Shell shape plasticity in Late Pennsylvanian myalinids (Bivalvia)

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.

BackgroundWithin-species skull shape variation of marsupial mammals is widely considered low and strongly size-dependent (allometric), possibly due to developmental constraints arising from the altricial birth of marsupials. However, species whose skulls are impacted by strong muscular stresses – particularly those produced through mastication of tough food items – may not display such intrinsic patterns very clearly because of the known plastic response of bone to muscle activity of the individual. In such cases, allometry may not dominate within-species shape variation, even if it is a driver of evolutionary shape divergence; ordination of shape in a geometric morphometric context through principal component analysis (PCA) should reveal main variation in areas under masticatory stress (incisor region/zygomatic arches/mandibular ramus); but this main variation should emerge from high individual variability and thus have low eigenvalues.ResultsWe assessed the evidence for high individual variation through 3D geometric morphometric shape analysis of crania and mandibles of three species of grazing-specialized wombats, whose diet of tough grasses puts considerable strain on their masticatory system. As expected, we found little allometry and low Principal Component 1 (PC1) eigenvalues within crania and mandibles of all three species. Also as expected, the main variation was in the muzzle, zygomatic arches, and masticatory muscle attachments of the mandibular ramus. We then implemented a new test to ask if the landmark variation reflected on PC1 was reflected in individuals with opposite PC1 scores and with opposite shapes in Procrustes space. This showed that correspondence between individual and ordinated shape variation was limited, indicating high levels of individual variability in the masticatory apparatus.DiscussionOur results are inconsistent with hypotheses that skull shape variation within marsupial species reflects a constraint pattern. Rather, they support suggestions that individual plasticity can be an important determinant of within-species shape variation in marsupials (and possibly other mammals) with high masticatory stresses, making it difficult to understand the degree to which intrinsic constraints act on shape variation at the within-species level. We conclude that studies that link micro- and macroevolutionary patterns of shape variation might benefit from a focus on species with low-impact mastication, such as carnivorous or frugivorous species.

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.

A key question in ecological speciation is to understand the causes and consequences of phenotypic divergence among populations. In this work, we analyzed the body shape and size variation in Jenynsia lineata across different coastal habitats along the Atlantic coast of South America. We hypothesized that J. lineata presents morphological variations to inhabit contrasting environments and that these adaptations are sex specific. We analyzed 13 populations from five coastal habitats, using linear and geometric morphometry, and tested the correlation of body shape variation with environmental variables to understand which environmental factors may influence body shape and size variation. Jenynsia lineata showed differences in body shape and size among populations, and these differences are specific to each sex. While females showed a variation in the caudal peduncle correlated with water current, we did not find such trait variation and correlation in males. Alternatively, individuals from marine rocky pools have a convex body curvature along the dorsal profile and larger body sizes, in both sexes. With these results, we describe the shape and size morphological variation of J. lineata and discuss this uncommon habitat-dependent sexual dimorphism in a Neotropical livebearer fish.

