Geographical variation in shell shape of the pod razor shell Ensis siliqua (Bivalvia: Pharidae)

The present study assessed the existence of variation in the shell shape of the pod razor shell (Ensis siliqua) throughout its distributional range in the north-eastern Atlantic. Shells of E. siliqua caught at seven collecting sites (three in Portugal, three in Spain and one in Ireland) were studied by geometric morphometric methods, using both landmark- and contour-based methods. Both approaches (landmarks inside the valves and shell outline) discriminated the shells from Aveiro (centre of Portugal) and Strangford Lough (Ireland) from those caught in the nearby localities (remaining Portuguese and Spanish sites, maximum distance of 550 km by sea). Landmark analysis revealed that shells from Aveiro were more similar to shells from Ireland (~1,500 km far away). Contour analysis revealed that shells from Aveiro had a shape with a comparatively larger height-to-width ratio, whereas shells from Ireland showed a slightly more curved outline than in the remaining sites. Landmark- and contour-based methods provided coherent complementary information, confirming the usefulness of geometric morphometric analyses for discerning differences in shell shape among populations of E. siliqua. A brief review of previous applications of geometric morphometric methods to modern bivalve species is also provided.     

Genome‐wide association analyses reveal polygenic genomic architecture underlying divergent shell morphology in Spanish Littorina saxatilis ecotypes

Gene flow between diverging populations experiencing dissimilar ecological conditions can theoretically constrain adaptive evolution. To minimize the effect of gene flow, alleles underlying traits essential for local adaptation are predicted to be located in linked genome regions with reduced recombination. Local reduction in gene flow caused by selection is expected to produce elevated divergence in these regions. The highly divergent crab‐adapted and wave‐adapted ecotypes of the marine snail Littorina saxatilis present a model system to test these predictions. We used genome‐wide association (GWA) analysis of geometric morphometric shell traits associated with microgeographic divergence between the two L. saxatilis ecotypes within three separate sampling sites. A total of 477 snails that had individual geometric morphometric data and individual genotypes at 4,066 single nucleotide polymorphisms (SNPs) were analyzed using GWA methods that corrected for population structure among the three sites. This approach allowed dissection of the genomic architecture of shell shape divergence between ecotypes across a wide geographic range, spanning two glacial lineages. GWA revealed 216 quantitative trait loci (QTL) with shell size or shape differences between ecotypes, with most loci explaining a small proportion of phenotypic variation. We found that QTL were evenly distributed across 17 linkage groups, and exhibited elevated interchromosomal linkage, suggesting a genome‐wide response to divergent selection on shell shape between the two ecotypes. Shell shape trait‐associated loci showed partial overlap with previously identified outlier loci under divergent selection between the two ecotypes, supporting the hypothesis of diversifying selection on these genomic regions. These results suggest that divergence in shell shape between the crab‐adapted and wave‐adapted ecotypes is produced predominantly by a polygenic genomic architecture with positive linkage disequilibrium among loci of small effect.     

Does allometry account for shape variability in <Emphasis Type="Italic">Ephedrus persicae</Emphasis> Froggatt (Hymenoptera: Braconidae: Aphidiinae) parasitic wasps?

We analysed linear measurements on various parts of the body and the configuration of 11 landmarks on the wing in a large sample of Ephedrus persicae that had emerged from 13 aphid host species, to assess whether static allometry (a measure of the scaling relationship between traits in a population of individuals at the same ontogenetic stage) accounts for variation in body shape. The analysed specimens came from several localities in Europe, Asia Minor, Japan and South America, and cover a large portion of the distribution area of E. persicae. We found that allometry accounts for variation in body shape among different biotypes within the E. persicae group. The allometric slopes for head size (HD), petiolus width (PETW), mesoscutum width (MSC), and ovipositor sheath length (OVPL) diverged significantly among biotypes, indicating biotype-specific allometries. The analysis of allometric variation in wing shape showed that the pattern and direction of allometric changes also differed among individuals that had emerged from different hosts. Our results (observed divergences in the directions of allometric slopes of particular morphometric traits and wing shape) suggest that allometric relations within E. persicae are not conserved, so that allometry itself changes, evolving differently in aphid parasitoids that emerge from different hosts.     

