Inheritance of longitudinal white shell bands in the snail <Emphasis Type="Italic">Littorina obtusata</Emphasis> (Gastropoda,Prosobranchia)

The hypothesis on monogenic inheritance of white longitudinal shell bands in the snail Littorina obtusata was tested. The data showed that a single gene with two alleles was responsible for the presence of the bands on the snail shell. The presence of bands was the dominant character.     

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.     

Distribution of pigments in the shell of the gastropod <Emphasis Type="Italic">Littorina obtusata</Emphasis> (Linnaeus, 1758)

The patterns of pigment distribution in the shell of the gastropod Littorina obtusata were studied. It was determined that the background coloration of the periwinkle shell resulted from the coloration interaction of the external and internal parts of the ostracum. The coloration of the latter can be monochromatic or twolayer. The external and/or internal zones of the ostracum can be colored purple, white, orange, yellow and greenish. Only 11 of 25 possible color combinations were recorded. Four additional versions of shell background coloration resulted from partial depigmentation of external or internal parts of ostracum. The mottling pattern corresponded to white or depigmented lenticular sites in the external part of the ostracum. The bands on the shell may be subdivided into two groups according to their means of formation. The bands that appeared due to redistribution of pigments forming background coloration belonged to the first group, the bands arising as a layer of additional (white, yellow, orange) pigment belonged to the second group. Various band versions could occur simultaneously in the same individual.     

Negative frequency‐dependent selection maintains shell banding polymorphisms in two marine snails (Littorina fabalis and Littorina saxatilis)

The presence of shell bands is common in gastropods. Both the marine snails Littorina fabalis and Lttorina saxatilis are polymorphic for this trait. Such polymorphism would be expected to be lost by the action of genetic drift or directional selection, but it appears to be widespread at relatively constant frequencies. This suggests it is maintained by balancing selection on the trait or on a genetically linked trait. Using long time series of empirical data, we compared potential effects of genetic drift and negative frequency‐dependent selection (NFDS) in the two species. The contribution of genetic drift to changes in the frequency of bands in L. fabalis was estimated using the effective population size estimated from microsatellite data, while the effect of genetic drift in L. saxatilis was derived from previously published study. Frequency‐dependent selection was assessed by comparing the cross‐product estimator of fitness with the frequency of the polymorphism across years using a regression analysis. Both studied species showed patterns of NFDS. In addition, in L. fabalis, contributions from genetic drift could explain some of the changes in banding frequency. Overdominance and heterogeneous selection did not fit well to our data. The possible biological explanations resulting in the maintenance of the banding polymorphism are discussed.     

Inheritance of longitudinal shell bands in the snails <Emphasis Type="Italic">Littorina obtusata</Emphasis> and <Emphasis Type="Italic">L. saxatilis</Emphasis> (Gastropoda,Prosobranchia)

The hypothesis of a monogenic inheritance of dark longitudinal bands on the shell in the gastropods Littorina obtusata and L. saxatilis was checked. One gene having two alleles proved to be responsible for the shell banding pattern in both of the species. The presence of bands was a dominant character in either case.     

South African populations of Littorina rudis

Littorina rudis was collected from stands of Spartina capensis in saltmarsh habitats within Langebaan and knysna lagoon, Cape Province. It was absent from rocky sores and from saltmarshes east of Knysna. The shell, radular and penial morphology are described. L. rudis from both localities bore tessellated colour patterns similar to those of British saltmarsh ecotypes. The Knysna population contained individuals with a unique amber background colour. The shells from Knysna and Langebaan were very thin in accordance with the sheltered habitats. The number of embryos carried in the brood puch was similar in both countries. Sexual maturity was attained at about 4 mm shell length; the male and female modal sizes were 4.9 mm and 5.1 mm respectively at Langebaan. No. published records were found for L. rudis in South West Africa. Some possible origins of the South African L. rudis populations are discussed.     

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

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

Colour polymorphism in the intertidal snail Littorina rudis Maton

The tictors affecting the variation of shell colour morph frequencies of the intertidal snail Littorina rudis Maton have been examined using ordination and correlation techniques. The major gradient underlying variation in shell colour is related to habitat type and structure. Dark, patterned shells are characteristic of cliff and salt marsh habitats whilst light, unpatterned shells reach high frequencies on boulder shores. Colour morphs do not differ significantly in shell thickness and between-shore variation in colour morph frequencies cannot be directly attributed to habitat dilterences in the risk of shell injury, but to other factors associated with habitat type and structure, e.g. parasitism. It is suggested that the maintenance of shell colour polymorphism in L. rudis is probably mediated by selection on pleiotropic characters rather than shell colour alone.     

