A striking example of the founder effect in the mollusc Littorina saxatilis

Two South African populations of Littorina saxatilis were examined by starch-gel electrophoresis at 16 enzyme loci and compared with 13 populations of North Atlantic saxatilis from both American and European coasts, and with six British populations of the closely related species Littorina arcana. The South African animals showed a severely reduced heterozygosity (– 0.052) compared with Atlantic populations of saxatilis ( = 0.181), and the mean genetic distance between the two areas was high ( = 0.203) compared with distances within the North Atlantic saxatilis populations (D = 0.034). In fact, the saxatilis from South Africa were genetically more distant from the North Atlantic samples of L. saxatilis than were the arcana from British shores. The reduced genetic heterozygosity and genetic divergence of the South African populations is attributed to founder effects following a postulated recent introduction by man.     

A preliminary study on genetic differentiation in Littorina saxatilis from Galway Bay,Ireland: Littorina tenebrosa Montagu – a valid species or ecotype?

Littorina tenebrosa is a small fragile-shelled periwinkle which lives on permanently submerged algae in coastal lagoons and non-tidal brackish pools. This periwinkle is a member of the rough periwinkle group which also comprises Littorina saxatilis, L. arcana, L. compressa and L. neglecta and is most closely related to L. saxatilis although its exact systematic status is in some doubt. Based on its unique ecology many believe L. tenebrosa to be a valid species. However, shell morphometric and allozyme analyses on Scottish and Swedish populations of L. tenebrosa and L. saxatilis have indicated that the two periwinkles are virtually identical. Preliminary results on five allozyme loci (AAT-1, GPI, PGM-1, PGM-2 and PNP) in samples of L. tenebrosa and L. saxatilis from Golam Head, Lettermullen, and other locations on the west coast of Ireland show L. tenebrosa to be genetically differentiated from L. saxatilis At Golam Head, where opportunities for gene flow occur between the two taxa, L. tenebrosa is as genetically differentiated from local L. saxatilis as it is from L. saxatilis from more geographically distant locations.     

A morphologic and genetic analysis of Littorina saxatilis (Prosobranchia) from Venice, and on the problem of saxatilis—rudis nomenclature

Shell morphology and allozyme patterns of 17 enzymes are analysed within the isolated population of L. saxatilis Olivi (1792) from Venice, the type locality of the species. The shell morphology is compared to three different L. saxatilis phenotypes from Sweden, and showed to be similar to the brackish water 'tenebrosa' type. The Venice population is genetically more distant from British and Swedish populations than are the latter from each other. Less variability (heterozygosity) within the Venice compared to the Atlantic populations is probably a consequence of random genetic drift during a period of small population size.
The nomenclature of L. saxatilis in the Atlantic is discussed and it is suggested that until the exact taxonomic relationship between the Mediterranean and the Atlantic populations has been confirmed by, for example, cross-breeding experiments, the name 'saxatilis' should be kept for the non-Venetian populations (described by Maton (1797) under the name 'rudis').     

Species relationships and population structure of Littorina saxatilis Olivi and L. tenebrosa Montagu in Ireland using single-strand conformational polymorphisms (SSCPs) of cytochrome b fragments

Littorina saxatilis is a ubiquitous snail of intertidal habitats in the North Atlantic. Shell type in littorinids is extremely polymorphic and defined by habitat. Taxonomy based upon shell type has been revised in the light of anatomic and genetic information, but uncertainties remain. In this study, the population structure of L. saxatilis and L. tenebrosa was studied at 11 sites in Ireland using single-strand conformational polymorphisms of a 375-bp portion of the cytochrome b gene, and the status of L. tenebrosa, the small, fragile-shelled, brackish water type, was considered. The genetic patterns among L. saxatilis and L. tenebrosa populations were examined over varying distances and L. tenebrosa was compared with adjacent L. saxatilis populations at four sites on the west coast of Ireland and one site on the east coast. Haplotype diversity was high with 32 haplotypes present among 995 individuals. Pairwise tests suggest gene flow over small scales among and between habitat types and may reflect the stochastic legacy of postglacial recolonization over larger scales. In AMOVA tests, geography explained nearly twice as much of the variance (30%) as habitat type (18%), indicating that gene flow is more restricted by distance than by habitat type, and supporting the status of L. tenebrosa as an ecotype of L. saxatilis rather than a separate species.     