SIZE, SHAPE AND SHELL MORPHOLOGY IN THE ANTARCTIC LIMPET NACELLA CONCINNA AT SIGNY ISLAND, SOUTH ORKNEY ISLANDS

Comparison of the size, shape and shell morphology in littoraland sub-littoral morphs of the Antarctic limpet Nacella concinnareveal differences in shell morphology which are enhanced bystructural anomalies within the shells of the two types. Infestationof sub-littoral shells by the conchocelis phase of an endolithicalga significantly affects shell density and total chlorophylllevels in the two shell morphs. The surface sculpture of sub-littoralshells is characterised by a series of grooves, the configurationof which closely resembles that of the radular teeth in N. concinna.Limpets utilise the available food supply within the shell matrixof other limpets by grazing the shell material. Epibiotic growthof calcareous algae prevent erosion and preserve underlyingshell layers. In severe cases, where protection is lacking,intraspecific shell grazing may remove parts of the shell exposingthe internal tissues. The Dominican Gull, Larus dominicanus, is a major shore predatorof both shell morphs. Gull middens contain both shell typesbut are dominated by the more accessible littoral shells. Comparisonof living populations and midden assemblages indicates thatsize and shape selection of prey occurs, with pear-shaped limpetsbetween 21 mm and 29 mm in length being taken preferentially. Apparent differences in shell form are induced by physical,biological and behavioural influences. Littoral animals arerobust in nature, resist avian pre-dation and are not extensivelygrazed whereas those of the sub-littoral are not subject tothe same degree of predatory attention but suffer a gradualdepletion of their shallower shell form through a combinationof algal infection and intraspecific shell grazing. (Received 21 February 1990; accepted 5 July 1990)     

Geometric Morphometric Analysis of Body-Form Variability in Cichlasoma minckleyi, the Cuatro Cienegas cichlid

In this study, I investigate body-form variability in the trophically polymorphic Cuatro Cienegas cichlid, Cichlasoma minckleyi. I use geometric morphometrics to assess and compare size, shape, and allometric patterns within and between dental morphs (distinguished by differences in pharyngeal bones and papilliform or molariform dentitions). I find that dental morphs do not differ significantly in size, shape, or allometric slopes, but do differ in allometric intercepts; thus, dental morphs exhibit different patterns of body-form variability. Within each morph, body-form varies between more fusiform 'piscivores' and deeper-bodied dietary generalists. However, papilliform individuals show a continuous distribution of body-forms, whereas molariform individuals exhibit a discontinuity. I compare results from geometric analyses with those from a traditional (distance-based) morphometric analysis, finding that geometric techniques more clearly recognize discontinuities in shape. Finally, I suggest explanations for observed differences in body-form variability between dental morphs, concluding that the best explanation hinges on the relative importance of genetic and environmental factors in influencing body-form.     

Description of Harpagifer permitini sp. nova (Harpagiferidae) from the sublittoral zone of South Georgia and redescription of the littoral H. georgianus Nybelin

Comparative morphology of Antarctic spiny plunderfishes of the genus Harpagifer from the littoral and sublittoral zones off South Georgia (Scotia Sea) is investigated. A new species H. permitini from the depth 65–80 m is described and the littoral H. georgianus Nybelin, 1947 is redescribed. The significant differences in their morphology are shown. Two morphological and bathopathic forms (“soft” and “spiny”) of spiny plunder-fishes are characterized.     

