Species Of Littorina Research Articles

The molecular background of the aspartate aminotransferase polymorphism in Littorina snails maintained by strong selection on small spatial scales

Allozymes present several classical examples of divergent selection, including the variation in the cytosolic aspartate aminotransferase (AAT) in the intertidal snails Littorina saxatilis. AAT is a part of the asparate-malate shuttle, in the interidal molluscs involved in the anaerobic respiration during desiccation. Previous allozyme studies reported the sharp gradient in the frequencies of the AAT100and the AAT120 alleles between the low and high shores in the Northern Europe and the differences in their enzymatic activity, supporting the role of AAT in adaptation to desiccation. However, the populations in the Iberian Peninsula showed the opposite allele cline. Using the mRNA sequencing and the genome pool-seq analyses we characterize DNA sequences of the different AAT alleles, report the amino acid replacements behind the allozyme variation and show that same allozyme alleles in Northern and Southern populations have different protein sequences. Gene phylogeny reveals that the AAT100 and the northern AAT120 alleles represent the old polymorphism, shared among the closely related species of Littorina, while the southern AAT120 allele is more recently derived from AAT100. Further, we show that the Aat gene is expressed at constitutive level in different genotypes and conditions, supporting the role of structural variation in regulation of enzyme activity. Finally, we report the location and the structure of the gene in the L. saxatilis genome and the presence of two additional non-functional gene copies. Altogether, we provide a missing link between the classical allozyme studies and the genome scans and bring together the results produced over decades of the genetic research.

Phenotypic plasticity in gastropod shell remodeling

AbstractA laboratory experiment was conducted with the marine gastropodLittorina littoreafrom three different sites in Long Island Sound to test for inducible responses to the non‐native predatory crabHemigrapsus sanguineus. Individuals ofL. littoreadid not exhibit decreased shell growth in response to increased predation risk, which differs from the response seen in other species ofLittorina.However, snails did exhibit slower growth in response to reduced food availability, as seen in other gastropods. Surprisingly, snails from two sites exhibited differences between the crab‐exposed and reduced food‐availability treatments in how they remodeled the thickness of different microstructural layers of shell material in apical parts of the shell. Although individuals ofL. littoreadid not show the typical shell thickening response seen in other gastropods, they did show differences among populations in remodeling their shell. This underlies the importance of examining how the shell is constructed and remodeled in studies of inducible defenses in gastropods, as well as the importance of examining animals from multiple populations.

A global molecular phylogeny of 147 periwinkle species (Gastropoda, Littorininae)

Reid, D. G., Dyal, P. & Williams, S.T. (2012) A global molecular phylogeny of 147 periwinkle species (Gastropoda, Littorininae). —Zoologica Scripta, 41, 125–136.Complete species‐level molecular phylogenies have been published for several genera of Littorinidae (e.g. Echinolittorina, Littoraria). Here we add new sequence data from three genes (28S rRNA, 12S rRNA, cytochrome oxidase c subunit I) for single specimens of an additional 24 species, to make a data set of 147 (97%) of the 152 recognized species of the subfamily Littorininae. This three‐gene data set is analysed to produce a phylogenetic hypothesis for the subfamily, which includes the first complete species‐level phylogeny of the genus Peasiella and the first three‐gene phylogeny of all Littorina species. The non‐planktotrophic species of Littorina have previously been classified together (as subgenus Neritrema), implying a single origin of this developmental mode. Tests of this hypothesis with the new data are inconclusive, and resolution is not improved in a tree constructed from five genes (adding previously published sequences of 16S rRNA and cytochrome b). Using available fossils for calibration we generate a BEAST chronogram, which emphasizes that the radiation of Littorina is more recent than that of other littorinine genera. A database is provided, listing all known species of Littorininae, with their distributions, development, ecology and gene sequences, as a tool for future evolutionary studies of this model group.

