Variation in dog-whelk shells in relation to wave action and crab predation

In Nucella lapillus there is usually a progressive gradation in mean shell shape from short squat individuals on exposed headlands to more elongated ones in shelter. This pattern has been attributed to the differential selective effects of wave action and crab predation, although it would almost certainly be an oversimplification to imagine these as the only significant forces involved, especially near the limits of the species' distribution.
However, whilst this pattern of shell-shape variation with exposure is generally true over most of the animal's range in both Europe and North America, there are exceptions. There are occasional enclaves with much more elongated shells than would normally have been expected. Sometimes these are so abundant, as in the Severn Estuary, that all dog-whelk enclaves are unusually elongated, regardless of the exposure of their habitat.
In other areas, such as south-eastern England, the Solway Firth and the area around the mouth of the Conway the shells are not unusually elongated, but the enclaves show very little variation: a normal sheltered-shore form is seen wherever the species occurs. Shetland samples appear to be a combination of this pattern (predominating) with the 'normal' one showing the full range of shellshape variation.
Nucella appears in the Atlantic fossil record at the end of the Pliocene without any antecedent forms. It is assumed to have colonized from the Pacific and exploited the vacant niche before the onset of the Ice Age. Successive advances and retreats of the ice, with attendant changes in sea level, will have served to break up and recombine those dog-whelk populations that were not eliminated. It is postulated that the modern Nucella lapillus is a combination of several genetically distinct populations, some of which show the full range of variation and may respond to the selective effects of crabs and waves, and some of which do not and show little variation.     

Corrigendum

A re-evaluation of shell shape variation in Shetland dog-whelks.Mucelbi lapillus (I.) and their use as biological exposure indicators.Journal of Molluscan Studies.58: 315–328. Figure 6 and Figure 9 arc transposed. The caption of Figure6 is correct, but the illustration is that of Figure 9 and viceversa.     

Some observations on shell-shape variation in North American populations of Nucella lapillus (L.)

This paper considers shell-shape variation in samples of the dog-whelk/dog-winkle, ucella lapillus , from the southern half of its North American range, between 41 and 46N, and compares it with the pattern seen in Europe. At the extreme southern limit of its American range, to the south of Cape Cod, the species exhibits features to be expected in an animal close to an environmental limit. It is generally rare and has a patchy distribution. There is little variation in the shell: almost all adult individuals have rather large, thick, white, elongated shells. This contrasts sharply with the situation in Portugal, at the southern limit in Europe, where the animals have small thin, coloured shells of intermediate shape.
Away from the marginal situation, north of Nahant (42^CN) in America, the species shows much the same range of variability in shell size, shape and colour on both sides of the Atlantic. Most populations appear to show the same pattern of shape variation in asociation with the exposure of their habitat to wave action. Animals from exposed sites tend to have shorter, squatter shells than their compatriots in shelter. However, in America as in some parts of Europe, there are occasional enclaves which do not fit the usual pattern. It is interesting to note that the form normally associated with extremely exposed shores from Brittany to Faroe in Europe is found in Atlantic Canada but is apparently absent from southern populations in both the U.S.A and the Iberian Peninsula.     

Shell shape variation in dog-whelks (Nucella lapillus (L.)) from the West Coast of Scotland

European populations of the common dog-whelk, Nucella lapillus (L.), usually show a neat and precise pattern of shell shape variation with the exposure of their habitat to wave action. Whilst this is the case along much of the west coast of Scotland, there are a number of instances where unusually elongated shells occur, somewhat reminiscent of the form described for the Severn Estuary. There are no obvious environmental or geographical correlates to explain the occurrence of these forms and a genetical interpretation is sought.     

Rapid evolutionary responses in a translocated population of intertidal snail (<Emphasis Type="Italic">Bembicium vittatum</Emphasis>) utilise variation from different source populations

The artificial movement of individuals between populations (translocation) can be an effective way to increase genetic diversity within populations, but few studies have undertaken long term genetic monitoring to determine if variation introduced by translocation is maintained over many generations or whether it can be used to adapt to local conditions. Here, we report on the changes in morphological and molecular variation over a 12-year period in a population of an intertidal littorine snail (Bembicium vittatum) that was created by mixing individuals from three geographically disjunct populations. These source populations differ genetically in shell shape and in allele frequency at several allozyme loci. We found that the translocated population had higher allozyme diversity than any of the source populations and that this pattern was maintained over multiple generations. Variation in shell shape also increased, but this declined over time as shells became taller. Some allozyme loci also showed significant changes in frequency over time. These changes were not consistently towards the genetic makeup of a single source population, and in the case of shell shape, were towards a phenotype that was most suited to the local environment. Our results suggest that genetic variation introduced into a population by translocation can be rapidly incorporated and used to adapt to local conditions without domination by a single source population’s genome. However, more studies are needed before generalisations on the benefits of mixing individuals from disjunct populations can be made.     

