Using 3D virtual surfaces to investigate molluscan shell shape

Analyses of shell shape variation in epifaunal and semi-infaunal myalinids from the LaSalle “cyclothem” attest to the extensive shape plasticity of some Late Pennsylvanian myalinids. Mean shell shapes differ significantly within and among species across three nearshore facies. Discriminant analyses of Fourier biometric data categorized by taxonomic, populational (habitat), life-mode, and “multi-species habitat assemblage”” discriminant groups reveal patterns of shape change and variation across an inferred environmental stress gradient in addition to taxonomic and life-mode shape differences. Fourier harmonic data are good indicators of shape differences between epifaunal and semi-infaunal life modes. Mean shapes of epifaunal species vary among habitats. Within-habitat shell shape convergence occurred between Myalina glossoidea and M. (Orthomyalina) slocomi. Interpopulational shape divergence occurred among all populations of M. (Orthomyalina) slocomi, M. glossoidea, and M. wyomingensis. Within-species variation among habitats produced significant mean shape differences among each of three “multi-species habitat assemblages.” Results indicate that a portion of the variation is of ecophenotypic origin.Mean interspecific shape differences reflect internal functional organization and life modes. Intraspecific shape differences among epifaunal species could reflect physical and biotic habitat variables. Several harmonic amplitudes vary in concert with inferred environmental variation among habitats. Although biologic interpretation of harmonic data is problematic, parallel trends in fifth harmonic amplitudes of epifaunal species mirror inferred differences in water turbulence among habitats and could reflect shape variation related to byssal attachment.Stress gradient trends in intraspecific shape variability cannot be collectively explained by either of the antithetical stability-diversity-variation hypotheses. Each species exhibits a different nonmonotonic trend. Maximal levels of intraspecific variation within each species occurred within different habitats. Within-species differences in levels of variation could result from genetic variation among populations as well as ecophenotypic influences. Comparative studies of ontogenetic shape change between valves of individuals, among individuals, and among populations are necessary to determine the relative influence of environmental variables on shape variation and evolutionary significance of phenotypic plasticity.

A multivariate study of geographic variation in the whelk Dicathais

Despite its widely recognized conchological variation, studies on the shell variability of Pomacea canaliculata are limited to its sexual and ontogenetic components. Here, we analyse the interpopulation variation in conchological and somatic traits, and sex-related growth patterns of P. canaliculata to discover if it is ecophenotypically or genetically determined. Pomacea canaliculata showed variation in shell shape, shell and body weight, and body ash content among populations from three environmentally different sites. Shell shape was also different when snails from the three sources were reared under homogeneous laboratory conditions, indicating a genetic basis for the differences. Shell shapes of laboratory snails differed from their field counterparts, suggesting an environmental influence on shape. The genetic differentiation of shell shape among the studied populations does not seem to be the outcome of adaptation to local conditions or of genetic drift, but is probably a side effect of adaptive differentiation in some life-history traits. On the other hand, weight and ash content differences disappeared under homogeneous conditions, suggesting that their variation is mainly ecophenotypic. Variability in shell thickness, body weight and ash content seems to be more related to trophic availability than to water chemistry. In the laboratory, females showed slightly higher growth rates than males, but these inter-sex differences varied among snails from the three sources. However, shell length was not different between sexes in the field populations, probably due to a greater effect of food shortage on female growth rates. The widespread pattern that shells of freshwater snails from contrasting environments are different has been attributed mostly to cumulative environmental effects or to adaptation to local conditions. However, we suggest that different shell shapes could arise as a collateral outcome of genetically different reproductive behaviours and that it would be misleading to study the shell as a trait exposed separately to selective pressures or environmental influences.

