Novel colour polymorphisms in a hybrid zone of Mandarina (Gastropoda: Pulmonata)

A hybrid zone of the land snails Mandarina mandarina and Mandarina chichijimana in the oceanic Bonin Islands was studied morphologically and genetically to show the potential of hybridization as a source of morphological novelties. These species are clearly distinguishable on the basis of allozymes, colour polymorphisms, shell form and genital morphology, but exhibit a hybrid zone from the northeast to southwest of Chichijima island. There is a cline in the frequency of the colour patterns characteristic of each of the species, and specimens with intermediate colour pattern on the shells appear in the hybrid zone. Not only specimens with colour patterns that are overlapping of the patterns of the two species, but also specimens with unique colour patterns appear in the hybrid populations. These unique colour patterns are not found in the pure populations of both species or other Mandarina species in Chichijima Islands, and it is suggested that these are produced by the hybridization. Because of the appearance of many types of unique colour patterns, variability of the colour polymorphism in the hybrid populations are remarkably higher than that in the pure populations. This result suggests that the novel morphology is produced by the hybridization between species with distinctive morphology. This reveals the importance of hybridization as a source of morphological variation, diversity and evolutionary novelty.     

Larval and juvenile gastropods from a Carboniferous black shale: palaeoecology and implications for the evolution of the Gastropoda

A horizon in the late Visean Ruddle Shale from Arkansas contains the oldest well-preserved gastropod protoconchs known from the Americas. The gastropod fauna consists of a diverse larval shell assemblage and a low diversity assemblage of juvenile gastropods that probably had a benthic life habit. Gastropod larval shells are always isolated, i.e. the gastropods did not complete their life cycle (no metamorphosis) and were unable to become benthic. This was caused by unfavorable environmental conditions on the soft muddy bottom that was probably due to anaerobic to exaerobic conditions. The absence or scarcity of bioturbation caused by invertebrate detritus or sediment feeders in both shale and concretions (formed before compaction) favored preservation of the delicate larval shells. The lack or scarcity of infauna and bioturbation as well as the low diversity of the presumed benthos supports an interpretation of a quasi-anaerobic to exaerobic benthic environment. The superbly preserved larval shells demonstrate that there are more caenogastropod clades present in the late Palaeozoic than suggested previously. Some larval shell types have an openly coiled first whorl followed by a planktotrophic larval shell; openly coiled initial whorls are unknown from modern caenogastropods. The vetigastropods have a smooth protoconch of two whorls clearly demarked from the following whorls - a pattern unknown in modern vetigastropods which have a protoconch of less than one whorl and build no larval shell during their planktonic stage. This could indicate a link between Palaeozoic vetigastropods and the caenogastropods.     

Crayfish induce a defensive shell shape in a freshwater snail

Abstract. This study was undertaken to determine whether intraspecific variation in shell morphology of the freshwater snail Elimia (= Goniobasis ) livescens is caused by predator-induced morphological changes. Juvenile snails from 3 populations were grown in the presence of effluent from predatory crayfish feeding on conspecific snails or in effluent from conspecific snails only. Snails from one population, Clear Creek, exhibited a predator-induced morphology; they grew a narrow body whorl when exposed to the effluent from crayfish. Experimental feeding trials with crayfish and snails from Clear Creek were conducted to determine whether a narrow body whorl reduced predation in the presence of the crayfish Orconectes propinquus . In the feeding trials, snails with a narrow body whorl were eaten less frequently than snails with wider shells. However, there was no difference in overall size (length) between snails that were eaten and those that were not. Thus, juvenile snails from Clear Creek exposed to the presence of crayfish were induced to develop relatively narrow shells, which reduce the risk of successful attack by these crayfish. Hence the induced shell morphology is probably a defense against predation by crayfish.     

Usefulness of the opercular nucleus for inferring early development in neritimorph gastropods