Phylogeographic analysis reveals a deep lineage split within North Atlantic Littorina saxatilis

Phylogeographic studies provide critical insight into the evolutionary histories of model organisms; yet, to date, range-wide data are lacking for the rough periwinkle Littorina saxatilis, a classic example of marine sympatric speciation. Here, we use mitochondrial DNA (mtDNA) sequence data to demonstrate that L. saxatilis is not monophyletic for this marker, but is composed of two distinct mtDNA lineages (I and II) that are shared with sister species Littorina arcana and Littorina compressa. Bayesian coalescent dating and phylogeographic patterns indicate that both L. saxatilis lineages originated in the eastern North Atlantic, around the British Isles, at approximately 0.64 Ma. Both lineages are now distributed broadly across the eastern, central and western North Atlantic, and show strong phylogeographic structure among regions. The Iberian Peninsula is genetically distinct, suggesting prolonged isolation from northeastern North Atlantic populations. Western North Atlantic populations of L. saxatilis lineages I and II predate the last glacial maximum and have been isolated from eastern North Atlantic populations since that time. This identification of two distinct, broadly distributed mtDNA lineages further complicates observed patterns of repeated incipient ecological speciation in L. saxatilis, because the sympatric origins of distinct ecotype pairs on eastern North Atlantic shores may be confounded by admixture of divergent lineages.     

Infection of the mollusc Littorina saxatilis with parthenites of trematodes and their impact on a shell form: analysis of populations inhabiting the littoral coast of the White Sea

12 rocky shore populations of Littorina saxatilis from three islands of Chupa Inlet (Kandalaksha Bay, White Sea) were examined for infection with trematodes. Morphometric characters (6 indexes of the shell and aperture shape) of molluscs were investigated for all these populations. Exposed and sheltered sites were considered at every island and high and low littoral samples were fulfilled at every site. Seven species of trematodes, Podocotyle atomon, Cryptocotyle lingua, Renicola sp., Himasthla sp., Microphallus piriformes, M. pygmaeus, M. pseudopygmaeus, were found. Uneven distribution of trematodes was confirmed by log-linear analysis. Sheltered populations of L. saxatilis have the greater infection prevalence than exposed ones. This is due to the heavy infection with M. piriformes and M. pygmaeus. The prevalences by these trematodes are up to 52.97% and 27.16% respectively in sheltered populations of the host. The prevalence of M. piriformes tend to be higher at the upper shore level of sheltered sites. In a contrast, the prevalence of M. pygmaeus is significantly higher at the low part of such sites. Factor analysis shows a significant association of the indices of L. saxatilis shell shape with three factors. The first one is associated with the "elongation" of a shell and reveals L. saxatilis from the exposed rocky shore to be more elongated than the molluscs from sheltered sites. The second one is connected with the "aperture shape" index. There is an association of this factor with the shore level position of samples. The third factor reflects the affect of trematodes on the shell shape. The molluscs infected with M. piriformes show "elongated" shell shape and relatively smaller aperture. Shall peculiarities of the hosts infected with M. piriformes and M. pygmaeus are somewhat different. The results of the factor analysis is justified by the series of analysis of variances on the values of shell indices (MANOVA) according to the factors "exposure", "shore level" and "infection".     

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.     