Shell shape variation in the Nassariid Buccinanops globulosus in northern Patagonia

Morphological variation among natural populations is a phenomenon commonly observed in marine invertebrates and well studied, particularly, in shelled gastropods. The nassariid Buccinanops globulosus is interesting to study shell shape variation because it exhibits strong interpopulation differences in life history features, including maximum size, fecundity and growth rate. In this study, we examined the pattern of variation in size and shell shape among populations and between sexes of B. globulosus (Bahía San Antonio 40°29′S 63°01′W, Playa Villarino 40°45′S 64°40′W and Bahía Nueva 42°46′S 65°02′W). In particular, we used geometric morphometric techniques to test: (1) whether the two components of shell morphology (size and shape) are independent and (2) whether shape differences between sexes are consistently found among populations, regardless of their body sizes. Our results show shell shape variation between the populations of B. globulosus of northern Patagonia. Intra-specific shell shape variation is affected by body size, indicating allometry. Regardless of the size differences, individuals from Playa Villarino have high-spired shells, and shorter apertures and wider columellar area than individuals from the other populations. Also, sex-related shape differences were consistently found at each population, thus suggesting a common sexual dimorphism in shell morphology for this species. The functional significance of the variability found is discussed in terms of the flexibility of developmental programmes for morphology as well as the evolution of phenotypic plasticity.     

Phenotypic variation along a cline in allozyme and karyotype frequencies, and its relationship with habitat, in the dog-whelk Nucella lapillus, L.

The dog-whelk Nucella lapillus exhibits a number of phenotypic variations and genetic polymorphisms which correlate with habitat-specific environmental pressures, especially those associated with wave action and temperature. This study investigates the relationship between genetic composition (karyotypic and electrophoretic variation) and phenotypic differentiation in N. lapillus sampled at 15 points along an 8 km stretch of coastline. Coincident clinal variation in gene frequencies and shell shape is described; they covary with differences in karyotype and also with growth. Laboratory-reared young show that differences in phenotype (shell shape and growth) are inherited. Experimental evidence is presented that specific shell shapes are adaptive under conditions of thermal stress. Together with the well-established correlations between shell shape and shore exposure, this association provides an argument for a relationship between genetic composition, phenotype and habitat in this species.     

MORPHOLOGICAL VARIATION IN THE PAINTED TOPSHELL, CALL1OSTOMA ZIZYPHINUM (L.) (PROSOBRANCHIA: TROCHIDAE), FROM INTER-TIDAL RAPIDS ON THE IRISH COAST

Shell shape variation in Calliostoma zizyphinum (L.) is examinedwith different techniques of analysis (profile, discriminant,canonical variate and ratio analysis). Shells are collectedfrom five tidal rapids systems, all in very sheltered inlets,where C. zizyphinum is to be found at shore levels higher thanthe normal shore distribution of LWNT to the sublittoral. Shellmorphology varies consistently with the local habitat conditions,such as water flow rate, relative exposure and shore level.populations can be distinguished using various shell parametersand shell profiles. Shells from two different levels in thesame rapids show the effects of their subtly different habitats. (Received 5 February 1987;     

Shell shape of the land snail Cornu aspersum in North Africa: unexpected evidence of a phylogeographical splitting

Anatomical and molecular characters used to differentiate populations of the land snail Cornu aspersum (Helix aspersa) exhibit, in the western Mediterranean, definite and concordant patterns of correlation with geography. Scenarios involving Pliocene geological changes and postglacial expansion during the Pleistocene were proposed in previous studies to account for the establishment of this geographical structure. In the present work, we have performed a spatial analysis of variation in shell morphometrics, after the partitioning of the overall variation into size and shape components by means of a principal component-based approach (Cadima and Jolliffe, 1996). In order to know if the same historical events have also structured shell variation, the analysis includes all the populations from North Africa which were investigated for anatomical and molecular surveys. Contrary to shell size, which shows a significant spatial heterogeneity essentially related to environmental pressures, variation in shell shape components splits the populations according to a geographical pattern reflective of hypotheses suggested for molecular markers and genital anatomy. This implies that the selective forces often invoked to explain spatial changes in shell shape are not the deciding factors in the present case. Moreover, within each of the two geographical clusters defined, Mantel correlograms show that the similarity between populations declines according to an isolation by distance model. Because of the different allometric relationships between shell size and genitalia measurements in Western and Eastern entities of North Africa, mechanical constraints, possibly leading to a precopulatory isolation in the contact zone, are involved.     