Environment-specific shell shape variation in the boring mytilid Leiosolenus patagonicus

Environmental conditions induce phenotypic responses (behavioural, morphological and physiological) in many marine species. The boring mytilid Leiosolenus patagonicus inhabits different types of substrata, such as sandstone intertidal and hard subtidal substrata (here called ‘lifeless-substratum’) and shells of bivalve species (here called ‘live-substratum’), where they are exposed to different restrictions in their growth. We used geometric morphometric methods to compare the contour shell shapes from each type of substratum (live and lifeless) since we expected the body shape to differ between individuals from these different substrata. The results showed that the shell shape depends on the type of substratum where the larvae recruit. The mean shell shapes of individuals from the live-substratum are more slender than those of the individuals growing inside the lifeless-substratum. Individuals from live-substratum can adapt their phenotype depending on the oyster’s anti-parasitism responses, while in lifeless-substratum they are able to build their own refuges.     

Environmental Heterogeneity and Morphological Variability in Pisidium subtruncatum (Sphaeriidae,Bivalvia)

On the example of P. subtruncatum, a widely distributed bivalve, we studied the relationship between environmental and morphological variability. The main questions addressed were: (1) are there differences in shell morphology between different populations, (2) are they dependent on sediment composition, hydrology or water chemistry, (3) are there differences in morphological variability between stable and unstable environments. Measurements of specimen from nine sampling sites showed high variability in shell shape and size. Length‐height relation, symmetry, width and dry weight of the shell all varied significantly. Shell shape was significantly correlated with sediment composition and dependent on hydrological connectivity, with higher, asymmetric shells occurring in sandy sediments and lower, rounded shells in muddy sediments. Analysis of intrapopulation morphometric variation showed that variability is higher in habitats with high temporal and spatial environmental variability and lower in habitats with low or reduced environmental heterogeneity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphological differences among three color morphotypes of Sebastes inermis (Scorpaenidae)

 Morphological differences among three color morphotypes occurring almost sympatrically (A, reddish type; B, blackish type; C, brownish type) of Sebastes inermis are clarified. Twenty-six morphometric characters and seven meristic characters were examined on about 20 specimens of each morphotype, 15 of the morphometric characters differing significantly. Principal component analysis (PCA) of the morphometric characters showed morphotype C to differ from the other two morphotypes. Canonical discriminant analysis (CDA) of the morphometric characters showed clear separation of each morphotype. Four meristic characters also differed significantly. Modes of pectoral fin ray counts of morphotypes A, B, and C were 15, 16, and 17, respectively, and the modes of anal fin ray counts of morphotypes B and C were 7 and 8, respectively. Many individuals of morphotype A had seven or eight anal fin rays. These differences suggested that the three morphotypes may be reproductively isolated from each other. Received: October 15, 2001 / Revised: March 14, 2002 / Accepted: April 9, 2002     

Taxonomy of the genus Megacyclops (Crustacea,Copepoda): morphometry and the use of enzyme electrophoresis

Morphometric investigations on the populations of Megacyclops in Lake Constance showed, that in a sublittoral region from about 20 to 50 m depth an intermediate type between the two species M. viridis (littoral) and M. gigas (benthal) could be distinguished. Using enzyme electrophoresis it was evident, that these specimens belonged to M. viridis.The annual range of water temperatures in the sub-littoral region corresponds to that found in the shallow areas of the lake or in ponds during winter and early spring, when the adults of M. viridis are markedly larger than in summer and fall (seasonal variation). Nevertheless the sublittoral population represents a different local type with special characteristics in morphometric details.     

Functional phenotypic plasticity of the endolithic mytilid Leiosolenus curtus (Lischke, 1874) (Bivalvia: Mytilidae)

In Sagami Bay, Japan, Leiosolenus curtus (Lischke, 1874) bores into dead shells of rock barnacles (Tetraclita japonica) in Manazuru, calcareous sand-rock in Oiso, weathered tuff in Miura and agglomerates formed by the tubes of clam worms (Spirobranchus kraussii) in Kanaya. In order to understand the relationships between morphology and environment, the shell morphologies and genetic characteristics of L. curtus, together with the calcium content of the different bored substrata, were analysed. Individuals from Kanaya and Manazuru were characterised by a chalk-like encrustation and a more rounded and inflated shell shape, while those from Oiso and Miura had a striated encrustation. The molecular phylogenetic tree and haplotype network showed that the genetic sequences of the individuals from all investigated localities were very similar, and implied that they belong to the same species. The striated encrustation of the individuals from Oiso and Miura presumably enables them to scrape those residues not dissolved by the mucus secreted by the mantle gland, and thereby to bore into non-calcareous substrata. The more rounded and inflated shell shape of the individuals from Manazuru is thought to be advantageous for boring into thin substrata.     

