host for the medically important trematode Schistosoma intercalatum. Characterization in 60 snails from three populations of B. forskalii from Cameroon revealed 4 to 18 alleles per locus. Low observed heterozygosity but higher expected heterozygosity, high F_IS estimates, significant departures from Hardy–Weinberg equilibrium and genotypic linkage disequilibria all indicate that B. forskalii is a preferential selfer. High F_ST estimates suggest that effective dispersal is limited and genetic drift is an important determinant of genetic structure. The potential utility of the microsatellite primers in other closely related Bulinus species was explored.     

Homing behaviour of the giant African snail,Achatina fulica (Ferussac) (Gastropoda; Pulmonata)

Homing ability ofAchatina fulica was studied in the field by mark-recapture experiments. Snails hid in resting sites in the day and crept out in the rain or at night. Young adults dispersed for longer distance than old adults. Old adults seldom changed resting sites, while young adults changed them left almost every day. It was only young adults that the study site or came in from other areas. In transfer experiments, old adults returned to the initial resting sites in 5 days. These results suggest that old adults have a homing ability, while young adults do not show it clearly. The homing ability ofA. fulica is age-dependent. These differences may be related to the maturation pattern of the reproductive organ.     

The effect of antibiotic treatment on the supercooling ability of the land snail Helix aspersa (Gastropoda: Pulmonata)

The land snail Helix aspersa is a partially freezing tolerant species whose supercooling ability is limited to ca. -3 to -5 degrees C. One hundred adult snails were subjected to the following two experimental conditions: (i) a starved group, provided with water; (ii) an antibiotic-treated group that was provided with a solution containing a mixture of two antibiotics. The antibiotic group exhibited a T(c) significantly lower than the starved group (-3.94 +/- 1.32 degrees C, n = 40 and -3.07 +/- 0.99, n = 30, t test, p < 0.005). This study showed that bacteria of the gut are likely to elevate animal supercooling points. It is also the first report in which a possible ice-nucleating activity of the gut microflora in a land snail has been suggested by the action of antibiotics on the T(c).     

The pedal muscle of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata): an ultrastructure approach

M. Cristina Faccioni-Heuser, Denise M. Zancan, Christiane Q. Lopes and Matilde Achaval. 1999. The pedal muscle of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata): an ultrastructure approach. — Acta Zoologica (Stockholm) 80: 325–337
The ultrastructure of the pedal muscle of the Megalobulimus oblongus is described. This muscle consists of transverse, longitudinal and oblique bundles ensheathed in collagenous tissue. Each muscle cell is also ensheathed by collagen. The smooth muscle cells contain thin and thick filaments; the thin filaments are attached to dense bodies. These cells contain a simple system of sarcoplasmic reticulum, subsarcolemmal caveolae and mitochondria with dense granules in the matrix, and glycogen. Three types of muscle cells were identified. Type A cells exhibited densely packed myofilaments, abundant glycogen rosettes, numerous mitochondria and sarcoplasmic reticulum profiles. Type B cells exhibited scanty glycogen and mitochondria, few cisternae of sarcoplasmic reticulum and large intermyofibrillar spaces. Type C cells exhibited intermediate characteristics between type A and type B cells. Neither nexus nor desmosomes were observed between the muscle cell membranes. The muscle contains well developed connective tissue and blood vessels. These structures and the distribution of muscle cells are probably involved in the muscular-hydrostat system. The muscle is richly innervated, having neuromuscular junctions with clear and electron-dense synaptic vesicles. The clear vesicles probably contain acetylcholine because the axons to which they are connected arise from acetylcholinesterase positive neurones of the pedal ganglion. The other vesicles may secrete monoamines such as serotonin and/or neuropeptides such as substance P.     

Correspondence analysis of shell morphology in the African freshwater snail Biomphalaria pfeifferi (Kraus 1848) (Pulmonata: Gastropoda)

A correspondence analysis of shell measurements taken from 521 widely dispersed specimens of the African aquatic pulmonate snail Biomphalaria pfeifferi suggests the existence of eight morphological groups. These groups appear to relate to either ecophysiological factors or to factors associated with the stability of the freshwater system rather than to aspects of geographic distribution and genetic isolation.     