Introduction While IVD strain can be measured, the stress distribution within the disk is dependent on the IVD's material properties and its geometry and must be calculated. The IVD geometry remains incompletely characterized. Prior whole-disk models have been constructed from single IVDs. While this approach ensures that the geometry has a physiological basis, the degree to which results from a single IVD shape can be generalized to the entire population is uncertain. The primary objective of this study, therefore, was to develop a quantitative representation of IVD shape for the population. The approach incorporates a representative average IVD shape from multiple individuals and uses principal components analysis (PCA) to partition the variability of IVD shape into a small number of orthogonal modes of variation. These modes of variation can be adjusted parametrically. Since IVD shape is expected to change with degeneration, each mode of variation was correlated with degeneration grade to isolate degeneration-related shape variability from normal variability between individuals. Materials and Methods Magnetic Resonance Imaging Total 14 postmortem, agarose embedded L3-L4 motion segments (13 M, 1 F - age 69 ± 12 years - Pfirrmann grade 3.3 ± 0.8) underwent MRI (7T Siemens scanner - custom RF coil) using a 3D FLASH sequence (TE/TR = 3.7/9 ms) with ∼200 mm isotropic resolution.1 Image SegmentationBefore segmentation, the MRI images underwent intensity inhomogeneity correction, 50% downsampling, and Gaussian smoothing. Segmentation of the intervertebral disk was done in SNAP.2 The intensity-based region competition active contour tool was used to provide an initial segmentation. Errors in the automatic result were corrected by manual slice-by-slice segmentation. Shape Quantification The shape model was created by PCA of the disk segmentations.3 To make the images amenable to PCA, they were embedded as signed distance functions.3 The mean of the signed distance functions represents the average disk shape, with the residual shape variability captured as offsets from the mean signed distance function. These residuals were analyzed by PCA to produce a new 13-dimensional orthonormal basis for IVD shape. Each basis vector represents a different mode of shape variation. The basis vectors were ordered by the amount of variability that they capture from the original dataset. Following PCA, each disk in the dataset was projected into the new basis as a linear combination of the mean shape and the shape variation basis vectors. To determine which basis vectors represented degeneration-related shape variation, the projection coefficients were then correlated to Pfirrmann grade. Finally, to supplement the PCA-derived representation, the disk shapes were summarized by geometric measures: width, (left to right), depth (anterior-posterior distance at the mid-sagittal plane), height (averaged over the entire IVD), and axial-plane area. Results The first basis vector M1 represented an order of magnitude more variance than any other basis vector. Relative to M1, the second basis vector M2 captured 11% as much variance; M3, 9%; M4, 7%; M5, 5%; M6, 3%. Basis vectors 7 to 13 captured less than 2% as much variance as basis vector M1. Given the low variances associated with basis vectors M2 to M13 relative to basis vector M1, it is possible to represent IVD shape by the basis vector M1 coefficient w1 alone and still capture the majority of the population's IVD shape variation. Examination of the shapes produced by adding and subtracting one standard deviation of basis vector M1 from the mean shape reveals that basis vector M1 primarily controls lateral disk bulging, with little change in disk height. The basis vector M1 was also the only basis vector to show a significant correlation to degeneration as measured by Pfirrmann grade. None of the basis vectors were significantly correlated with age. Conclusion The objective of obtaining a low-dimensional representation of IVD shape was met. IVD shape variation was dominated by a single lateral bulging-related component, which was also correlated to degeneration grade. The low dimensionality of the shape representation makes it amenable to human manipulation, permitting straightforward modeling of a representative sampling of IVD shapes for example, for finite element analysis. The shape representation can also be used directly as a shape before Bayesian image segmentation of low-detail IVD images, for example, in vivo MRI. Further work is required to establish causation between degeneration and shape changes. Acknowledgment NIH grant R01 AR05005. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared Pfirrmann CW, et al. Spine 2001;26:1873 Yushkevich PA, et al. NeuroImage 2006;31:1116 Tsai A, et al. IEEE Transaction Medical Imaging 2003;22:137

Another look at shape variation in the distal femur of Australopithecus afarensis: implications for taxonomic and functional diversity at Hadar.

In Russia, Western Siberia, Anopheles from maculipennis subgroup comprises three vector species: An. messeae, An. daciae, An. beklemishevi, and the hybrid between An. messeae and An. daciae (Anopheles m-d), which exhibit complex cryptic morphological traits. Traditional morphological methods, such as egg morphology and exochorion coloration, have proven insufficient for reliably distinguishing these closely related species due to overlapping characteristics and high intra-species variability. To overcome these limitations, geometric morphometrics (GM) has emerged as a powerful tool for analyzing cryptic morphology. This article focuses on wing venation patterns, where GM provides precise, quantitative data based on defined anatomical landmarks, enabling detailed assessment of size and shape variation among species. Procrustes ANOVA, principal component analysis (PCA), and canonical variate analysis (CVA) were employed to assess shape variation and species differentiation. Centroid size and its relationship to shape variation were examined using multivariate regression. Despite significant morphological differences, the overlap observed in hybrids (An. m-d) reflects their intermediate position between the parental species. Our analyses revealed significant differences in wing shape and size among An. messeae, An. daciae, An. beklemishevi, and their hybrids, with hybrids showing intermediate morphologies. Landmarks on radial and medial veins were the most consistent contributors to species separation. No evidence of static allometry was detected, and wing shape differences were not explained by size. These findings demonstrate that wing morphometrics, combined with molecular identification, provides a reliable framework for species delimitation and surveillance of malaria vectors in temperate regions.

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.