Shell shape and tissue withdrawal depth in 14 species of temperate intertidal snail

Predation is an important selective agent in the evolution of gastropod shell form. The adaptive value of shell elongation, however, has received relatively little attention in the context of predation. Arguments for its functional significance include the hypothesis that shell elongation facilitates soft tissue withdrawal inside the shell, thus elongation may improve fitness when predation by shell entry is intense. Here we ask whether species with more elongate shells also retract more deeply, a longheld assertion that until now lacked empirical support. To do this, we measured shell elongation (shell height divided by breadth) and soft-tissue withdrawal depth (angular retraction) in 14 species of temperate marine snail, covering a wide range of shell forms. Results show shell elongation to be a good predictor of angular retraction across genera and distantly related species. However, the elongation-retraction relationship was not apparent in comparisons among closely related species (specifically, species of Nucella and Littorina), nor were there such relationships among individuals within species (except, perhaps, within a few species having high shell-shape diversity). Competing arguments for the functional significance of shell elongation are discussed, as is the potential role of environment in affecting shell elongation and withdrawal depth independently.

LENGTH POLYMORPHISMS IN AN INTRON OF AMINOPEPTIDASE N PROVIDE A USEFUL NUCLEAR DNA MARKER FOR LITTORINA SPECIES (CAENOGASTROPODA)

Studies of population genetic structure and hybridization among ecotypes of Littorina require both mitochondrial and nuclear markers, yet relatively few nuclear markers have been described. We used universal PCR primers for aminopeptidase N that were originally developed for the Pacific oyster, to amplify the gene in Littorina subrotundata. Three different sized PCR products (320 bp, 460 bp and 720 bp) were obtained and identified as three different proteins containing the zinc-binding motif HEXXHXW, characteristic of aminopeptidase N. We chose the locus with the longest intron, APN54, and designed primers in the flanking exon sequence that were specific for locus APN54 for Littorina species. The Littorina-specific primers successfully amplified only the APN54 locus in all seven species of Littorina tested and showed considerable between- and within-species polymorphism in intron sequence and length. We present data from two populations of two different species, L. subrotundata and L. sitkana, that show our new nuclear marker has two or three common alleles per locus and is therefore an especially efficient population marker for small sample sizes.

A neutral DNA marker suggests that parallel physiological adaptations to open shore and salt marsh habitats have evolved more than once within two different species of gastropods

Local adaptation is an important mechanism generating physiological diversity and can be especially pronounced in species with restricted dispersal and gene flow such as direct developing snails of the genus Littorina. We compared physiological responses to salinity and desiccation stress in two co-occurring species of northeastern Pacific Littorina (L. subrotundata and L. sitkana) with salt marsh and open shore ecotypes. The animals from salt marsh populations were significantly more tolerant to low salinities and significantly less resistant to desiccation stress than their open shore counterparts. The lower resistance to desiccation in salt marsh animals was not associated with a higher rate of water loss during air exposure or with lower body water reserves, but instead reflected a lower tolerance to high salinities. These habitat-related physiological differences occurred in parallel in the two studied species of Littorina and persisted after prolonged laboratory acclimation, suggesting that they may reflect selection for markedly different local optima in the salt marsh habitats than in the open shore habitats. We used a neutral polymorphic nuclear DNA marker (intron of aminopeptidase N) to estimate the level of gene flow between the populations from different habitats and found isolation by distance regardless of the habitat from which the snails were collected. Our molecular data suggest that physiological cohesiveness of ecotypes can arise despite different genetic backgrounds, and could potentially be due to parallel evolution of convergent phenotypes in similar habitats.

Evolution in Littorina: ecology matters

Organisms of marine rocky shores are exposed to physical stress from abiotic factors, such as temperature, salinity and wave action. These factors vary over compressed temporal and spatial scales, producing an exceedingly heterogeneous habitat with steep gradients of selection, and it seems likely that this has a strong influence on the evolution of populations of rocky shore organisms. With the periwinkles (genus Littorina) as a model group, I review strategies for coping with small-scale heterogeneous environments and what implications these strategies have on the evolution of these species. Some species of Littorina have long-lived pelagic larvae and sites of various habitats are thus recruited from a common gene pool. This largely prevents local adaptation but minor adjustments are possible through a plastic phenotype. Other species of the genus are directly developing with no larval dispersal and among these there is evidence of strong local adaptation forming distinct ecotypes in contrasting habitats by parallel evolution. In at least one of the directly developing species ( L. saxatilis) divergent selection among ecotypes has resulted in partial reproductive barriers that further impede gene flow among ecotypes. Furthermore, convergent evolution among species has produced superficially similar morphs in different habitats. Ecotype formation, ecological reproductive barriers and convergence among species all indicate that ecological processes are critical for evolution of Littorina species.