A component of shell shape was measured in two populations of the freshwater gastropod Lymnaea stagnalis and a highly significant difference noted. Samples of juveniles were then taken from both natural populations, and cultured in the laboratory in tanks containing water from the pond inhabited by one of the natural populations; that is, one of the laboratory populations can be regarded as a transplant, the other as a control. These two cultures were maintained until substantial growth of the juvenile snails had occurred. The shell shapes of the snails were then measured and the two populations found to be almost identical. The difference between this comparison and the one involving the two natural populations was caused entirely by a marked shift in the shell shape distribution of the transplanted population. Thus the large difference in shape observed between the populations in the wild was completely due to direct environmental effects on the phenotype, or any genetic component of the variation was so small as to be undetectable by the method employed. It is noted that this result cautions against acceptance of the evolutionary inferences that are often drawn from studies of phenotypic variation in shell shape where these are unaccompanied by demonstration of an inherited component of the variation.

Morphology reflects ecological pressures, phylogeny, and genetic and biophysical constraints. Disentangling their influence is fundamental to understanding selection and trait evolution. Here, we assess the contributions of function, phylogeny, and habitat to patterns of plastron (ventral shell) shape variation in emydine turtles. We quantify shape variation using geometric morphometrics, and determine the influence of several variables on shape using path analysis. Factors influencing plastron shape variation are similar between emydine turtles and the more inclusive Testudinoidea. We evaluate the fit of various evolutionary models to the shape data to investigate the selective landscape responsible for the observed morphological patterns. The presence of a hinge on the plastron accounts for most morphological variance, but phylogeny and habitat also correlate with shape. The distribution of shape variance across emydine phylogeny is most consistent with an evolutionary model containing two adaptive zones--one for turtles with kinetic plastra, and one for turtles with rigid plastra. Models with more complex adaptive landscapes often fit the data only as well as the null model (purely stochastic evolution). The adaptive landscape of plastron shape in Emydinae may be relatively simple because plastral kinesis imposes overriding mechanical constraints on the evolution of form.

Here we investigate spatial variation in shell shape of Littorina saxatilis, an ovoviviparous species, and Melarhaphe neritoides, a species with planktonic eggs and larvae. Populations of both species were sampled in 6 sites located along the west coast of the Iberian Peninsula. Shell shape was studied using landmarkbased morphometric methods. Landmark data was used to estimate individual size and to describe shell shape. Prior to statistical analysis specimens were aligned using Generalised Procrustes Analysis (GPA). Ordinations based on GPA of landmark data and using Principal Components Analysis (PCA), revealed a clear spatial segregation of sites for both species, although this was more evident for L. saxatilis than M. neritoides. Statistical analysis revealed significant multivariate variation in shape among sites and a significant effect of allometry on shape for both species. In contrast to this last result, there was no significant linear relationship between any of the first three PC axes and size in L. saxatilis but highly significant associations between the first and third PC axes and size for M. neritoides. Spatial variation in the shape of both species was primarily related to variation in the height of the apical whorls and in the width of the aperture for L. saxatilis. Further variation in shape was related to the shape of the last whorl in L. saxatilis and shell elongation and a change in aperture shape in M. neritoides.

Northern Map Turtles, Graptemys geographica, are a long-lived riverine species of conservation concern. We examined carapace morphology of Northern Map Turtles at Mount Union, a major turtle nesting area and former industrial site along the Juniata River in central Pennsylvania, USA. Among 535 reproductive adult female G. geographica, 29% exhibited abnormal shell shape, often in the form of an indentation in one or both sides of the carapace. Older females had a higher incidence of abnormalities than younger females. We quantified variation in shell shape, compared morphology among life history stages, and assessed thermal incubation environments of embryos to determine the magnitude and potential source of shell shape abnormalities. Geometric morphometric analysis showed significant differences among several carapace shape categories of adult turtles. No shell shape abnormalities were observed among 703 hatchlings collected from nests, and no shell shape abnormalities were observed among seven o...

