Shell Coiling Research Articles

Assessing phenotypic variation and plasticity of endemic gastropods from thermal water refugia using complex morphometric techniques: A case study of Lake Pețea melanopsids.

Understanding the underlying reasons for phenotypic plasticity and resulting morphological disparity is one of the key topics of evolutionary research. The phenotypic plasticity of extant and fossil melanopsids has been widely documented. Yet millennial-resolution, well-dated records from small aquatic habitats harboring endemics are scarce. The thermal spring-fed Lake Pețea is an ice age refugia harboring a unique endemic warm-water fauna. Subfossil melanopsids display incredible morphological variability from smooth to keeled, elongated to ribbed, shouldered forms. Numerous morphotypes have been considered as individual taxa with a fluent succession from the smooth elongated to the ribbed, shouldered types. This study presents an extensive morphometric analysis of subfossil melanopsids (ca. 3500 specimens) derived from stratified samples with an independent chronology. The aim was to separate morphotypes for investigations of temporal morphological disparity. Our results challenge the widely accepted hypothesis that proposes the evolution of shouldered, compressed, ribbed shells through a two-step process from smooth elongated spindle-shaped shells. Instead, it suggests that the subfossil shells belong to two distinct taxa present throughout the available stratigraphic data. The main components of shape variation, shape globularity, and shell coiling seem allometry-related. Ribs, striation, and keels appear randomly. High-spired spindle-shaped forms were considered to represent specimens of Microcolpia daudebartii hazayi. Bulkier low-spired and shouldered specimens represent phenotypes of Mi. parreyssii parreyssii. The collective and random distribution of morphotypes from the early stages of the lake's history also refutes the idea of a continuous transformation of the elongated forms into compressed, shouldered ones. Rather points to multiple events and environmental stimuli triggering development. Melanopsids appear in Late Glacial horizons, with Theodoxus prevostianus preferring temperatures above 23°C which may indicate the subordinate presence of hot water microhabitats in cooler waters.

Single-gene speciation: Mating and gene flow between mirror-image snails.

Variation in the shell coiling, or chirality, of land snails provides an opportunity to investigate the potential for “single‐gene” speciation, because mating between individuals of opposite chirality is believed not possible if the snails mate in a face‐to‐face position. However, the evidence in support of single‐gene speciation is sparse, mostly based upon single‐gene mitochondrial studies and patterns of chiral variation between species. Previously, we used a theoretical model to show that as the chiral phenotype of offspring is determined by the maternal genotype, occasional chiral reversals may take place and enable gene flow between mirror image morphs, preventing speciation. Here, we show empirically that there is recent or ongoing gene flow between the different chiral types of Japanese Euhadra species. We also report evidence of mating between mirror‐image morphs, directly showing the potential for gene flow. Thus, theoretical models are suggestive of gene flow between oppositely coiled snails, and our empirical study shows that they can mate and that there is gene flow in Euhadra. More than a single gene is required before chiral variation in shell coiling can be considered to have created a new species.

Fine Mapping of the Pond Snail Left-Right Asymmetry (Chirality) Locus Using RAD-Seq and Fibre-FISH

The left-right asymmetry of snails, including the direction of shell coiling, is determined by the delayed effect of a maternal gene on the chiral twist that takes place during early embryonic cell divisions. Yet, despite being a well-established classical problem, the identity of the gene and the means by which left-right asymmetry is established in snails remain unknown. We here demonstrate the power of new genomic approaches for identification of the chirality gene, “D”. First, heterozygous (Dd) pond snails Lymnaea stagnalis were self-fertilised or backcrossed, and the genotype of more than six thousand offspring inferred, either dextral (DD/Dd) or sinistral (dd). Then, twenty of the offspring were used for Restriction-site-Associated DNA Sequencing (RAD-Seq) to identify anonymous molecular markers that are linked to the chirality locus. A local genetic map was constructed by genotyping three flanking markers in over three thousand snails. The three markers lie either side of the chirality locus, with one very tightly linked (<0.1 cM). Finally, bacterial artificial chromosomes (BACs) were isolated that contained the three loci. Fluorescent in situ hybridization (FISH) of pachytene cells showed that the three BACs tightly cluster on the same bivalent chromosome. Fibre-FISH identified a region of greater that ∼0.4 Mb between two BAC clone markers that must contain D. This work therefore establishes the resources for molecular identification of the chirality gene and the variation that underpins sinistral and dextral coiling. More generally, the results also show that combining genomic technologies, such as RAD-Seq and high resolution FISH, is a robust approach for mapping key loci in non-model systems.