Phenotypic plasticity in two marine snails: constraints superseding life history

In organisms encountering predictable environments, fixed development is expected, whereas in organisms that cannot predict their future environment, phenotypic plasticity would be optimal to increase local adaptation. To test this prediction we experimentally compared phenotypic plasticity in two rocky-shore snail species; Littorina saxatilis releasing miniature snails on the shore, and Littorina littorea releasing drifting larvae settling on various shores, expecting L. littorea to show more phenotypic plasticity than L. saxatilis. We compared magnitude and direction of vectors of phenotypic difference in juvenile shell traits after 3 months exposure to different stimuli simulating sheltered and crab-rich shores, or wave-exposed and crab-free shores. Both species showed similar direction and magnitude of vectors of phenotypic difference with minor differences only between ecotypes of the nondispersing species, indicating that plasticity is an evolving trait in L. saxatilis. The lack of a strong plastic response in L. littorea might be explained by limits rather than costs to plasticity.     

Screening the antioxidant and antimicrobial properties of the lichens Parmelia saxatilis, Platismatia glauca, Ramalina pollinaria, Ramalina polymorpha and Umbilicaria nylanderiana.

The aim of this study is to investigate in vitro antimicrobial and antioxidant activities of the methanol extracts of Parmelia saxatilis (L) Ach., Platismatia glauca (L.) W.L. Club. & C.F. Culb., Ramalina pollinaria (Wesstr.) Ach., Ramalina polymorpha (Liljeblad) Ach. and Umbilicaria nylanderiana (Zahlbr.) H. Magn. Antioxidant activity was evaluated by two separate methods: scavenging of free radical DPPH and the inhibition of linoleic acid oxidation. Extracts of Parmelia saxatilis, Platismatia glauca., Ramalina pollinaria and Ramalina polymorpha did not exert any activity in both assays, whereas those of Umbilicaria nylanderiana provided 50% inhibition at 400.2 microg/ml concentration in the former and gave 53% inhibition at 2g/l concentration. Total phenolic constituents of extracts from lichen species tested (P. saxatilis, P. glauca, R. pollinaria, R. polymorpha and U. nylanderiana) were 1.0% (w/w), 1.1% (w/w), 1.0% (w/w), 0.8% (w/w) and (3.0% w/w), respectively (as gallic acid equivalent); implying that the observed activity could be related to the amount of polar phenolics. Extracts were also found to possess antimicrobial activity against some test bacteria and fungi and yeast.     

Distribution patterns of marine bird digenean larvae in periwinkles along the southern coast of the Barents Sea.

An important component of the parasite fauna of seabirds in arctic regions are the flukes (Digena). Different species of digeneans have life cycles which may consist of 1 intermediate host and no free-living larval stages, 2 intermediate hosts and 1 free-living stage, or 2 intermediate hosts and 2 free-living larval stages. This study examined the distribution of such parasites in the intertidal zones of the southern coast of the Barents Sea (northwestern Russia and northern Norway) by investigating 2 species of periwinkles (Littorina saxatilis and L. obtusata) which are intermediate hosts of many species of digeneans. A total of 26,020 snails from 134 sampling stations were collected. The study area was divided into 5 regions, and the number of species, frequency of occurrence and prevalence of different digenean species and groups of species (depending on life cycle complexity) were compared among these regions, statistically controlling for environmental exposure. We found 14 species of digeneans, of which 13 have marine birds as final hosts. The number of species per sampling station increased westwards, and was higher on the Norwegian coast than on the Russian coast. The frequency of occurrence of digeneans with more than 1 intermediate host increased westwards, making up a larger proportion of the digeneans among infected snails. This was significant in L. saxatilis. The prevalence of different species showed the same pattern, and significantly more snails of both species were infected with digeneans with complicated life cycles in the western regions. In L. saxatilis, environmental exposure had a statistically significant effect on the distribution of the most common digenean species. This was less obvious in L. obtusata. The causes of changing species composition between regions are probably (1) the harsh climate in the eastern part of the study area reducing the probability of successful transmission of digeneans with complicated life cycles, and (2) the distribution of different final hosts.     