Do reproductive strategy and breeding season influence the presence of mate recognition in the intertidal snail Littorina?

Summary To test the assumption that littorinid snails do not discriminate between mates prior to copulation, I studied mate recognition behaviour in two species of Littorina on Swedish rocky shores: L. littorea (which has pelagic development and a distinct reproductive season) and L. saxatilis (non-pelagic development; reproduces throughout the year). Copulating pairs were sampled in five populations of each species to determine the occurrence of maladaptive (male-male or male-juvenile) matings. To complement the field sampling I did laboratory experiments on mucous-trail following, to reveal possible discriminating behaviour among potential mates when following mucous trails. In L. littorea about 5% of copulating pairs were between males, while in L. saxatilis, 33–38% were male-male or male-juvenile pairs. In contrast to L. littorea, this study showed that males of L. saxatilis did not discriminate between sexes from mucous trails, but followed any mucous trail (irrespective of sex of marker snail) for longer than females did. Thus, mate recognition behaviour by trail following can explain the difference in the proportion of maladaptive mating pairs between the two species studied. Among the 19 recognized species of the snail Littorina, nine show pelagic larval development, while 10 have direct, non-pelagic development. From the literature a clear relationship between reproductive strategy and the presence or absence of breeding season in Littorina snails was found. Species with pelagic development always have a distinct reproductive season, while species with non-pelagic development either reproduce throughout the year, or show variation between regions in seasonal or non-seasonal reproduction. The hypothesis that reproductive strategy and breeding season may influence the presence of mate recognition in Littorina snails is discussed.

DNA sequence evidence for speciation, paraphyly and a Mesozoic dispersal of cancellothyridid articulate brachiopods

Because the classification of extant and fossil articulate brachiopods is based largely upon shell characters observable in fossils, it identifies morphotaxa whose biological status can, in practice, best be inferred from estimates of genetic divergence. Allozyme polymorphism and restriction fragment length polymorphism of mitochondrial DNA (mtDNA RFLP) have been used to show that nuclear and mitochondrial genetic divergence between samples of the cancellothyridid brachiopods Terebratulina septentrionalis from Canada and T. retusa from Europe is compatible with biological speciation, but the genetic distances obtained were biased by methodological limitations. Here, we report estimates of divergence in 12S rDNA mitochondrial sequences within and between samples of these brachiopods. The sequence-based genetic distance between these samples (5.98±0.07% SE) is at least 10 times greater than within them and, since they also differ in a complex life-history trait, their species status is considered to be securely established. Divergence levels between 12S rDNA genes of three other cancellothyridids, T. unguicula from Alaska, T. crossei from near Japan, and Cancellothyris hedleyi from near Australia are higher than between the two North Atlantic species, and the mean nucleotide distance between all these cancellothyrids is similar to the mean distance between species of Littorina (Mollusca: Gastropoda). Sequences of both 12S and 16S genes from cancellothyridids and other short-looped brachiopod species show neither saturation nor lineage-specific rate differences and, when analysed with different outgroups, either separately or together, yield one unexpected, but well-supported, tree with Alaskan T. unguicula basal and C. hedleyi nested within Terebratulina, i.e. these genera are paraphyletic. A geologically dated divergence between Antarctic and New Zealand species of the short-looped brachiopod Liothyrella is used to calibrate the rate of 12S divergence at ca. 0.1% per million years (MY), and this rate is used to infer that T. septentrionalis and T. retusa have been diverging for ca. 60 MY and that they and T. unguicula have been diverging from their last common ancestor for ca. 100 MY. This indicates a Mesozoic origin for the present-day distribution of cancellothyridids and the basal position of T. unguicula suggests a possible North Pacific centre of origin, with separate Atlantic and Pacific radiations. The inclusion of Cancellothyris within Terebratulina also shows that adult shell characters such as umbo, foramen and symphytium shape, whilst probably indispensible for the practical classification of fossils, are not reliable guides to genealogy.