CORRELATIONS BETWEEN SHAPE, WEIGHT AND THICKNESS OF SHELL IN FOUR POPULATIONS OF VENERUPIS RHOMBOIDES (PENNANT)

Three populations of Venerupis rhomboides in the Plymouth regionand one from the Firth of Clyde show remarkably wide, overlappingranges of variation in shape of shell. Shells of each populationare defined statistically in terms of relationships of shelllength, convexity and mean shell thickness, the latter measuredby a new technique utilizing sections of the shell cut radiallyfrom the umbo. Differences in shell shape are considered againstthe differences in the environments of the four populations,although environmental factors are incompletely known. The results of a transplantation experiment with mature bivalvesare also presented. The survivors showed correlated changesin shell shape and weight in the same direction and with thesame order of magnitude as those found in the distribution ofthe four populations. Detailed examination of the microscopicgrowth line patterns within the shells suggests that the increasedheight/length and convexity/length ratios and increased meanshell thickness were the results of reduced mantle exposureat the shell periphery in conditions of high concentration ofsuspended silt and clay. (Received 12 September 1982;     

INTRA-POPULATION VARIATION OF THE SHELL OF LITTORINA RUDIS (MATON) (MOLLUSCA: PROSOBRANCHIA)

Shell variation within a single population of Litlorina rudis(Maton), collected near the Biological Station at Trondheim,Norway, was studied. The variation of the shell features studied, which includedthe shape of the basal part of the outer lip, and the relativeheight of the spire, width of the shell and of the aperture,were found to be at least partially related to shell height.Furthermore, the shell shape in the population investigated,besides varying with age, also varies due to shell damage. Nosignificant differences were found between the shapes of theshells of males and females. The shell characteristics of different populations of L. rudisalso vary greatly, one of the causes being differences in theenvironmental conditions. When studying this variation betweendifferent populations, however, it must be emphasized that onlyundamaged shells of individuals of about the same age shouldbe compared, in order to obviate the effects of the variabilitydue to age differences and shell damage within each individualpopulation. ^*Contribution from Trondhjem Biological Station no. 199 (Received 18 February 1980;     

A multivariate study of geographic variation in the whelk Dicathais

An analysis has been made of the variation in shell shape and shell characteristics of 889 Australian and New Zealand specimens of the genus Dicathais, using multivariate techniques. Shell measurements taken were: the overall length, length of spire, length of aperture, and width of aperture. Weight of the shell plus the preserved animal was also recorded. The sculpture of the shell, thickness of the lip, and the presence or absence of a reddish or purplish colouration or banding on the inside of the lip, were assessed qualitatively.Principal component analyses of the size measurements for each site showed that the first principal component, which accounted for greater than 95% of the variation at each site, was associated with variation in the ‘size’ of the animal. Canonical analysis of the size measurements showed a cline in shell shape from the animals on the western side of Australia to those on the eastern side of Australia and New Zealand. The resulting canonical variates were associated with variation in the ‘shape’ of the shell. Principal component analyses of the between-group matrix and of the within-group matrix of the size measurements showed that the site means exhibited a similar pattern of dispersion to that of the animals within each site.Canonical analysis of the shell characteristics showed that variation along the first canonical axis was largely produced by shell sculpture, while variation along the second resulted from differences in colouration/banding.The generalized variances of the correlation matrices for the size measurements showed that groups with similar shell shape were associated with the presence of granite substrata and/or mussel beds or, alternatively, with limestone substrata, but canonical correlation analysis of the relationship between the size measurements and shell characteristics showed that no consistent trend was evident over all sites.A subjective examination of the structure of the radula of 84 animals showed that two distinct morphological forms were present, but that they were not correlated either with sex or any of the named shell forms or site groupings.An analysis of the growth curves of 27 animals of the two forms from the eastern and western coasts of Australia, held in the laboratory, was carried out. The eastern coast form showed a loss of sculpturing and a change in shell shape when kept under west coast conditions and on a mussel diet.Water temperature, diet, substratum, and degree of exposure to wave action were all found to show associations with variations in either shell shape or shell characteristics. It is suggested that the selective force of the habitat which produces changes in shell shape and shell characteristics of the animals at any site is a complex of factors, many of which are interrelated. The genetic basis for the development of shell shape and production of the shell characteristics in Dicathais may be similar to that found in Nucella lapillus (L.) in the Northern Hemisphere.These data suggest that the Dicathais found at the sites studied in this investigation are all part of the same ‘population’, the shell shape and shell characteristics of the adult populations being determined both by selection and phenotypic expressions caused by the selective force of the habitat at each site. It is concluded that the genus consists of a single highly variable species.The value of the application of multivariate analyses to this type of study is shown to lie in the way in which the techniques provide an overall picture of the variation within sites and of the variation between sites.     