The early ontogeny of gastropods (i.e., planktotrophic vs. nonplanktotrophic) may be inferable from the morphology of the protoconch in adult shells. The protoconch consists of both embryonic and larval shells in species with planktotrophic development; the embryonic shell forms in the intracapsular period and the succeeding larval shell gradually develops during the larval period. In nonplanktotrophic species, on the other hand, there is no additional growth of the larval shell and the protoconch consists exclusively of a relatively large embryonic shell formed prior to hatching. This "shell apex theory" has been applied to many species of shell-bearing gastropods, but biotic and abiotic erosion of the apex often prevents detailed examination of the protoconch and subsequent inferences about ontogeny. I examined the gastropod operculum to test its utility for predicting developmental mode, drawing on the Neritimorpha as model taxa. Most aquatic members of Neritimorpha were found to bear an operculum with a clearly demarcated nucleus; SEM observations reveal four types of nuclei, which correspond to different types of protoconch morphologies and observed ontogenies for the study species. The nucleus is secreted before metamorphosis, fits into the shell aperture of the larva, and reflects early ontogeny as morphology, as does the protoconch. Moreover, the apparently organic (rather than calcareous) composition of the nucleus makes it nearly invulnerable to erosion and very advantageous, compared to the protoconch, in this ecologically diverse group, whose habitats range from freshwater streams and mangrove swamps to rocky shores and deep-sea hydrothermal vents. The measurements of the nucleus are also valuable for taxonomic purposes, especially in the species identification of veliger larvae and juvenile snails. On the other hand, the opercular nuclei of the Caenogastropoda and Heterobranchia are often eroded away in adult individuals; even if present, the morphology of the nuclei does not seem to clearly reflect early ontogeny in those groups.     

Sexual dimorphism in shells of Cochlostoma septemspirale (Caenogastropoda, Cyclophoroidea, Diplommatinidae, Cochlostomatinae)

Sexual dimorphisms in shell-bearing snails expressed by characteristic traits of their respective shells would offer the possibility for a lot of studies about gender distribution in populations, species, etc. In this study, the seven main shell characters of the snail Cochlostoma septemspirale were measured in both sexes: (1) height and (2) width of the shell, (3) height and (4) width of the aperture, (5) width of the last whorl, (6) rib density on the last whorl, and (7) intensity of the reddish or brown pigments forming three bands over the shell. The variation of size and shape was explored with statistical methods adapted to principal components analysis (PCA) and linear discriminant analysis (LDA). In particular, we applied some multivariate morphometric tools for the analysis of ratios that have been developed only recently, that is, the PCA ratio spectrum, allometry ratio spectrum, and LDA ratio extractor. The overall separation of the two sexes was tested with LDA cross validation.The results show that there is a sexual dimorphism in the size and shape of shells. Females are more slender than males and are characterised by larger size, a slightly reduced aperture height but larger shell height and whorl width. Therefore they have a considerable larger shell volume (about one fifth) in the part above the aperture. Furthermore, the last whorl of females is slightly less strongly pigmented and mean rib density slightly higher. All characters overlap quite considerably between sexes. However, by using cross validation based on the 5 continuous shell characters more than 90% of the shells can be correctly assigned to each sex.     

Larval shells of Late Palaeozoic naticopsid gastropods (Neritopsoidea: Neritimorpha) with a discussion of the early neritimorph evolution

Extant neritimorphs with planktotrophic larval development have a convolute smooth larval shell which is internally resorbed. The oldest known larval shells of this type are of Triassic age. Well-preserved Late Palaeozoic neritimorph specimens have larval shells of two or more rapidly increasing well separated whorls. These larval shells resemble planktotrophic caenogastropod larval shells. This type of larval shell is possibly plesiomorphic in neritimorphs and caenogastropods. Permian/Pennsylvanian neritimorphs (Naticopsis, Trachyspird) have smooth larval shells (Naticopsidae) or larval shells with strong axial ribs (Trachyspiridae new family). The convolute low-spired round shell shape of modern neritimorphs is causally linked with the resorption of the inner teleoconch and protoconch whorls. Modern neritimorph shells with a uniform, undifferentiated inner lumen have probably evolved from naticopsid ancestors which lack resorption. It is possible that an elevated spire, deep sutures and protruding spiral larval shells would have made such internally undifferentiated shells more vulnerable for mechanical destruction and prédation. Suggestions that coiling evolved independently in neritimorphs and other Gastropoda are unlikely and contrast with the fossil record. The modern neritid larval shell has probably evolved from relatively low-spired smooth naticopsid larval shells like those reported here.     

Morphological Features of Supralittoral Mollusks of the Genus Cecina (Gastropoda: Pomatiopsidae) from Peter the Great Bay, Sea of Japan

Amphibious mollusks of the genus Cecina in the littoral and supralittoral of the Sea of Japan are represented by three species: Cecina manchurica A. Adams, 1861; C. tatarica (Schrenck, 1867); and C. scarlatoi Prozorova, 1996. Additional data on the morphology of the shell and radula are provided. Indices for the identification of adult decollated and corroded shells of these species which retained only 3.0–2.5 last whorls are proposed: (1) the ratio of the width of the ultimate whorl (without aperture) to the length of the ultimate and penultimate whorls and (2) the ratio of the width of the third whorl (from the bottom) to the width of the ultimate whorl (without aperture).     