Digenetic trematodes in four species oflittorinafrom the West Coast of Sweden

Abstract Four species of Littorina (L. littorea, L. obtusata, L. fabalis and L. saxatilis) from the Swedish west coast were examined for infections with digenetic trematodes. Four species of the genus Microphallus, Podocotyle alomon, Renicola roscovita, Himasthla elongata, Cryptocotyle lingua, and Paramonostomum chabaudi were found, in addition to encysted metacercariae of the two genera Himasthla and Renicola. The prevalence of sporocysts and rediae varied substantially among sampled populations (0-44%), as did the prevalence of metacercarial infections (3-100%), and this variation was due both to different microhabitats and to different host species. In L. saxatilis exposed rocky shore populations had lower prevalence than sheltered boulder shore populations. Within the same shores, L. littorea was the host species with highest prevalence of digeneans. The general pattern of distribution of the nine trematode species over host species was similar to what have been reported from other North Atlantic area...

Intracapsular Feeding by Embryos of the Gastropod Genus Littorina.

Many gastropod species develop within egg capsules within which larvae are provided with extraembryonic nutrients. Species with encapsulated development frequently have transitory embryonic organs, such as "larval kidneys," that may represent specializations for consumption of intracapsular nutrition. Larvae of Littorina species with nonplanktonic, encapsulated development consume intracapsular albumen, but they lack obvious morphological modifications for albumen consumption. To determine the mechanism and location of protein uptake, larvae of seven species of Littorina (L. keenae, L. littorea, L. plena, L. saxatilis, L. scutulata, L. sitkana, L. subrotundata) were exposed to solutions of either fluorescently labeled protein (FITC-bovine serum albumen) or ferritin. Under fluorescence microscopy, larvae of all species with encapsulated, nonplanktonic development displayed strong regional affinity for FITC in the ciliated cells of the velum, whereas hatched larvae of planktotrophic Littorina species did not. Transmission electron microscopy of epithelial cells of nonplanktotrophic veligers exposed to ferritin supported the interpretation that localized affinity for labeled protein indicated endocytotic protein uptake. Planktotrophic Littorina and Littorina with encapsulated, nonplanktonic development were shown to share equivalent velar width/larval length ratios during early embryonic development, whereas a literature search suggested that in other nonplanktotrophic prosobranchs the velum is relatively smaller than in planktotrophs. Retention of a large velum in Littorina that develop entirely within egg capsules may facilitate feeding on intracapsular protein, in the absence of specialized assimilative organs found in other species with encapsulated development.

The apical complex of the gregarineDigyalum oweni(Protozoa: Apicomplexa)

The aseptate gregarine Digyalum oweni is parasitic in the gut of some species of Littorina, marine gastropod molluscs. The apical complex comprises a mucron (described previously), polar ring, polar ring extension, subpellicular microtubules and rhoptries; and possibly also micronemes and dense granules. The polar ring extension is possibly a microtubule organizing centre. The method by which D. oweni attaches to its host cell may be similar to that used by intracellular apicomplexans to invade host cells. After attachment of D. oweni to the host cell the rhoptries may secrete enzymes into the host cell prior to digestion.

Mechanisms of differential survival and growth of two species of Littorina on wave-exposed and on protected shores

Gastropods from wave-exposed shores are typically smaller and thinner-shelled with a larger foot than gastropods from protected shores. We investigated the value of these adaptations by comparing the survival and growth of one exposed shore species and one protected shore species of Littorina transplanted to wave-exposed and protected shores. Adult snails on exposed shores with heavy surf had reduced winter growth rates and low over-winter survival rates. Demographic studies of the exposed shore species showed that few adults survived the winter at the most exposed sites; recruitment to the following year's population was primarily from small juveniles that overwintered between barnacles and within algae. The protected shore species had slightly lower survival than the exposed shore species on exposed shores. The protected shore species was also more susceptible to dislodgement in a flow tank. The low winter growth rates and survival may explain why the large, late-maturing, protected shore species does not occur on exposed shores. Adult snails on protected shores had moderate winter growth rates and moderate over-winter survival. The exposed shore species survived very poorly on protected shores, in part because of predation by shore crabs. In laboratory experiments the thin-shelled exposed shore species was significantly preferred by shore crabs and withstood significantly less force. The small size, thin shell, and large shell aperture of the exposed shore species makes it vulnerable to the shore crabs and may explain why it does not occur on protected shores.