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.     

Shell occupation as a limiting factor for Pagurus brevidactylus (Stimpson, 1859) in the Marine State Park of Laje de Santos,Brazil

Hermit crabs are soft-abdomen decapods. They depend upon gastropod shells for protection, with the latter influencing growth rate, mortality and reproduction. The present work aims to provide information about the biology and reproduction of Pagurus brevidactylus in a conservation area in Brazilian southeastern littoral. The material was sampled through active searching by SCUBA diving and refuge traps. We analyzed population structure, fecundity, reproductive output (RO), reproductive periodicity and shell utilization for P. brevidactylus. In total, 54 males, 18 non-ovigerous females, 34 ovigerous females and 10 juveniles were sampled. The females had a fecundity of 69.5 ± 65 eggs, which was lower than that compared to other populations from different localities. The RO ranged from 1.48% to 63.04%, with a mean of 26.75 ± 18.23%. There was a positive relationship between egg number and female size. The animals had continuous reproduction, but they reached smaller maximum sizes when compared with other populations. The shell occupation pattern was also different from other localities, the shells of Phrontis alba being the most occupied; this shell has a smaller size when compared to others that P. brevidactylus occupies in other localities. Therefore, P. alba could have had limited the growth and reproduction of P. brevidactylus.     

Growth Characteristics of the Common Mussel Mytilus edulis from the White Sea

We studied the following growth indices of the White Sea mussels Mytilus edulis: shell length, total weight, soft tissue weight, and shell weight. The coefficients of allometric relationships between the indices were determined. Age-related changes in the indices could be approximated by the Bertalanffy equation. The maximum age of mollusks in the studied population equaled 13 years (with the maximum shell length of 66.2 mm). Growth rate of littoral mussels in the region of Umba Settlement (Northern Kandalaksha Bay) was lower as compared to those published for other littoral White Sea populations (Chupa Bay).     

Bioerosion of oyster shells in brackish modern mangrove swamps,Nigeria

Studies on shells of the intertidal oyster Crassostrea tulipa cemented to aerial prop roots and stems of the mangroves Rhizo‐phora racemosa, R. harrisonii, and R. mangle from the Cross River and Qua Iboe River estuaries show that photosynthetic en‐dolithic cyanobacteria (blue‐greens) are the major bioerosional agent, affecting about 94% of the shells examined. Pblychaetes attacked less than 10% of these materials and thus have low bioerosional impact.

Herbivorous gastropods play a secondary role because, by grazing, they clear the shell surface of encrusting cyanobacteria and thus enhance the activity of boring forms. Where epilithic cyanobacteria have been removed, the gastropods sometimes leave faint grazing traces of low fossilization potential.

Microfloral boring activity is high at all stations, but the assemblage is of markedly low diversity compared with those of littoral and shallow sublittoral marine communities and may thus be useful as a paleoenvironmental tool. Bioerosional agents are directly or indirectly responsible for the disintegration of oyster shells, whose fragments are incorporated in muddy intertidal sediments.     