Modification of the structure of penial glands in males of the intertidal molluscs Littorina saxatilis and L. obtusata under the influence of the infestation by trematode parthenites

Sporocycts of the "pygmaeus" microphallides (Microphallus piriformes) are localized in hepatopancreas and gonads of Littorina molluscs causing total parasitic castration. A histological study of penial glands in Littorina saxatilis and L. obtusata males infested with trematodes M. piriformes has been made. Copulatory organs of noninfested molluscs, molluscs after recent contamination (with not completely formed daughter sporocysts), and molluscs containing mature metacercariae inside daughter sporocysts were examined. Based on the data obtained, probable dynamics of the histological structure of infested glandular apparatus was established. It was shown, that the trematode infestation have an influence on the muscular and secretory parts of penial glands. The wall of the penial gland muscular capsule becomes more fine in infested L. saxatilis. On the contrary, this wall is vastly thicker in infested L. obtusata, as compared with noninfested individuals. Glandular cells of the molluscs' penial glands decreases the amount of granular secret in both species. In L. obtusata the number of secretory cells is shown to be reduced up to their total disappearance. The above pathological changes probably prevent normal function of penial glands.     

A phylogeny of the temperate seabasses (Moronidae) characterized by a translocation of the mt-nd6 gene

The entire mitochondrial genome of the striped bass Morone saxatilis was sequenced together with the mitochondrial (mt) control regions of the white bass Morone chrysops, white perch Morone americana, yellow bass Morone mississippiensis, spotted seabass Dicentrarchus punctatus, European seabass Dicentrarchus labrax and the Japanese seabass Lateolabrax japonicus. The resultant 17 580 base pair circular genome of M. saxatilis contains 38 genes (13 proteins, 23 transfer RNAs and two ribosomal RNAs) and a control region bordered by the proline and phenylalanine mitochondrial tRNAs. Gene arrangement was similar to other vertebrates, except that the mt-nd6 gene was found within the control region rather than the canonical position between the mt-nd5 and mt-cyb genes. This translocation was found in all the Morone and Dicentrarchus species studied without functional copies or pseudogenes in the ancestral position. In L. japonicus, the mt-nd6 gene was found in the canonical position without evidence of an mt-nd6 gene in the control region. A Bayesian analysis of these and published mt-nd6 sequences from 45 other Perciformes grouped the Morone and Dicentrarchus species monophyletically with a probability of 1·00 with respect to L. japonicus and all other perciforms, and placed the Dicentrarchus species in the basal position. These data reinforce current placement of L. japonicus outside the Moronidae and provide a clear evolutionary character to define this family. The phylogeny of the Moronidae presented here also supports the hypothesis of an anadromous common ancestor to this family that gave rise to the North American estuarine and freshwater species. A series of tandem repeats previously reported in M. saxatilis was found in the control region of all Morone species between the mt-nd6 and mt-rnr1 genes, but not in either Dicentrarchus species, which reinforces the continued use of these two separate genera.     

Radular myoglobin and protein variation within and among some littorinid species (Mollusca: Gastropoda)

The radular muscles of several littorinid species, including Littorina littorea, L. saxatilis, L. obtusata, L. striata and Melarhaphe neritoides, contain myoglobin (Mb). Here we report on the presence of radular Mb in eight other littorinids: L. compressa, L. arcana, L. fabalis, Nodilittorina punctata, N. trochoides, N. radiata, Littoraria undulata and Littoraria cingulifera. Using native polyacrylamide gel electrophoresis (PAGE) and isoelectric focusing (IEF) we compared the Mb and soluble protein (SP) profiles of these species. This suggested that: (1) L. saxatilis and L. arcana may have specific Mb/SP profiles, (2) Littoraria spp., Nodilittorina spp. and L. striata share similar Mb patterns, (3) Mb is remarkably diverse in the genus Littorina, (4) L. littorea shows intraspecific Mb/SP variation, (5) L. saxatilis does not show geographic Mb/SP differences, and (6) IEF uncovers substantial hidden Mb/SP heterogeneity not shown by PAGE (particularly for Melarhaphe neritoides). Hence, littorinid Mb/SP may be a useful taxonomic marker whose ecophysiological significance deserves further study, even if its genetic basis remains unclear.     