The mucron of the gregarineDigyalum oweni(Protozoa: Apicomplexa), parasitic inLittorinaspecies (Mollusca: Gastropoda)

The aseptate gregarine Digyalum oweni is parasitic in the gut of some species of Littorina, marine gastropod molluscs. The mucron, its anterior attachment organelle, comprises a ring of lobes and a ring of granular material embedded in the apex of the host intestinal cell. Some tissue damage is caused to the host cell but this is confined to the apical tip and otherwise the parasite does not appear to harm the snail. In addition to securing the parasite to its host, the mucron may have a role as a microtubule organizing centre and in nutrition.

Use of random amplified polymorphic DNAs (RAPDs) in separating two sibling species of Littorina

Use of random amplified polymorphic DNAs (RAPDs) in separating two sibling species of Littorina

Feeding preferences of periwinkles among four species ofFucus

A widely accepted view of intertidal community organizatiton in the NW Atlantic proposes that fucoid vegetation is maintained by the actions of predators which remove species competitively superior toFucus species. Herbivory is an important component of these predatory interactions, but has been studied largely with reference to the interaction betweenF. vesiculosus andLittorina littorea. There are many species of fucoids and herbivorous invertebrates on the shores of the NW Atlantic and this paper reports field and laboratory experiments performed in 1987 (in or near Halifax, Canada) on the effects of grazing by three species ofLittorina on adults and juveniles of four species ofFucus. In laboratory experiments, portions ofFucus species were presented singly (no choice) or together (multiple choice) to single species ofLittorina. AdultF. distichus was grazed to only a small extent.F. evanescens andF. vesiculosus adult tissues were heavily grazed in most laboratory experiments.F. spiralis adults were heavily grazed in no choice experiments, but were grazed only slightly in multiple choice tests. When adult fucoid tissues were exposed to a mixed suite of grazers in the field, onlyF. vesiculosus was grazed heavily. Tests done on adult tissues showed clearly thatFucus species are highly variable in their vunerability to grazers. Moreover, we found variability among periwinkle species in grazing rates on adultFucus. We also found a great disparity in the feeding rates of different periwinkle species on juveniles ofFucus. Therefore, it is not possible to extrapolate from a single periwinkle species/single fucoid species interaction when attempting to identify vegetation structuring processes.

Allozyme and shell variation in two marine snails (Littorina, Prosobranchia) with different dispersal abilities

As adults Littorina littorea and L. saxatilis have overlapping distributions, both on the shore and on a geographical scale, this implies that they are affected by similar selective forces to a large extent. The two species have, however, different reproductive strategies. Littorina littorea has a planktonic larval stage lasting 4–6 weeks, whereas L. saxatilis has direct development. This influences dispersal rates of the two species, and gene flow and effective population size are both assumed to be much smaller in L. saxatilis than in L. littorea. Intraspeciflc variation in morphological and allozyme characters differs between the two species. Littorina littorea is homogeneous for both types of characters, both within and between populations, while populations of L. saxatilis may show pronounced differences in shell characters with micro-environment changes, and allozyme variation is markedly higher in this species too. The polymorphism in shell characters of L. saxatilis and the monomorphism of L. littorea, may be expected from theory as a consequence of the different dispersal strategies. Also the lack of divergence between geographically separate populations of L. littorea, but the high degree of between population differentiation in L. saxatilis, could be attributed to the high and low dispersal rates, respectively. However, the larger allozyme variation within subpopulations of L. saxatilis compared to that of L. littorea, is surprising as the former species has a smaller effective population size than the latter. Furthermore, there is no correlation between dispersal ability and overall allozyme heterozygosity, when 10 additional species of Littorina are included in the comparison. It is suggested that different evolutionary backgrounds of different species will possibly explain this, as neither selectionist nor neutralist models alone account for the observation. That is, an earlier passage through a population bottleneck may, for example, influence the genetic variation for a long period of time.