Pyramidellid Protoconchs, Eggs, Embryos and Larval Ecology: An Introductory Survey

Abstract: Major papers on pyramidellid protoconchs, larval development, and larval ecology are reviewed, partly with illustrations. Larval shell coiling (including hyperstrophy and heterostrophy) is discussed. A-, B-, and C-type protoconchs are figured and distinguished. Those of species in the Baltic and northwestern France have been used to infer modes of larval development. The reliability of many of these inferences is questioned. Egg, embryo, and veliger morphologies, functions and terminologies are then discussed, including confusions of the terms “egg” and “capsule.” The pigmented mantle organ or gland (“PMO”) is briefly reviewed. Previously published data on the eggs and larvae of Brachystomia “scalaris” (Macgillivray, 1843), of“Turbonilla” cummingi Hori and Okutani, 1997, and of “Odostomia columbiana Dall and Bartsch, 1907”, are reviewed in detail with some new interpretations. In the first of these species, egg size variation is possibly salinity-induced and may cause variation in mode of develo...

Possible functions of Dpp in gastropod shell formation and shell coiling

We examined dpp expression patterns in the pulmonate snail Lymnaea stagnalis and analyzed the functions of dpp using the Dpp signal inhibitor dorsomorphin in order to understand developmental mechanisms and evolution of shell formation in gastropods. The dpp gene is expressed in the right half of the circular area around the shell gland at the trochophore stage and at the right-hand side of the mantle at the veliger stage in the dextral snails. Two types of shell malformations were observed when the Dpp signals were inhibited by dorsomorphin. When the embryos were treated with dorsomorphin at the 2-cell and blastula stages before the shell gland is formed, the juvenile shells grew imperfectly and were not mineralized. On the other hand, when treated at the trochophore and veliger stage after the shell gland formation, juvenile shells grew to show a cone-like form rather than a normal coiled form. These results indicated that dpp plays important roles in the formation and coiling of the shell in this gastropod species.

Is water temperature responsible for geographic variation in shell mass of Littorina obtusata (L.) snails in the Gulf of Maine?

A latitudinal cline in length-standardized shell thickness and mass of Littorina obtusata in the Gulf of Maine (GoM) has been attributed to predation by the European green crab Carcinus maenas. However, latitudinal variation in water temperature may also contribute to this cline. To investigate this hypothesis, we grew snails from 2 thin-shelled and 2 thick-shelled populations in warm (16 °C) and cold (10 °C) water treatments, representing summer temperatures of southern and northern parts of the GoM, respectively. Snails grown in warmer water showed a markedly greater increase in shell length (50%) and shell mass (60%) compared to conspecifics grown in colder water, but shell mass standardized for length was only weakly (relative to clinal variation) and not consistently (among populations) affected by temperature. Our experimental snails made shells that were lighter, when standardized for length, than conspecifics grown in nature. This “shell thinning” was particularly apparent in warmer water, because experimental snails diverted the increased calcification afforded by warmer water parallel (shell elongation) rather than perpendicular (shell thickening) to the axis of shell coiling. Interestingly, we estimate that if 16 °C snails had only grown to the length of 10 °C snails and used the additional shell material they produced to make their shell thicker rather than longer, they would have produced a shell comparable in mass to that of snails from the same population and shell length grown in nature. We conclude that water temperature is not directly responsible for natural variation in size-standardized shell thickness of L. obtusata in the GoM, but that it may contribute to this pattern indirectly, for example by facilitating the snail's plastic phenotypic response (shell thickening) to predation risk.

Hermit arthropods 500 million years ago?