We studied shell strength and appearance in six mixed-species cultured populations of blue mussels (Mytilus edulis, M. trossulus, and their hybrids) from Newfoundland, Canada. Shell strength (defined as the force causing shell breakage) and shell thickness both increased with shell length. Strength and thickness varied significantly among sites and among genotypes within sites; M. edulis had stronger and thicker shells than M. trossulus. Shell strength was related to shell thickness and shell width, both of which were greater in M. edulis than in M. trossulus. Hybrids tended to be intermediate. Appearance was studied on the basis of shell color and shape. The frequency of brown-shelled mussels varied considerably among sites and was higher in M. trossulus than in M. edulis. Shell width, depth, width-length ratio, and depth-length ratio all varied significantly among sites. Within sites, M. edulis had a greater shell width, width-length ratio, and width-depth ratio than M. trossulus, whereas hybrids were intermediate. Intrasite variability in shell shape characteristics, expressed as total population variance, was greater in the mixed-species populations from Newfoundland than in a unispecific M. edulis population from Prince Edward Island. In a reciprocal seed transfer experiment, shell width, width-length ratio, and width-depth ratio were greater in M. edulis than in M. trossulus of similar age. Width-length and width-depth ratios decreased with increasing age. Fourteen months after transfer, shell width and depth differed between transferred and nontransferred stocks. Interspecific relationships between M. edulis and M. trossulus in shell width and width-length ratio were similar in transferred and nontransferred stocks, while the other shell dimensions and their ratios differed. We infer that genotype-related differences in shell shape may change after stock transfer owing to site-related effects on shell growth. We discuss these results in terms of their potential impact on future industry development, particularly expansion of the seed supply for further industry growth.

The cranial morphology of the African Old World monkeys Mandrillus, Papio, and Theropithecus (i.e., baboons) has been the subject of a number of studies investigating their systematic relationships, patterns of scaling, and growth. In this study, we use landmark-based geometric morphometrics and multivariate analysis to assess the effects of size, sex, taxonomy, and geographic location on cranial shape. Forty-five landmarks were digitized in three dimensions on 452 baboon crania and subjected to generalized Procrustes analysis (GPA), which standardizes geometric size but leaves scaling-based shape differences in the data. The resulting shape coordinates were submitted to regression analysis, principal components analysis (PCA), partial least-squares (PLS) analysis, and various clustering techniques. Scaling (shape differences correlated with size) was the largest single factor explaining cranial shape variation. For instance, most (but not all) of the shape differences between the sexes were explained by size dimorphism. However, central tendencies of shape clearly varied by taxon (both specific and subspecific) even after variations in size and sex were adjusted out. Within Papio, about 60% of the size- and sex-adjusted shape variations were explained by the geographic coordinates of the specimen's provenance, revealing a stepped cline in cranial morphology, with the greatest separation between northern and southern populations. Based on evidence from genetic studies, and the presence of at least two major hybrid/interbreeding zones, we interpret the phylogeographic pattern of cranial variation as indicating that these populations are best ranked as subspecies of a single species, rather than as two or more distinct biological species. This objective approach can be applied to other vertebrate species or species groups to help determine the taxonomic rank of problematic taxa.

Resumen. La variación de forma entre una especie viviente y dos extintas de caracoles acuáticos del género Aylacostoma, fue investigada mediante los métodos de morfometría geométrica de thin plate splines y análisis multivariado de partial warp scores. El análisis se realizó para evaluar la capacidad diagnóstica del método y explorar las diferencias de forma de conchilla, debido a la falta de otros datos, en un intento por responder por qué sólo una de las especies persistió en la naturaleza. Para ello, 16 landmarks fueron definidos en un espacio bi-dimensional para 32 conchillas de especímenes tipo, paratipo y de referencia depositados en museos de Argentina. La variación fue descompuesta en componentes uniformes y no uniformes. Diferencias significativas en apertura y espira fueron encontradas respecto de los 9 primeros componentes no uniformes, que explicaron más del 95% de la variación observada. Estas diferencias podrían vincularse a un uso diferencial del hábitat y/o al grado de exposición a la corriente. La apertura más grande en las especies extintas pudo estar asociada a hábitats y sustratos con mayores corrientes, mientras que la abertura más pequeña en la tercera especie podría estar relacionada con la preferencia por hábitats más protegidos, como los que actualmente habita.

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.

Using 3D virtual surfaces to investigate molluscan shell shape

Elliptical Fourier analysis of symphyseal shape in great ape mandibles

Bone shape difference between control and osteochondral defect groups of the ankle joint