Shell shape and habitat use in the North‐west Pacific land snail Mandarina polita from Hahajima,Ogasawara: current adaptation or ghost of species past?

The endemic land snail genus Mandarina of the Ogasawara Islands provides an excellent model system to investigate adaptive radiation. Previously, it has been shown that coexisting species of the islands segregate by microhabitat, so that they are either predominantly found on the ground in relatively wet and sheltered sites, dry and exposed sites, or else are arboreal. Moreover, shell morphology correlates with microhabitat, so that species in wet and sheltered sites tend to have high-spired shells with a high aperture, and those in dry and exposed sites tend to have relatively low-spired shells with a wide aperture. We have now found that on Hahajima, Mandarina polita have variable shell morphology, and there is a correlation between morphology and the depth of leaf litter, as well as the presence/absence of other terrestrial species. Specifically, when high-spired terrestrial Mandarina ponderosa is present, M. polita tend to be low-spired and have a large aperture, indicative of character displacement. When M. ponderosa is absent, the shell shape of M. polita is much more variable, the overall spire is higher, individuals are found in deeper litter, and there is a strong correlation between litter depth and spire height. We argue that these patterns are due to local adaptation, but it remains possible that they are an artefact due to the 'ghost of species past'.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 149–159.     

Ontogeny of shell morphology and shell strength of the marine snails Littorina obtusata and Littorina mariae: different defence strategies in a pair of sympatric, sibling species

The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi & Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata , however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime.     

How do gastropods grow synchronized shell sculpture? Effect of experimental varix manipulations on shell growth by Ceratostoma foliatum (Muricidae: Ocenebrinae)

Gastropod shells display striking patterns in both color and sculpture, but rather little is known about the developmental mechanisms that produce those patterns. Here, we tested a physical feedback hypothesis for how snails control spatial patterning of shell sculpture. Varices—a form of synchronized, blade‐like axial sculpture—are produced at regular intervals around the shell and often aligned closely between adjacent whorls. Older varices were believed to provide a spatial cue about where to position a new varix. To test this hypothesis, we manipulated physical cues by cutting off varices or attaching new ones to the body whorl of individuals of Ceratostoma foliatum, and then allowing snails to grow a new varix. We found that previous varices on the shell were neither necessary nor sufficient to induce a new varix at a particular location. However, the position of older varices did appear to affect the fine tuning of subsequent varix placement. The results of our experiments therefore suggest that varix synchrony arises mainly from some internal mechanism that yields a standardized amount of spiral growth per growth spurt. We also found that shell damage can induce varix production in unusual or aberrant locations during subsequent shell growth.     

Associations between shell morphology and land crab predation in the land snail Cerion

1. The land crab Gecarcinus lateralis is a significant predator of the abundant Bahamian land snails of the genus Cerion . The crabs typically 'scissor' the cylindrical shells in half or break the lip and peel back the shell to reach the animal which withdraws two or three whorls into the shell. Scars on shells of live adults at 73 sites in the Bahamas and Florida Keys show that about 8% (range: 0–44%) of the snails have survived attacks of this type.
2. An artificial crab claw was used to investigate the compressive force required to break Cerion shells of different morphotypes. Defining shell strength as the ability to withstand compressive forces, 10 morphotypes were found that exhibited mean relative strengths of between 30 and 300 newtons. Feeding trials with one adult crab showed that snails whose shells could withstand compressive forces of > 95 N were safe from this individual predator.
3. Multiple linear regression analyses showed that both shell size (length and width) and shell wall thickness were the ultimate determinants of shell strength. Ribs strengthen the shell by contributing to wall thickness and also by increasing overall shell width. The thickened adult shell lip and collabral ribs provide effective protection from attack by peeling.     