Spatial and temporal variation of trematode infection in coexisting populations of intertidal gastropods Littorina saxatilis and L. obtusata in the White Sea

Trematode infection was studied in sympatric populations of the periwinkles Littorina saxatilis and L. obtusata in 2 regions of Kandalaksha Bay of the White Sea to assess host-parasite interactions at the population level. Twenty-seven spatially separated populations were each surveyed in 1984-1994; 2 heavily infected populations were investigated annually over a 16 yr period. Ten trematode species were found in the periwinkle populations. The closest association in spatial distribution and temporal dynamics was observed between 3 ecologically and morphologically similar trematodes of the 'pygmaeus' group: Microphallus piriformes, M. pygmaeus and M. pseudopygmaeus. For these 3 species, the prevalences were closely associated in the 2 host species when spatially separated sites from the 2 studied regions were considered, while in the 2 populations studied over the 16 yr period, a correlation was only observed between the infection levels of L. saxatilis and L. obtusata by either M. piriformes and immature microphallids. Likewise, within each host species, significant correlations were revealed between the prevalence of the different microphallids of the 'pygmaeus' groups. However, they were fewer and weaker when the long-term dynamics of infection in the 2 heavily infected populations were considered. Most other trematodes did not show significant association in prevalence either within or between the 2 host species on spatial and temporal scales.     

Alkane composition diversity among populations of dwarf forms of Juniperus communis L.: comparison between western Europe and northern American populations

The cuticular wax composition of leaves has been analysed in three western European populations (Corsica, central Pyrenees, northern Alps) of Juniperus communis var. saxatilis Pall. (=  J. nana Willd., nom illeg.) and in one population of J. communis L. var. depressa Pursh. from North America (Sierra Nevada). Gas chromatography shows the presence of 13 alkanes in all samples ranging from C₂₃ to C₃₅ with important intraspecific polymorphism in alkane content. The dominant alkanes range from C₃₃ to C₃₅. Alkanes C₂₁ and C₂₂ were found only in Corsica and Sierra Nevada populations. Canonical discriminant analysis separated the J. communis L. var. depressa Pursh. of the population of Sierra Nevada from other populations of J. communis var. saxatilis Pall. on the basis of their higher C₃₁ content and the constant presence of C₂₁ and C₂₂ alkanes. J. communis var. saxatilis Pall. populations from the Pyrenees are close to northern Alps populations characterized by high concentrations of C₃₃, C₃₄ and C₃₅ alkanes. This paper confirms the existence of Juniperus var. saxatilis Pall. in the Pyrenees (France).  © The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 140 , 165–168.     

Case studies and mathematical models of ecological speciation. 3: Ecotype formation in a Swedish snail

Formation of partially reproductively isolated ecotypes in the rough periwinkle, Littorina saxatilis , may be a case of incipient nonallopatric ecological speciation. To better understand the dynamics of ecotype formation, its timescale, driving forces and evolutionary consequences, we developed a spatially explicit, individual-based model incorporating relevant ecological, spatial and mate selection data for Swedish L. saxatilis . We explore the impact of bounded hybrid superiority, ecological scenarios and mate selection systems on ecotype formation, gene flow and the evolution of prezygotic isolation. Our model shows that ecotypes are expected to form rapidly in parapatry under conditions applicable to Swedish L. saxatilis and may proceed to speciation. However, evolution of nonrandom mating had complex behaviour. Ecotype evolution was inhibited by pre-existing mating preferences, but facilitated by the evolution of novel preferences. While in many scenarios positive assortative mating reduced gene flow between ecotypes, in others negative assortative mating arose, preferences were lost after ecotype formation, preferences were confined to one ecotype or the ancestral ecotype became extinct through sexual selection. Bounded hybrid superiority (as observed in nature) enhanced ecotype formation but increased gene flow. Our results highlight that ecotype formation and speciation are distinct processes: factors that contribute to ecotype formation can be detrimental to speciation and vice versa. The complex interactions observed between local adaptation and nonrandom mating imply that generalization from data is unreliable without quantitative theory for speciation.     

