scispace - formally typeset
Search or ask a question
Journal ArticleDOI

Variation in Land-snail Shell form and Size and its Causes: a Review

01 Jun 1986-Systematic Biology (Oxford University Press)-Vol. 35, Iss: 2, pp 204-223
TL;DR: Variation in land-snail shell form has been extensively documented, but its causes are poorly understood and identification of nonadaptive variation which results from developmental dependence on another character is dependent on the study of the selective and direct-environmental causes of variation.
Abstract: Variation in land-snail shell form has been extensively documented, but its causes are poorly understood. For no character are there general rules relating shell form to environ- mental characteristics, although certain correlations are common. Size variation generally has a large genetic component. Larger snails are often associated with moister conditions; the effect may be inductive (direct) or selective, but the mechanism is not documented. Snails may attain smaller adult sizes at higher population densities, apparently through the effects of pheromones on growth rate. Relative aperture area tends to be smaller under drier conditions, probably because of selection for smaller whorl cross-sectional area to reduce water loss. Larger snails tend to have higher whorl expansion rates. This pattern is variously interpreted as relating to the maintenance of constant attachment area/weight, whether of foot surface area when the snail is active or when attached to a substrate or of aperture perimeter when attached. Apertural denticles are generally thought to represent adaptations to reduce predation. Relative shell height of snail species relates to the angle of the substrate on which activity occurs; this could be related to the mechanics of shell balance. For unknown reasons, helicid species in the Med- iterranean area frequently have forms with keeled and with rounded shell peripheries. Snails living on calcareous substrates sometimes have thicker shells; the effect is not necessarily direct. Surprisingly, only a weak relationship exists between shell thickness and moisture conditions. Shell coiling sometimes occurs in the opposite direction between sympatric species, probably as a result of selection for reproductive isolation. A recurring problem in the explanation of shell form is the interpretation of covarying shell characters. Identification of nonadaptive variation which results from developmental dependence on another character is dependent on the study of the selective and direct-environmental causes of variation in land snail shell form. (Snail; gastropod; shell; form; shape; size; denticles; variation.)
Citations
More filters
Journal ArticleDOI
TL;DR: A physics-based model is presented that describes the appearances of scenes in uniform bad weather conditions and a fast algorithm to restore scene contrast, which is effective under a wide range of weather conditions including haze, mist, fog, and conditions arising due to other aerosols.
Abstract: Images of outdoor scenes captured in bad weather suffer from poor contrast. Under bad weather conditions, the light reaching a camera is severely scattered by the atmosphere. The resulting decay in contrast varies across the scene and is exponential in the depths of scene points. Therefore, traditional space invariant image processing techniques are not sufficient to remove weather effects from images. We present a physics-based model that describes the appearances of scenes in uniform bad weather conditions. Changes in intensities of scene points under different weather conditions provide simple constraints to detect depth discontinuities in the scene and also to compute scene structure. Then, a fast algorithm to restore scene contrast is presented. In contrast to previous techniques, our weather removal algorithm does not require any a priori scene structure, distributions of scene reflectances, or detailed knowledge about the particular weather condition. All the methods described in this paper are effective under a wide range of weather conditions including haze, mist, fog, and conditions arising due to other aerosols. Further, our methods can be applied to gray scale, RGB color, multispectral and even IR images. We also extend our techniques to restore contrast of scenes with moving objects, captured using a video camera.

1,393 citations


Cites background from "Variation in Land-snail Shell form ..."

  • ...This was done because high humidity and whether rain has fallen recently affects many aspects of land snail behaviour, including feeding, and erratic growth can result if suitable conditions occur only intermittently (Solem & Christensen 1984; Baba 1985; Goodfriend 1986)....

    [...]

Journal ArticleDOI
TL;DR: Although proximate factors like temperature explain most of the variation in life history, evidence was obtained also for genetic divergence among populations and the results match the predictions of the adversity selection model well.
Abstract: SUMMARY (1) The life history of the polymorphic land snail Arianta arbustorum was investigated in five natural populations along an altitudinal gradient from 1220 to 2600 m in the Swiss Alps. In order to assess the environmentally-induced variation of reproductive characters snails were transplanted to the lowlands. (2) Adult size decreased from 19-9 to 16-0 mm (mean shell breadth) with increasing altitude, age at maturity increased from 1-9 to 5-0 years, but median adult longevity and adult survival rates were approximately the same at all altitudes. (3) Clutch number, clutch size, egg size and reproductive investment per year decreased with increasing altitude in resident snails, whereas reproductive investment per egg increased. Within populations, clutch number, number of eggs produced per year and egg size scaled allometrically with shell size indicating a size-specific fecundity. (4) Transplanted snails laid 1-3-11-5 times more clutches than did resident snails. However, no differences in clutch size, egg dry weight and hatching success were found between transplanted and resident snails, except in one population, in which transplanted snails laid smaller clutches with larger eggs and scored a lower hatching success. (5) Transplanted snails were less variable in reproductive characters than resident ones, except for hatching success. (6) In general, life-history characters did not appear to be correlated with shell or body colour. (7) Although proximate factors like temperature explain most of the variation in life history, evidence was obtained also for genetic divergence among populations. The results match the predictions of the adversity selection model well, rather than those of bet