ADAPTATION FROM RESTRICTED GEOMETRIES: THE SHELL INCLINATION OF TERRESTRIAL GASTROPODS

The adaptations that occur for support and protection can be studied with regard to the optimal structure that balances these objectives with any imposed constraints. The shell inclination of terrestrial gastropods is an appropriate model to address this problem. In this study, we examined how gastropods improve shell angles to well‐balanced ones from geometrically constrained shapes. Our geometric analysis and physical analysis showed that constantly coiled shells are constrained from adopting a well‐balanced angle; the shell angle of such basic shells tends to increase as the spire index (shell height/width) increases, although the optimum angle for stability is 90° for flat shells and 0° for tall shells. Furthermore, we estimated the influences of the geometric rule and the functional demands on actual shells by measuring the shell angles of both resting and active snails. We found that terrestrial gastropods have shell angles that are suited for balance. The growth lines of the shells indicated that this adaptation depends on the deflection of the last whorl: the apertures of flat shells are deflected downward, whereas those of tall shells are deflected upward. Our observations of active snails demonstrated that the animals hold their shells at better balanced angles than inactive snails.     

A DEVELOPMENTAL CONSTRAINT IN CERION,WITH COMMENTS ON THE DEFINITION AND INTERPRETATION OF CONSTRAINT IN EVOLUTION

Since orthodox evolutionary theory is functionalist, constraints attain their most important positive meaning as channels of change imposed by historical and formal determinants, rather than by immediate natural selection. Since ontogeny is the usual locus of expression for these determinants, developmental constraint is an appropriate, general term. A particular developmental constraint in Cerion, most variable of West Indian land snails, stands out for two reasons: 1) it is simply and inexorably defined as a consequence both of formal principles (coiling of tube about axis) and of historical contingencies in Cerion's invariant allometry of growth; 2) it is pervasive in its influence, underlying major patterns of variation in every Cerion study I have ever undertaken. I refer to this pattern as the “jigsaw constraint.” When whorls are large and final size is limited, adult shells must grow fewer whorls. In Cerion, this obvious fact is promoted from trivial to important because complex allometries impose a substantial set of further consequences for form upon this basic trade-off of whorl size and whorl number. I show that this complex of consequences dominates patterns of natural variation in Cerion at all levels (among shells within samples, between samples in the geographic variation of single species, and between species in multitaxon faunas). It also sets patterns of hybridization between taxa. This paper is primarily a compendium of such examples. It is designed to illustrate the importance of this constraint by the fundamental criterion of relative frequency.     

Microstructural details in shells of the gastropod genera Carychiella and Carychium of the Middle Miocene

Microstructural details are revealed via scanning electron microscopy (SEM) in two carychiid species from the early Middle Miocene of Styria, SE Austria. The protoconchs of the shells of Carychiella eumicrum (Bourguignat 1857) and Carychium gibbum (Sandberger 1875) show different types of microstructure on the embryonic shell during ontogeny. Total, superficial punctate structure on the shell of Carychiella eumicrum contrasts with the protoconch–teleoconch demarcation (p/t boundary) observed on the protoconch of Carychium gibbum. Both species exhibit aragonitic microstructure. Diagenetic effects, prismatic, homogeneous and crossed lamellar microstructures are detectible in both species. Rheomorphic folding and dense pitting within the columella of Carychiella eumicrum suggest a structure–function relationship for tensile strength and bulk weight reduction in carychiid snails. We hypothesize that total superficial pitting on the shell of C. eumicum, seen here for the first time in the Carychiidae, suggests paedomorphosis as a life‐history strategy to palaeoecological conditions of the Rein Basin during the early Middle Miocene.     

Shell structure of two polar pelagic molluscs, Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica

The purpose of this study was to investigate shell growth performance in two thin-shelled pelagic gastropods from cold seawater habitats. The shells of Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica are very thin, approximately 2–9 μm for shells of 0.5–6 mm in diameter. Many axial ribbed growth lines were observed on the surface of the shell of both Limacina species. Distinct axial ribs were observed on the outermost whorl, while weak or no rib-like structures were observed on the inner whorls in the larger shell of L. helicina antarctica forma antarctica. For L. helicina, no ribs were observed on small individuals with three whorls, while larger individuals had distinct ribs on the outer whorls. Shell microstructure was examined in both species. There is an inner crossed-lamellar and extremely thin outer prismatic layer in small individuals of both species, and a distinct thick inner prismatic layer was observed beneath the crossed-lamellar layer in large Antarctic individuals. Various orientations of the crossed-lamellar structure were observed in one individual. Shell structure appeared to be different between the Antarctic and Arctic species and among shells of different size.     