Cambrian intertidal sandstones of North America record early excursions of large animals onto tidal flats, where continuous microbial films served as preservational agents for surface tracks. Whereas biomineralized fossils are rare in such lithofacies, trace fossils from the Late Cambrian Elk Mound Group of Wisconsin illustrate how some arthropods might have managed to withstand the vicissitudes of subaerial exposure—by using foreign shells like hermit crabs. This behavior is suggested by trackways (Protichnites eremita isp. nov.), which have “tail” impressions that are obliquely segmented and always shingled to the left side. These anomalous impressions are best explained by a dextrally coiled shell intermittently touching the sediment. However, unlike in modern hermit crabs, this shell was too small to house the whole animal. It probably served only to provide a humid chamber that reduced desiccation of the animal9s abdominal gills. The dorsal flexure of the tail, in connection with dextral shell coiling, resulted in left-hand shingling of the touch marks.

Lophotrochozoa Get into the Game: The Nodal Pathway and Left/Right Asymmetry in Bilateria

Animals as diverse as humans, flies, crabs, and snails show overall bilateral symmetry, but each species has specific structures and organs that display left/right asymmetry, and the presence of these asymmetries is vital to the organism. Here, we review recent results showing that part of the molecular pathway that sets left/right asymmetry in vertebrates is also conserved in snails, suggesting that left/right asymmetry was present in the common ancestor of all bilaterians. More specifically, we can now predict that the signaling molecule Nodal and the transcription factor Pitx were expressed on the right side of the bilaterian ancestor. These results also allow us to understand how the direction of shell coiling (chirality) is regulated in snails and provides interesting insights into the possible inversion of the dorsoventral axis in the lineage leading to chordates.

Snail shell coiling (re-)evolution and the evo-devo revolution

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APPEARANCE OF MORPHOLOGICAL NOVELTY IN A HYBRID ZONE BETWEEN TWO SPECIES OF LAND SNAIL

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.

APPEARANCE OF MORPHOLOGICAL NOVELTY IN A HYBRID ZONE BETWEEN TWO SPECIES OF LAND SNAIL

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.

Morphostructural constraints and phylogenetic overprint on sutural frilling in Late Jurassic ammonites

The functional significance of frilled septa and complex sutures in ammonoids has generated ongoing debate. The ‘classic’ hypothesis envisages ammonoid shells and septa as designed for resisting ambient hydrostatic pressure, complex sutures being the evidence of strength in shells for colonization of deep habitats. Here we address the ‘suture problem’, focusing on the analysis and interpretation of variables in our database of Late Jurassic ammonites not included in previous studies, such as whorl height (W h), whorl shape (S), shell coiling (WD), taxonomic grouping and basic planispiral shell shape. The results indicate that sutural complexity, as measured by the fractal dimension (D f) value of the suture line, is positively correlated with W h, and that the sutures of oceanic shells tend to provide, for a given W h value, lower D f estimates than do those of neritic shells. No general trend of increase in sutural complexity was noted for specimens recovered from swell areas belonging to oceanic fringes with respect to those that inhabited epicontinental shelves. In fact, Perisphinctoidea, the clade best represented in the database analysed, shows a higher D f mean value in neritic species than in epioceanic ones. Significant differences in sutural complexity were detected for groups of ammonites classified according to shell shapes (WD, S). Oxycones and discocones, streamlined potential swimmers, show the highest D f mean values, while spherocones and cadicones, which were presumably vertical vagrants, present the lowest ones. This indicates that sutural complexity was more related to shell geometry than to bathymetry.