Identification of ecophenotypic trends within three European freshwater mussel species (Bivalvia: Unionoida) using traditional and modern morphometric techniques

Most species of freshwater mussels (Unionoida) show a wide variability in shell form and size but an understanding of which factors determine unionoid morphology is poor. We identified ecophenotypic trends in shell and internal characters within three unionoid species from two habitat types (marinas and river) of the River Thames, UK, using traditional and modern morphometric techniques. In marinas, all species grew to larger maximum sizes than in the river, which might be a result of higher temperatures and phytoplankton densities in marinas. Unio pictorum in marinas was more elongated than in the river and Fourier shape analysis revealed a trend from dorsally arched river specimens to straight dorsal and pointed posterior margins in marina individuals. The degree of shell elongation and shape of dorso‐posterior margin were not associated with sediment composition, but were associated with the different hydrological characters of the two habitat types. Relative shell width was a poor indicator of collection site and influenced by allometric growth. Unlike U. pictorum, a difference in shell elongation of marina and river mussels could not be detected in Unio tumidus and Anodonta anatina. However, all three species showed the same trends regarding the shape of the dorso‐posterior shell margin. This shell character may thus have broad ecological significance and could have considerable utility to palaeontologists, taxonomists, and conservation biologists. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 814–825.     

Microgeographical variation in shell strength in the flat periwinkles Littorina obtusata and Littorina mariae

The strength of molluscan shells has been shown to vary in adaptive ways in a number of species and one of the main factors thought to be involved is shell-crushing by predators. A recent study found that the sibling species of flat periwinkle Littorina obtusata and Littorina mariae showed significant differences in the rates at which shell strength increased with shell length in specimens which had been collected from the same location, where the species were sympatric. This paper describes differences between the shells of the two species from a number of localities around Milford Haven in Dyfed, Wales, and local geographical variation in the shells. Littorina mariae, which is normally found at lower tidal levels than L. obtusata, matures at a smaller shell length. Both species reinforce the shell as they grow since shell strength, determined as the maximum force applied by a hydraulic tensile testing machine before the shell cracked, is strongly positively allometric; it increases at a rate close to the cube of shell length whilst isometric growth would result in strength increasing in proportion to the square of shell length. Because L. mariae matures earlier and reinforces the shell at a smaller size, the mature shell of L. mariae is substantially stronger on average than that of a similar sized but immature L. obtusata. At maturity the shell strengths of the two species are not very different despite the substantial difference in mean shell length. Strength varies significantly from shore to shore, and with the level of the shore from which the animals were collected. Strength increases down the shore in both species. Shell strength decreases with exposure to wave action in L. mariae but increases with exposure in L. obtusata; there is also substantial shore-to-shore variation which is not explained by exposure. Path analysis was used to explore the relationship between shell strength and other measured shell parameters (mass, length, height, thickness). The best predictor of shell strength in both species is a parameter which is heavily positively loaded on LN (shell mass) and strongly offset by negative loadings on LN (shell length) and LN (shell height). This is logical because for a given shell length a heavier shell will be thicker and stronger, whilst for a given shell mass a bigger shell will be thinner and therefore weaker. Such differential variation of shell mass and shell length explains most of the geographical variation observed in shell strength; shells are stronger in snails collected from one place than from another because, for the same shell length they are heavier or, to put it the other way, because at the same shell mass, they are smaller.     

Comparative morphometric study of the invasive pearl oyster <Emphasis Type="Italic">Pinctada radiata</Emphasis> along the Tunisian coastline

In order to study the relative growth of the pearl oyster Pinctada radiata in Tunisia, a total of 330 individuals of this species were collected from six sites along the Tunisian coastline. Quantitative measurements of collected oysters were conducted for shell height, shell length, shell width, hinge length, height and width of the nacreous part and wet weight. The size structure of the sampled populations was described and the relative growth between different morphometric characteristics was estimated as allometric growth lines for the six P. radiata samples. It appeared that the majority of examined samples were dominated by large individuals that exceed a shell height of 42 mm. The maximum size (100.5 mm), recorded in Bizerta lagoon, is bigger than that recorded elsewhere in particular in the Red Sea. Size distribution analysis also showed that the majority of P. radiata samples were dominated by two or more size groups. Differences of allometric regression were found between the examined samples for the tested relationships. Moreover, the Factorial Discriminant Analysis, coupled with Ascending Hierarchic Classification, classified the sub-populations according to geographic locations.