Ecological and morphological patterns in communities of land snails of the genus Mandarina from the Bonin Islands

The land snail genus Mandarina has undergone extensive radiation within the Bonin Islands in the west Pacific. The preferred above-ground vegetation heights of sympatric species were clearly different. They separated into arboreal, semi-arboreal, exposed ground and sheltered ground ecotypes. Shells of species with different ecotypes differ markedly, but shells of species with the same ecotype are very similar to each other. Shell morphologies of some phylogenetically distantly related species with the same ecotype were indistinguishable. Character evolution estimated parsimoniously using a phylogenetic tree suggests that the speciation among sympatric species is accompanied by ecological and morphological diversification. In addition, species coexistence of Mandarina is related to niche differentiation. The above findings suggest that ecological interactions among species contribute to the ecological and morphological diversification and radiation of these land snails in this depauperate environment.     

Variation in foot size and shape in some British land snails and its functional significance

Live weight, and the length, breadth and surface area of the extended foot sole were measured in 24 species of British terrestrial snails. Allometric relationships between these variables are described, concentrating on the relationship between foot surface area and weight. Intraspecific foot area/weight relationships deviate from isometric expectation for several species; this may be attributed to density changes, foot-shape variation or the physical constraint imposed by shell aperture size.
The rate of increase of foot sole area with live weight among species is greater than expected under isometry, indicating that larger species partially compensate for the increase in foot loading (weight per unit foot area). The rate of whorl expansion of the shell is related to the deviations from the interspecific foot area/weight relationship, reinforcing the possibility that shell aperture area may limit foot size in some species. Foot ratio (foot length:foot width) is negatively related to live weight and foot loading.
The findings are discussed in relation to the known behaviour and microdistribution patterns of the species.     

Ecological speciation in an island snail: evidence for the parallel evolution of a novel ecotype and maintenance by ecologically dependent postzygotic isolation

Speciation is the process by which reproductive isolation evolves between populations. Two general models of speciation have been proposed: ecological speciation, where reproductive barriers evolve due to ecologically based divergent selection, and mutation‐order speciation, where populations fix different mutations as they adapt to similar selection pressures. I evaluate these alternative models and determine the progress of speciation in a diverse group of land snails, genus Rhagada, inhabiting Rosemary Island. A recently derived keeled‐flat morphotype occupies two isolated rocky hills, while globose‐shelled snails inhabit the surrounding plains. The study of one hill reveals that they are separated by a narrow hybrid zone. As predicted by ecological speciation theory, there are local and landscape level associations between shell shape and habitat, and the morphological transition coincides with a narrow ecotone between the two distinct environments. Microsatellite DNA revealed a cline of hybrid index scores much wider than the morphological cline, further supporting the ecological maintenance of the morphotypes. The hybrid zone does not run through an area of low population density, as is expected for mutation‐order hybrid zones, and there is a unimodal distribution of phenotypes at the centre, suggesting that there is little or no prezygotic isolation. Instead, these data suggest that the ecotypes are maintained by ecologically dependent postzygotic isolation (i.e. ecological selection against hybrids). Mitochondrial and Microsatellite DNA indicate that the keeled‐flat form evolved recently, and without major historical disruptions to gene flow. The data also suggest that the two keeled‐flat populations, inhabiting similar rocky hills, have evolved in parallel. These snails provide a complex example of ecological speciation in its early stages.     

不同壳色福寿螺形态性状与体质量的关系

【背景】福寿螺因其食性杂、抗逆性和繁殖力强以及自然天敌少等不断扩散,侵害农作物,被列为我国首批外来入侵物种。国内外学者一直致力于研究对其的防治与监控。自然界中福寿螺存在2种壳色——黄色和黑色,壳色受遗传因素和环境因素的双重影响。广东省福寿螺多以黑色为主,福寿螺倾向于与不同壳色的螺交配。壳色在一定程度上影响其交配的选择性,但2种壳色的福寿螺繁殖力指标差异不显著。而关于这2种壳色的螺在形态学上的差异鲜有报道。【方法】利用生物统计软件和分析方法进行相关性分析、通径分析及多元回归分析,计算相关系数、通径系数和决定系数,研究2种壳色福寿螺形态性状与体质量的关系。【结果】2种壳色福寿螺的体质量、层高的变异系数较大,且黄色比黑色变异系数大。对黄色福寿螺体质量影响较大的依次为壳高、口宽;对黑色福寿螺体质量影响较大的依次为口宽、层高。【结论与意义】2种壳色福寿螺在形态性状方面差异显著,可以将壳色作为特征标记,为福寿螺的监测与灾害评估提供参考。