Shell colour variation in Littorina saxatilis Olivi (Prosobranchia: Littorinidae): a multi-factor approach

The marine snail Littorina saxatilis is highly polymorphic for shell colour. It lives in the heterogeneous intertidal zone, where there are sharp transitions in a number of abiotic factors that may influence the relative fitness of morphs. We investigated the hypothesis of selected variation by relating the colour distribution to five factors (wave exposure, substratum, shore level, sex, snail age), and to interactions between them. We compared patterns from geographical areas in Sweden, Iceland and Russia. Cryptic morphs (tessellated and different dark colours) generally dominated (80–98%) while conspicuous morphs (white, yellow, red and banded) were less common (2–20%). The colour frequencies were often related to wave exposure, substratum and shore level. Frequencies rarely varied with age and never with sex. In order to test the assumption that the different colours are genetically determined we cross-bred snails from Iceland in the laboratory. Both the presence of bands and the ground colours of the shell were inherited, and we have tentative support for a one-locus two-allele model for banding. Our results support a model of selected inherited colour variation, involving a number of different selective agents, the importance of which may vary between populations on local and geographical scales.     

Gills and size in some small Littorina

Gills were examined in Littorina saxatilis, L. arcana and L. neglecta, using animals of similar size. The form of the gills in the three groups is very similar, although there is an indication of qualitative differences between them. The relationship of number of gill lamellae to shell length shows that L. saxatilis and L. arcana share the same allometric relationship, while in L. neglecta the slope is the same but the intercept is less: at equivalent shell sizes, these animals have about three fewer lamellae.     

Lack of significant esterase and myoglobin differentiation in the periwinkle,Littorina striata (Gastropoda,Prosobranchia)

The relationship between gene flow and the maintenance of geographic or morphology-related variation in the polymorphic Macaronesian periwinkle, Littorina striata, was investigated by means of isoelectric focusing of esterases (EST) and myoglobin (Mb). This revealed that: (1) individual EST variation is very high, (2) there is no EST differentiation between sexes, shell morphotypes or wave-exposure regimes, (3) there is no clear macrogeographic patterning of EST variability, although both Cape Verde Islands have the highest, with exception of Flores, mean number of EST bands, and (4) there is no Mb variation, not even between islands separated by more than 2000 km. These results indicate that L. striata shows a high degree of genetic homogeneity among geographic populations and that the morphological patterning in this species persists in the presence of intense gene flow.     

Ontogeny of shell morphology and shell strength of the marine snails Littorina obtusata and Littorina mariae: different defence strategies in a pair of sympatric, sibling species

The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi & Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata , however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime.     

Maintenance of clinal variation for shell colour phenotype in the flat periwinkle Littorina obtusata

Clines can signal spatially varying selection and therefore have long been used to investigate the role of environmental heterogeneity in maintaining genetic variation. However, clinal patterns alone are not sufficient to reject neutrality or to establish the mechanism of selection. Indirect, inferential methods can be used to address neutrality and mechanism, but fully understanding the adaptive significance of clinal variation ultimately requires a direct approach. Ecological model systems such as the rocky intertidal provide a useful context for direct experimentation and can serve as a complement to studies in more traditional genetic model systems. In this study, we use indirect and direct approaches to investigate the role of environmental heterogeneity in the maintenance of shell colour polymorphism in the flat periwinkle snail, Littorina obtusata. We document replicated clines in shell colour morph frequencies over thermal gradients at two spatial scales, contrasting with patterns at previously reported microsatellite loci. In addition, experimental results demonstrate that that shell colour has predictable effects on shell temperature and that these differences in temperature, in turn, coincide with patterns of survivorship under episodic thermal stress. Direct manipulation of shell colour revealed that shell colour, and not a correlated character, was the target of selection. Our study provides evidence that spatially varying selection via thermal regime contributes to the maintenance of shell colour phenotype variation in L. obtusata in the sampled areas of the Gulf of Maine.     

On the taxonomy of Littorina africana (Mollusca: Gastropoda)

L. africana and L. knysnaensis are regarded as two morphs of a single species which exhibits a genetic cline along the south-eastern coast of southern Africa. The dark brown morph knysnaensis dominates the western, cooler end of the cline and is replaced by the pale blue morph africana at the warmer end of the cline. These conclusions are based on evidence from the latitudinal distributions, the complete range of intermediate forms regarding shell colour and shell morphology and the lack of differences in redular morphology, penial morphology or habitat.     

The effects of environment upon shell construction and strength in the Rough periwinkle Littorina rudis Maton (Mollusca: Gastropoda)

The interrelationships between environment, shell construction and shell strength of the Rough periwinkle Littorina rudis are investigated in samples of shells taken from a range of shores differing in exposure and salinity level. The effects of different aspects of shell construction upon shell strength are evaluated. Shell strength is compared to chelar bite force of the predatory crab Carcinus maenas. The results are discussed in terms of a theory of factors of safety, and the observed patterns of variation in shell strength and construction with respect to environment are found to be consistent with the predictions of the theory.     