156 citations

Journal ArticleDOI
TL;DR: The use of land snail shells for paleoenvironmental reconstruction has been studied extensively as mentioned in this paper. But, there are a number of problems involved in using this approach and these are discussed.

124 citations

Journal ArticleDOI
TL;DR: Recent progress in understanding snail chirality is reviewed, suggesting that chiral reversal must sometimes occur, but it is rarely likely to lead to so-called ‘single-gene’ speciation and is an important target for future research.
Abstract: The direction that a snail (Mollusca: Gastropoda) coils, whether dextral (right-handed) or sinistral (left-handed), originates in early development but is most easily observed in the shell form of the adult. Here, we review recent progress in understanding snail chirality from genetic, developmental and ecological perspectives. In the few species that have been characterized, chirality is determined by a single genetic locus with delayed inheritance, which means that the genotype is expressed in the mother's offspring. Although research lags behind the studies of asymmetry in the mouse and nematode, attempts to isolate the loci involved in snail chirality have begun, with the final aim of understanding how the axis of left–right asymmetry is established. In nature, most snail taxa (>90%) are dextral, but sinistrality is known from mutant individuals, populations within dextral species, entirely sinistral species, genera and even families. Ordinarily, it is expected that strong frequency-dependent selection should act against the establishment of new chiral types because the chiral minority have difficulty finding a suitable mating partner (their genitalia are on the ‘wrong’ side). Mixed populations should therefore not persist. Intriguingly, however, a very few land snail species, notably the subgenus Amphidromus sensu stricto, not only appear to mate randomly between different chiral types, but also have a stable, within-population chiral dimorphism, which suggests the involvement of a balancing factor. At the other end of the spectrum, in many species, different chiral types are unable to mate and so could be reproductively isolated from one another. However, while empirical data, models and simulations have indicated that chiral reversal must sometimes occur, it is rarely likely to lead to so-called ‘single-gene’ speciation. Nevertheless, chiral reversal could still be a contributing factor to speciation (or to divergence after speciation) when reproductive character displacement is involved. Understanding the establishment of chirality, the preponderance of dextral species and the rare instances of stable dimorphism is an important target for future research. Since the genetics of chirality have been studied in only a few pulmonate species, we also urge that more taxa, especially those from the sea, should be investigated.

116 citations

Journal ArticleDOI
TL;DR: Counter to predictions, water loss did not relate to shell biometric measures but was negatively correlated with adeno- sine triphosphate (ATP) concentrations, confirming the idea that negative effects of exposure to elevated pCO2/low pH and elevated temperature on shell morphology may occur through metabolic disruption.
Abstract: Phenotypic plasticity is a mechanism by which organisms can alter their morphology, life history or behaviour in response to environmental change. Here, we investigate shell plasticity in the intertidal gastropod Littorina littorea in response to the ocean acidification and elevated temperature values predicted for 2100, focusing on shell traits known to relate to protection from predators (size, shape and thickness) and resistance to desiccation (aperture shape). We also measured and desiccation rates (measured as percentage water loss). Ocean acidification was simulated by bubbling carbon dioxide into closed-circuit tanks at concentrations of 380 and 1000 ppm, giving respective pH levels of 8.0 and 7.7; temperatures were set at 15 or 20°C. Both low pH and elevated temperature disrupted the overall investment in shell material; snails in acid- ified seawater and elevated temperature in isolation or in combination had lower shell growth rates than control individuals. The percentage increase in shell length was also lower for individ- uals kept under combined acidified seawater and elevated temperature, and the percentage of shell thickness increase at the growing edge was lower under acidified and combined conditions. Shells were also more globular (i.e. had lower aspect ratios) under elevated temperature and lower pH. Desiccation rates were lower at low pH and high temperature. Counter to predictions, water loss did not relate to shell biometric measures but was negatively correlated with adeno- sine triphosphate (ATP) concentrations. Finally, ATP concentration was positively correlated with shell thickening and weight, confirming the idea that negative effects of exposure to elevated pCO2/low pH and elevated temperature on shell morphology may occur (at least in part) through metabolic disruption.