The eastern oyster Crassostrea virginica is a commercially and ecologically important organism found throughout the western North Atlantic and Gulf of Mexico. Morphological differences in shell shape of eastern oysters are known to arise from environmental, genetic, and husbandry-related factors. Here, live eastern oysters were collected from 17 sites along the U.S. Gulf of Mexico coast from Texas to Florida to examine morphological differences among geographic samples. Six morphological metrics were recorded for each individual, and four different composite ratios commonly used to describe oyster shape (fan, cup, volume ratio, and weight ratio) were calculated. Principal component analysis was used to demonstrate geographic differences in ordinated shape ratios, correlating roughly with eastern (Florida), northern (Alabama, Louisiana, north Texas), and western (south Texas) samples. In Texas, differences in shape were correlated with previously described genetic population boundaries, indicating that populations north versus south of Aransas Bay had different overall shell shapes. On a broader scale, shell shape variation correlated roughly with previously described genetic population boundaries throughout the Gulf of Mexico as well as tide depth (intertidal versus subtidal reefs). Among the various factors that might act as drivers of shell shape, individual variation is important, but population structure and tide height are also significant predictor variables of shape in this species.

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 (42CN) 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.

Pteropods, a group of holoplanktonic gastropods, are regarded as bioindicators of the effects of ocean acidification on open ocean ecosystems, because their thin aragonitic shells are susceptible to dissolution. While there have been recent efforts to address their capacity for physiological acclimation, it is also important to gain predictive understanding of their ability to adapt to future ocean conditions. However, little is known about the levels of genetic variation and large‐scale population structuring of pteropods, key characteristics enabling local adaptation. We examined the spatial distribution of genetic diversity in the mitochondrial cytochrome c oxidase I (COI) and nuclear 28S gene fragments, as well as shell shape variation, across a latitudinal transect in the Atlantic Ocean (35°N–36°S) for the pteropod Limacina bulimoides. We observed high levels of genetic variability (COI π = 0.034, 28S π = 0.0021) and strong spatial structuring (COI ΦST = 0.230, 28S ΦST = 0.255) across this transect. Based on the congruence of mitochondrial and nuclear differentiation, as well as differences in shell shape, we identified a primary dispersal barrier in the southern Atlantic subtropical gyre (15–18°S). This barrier is maintained despite the presence of expatriates, a gyral current system, and in the absence of any distinct oceanographic gradients in this region, suggesting that reproductive isolation between these populations must be strong. A secondary dispersal barrier supported only by 28S pairwise ΦST comparisons was identified in the equatorial upwelling region (between 15°N and 4°S), which is concordant with barriers observed in other zooplankton species. Both oceanic dispersal barriers were congruent with regions of low abundance reported for a similar basin‐scale transect that was sampled 2 years later. Our finding supports the hypothesis that low abundance indicates areas of suboptimal habitat that result in barriers to gene flow in widely distributed zooplankton species. Such species may in fact consist of several populations or (sub)species that are adapted to local environmental conditions, limiting their potential for adaptive responses to ocean changes. Future analyses of genome‐wide diversity in pteropods could provide further insight into the strength, formation and maintenance of oceanic dispersal barriers.

Shape variability represents an important direct response of organisms to selective environments. Here, we use a combination of geometric morphometrics and generalised additive mixed models (GAMMs) to identify spatial patterns of natural shell shape variation in the North Atlantic and Arctic blue mussels, Mytilus edulis and M. trossulus, with environmental gradients of temperature, salinity and food availability across 3980 km of coastlines. New statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the developmental and genetic contributions to shell shape and made it possible to identify general relationships between blue mussel shape variation and environment that are independent of age and species influences. We find salinity had the strongest effect on the latitudinal patterns of Mytilus shape, producing shells that were more elongated, narrower and with more parallel dorsoventral margins at lower salinities. Temperature and food supply, however, were the main drivers of mussel shape heterogeneity. Our findings revealed similar shell shape responses in Mytilus to less favourable environmental conditions across the different geographical scales analysed. Our results show how shell shape plasticity represents a powerful indicator to understand the alterations of blue mussel communities in rapidly changing environments.