Shape variation in the rough periwinkle Littorina saxatilis on the west and south coasts of Britain

Variation in the shape of the shell in Littorina saxatilis Olivi has been shown to be due largely to the same variables on both the west and the south coasts of Britain, and it exhibits various clines. Two important aspects are the size of the aperture, which becomes relatively larger from the Isle of Man southwards to Cornwall and eastwards from Devon to the Isle of Wight, and the jugosity of the shell, which increases with distance from Cornwall both northwards as far as the Isle of Man and eastwards as far as Kent. Superimposed on the clines are domains of shape, notably one in Lewis/Harris, where the shells have a relatively large aperture, which is long and narrow, coupled with a rather globose second whorl. The local and geographical aspects of shell shape variation are discussed.     

The role of selection in the colour polymorphism of Littorina rudis Maton and Littorina arcana Hannaford-Ellis (Prosobranchia: Littorinidae)

Forty-one mixed samples of winkles containing the closely related species, Littorina rudis and L. arcana , were collected from different parts of the British Isles. Littorina rudis was the more ubiquitous species, with L. arcana being more-or-less confined to vertical cliffs and rocks. The frequencies of different shell colour patterns were determined for both species in each sample. Several colour morphs were diagnostic of one or other of the species over large areas, which confirms that these are separate species. The frequencies, in the two species, of the two commonest morphs, brown and fawn, were strongly correlated; in several other morphs their presence and absence in the two species was significantly associated; the levels of phenotypic diversity in the two species were also correlated. These patterns could not have arisen if the variation between shores was the result of genetic drift or founder effects, so some form of selection is implicated. In the brown morph there is some evidence for frequency-dependent selection, and it is suggested that the polymorphism may be maintained by visual predators through apostatic selection.     

Mechanism of a plastic phenotypic response: predator-induced shell thickening in the intertidal gastropod Littorina obtusata

Phenotypic plasticity has been the object of considerable interest over the past several decades, but in few cases are mechanisms underlying plastic responses well understood. For example, it is unclear whether predator-induced changes in gastropod shell morphology represent an active physiological response or a by-product of reduced feeding. We address this question by manipulating feeding and growth of intertidal snails, Littorina obtusata, using two approaches: (i) exposure to predation cues from green crabs Carcinus maenas and (ii) reduced food availability, and quantifying growth in shell length, shell mass, and body mass, as well as production of faecal material and shell micro-structural characteristics (mineralogy and organic fraction) after 96 days. We demonstrate that L. obtusata actively increases calcification rate in response to predation threat, and that this response entails energetic and developmental costs. That this induced response is not strictly tied to the animal's behaviour should enhance its evolutionary potential.     

Hybridization patterns between two marine snails,Littorina fabalis and L. obtusata

Characterizing the patterns of hybridization between closely related species is crucial to understand the role of gene flow in speciation. In particular, systems comprising multiple contacts between sister species offer an outstanding opportunity to investigate how reproductive isolation varies with environmental conditions, demography and geographic contexts of divergence. The flat periwinkles, Littorina obtusata and L. fabalis (Gastropoda), are two intertidal sister species with marked ecological differences compatible with late stages of speciation. Although hybridization between the two was previously suggested, its extent across the Atlantic shores of Europe remained largely unknown. Here, we combined genetic (microsatellites and mtDNA) and morphological data (shell and male genital morphology) from multiple populations of flat periwinkles in north‐western Iberia to assess the extent of current and past hybridization between L. obtusata and L. fabalis under two contrasting geographic settings of divergence (sympatry and allopatry). Hybridization signatures based on both mtDNA and microsatellites were stronger in sympatric sites, although evidence for recent extensive admixture was found in a single location. Misidentification of individuals into species based on shell morphology was higher in sympatric than in allopatric sites. However, despite hybridization, species distinctiveness based on this phenotypic trait together with male genital morphology remained relatively high. The observed variation in the extent of hybridization among locations provides a rare opportunity for future studies on the consequences of different levels of gene flow for reinforcement, thus informing about the mechanisms underlying the completion of speciation.     

First record of colour polymorphism in the Neotropical apple snail Asolene platae: inheritance mechanism and evidence for multiple paternity

Asolene platae (Ampullariidae) is a dioecious freshwater snail with subaquatic gelatinous egg masses that dwells in the Río de la Plata basin (Argentina). The aim of this study was to describe the inheritance mechanism of the colour variations of the shell and soft parts of this snail, and to study their potential use as a genetic marker. The wild-type phenotype presents dark pigments in the soft parts and in shell bands, whereas the yellow phenotype lacks dark pigments in the soft parts and also most dark bands in the shell, except for a subsutural and a periumbilical band. The data showed that the lack of pigments in A. platae is a recessive homozygotic condition with a simple Mendelian inheritance mechanism. Females of the wild-type phenotype had a higher number of bands than the males. The pigment of the bands of both phenotypes is located in the calcareous matrix of the shell. Using the lack of pigments as a genetic marker we demonstrated the existence of biparental egg masses in A. platae, hitherto known in only one species within the Ampullariidae.