110 citations


Cites background from "Variation in Land-snail Shell form ..."

  • ...For example, the marine gastropod Thais lapillus increases operculum size (Gibson 1970), the land snail Cepaea spp. reduces aperture size (Goodfriend 1986) and the common limpet Patella spp. decreases base-aperture size (Cabral 2007) in order to maintain constant body temperatures and reduce…...

    [...]

References
More filters
Journal ArticleDOI
01 Dec 1979-Ecology
TL;DR: Ecology is currently published by The Ecological Society of America and your use of the JSTOR archive indicates your acceptance of JSTor's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html.
Abstract: Ecology is currently published by The Ecological Society of America. Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/esa.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is an independent not-for-profit organization dedicated to creating and preserving a digital archive of scholarly journals. For more information regarding JSTOR, please contact support@jstor.org. http://www.jstor.org/ Thu Feb 2 13:29:01 2006

1,089 citations

Journal Article
TL;DR: In this article, the spectrum of possible shell forms is shown by a block diagram, where functional and evolutionary groups are confined to discrete regions of the spectrum and three parameters are considered at a time.
Abstract: Among the shells of invertebrates that exhibit spiral growth, differences in form can be expressed by differences in geometric parameters. If three parameters are considered at a time, the spectrum of possible shell forms may be shown by a block diagram. Analog and digital computer constructions make it possible to visualize shell forms that are theoretically possible but do not occur in nature. Actual species are not randomly distributed in the total spectrum of theoretically possible forms. Functional and evolutionary groups are confined to discrete regions of the spectrum. For example, a bivalve must have non-overlapping whorls in order to have a functional hinge. This fact restricts the geometric range of both brachiopods and bivalved molluscs. Ontogenetic change in coiling geometry may be interpreted as compensation for effects of increase in absolute size during growth.

681 citations

Journal ArticleDOI
TL;DR: It is concluded that the evidence for the ecological aspect of character displacement is weak and the principal ideas in the original definition given by Brown & Wilson (1956) are retained.
Abstract: Consideration of the possibilities and difficulties of detecting character displacement leads to a re-definition of the phenomenon; character displacement is the process by which a morphological character state of a species changes under Natural Selection arising from the presence, in the same environment, of one or more species similar to it ecologically and/or reproductively. This incorporates the principal ideas in the original definition given by Brown & Wilson (1956), but eliminates the restriction of making comparisons of the character states of a species in sympatry and allopatry. The evidence for the ecological (competitive) aspect of character displacement is assessed by analyzing in detail the best documented and well publicized examples in the literature. Some of the examples either do not exhibit displaced characters or, if they do, the “displacement” can be interpreted in other and perhaps simpler ways; this applies to the so-called classical case of character displacement, Sitta tephronota and S. neumayer in Iran. Other examples, involving lizards and birds, constitute better evidence for character displacement, but in no single study is it entirely satisfactory. It is concluded that the evidence for the ecological aspect of character displacement is weak.

628 citations

Journal ArticleDOI
TL;DR: The most important lesson to be gained from an intensive study of the Cepaea polymorphism is that many types of evolutionary force act upon it and that their relative importance varies between different polymorphic loci, or even when the same locus is studied in different populations.
Abstract: When faced with a problem or a series of related problems, scientists tend to look for a single unifying solution. When alternative solutions present themselves, it is usually accepted that only one is likely to be true. Further research often justifies this assumption. The successes of this type of reasoning have tended to obscure the fact that not all biological observations must necessarily be explicable in a simple and unitary way and that not all hypotheses to be tested need be mutually exclusive. We hope to illustrate this by considering some of the evolutionary processes affecting one well known genetic polymorphism, that of shell pattern in the land snail Cepaea. rhe development of techniques for detecting molecular polymorphisms has led to considerable disagreement between those who believe that such genetic variation is actively maintained by natural selection and those who suggest that random processes are the only significant factor in its control. The most important lesson to be gained from an intensive study of the Cepaea polymorphism is that many types of evolutionary force act upon it and that their relative importance varies between different polymorphic loci, or even when the same locus is studied in different populations. It is largely meaningless to ask whether selection or drift explains the observed variation in gene frequency, or indeed to attempt to identify the single selective mechanism acting on the polymorphism. The nature of the evolutionary process means that the genetic structure of each Cepaea population usually requires a complex and perhaps a unique explanation. This is a point which has not been sufficiently emphasized by students of other polymorphic systems.

401 citations