Morphometrics Parallel Genetics in a Newly Discovered and Endangered Taxon of Galápagos Tortoise

Galápagos tortoises represent the only surviving lineage of giant tortoises that exhibit two different types of shell morphology. The taxonomy of Galápagos tortoises was initially based mainly on diagnostic morphological characters of the shell, but has been clarified by molecular studies indicating that most islands harbor monophyletic lineages, with the exception of Isabela and Santa Cruz. On Santa Cruz there is strong genetic differentiation between the two tortoise populations (Cerro Fatal and La Reserva) exhibiting domed shell morphology. Here we integrate nuclear microsatellite and mitochondrial data with statistical analyses of shell shape morphology to evaluate whether the genetic distinction and variability of the two domed tortoise populations is paralleled by differences in shell shape. Based on our results, morphometric analyses support the genetic distinction of the two populations and also reveal that the level of genetic variation is associated with morphological shell shape variation in both populations. The Cerro Fatal population possesses lower levels of morphological and genetic variation compared to the La Reserva population. Because the turtle shell is a complex heritable trait, our results suggest that, for the Cerro Fatal population, non-neutral loci have probably experienced a parallel decrease in variability as that observed for the genetic data.     

The taxonomy of the Littorina saxatilis speicies-complex, with particular reference to the systemaitc status of Littorina patula Jeffrys

Variation in shell shape and penis morphology of Littorina rudis Maton is examined using data from all parts of Britain. The shell shape variation within populations of L. rudis is shown to account for Liltorina patula Jeffrys at the only site where the latter species was recorded. In addition, the shell shape of I,. rudis varies with exposure, individuals on exposed shores having a relatively larger aperture than those on sheltered shores. Wave action and desiccation are considered the most likely factors maintaining this variation. The penis morphology of L. rudis varies within and between shores to an extent that renders the use of this character invalid for distinguishing L. patula from L. rudis. The radulae of adults of L. rudis, L. patula and Littorina nigrolineata (Gray) are similar in structure having blunt cusps, whilst adult Littorina neglecta Bean and juvenile L. rudis have pointed cusps. The possibility of a neotenous origin of L. neglecta from L. rudis is discussed. On the evidence presented here it is suggested that L. patula must be regarded as a synonym of L. rudis.     

Adaptive responses and invasion: the role of plasticity and evolution in snail shell morphology

Invasive species often exhibit either evolved or plastic adaptations in response to spatially varying environmental conditions. We investigated whether evolved or plastic adaptation was driving variation in shell morphology among invasive populations of the New Zealand mud snail (Potamopyrgus antipodarum) in the western United States. We found that invasive populations exhibit considerable shell shape variation and inhabit a variety of flow velocity habitats. We investigated the importance of evolution and plasticity by examining variation in shell morphological traits 1) between the parental and F₁ generations for each population and 2) among populations of the first lab generation (F₁) in a common garden, full‐sib design using Canonical Variate Analyses (CVA). We compared the F₁ generation to the parental lineages and found significant differences in overall shell shape indicating a plastic response. However, when examining differences among the F₁ populations, we found that they maintained among‐population shell shape differences, indicating a genetic response. The F₁ generation exhibited a smaller shell morph more suited to the low‐flow common garden environment within a single generation. Our results suggest that phenotypic plasticity in conjunction with evolution may be driving variation in shell morphology of this widespread invasive snail.     