Adaptive divergence in response to shared environmental pressures: examining gastropod shell exploitation by intertidal hermit crabs.

The anomalous atrypid brachiopod genusKerpina Struve (family AtrypidaeGill) is restricted in distribution to Eifelian (Middle Devonian) rocks of the Eifel region, Germany, and is found associated with, or attached to, the platy tabulate coralAlveolites. Both coral and brachiopod appear to be characteristics of a currentagitated shallow water environment. The marked variation in shell shape and beak incurvature, as well as the unique internal morphology of the type speciesKerpina vineta Struve are described for the first time.

Shape variation is widespread in nature and embodies both a response to and a source for evolution and natural selection. To detect patterns of shape evolution, one must assess the quantitative genetic underpinnings of shape variation as well as the selective environment that the organisms have experienced. Here we used geometric morphometrics to assess variation in plastron shell shape in 1314 neonatal slider turtles (Trachemys scripta) from 162 clutches of laboratory-incubated eggs from two nesting areas. Multivariate analysis of variance indicated that nesting area has a limited role in describing plastron shape variation among clutches, whereas differences between individual clutches were highly significant, suggesting a prominent clutch effect. The covariation between plastron shape and several possible maternal effect variables (yolk hormone levels and egg dimensions) was assessed for a subset of clutches and found to be negligible. We subsequently employed several recently proposed methods for estimating heritability from shape variables, and generalized a univariate approach to accommodate unequal sample sizes. Univariate estimates of shape heritability based on Procrustes distances yielded large values for both nesting populations (h2 approximately 0.86), and multivariate estimates of maximal additive heritability were also large for both nesting populations (h2max approximately 0.57). We also estimated the dominant trend in heritable shape change for each nesting population and found that the direction of shape evolution was not the same for the two sites. Therefore, although the magnitude of shape evolution was similar between nesting populations, the manner in which plastron shape is evolving is not. We conclude that the univariate approach for assessing quantitative genetic parameters from geometric morphometric data has limited utility, because it is unable to accurately describe how shape is evolving.

Cerion tva (L.) and Tudora megacheilos (Potiez and Michaud) are the dominant land snails of Aruba, Bonaire and Curacao; both display the high degree of intraspecific character variation so often noted in terrestrial gastropods. This study correlates patterns of morphologic diversity with geographic and environmental distribution. Projected upon factor axes, the samples of each species fall into four groups corresponding to degrees of geographic isolation: Aruba, Bonaire, eastern and western Curacao. Factor axes represent the major trends of variation in shell size and shape. Subgrouping within geographic clusters is influenced by environmental parameters. In Cerion, for example, shells collected from volcanic rocks tend to be larger than those from limestone. In all geographic regions, these volcanic samples are distinguished by their higher projection on an axis representing increased size and concomitant changes of shape. Pulmonates usually favor limestone, but the anomaly of Cerion's smaller size is related to the unfavorable climatic conditions associated with limestone areas in the Dutch West Indies (dry, windy, and poorly vegetated). If the morphologic differences among geographic regions are similar to ecologicallyinduced variation within regions, then interregional differences are probably adaptive, and not a fortuitous result of geographic isolation, as many have claimed. Cerion is small ancd Tudora large in eastern Curacao; Tudora is small and Cerion large in western Curacao. This interregional relationship is parallelled by significant negative correlations between sizes of the two species for samples within regions. Diversity patterns did not change significantly during 16 years separating the studies of Baker and Hummelinek. Baker's data show the same distinction of four geographic areas and similar patterns of distribution within areas; significant positive correlation exists between measures made by Baker and Hummelinck on samples collected from the same localities. Fossil samples from cave phosphates do not conform to modem distributions. The basic character diversity pattem-good (presumably genetic) distinction among regions and ecologically induced variation within regions-requires a revision of Baker's classification. Separate names for the four regions are warranted, but the cumbersome nomenclature for intraregional diversity should be dropped.