Abnormalities as natural experiments: a morphogenetic model for coiling regulation in planispiral ammonites

Exceptional examples of planispiral ammonites that were infested by epizoans during life display alterations of their normal coiling. Most commonly, the epizoan(s) settled on the venter of the ammonite and constituted an obstacle for the whorl tube grown one whorl later; this caused lateral deviation of the whorl tube and tilting of the ammonoid because of changes in the hydrostatic condition; from here on, the whorl tube periodically crossed the venter of the preceding whorl, thereby producing a zigzag coiling pattern. Some epizoans, which were particularly centered on the midventer, provoked detachment between whorls. In a few cases, lateral placement of the epizoan did not directly obstruct the normal growth path of the ammonite but induced a trochospiral coiling pattern. Both the zigzag and the trochospiral pattern were created when the ammonite tried to maintain the growth direction within the vertical plane at the same time as whorls remained in contact along a differentiated dorsal epithelium. The aperture reacted to changes in growth direction, to maintain also a permanent angle with the vertical direction. Growth direction, then, was a major morphogenetic parameter in ammonites, because it contained the necessary instructions for correct shell coiling. The model based on the observation of fabricational defects has been tested by a theoretical model by which the different situations so far observed are simulated and in which the parameters are the morphogenetic instructions inferred to have been present in the biological system.

Morphological significance of the orientation of shell coiling and the outline of “the aperture” in bivalve molluscs

Morphological significance of the orientation of shell coiling and the outline of “the aperture” in bivalve molluscs

Origine, évolution et systématique de Praechubbina n. gen., foraminifères alveolinacea du crétacé supérieur du Guatemala et du Mexique

The sub family Chubbininae is represented by many taxa known from the shallow campanian-maastrichtian carbonate platforms of south-east Mexico and Guatemala. The structure of this group, together with the origins of the genus Chubbina, can be understood through the description of 6 new species. One of them, Pseudonummoloculina pecheuxi n. sp., with typical Miliolid features (devoid of any internal partitions), seems to be the common ancestor of the others. It is especially that its aperture, a slit with a crenellated margin, give birth to the multiple rows of rounded apertures distinctive of the sub family, of which the new genus Praechubbina belongs. The 5 other new species, together with « Borelis cardenasensis Barker and Grimsdale, are gathered in the new genus. One of them, Praechubbina breviclaustra n. sp., displays a link between Pseudonummoloculina pecheuxi n. sp. and the five congeneric new species: its aperture, originally a slit with a crenellated margin, changes into rows of several rounded openings; the same species shows the advent of a faint partitioning. The other species, P. cardenasensis ( Barker et Grimsdale), P. compressa n. sp., P. obesa n. sp., P. ovoidea n. sp. et P. streptospira n. sp. share the same apertural features; they display a large range fvarious of shell coiling and endoskeletal organisation (inclusive of floors emergence) not connected to the proloculus size. The new genus corresponds to a very rich plexus in which the various species of Chabbina takes roots.

Variation of shell shape in the clonal snail Melanoides tuberculata and its consequences for the interpretation of fossil series.

Interpreting paleontological data is difficult because the genetic nature of observed morphological variation is generally unknown. Indeed, it is hardly possible to distinguish among several sources of morphological variation including phenotypic plasticity, sexual dimorphism, within-species genetic variation or differences among species. This can be addressed using fossil organisms with recent representatives. The freshwater snail Melanoides tuberculata ranks in this category. A fossil series of this and other species have been studied in the Turkana Basin (Kenya) and is presented as one of the best examples illustrating the punctuated pattern of evolution by the tenants of this theory. Melanoides tuberculata today occupies most of the tropics. We studied variation of shell shape in natural populations of this parthenogenetic snail using Raup's model of shell coiling. We considered different sources of variation on estimates of three relevant parameters of Raup's model: (1) variation in shell shape was detected among clones, and had both genetic and environmental bases; (2) sexual dimorphism, in those clones in which males occur, appeared as an additional source of shell variation; and (3) ecophenotypic variation was detected by comparing samples from different sites and years within two clones. We then tested the performance of discriminant function analyses, a classical tool in paleontological studies, using several datasets. Although the three sources of variation cited above contributed significantly to the observed morphological variance, they could not be detected without a priori knowledge of the biological entities studied. However, it was possible to distinguish between M. tuberculata and a related thiarid species using these analyses. Overall, this suggests that the tools classically used in paleontological studies are poorly efficient when distinguishing between important sources of within-species variation. Our study also gives some empirical bases to the doubts cast on the interpretation of the molluscan series of the Turkana Basin.