Diel and tidal movement of two co-occurring neritid snails; differences in grazing patterns on a tropical rocky shore

Nerita scabricosta (Lamarck, 1822) and Nerita funiculata (Menke, 1851) are common, herbivorous snails which occur together on the Pacific rocky coast of Panama. We show that the foraging patterns of both species are constrained by 1. (1) the probability of predation by fishes, 2. (2) overheating and desiccation. Their different responses to these factors control their effects on the rest of the intertidal community. Both species are found in a limited number of microhabitats while inactive. N. funiculata is restricted to mid-zone crevices. Individuals forage out from these crevices for short periods on ebbing or rising tides. Movement is limited to ≈50 cm on either side of occupied crevices. The rock within ≈10 cm from crevices containing N. funiculata is bare; crustose algal cover increases with distance from the crevice. There is no change in algal cover with distance around crevices which naturally lack N. funiculata and in experimental removals algal crusts settle or grow in to cover 80–100% of the rock. Thus N. funiculata affects local, discrete areas of intertidal substratum by causing small-scale patchiness in algal cover. N. scabricosta is found at upper tidal levels while inactive during high tide, aggregated above the high-water mark in crevices or on vertical surfaces. Individuals move down and graze from the high to the low zones as the tide falls. They retreat to the high intertidal zone as the tide rises and once again become inactive until the tide falls. Thus, unlike its congener, N. scabricosta exerts grazing effects throughout the intertidal zone. We demonstrate that this snail affects 1. (1) barnacle settlement, 2. (2) the density and size structure of co-occurring species of Littorina, 3. (3) the abundance of crustose algae from the high to the low mid-zones. These differences in community effects between species appear to be related to the larger body size reached by N. scabricosta.

Comparative Demography of Littorina rudis, L. Nigrolineata and L. Neritoides on Three Contrasted Shores in North Wales

(1) A possible relationship between reproductive mode and demography was sought between species of Littorina that brood young, lay benthic egg masses and broadcast planktonic larvae. Population size-frequency and density and cohort survivorship were monitored for Littorina rudis on a sheltered saltmarsh, L. rudis and L. nigrolineata on a boulder shore and for L. rudis and L. neritoides on an exposed cliff face. (2) Adult size classes formed a single broad peak and, except in L. neritoides, young recruits formed a second peak each summer. (3) In the saltmarsh, L. rudis population density fell from a maximum after summer recruitment to a minimum in spring; this was less evident in L. rudis and L. nigrolineata on the boulder shore and undetectable in the cliff populations of L. rudis and L. neritoides. (4) Spring densities fluctuated between years by maximum factors of 7, 6, 9, 9 and 2 in saltmarsh L. rudis, boulder shore L. rudis, cliff L. rudis, L. neritoides and L. nigrolineata respectively. (5) There was no evidence of density dependence in any of the population trajectories. (6) Artificial crevices increased the local density of L. rudis and L. neritoides on the cliff by attracting snails seeking shelter, but did not affect the total population size. (7) Mortality rate diminished progressively through the juvenile phase to a constant rate in adults. (8) Generation times were 24, 18, 18, 25 and 41 months for the saltmarsh L. rudis, boulder shore L. rudis, L. nigrolineata, cliff L. rudis and L. neritoides respectively. (9) Age-specific reproductive value increased beyond the age of first sexual maturity and declined only when growth had almost ceased. (10) All populations except that of L. nigrolineata had values of R0 < 1. Isoclines of juvenile versus adult survivorship for R0 = 1 were used to assess the reliability of the observed survivorship values. (11) Apart from a possible advantage of oviparity versus ovoviviparity during phases of population expansion, no relationship between demography and reproductive method could be found. Oviparity may be advantageous where egg survivorship is high and ovoviviparity in the converse situation. Broadcasting of planktonic larvae by L. neritoides may be correlated with the temporal and spatial unpredictability of microhabitat quality between shores.

GENE-ENZYME VARIATION IN THREE SYMPATRIC SPECIES OFLITTORINA. II. THE ROSCOFF POPULATION, WITH A NOTE ON THE ORIGIN OF NORTH AMERICANL. LITTOREA

Gene-enzyme variation was examined at ten to fifteen loci in three sympatric species of Littorina collected from Cape Cod, Massachusetts and Roscoff, Brittany. The North American and French populations of L. saxatilis and L. obtusata were, in general, quite similar in allele content and frequency. In contrast, the North American and French populations of L. littorea were genetically differentiated at a majority of their loci. This pattern of heterogeneity in the three species, along with calculated values of average heterozygosity, suggests that North American specimens of L. littorea are not recent colonists from Europe and have passed through a severe population bottleneck on the North American continent. Survey studies carried out over a four year period revealed a general pattern of temporal constancy in allele frequency in all three species.