Glacial history of the North Atlantic marine snail, Littorina saxatilis, inferred from distribution of mitochondrial DNA lineages

The North Atlantic intertidal gastropod, Littorina saxatilis (Olivi, 1792), exhibits extreme morphological variation between and within geographic regions and has become a model for studies of local adaptation; yet a comprehensive analysis of the species' phylogeography is lacking. Here, we examine phylogeographic patterns of the species' populations in the North Atlantic and one remote Mediterranean population using sequence variation in a fragment of the mitochondrial cytochrome b gene (607 bp). We found that, as opposed to many other rocky intertidal species, L. saxatilis has likely had a long and continuous history in the Northwest Atlantic, including survival during the last glacial maximum (LGM), possibly in two refugia. In the Northeast Atlantic, several areas likely harboured refugial populations that recolonized different parts of this region after glacial retreat, resulting in strong population structure. However, the outlying monomorphic Venetian population is likely a recent anthropogenic introduction from northern Europe and not a remnant of an earlier wider distribution in the Mediterranean Sea. Overall, our detailed phylogeography of L. saxatilis adds an important piece to the understanding of Pleistocene history in North Atlantic marine biota as well as being the first study to describe the species' evolutionary history in its natural range. The latter contribution is noteworthy because the snail has recently become an important model species for understanding evolutionary processes of speciation; thus our work provides integral information for such endeavours.     

Inheritance of longitudinal shell bands in the snails Littorina obtusata and Littorina saxatilis (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.     

新疆梅衣属地衣生态地理特征分析

根据多年的实地调查资料和相关研究资料,对新疆梅衣属（Parmelia Ach．）地衣的种类及分布区、区系特征和垂直分布特征进行了初步研究。结果表明,分布在新疆的梅衣属地衣共有6种,分别为破裂梅衣（Parmelia erumpens Kurok．）、稀生梅衣（P．meiophora Nyl．）、脐梅衣[P．omphalodes（L．）Ach．]、石梅衣[P．saxatilis（L．）Ach．]、槽梅衣（P．sulcata Tayl．）和亚广开梅衣（P．fertilis Muell．）,主要分布在新疆的天山和阿勒泰山。根据对环境的适应特征和选择性,将新疆梅衣属地衣的地理分布区类型划分为：1）环低北极及北方种,包括石梅衣和脐梅衣2种;2）环北方种,包括槽梅衣和稀生梅衣2种;3）温带亚洲种,包括亚广开梅衣和稀生梅衣2种。研究结果还显示,分布在阿勒泰山和天山的梅衣属种类的垂直分布有明显差异。     

Genetic differentiation in the molluscan species Littorina rudis and Littorina arcana (Prosobranchia: Littorinidae)

Six populations of Littorina rudis and ihree of L. arcana were screened for elcctrophorctically dctci table variation at 21 enzyme loci. These species reproduce by ovoviviparity and oviparily respectively, and both are members of the L. saxatilis species complex. The mean coefficient of genetic identity between the two species was high (0.957) and no diagnostic loci were observed. Allele frequency differences are maintained in sympatric populations of the two species. Intraspecific values of/ ranged from 0.944 to 0.995 and both species showed marked inter-population genetic heterogeneity, a feature consistent with their limited powers of dispersal. Heterozygosity is high in all populations of both species. Populations of I. rudis have higher mean heterozygosities (= 0.153) then L. arcane ( H = 0.132).