History of classification of Mesozoic ammonites

The ammonites were first subdivided at generic level on the basis of shell coiling, heteromorphs or ‘uncoiled’ shells being separated after 1799 from the normally coiled shells. The latter were referred to a single genus, Ammonites, until the late nineteenth century. After the publication of Charles Darwin's Origin of species in 1859, the more forward-looking workers adopted the ideal of a classification based on phylogenetic relationships. This necessitated the use of a wider range of characters. Increasing realization of the complexity of phylogenetic relationships brought a proliferation of names at lower taxonomic levels—genus and family. For a while, the belief that the growth stages of the shell provided a reliable key to ancestry dominated the work of several specialists. In the early twentieth century, Schindewolf favoured reliance on a single character, sutural ontogeny, as the decisive criterion in determining relationships. Other workers favoured a balanced appreciation of several characters. In the mid-twentieth century some felt that the detailed lineages, of which glimpses are seen through the fossil record, might never be disentangled, and that classification at generic and specific level must often be arbitrary. Recently, more abundant material and better stratigraphical information have led to the recognition of lineages based on closely-spaced assemblages which can be used as a basis for classification. The usefulness of the present classification is briefly considered.

Mechanical constraints on aperture form in gastropods.

Gastropod apertures reflect expanded states of their mantle edge under variable boundary conditions. The apertures are divided into two groups: apertures without distinct overlap zones (e.g., whorl overlap) and those with overlap zones. Each group follows a unique morphological rule. Apertures without overlap zones are generally circular in outline. Apertures with overlap zones are either elongated perpendicularly to the overlap zone or inflated abapically. Moreover, the latter abapically inflated apertures are generally accompanied by a straight section anterior to the overlap zone along the columellar axis (columellar part). Numerical analysis of an elastic double membrane tube whose main frame simulates the gastropod mantle indicates that these morphological rules are the products of mantle edge expansion under the condition that the head-foot mass presses against the mantle edge in both the overlap zone and the columellar part. The mantle edge in these two zones is thus in a completely or partly fixed boundary condition at the moment of shell growth. The rest of the mantle edge is free to expand either symmetrically or asymmetrically. It is hypothesized that the head-foot mass is a driving force for regulating the pattern of shell coiling and apertural shape.

Shell Coiling in Gastropods: Analysis by Stereographic Projection

Shell coiling is readily visualized as a revolution and expansion of the shell's aperture around the coiling axis. Stereographic projections provide an effective tool for plotting the orientations of the aperture through timelspace. In isometric growth, poles to the aperture (e.g., perpendiculars to the aperture plane) plot on a small circle on the stereographic projection, and the center of the small circle defines the orientation of the shell's coiling axis. The stereographic analysis requires no a priori determinations of the coiling axis position in the shell. Stereographic analyses consider shell growth in terms of rigid body motions of the entire aperture, as opposed to conventional methods that consider only the form of individual spiral trajectories (single points on the aperture). Stereographic projections offer a novel and instructive approach to the analysis of shell coiling.

Polymorphism for direction of coil in Partula suturalis: Behavioural isolation and positive frequency dependent selection

Partula suturalis is unusual among snails in being polymorphic for direction of shell coiling. Polymorphic populations of P. suturalis occur in narrow clines between areas of monomorphic dextral or sinistral populations. The distribution of dextral populations coincides with the distribution of the sinistral species P. mooreana, P. tohiveana and P. Olympia suggesting character displacement for reproductive isolation. Chirality affects courtship behaviour, and no-choice mating experiments show that pairs with opposite coil mate only 20 per cent as frequently as do pairs with the same coil. Fewer young are produced by these mixed pairs. Fertilities of dextral and sinistral P. suturalis are the same in a population composed of equal frequencies of the two morphs. However, among wild-caught snails from a population in which sinistrals occur at low frequency, the rare morph produces fewer young than the common morph. The effects of chirality on behavioural isolation result in selection against the rare morph, accounting for the rarity of polymorphic populations. Such positive frequency dependent selection is capable of maintaining the stability of hybrid zones.