Reproductive cycle of the Azorean endemic land snail Oxychilus (Drouetia) brincki Riedel, 1964 (Gastropoda: Pulmonata)

The life history of Oxychilus (Drouetia) brincki (Riedel, 1964), an endemic species from Santa Maria island (Azores), is addressed here for the first time. One population was analyzed for 1 year, to study the reproductive cycle and to assess the validity of three morphometric parameters as maturation diagnostic characters. The positive correlation between morphometric parameters (maximum diameter and number of whorls) can be useful to define the reproductive maturation status particularly during the active reproductive season. Oxychilus brincki is reproductively active between May and October/November; the residual values of mature oocytes between February and April allow reproduction to occur throughout the year. Our observations on gonadal maturation show that there is a functional protandric tendency and that photoperiod probably triggers the maturation process, whereas temperature acts mainly as a regulatory factor. Contrary to other terrestrial pulmonate snails that have a well demarcated reproductive cycle, Azorean species are gametogenically active throughout almost all the year.     

Dual strategy for immune defense in the land snail Cornu aspersum (Gastropoda, Pulmonata)

Immune defenses have been shown to be heavily involved in the evolution of physiological trade-offs. In this study, we compared the internal defense systems in two subspecies of the land snail Cornu aspersum that exhibit contrasting life-history strategies. The "fast-living" Cornu aspersum subsp. aspersa is widespread throughout the world, especially in ecosystems disturbed by man, whereas natural populations of the giant Cornu aspersum subsp. maxima, characterized by a longer life span, are present only in north Africa. Snails were experimentally challenged with Escherichia coli; the measurements used to assess their internal defense for cell- and humoral-mediated immune responses were bacterial clearance, hemocyte density, reactive oxygen species (ROS) production, and plasma antibacterial activity. Both subspecies showed a similar ability to clear bacteria from their hemolymph; however, they varied in the robustness of different individual immune components. Cornu aspersum aspersa had higher ROS activity than did C. a. maxima and lower plasma bactericidal activity. These results suggest that ecological factors can sculpt the immune response. One interpretation is that shorter life span selects for immune defenses such as ROS that, although effective, can cause long-term damage. Such different immune patterns obviously entail various costs involved in the strong intraspecific variation of life-history trade-offs we previously observed. We also have to consider that such variation might be related to intraspecific differences in the relative strength of resistance and tolerance mechanisms.     

Proximate factors influencing egg cannibalism in the land snail Arianta arbustorum (Pulmonata,Helicidae)

Summary Byttneria aculeata (Sterculiaceae), a subcanopy liane with a shrubby juvenile form has two distinct leaf color morphs in juvenile plants- a given juvenile has plain green leaves or leaves with whitish variegation. Both forms occur together in the forest and in clearings; however, the variegated morph is more common in open sites, and the plain morph predominates in the forest. Percent variegation per leaf for variegated plants increased from closed to open sites. Measurements of growth support the idea that variegation is favored in open habitats. Within a given habitat, rate of herbivory by leaf miners on a given morph increases with increasing relative frequency of that morph; however, at a given relative frequency, the variegated morph is less heavily attacked than is the plain morph when it occurs elsewhere at that same frequency.     

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.     

A question of time: the land snail Murella muralis (Gastropoda: Pulmonata) reveals constraints on past ecological speciation

The lively debate about speciation currently focuses on the relative importance of factors driving population differentiation. While many studies are increasingly producing results on the importance of selection, little is known about the interaction between drift and selection. Moreover, there is still little knowledge on the spatial‐temporal scales at which speciation occurs, that is, arrangement of habitat patches, abruptness of habitat transitions, climate and habitat changes interacting with selective forces. To investigate these questions, we quantified variation on a fine geographical scale analysing morphological (shell) and genetic data sets coupled with environmental data in the land snail Murella muralis, endemic to the Mediterranean island of Sicily. Analysis of a fragment of the mitochondrial DNA cytochrome oxidase I gene (COI) and eight nuclear microsatellite loci showed that genetic variation is highly structured at a very fine spatial scale by local palaeogeographical events and historical population dynamics. Molecular clock estimates, calibrated here specifically for Tyrrhenian land snails, provided a framework of palaeogeographical events responsible for the observed geographical variations and migration routes. Finally, we showed for the first time well‐documented lines of evidence of selection in the past, which explains divergence of land snail shell shapes. We suggest that time and palaeogeographical history acted as constraints in the progress along the ecological speciation continuum. Our study shows that testing for correlation among palaeogeography, morphology and genetic data on a fine geographical scale provides information fundamental for a detailed understanding of ecological speciation processes.