Spatial variation of shell morphometrics in the subantarctic land snail <Emphasis Type="Italic">Notodiscus hookeri</Emphasis> from Crozet and Kerguelen Islands

The land snail Notodiscus hookeri, widely distributed in subantarctic islands, shows a large intraspecific variation in shell morphology. In the present work, shell size and form of individuals from populations located in Crozet and Kerguelen archipelagos were investigated by means of multivariate statistics. Variation in shell morphometrics was analysed after the partitioning of the overall variation into size and shape components by means of a principal component-based approach. Shell size shows a significant spatial heterogeneity, which seems essentially related to environmental pressures. Previous works pointed to a greater conchological variation between populations from Kerguelen but present observations show that intra-island variances are not significantly different in the two islands studied. Variation in shell shape splits the populations into two main entities because of different allometric relationships between two shell height components and all other measurements. However, using geographical affinities of populations as instrumental variables shows that more complex environmental features interfered in population clustering.     

A RE-EVALUATION OF SHELL SHAPE VARIATION IN SHETLAND DOG-WHELKS, NUCELLA LAPILLUS (L.) AND THEIR USE AS BIOLOGICAL EXPOSURE INDICATORS

In an earlier investigation, Crothers (1979) thought that therewere only two forms of the dog-whelk in Shetland (in place ofthe continuum found in most other places); a short squat morphon truly exposed shores, and an elongated one everywhere else.It was concluded that the species was of little or no use inthe assessment of exposure around those islands. The present paper reconsiders the situation, following subsequentinvestigations elsewhere and in the light of further samplingduring 1983 and 1991. The confusion in the earlier paper arosefrom a failure to recognise the presence of different patternsin different parts of the archipelago. When the appropriatemodel is applied, satisfactory predictions of exposure are obtained.A model derived from observations near Mongstad (Norway) explainsthe variation in some parts of Shetland and a different one,based on observations near Conway (North Wales), in others—whilsta combination of the two is required for some intermediate sites.It is suggested that the confusion is real, and due to the comparatively-recentrecombination of populations that became isolated into geographically-discreteareas during the last Ice Age. (Received 5 August 1991; accepted 12 February 1992)     

Gene frequencies, genetic background and environment in Pyrenean populations of Cepaea nemoralis (L.)

Microgeographical genetic differentiation of populations of the polymorphic land snail Cepaea nemoralis (L.) has been the subject of intensive investigation. The importance of environmental selection acting at the loci which control the shell polymorphism in determining the distribution of genes in this species remains a matter of considerable controversy. Some populations of C. nemoralis in the central Pyrenees show striking local associations of morph frequency (and particularly of the frequency of unhanded shells) with topography which have been ascribed to the action of natural selection.
In the Ter valley (eastern Spanish Pyrenees) populations described here, however, the pattern of association of the frequencies of the shell morphs with topography is quite different from that found in the central Pyrenees. In particular, the distribution of unhanded shells is almost the inverse of that found in some previously studied populations. The general picture of variation in relation to topography in Pyrenean C. nemoralis populations is now seen to be one of great local consistency within valley systems, but of considerable differences between valley systems. This variation in population structure may be due either to cryptic and undetected environmental differences between valley systems or to evolutionary divergence between their C. nemoralis populations. The difficulty of deciding between these two conflicting hypotheses emphasizes the limitations of the technique of simply examining the distribution of individual genes in natural populations as a method of analysing their evolutionary history.     

Two different patterns of shell-shape variation in the dog-whelk Nucella lapillus (L.)

Populations of the common dog-whelk, Nucella lapillus (L.), usually show a gradation in mean shell shape related to the exposure of the shore on which they live. Enclaves from exposed headlands have shorter, squatter shells than do their compatriots in shelter. This paper describes the pattern of shell-shape variation shown by populations in the Isles of Scilly and on the north coast of Wales. In both places a gradation is present, but it is not numerically similar, the Welsh population being more elongated. It is concluded that the selective agents are the same in both places. Wave action always eliminates the most elongate-shelled whelks first, and crabs always find it easier to take the animals with the widest apertures to their shells. The reason for the observed differences between the patterns described here lies in the genetic differences between the populations: Scillonian whelks do not have the option of being as elongated as the Welsh ones, which, in turn, cannot show very short squat shells in exposure.     

The winged Antarctic midge Parochlus steinenii (Gerke) (Diptera: Chironomidae) in the South Shetland Islands

The morphological variation of adult Parochlus steinenii (Gerke) is described from measurements of two populations from the South Shetland Islands. Morphologically, the population from Ardley Island is significantly larger than the population from Livingston Island, and in both populations variation in forefemur length is generally greater than variation in either antennal or wing length. The final instar larva of P. steinenii is described in detail. A consideration of the species' distribution in three geographically isolated areas, as well as of the greater morphological variation in polar as opposed to temperate populations, indicates that a flexible life history strategy in the larval stage may be important for survival in extreme environments.