Maintenance of zonation patterns in two species of flat periwinkle,Littorina obtusata and L. mariae

The zonation patterns of Littorina obtusata (L.) and Littorina mariae Sacchi et Rastelli were shown to be quite distinct on a sheltered rocky shore. L. obtusata was found at all the heights sampled; it reached peak numbers at mid shore on the alga Ascophyllum nodosum L. (Le Jol). There was no difference in the tidal height occupied by adults or juveniles; or in the mean size of L. obtusata along the vertical gradient of the shore. In contrast L. mariae occurred exclusively low on the shore, on Fucus serratus L. Translocation of the two species within their respective levels resulted in random movement after 4 days, although initial movements after 1 and 2 days were sometimes directional. Animals transplanted to the normal level of the other species showed directional movement towards their home zone; this was most pronounced after 4 days. There was no difference in the distance moved by the two species, although the distance moved did vary with tidal height, both species moving further at mid shore than low shore. Distances moved by littorinids at replicate areas in the low shore were similar but those at mid shore did vary. There was an interaction between the species and the different tidal heights which revealed that transplanted species moved further than translocated species at the same tidal level. However, this was only significant in the case of L. mariae. It is suggested that the close relationship between the winkles and their host algae may direct the homing behaviour of displaced individuals.     

A geographically-based study of shell shape in small rough periwinkles

A study using principal component analysis and discriminant analysis was carried out on shell shape variation in 3093 specimens of rough periwinkles, 2500 of which were below 5.5 mm in columella length, from around the North Atlantic. Using a combination of colour plus sculpture, and life history trait, the snails were classified by inspection and examination into Littorina nigrolineata, L. arcana, L. saxatilis and L. neglecta. Principal component analyses indicated that similar aspects of variation were important in the different taxa, but these were sometimes of differing levels of importance between L. saxatilis and L. neglecta. Crossvalidation in a discriminant analysis showed classification of shells larger than 5.5 mm to have at least an 88% accuracy. That of shells below 5.5 mm showed an accuracy of 49% in L. arcana, increasing to 54% in L. saxatilis and 63% in L. neglecta, with 76% accuracy for small L. nigrolineata. This last was a special case as only one site was sampled, therefore comparative data are not available. This geographically-based study reveals that L. neglecta is more homogeneous over its range than recently reported by other workers and shows greater differences from L. saxatilis than the latter does from either L. nigrolineata or L. arcana. Size effects do not account for these differences because L. neglecta is morphometrically distinct from both large and small L. saxatilis. Furthermore, small, mid-shore L. saxatilis classify with large high-shore L. saxatilis in discriminant analysis, not with L. neglecta. These results provide evidence that the taxon L. neglecta is more distinct than has sometimes been suggested.     

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.     

Variation in populations of Littorina obtusata and L. mariae (Gastropoda) in the Severn Estuary

Populations of both species of flat periwinkle were examined at five discrete sites along the Severn Estuary, and at a marine site in Dyfed. The vertical distribution of the two species showed a clear partitioning of the shore – Littorina obtusata occupied the high/mid shore and L. mariae the low shore area. L. obtusata also extended further up the Severn Estuary than L. mariae. Maximum densities of both species were found in Milford Haven. Biometric investigations revealed intra and inter-specific differences in colour morphs, shell damage and size between populations. The possible influence of differential selection pressures at the various sites is discussed.     

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.     

Extreme morphological diversity between populations of Littorina obtusata (L.) from Iceland and the UK

Marine gastropods which do not disperse larvae in the plankton exhibit a relatively high degree of interpopulation morphological variability. This phenomenon has been the root of considerable taxonomic confusion, particularly in the Littorinidae. In the present study, specimens of Littorina obtusata from the UK were compared morphologically and genetically with two samples of a northern high-spired form from Iceland which has been referred to by some workers as Littorina palliata. A multivariate discriminant function analysis based on three measured shell dimensions clearly separated the three samples, correctly predicting the origin of shells on the basis of morphology alone in 96% of cases. However, genetic analysis revealed that the most distant relationship, based on allozyme data at 13 loci, was surprisingly close (Nei's I = 0.983) providing no evidence to suggest that L. palliata is not conspecific with L. obtusata.     

Hybridisation in the Littorina saxatilis species complex (Prosobranchia : Mollusca)

This paper details attempts to breed members of the L. saxatilis species complex in the laboratory and to construct interspecific hybrids between them. Success in reciprocal crosses between animals from the type localities of the species of Littorina saxatilis (Olivi) and Littorina rudis (Maton) indicates that these two taxa are synonymous. Six of the twelve possible reciprocal hybridisation crosses between the four species (L. saxatilis, L. arcana Hannaford Ellis, L. nigrolineata Gray and L. neglecta Bean) have been attempted, with only one proving successful, that between male L. saxatilis and female L. arcana. This hybrid cross produces viable offspring, although at a lower frequency than either of the parental crosses in the laboratory, while the reciprocal cross has, as yet, proved unsuccessful. Limited work on the F1 hybrids shows them to be inter-fertile and also capable of backcrossing with male L. saxatilis, but not with female L. saxatilis or L. arcana. Details are also given of attempts to find natural hybrids in sympatric populations of the species of the complex, using gel electrophoresis.     

On the nurse cell and the spermatozeugma in Littorina sitkana

Summary Nurse cells develop from diploid cells in the testis. Each cell undergoes a reduction division which leaves the nucleus with half the volume of a normal diploid cell. They send out pseudopodia which form desmosomelike junctions with developing spermatids. The nurse cells detach from the testicular wall, their nuclei degenerate and secretion droplets form in the cytoplasm. The pseudopodia are drawn in as the cytoplasmic secretions swell and the nurse cell becomes spherical. The eupyrene sperm become grouped unilaterally and at this stage are attached to the nurse cell by only the tips of their acrosomes. At maturity the nurse cells with their clumps of attached eupyrene sperm (spermatozeugmata) are released from the testis via ducts into the seminal vesicles, where they are stored prior to copulation. Nurse cells serve similar functions to those of apyrene sperm which are common among the Molluscs. We believe that the nurse cell and apyrene sperm are homologous.     

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.     

Locomotion of the filiform sperm of Littorina (Gastropoda,Prosobranchia)

Summary The filiform sperm of Littorina sitkana swims effectively in sea water and more viscous fluids, overcoming the problems of a non-uniform flagellar beat with an unusual mechanism, which involves three main events: (1) the sperm rotates anti-clockwise (looking from tail to head); then (2) stops rotating and stiffens itself to form a screw-shape, with the tail being held perpendicular to the middle piece, and finally; (3) reverses its rotation and propels itself forward in a clockwise spiral. The average velocity of sperm is approximately 185 ms with a rotational frequency of 24 revs. The mechanism of propulsion may involve two kinetic centers at opposite ends of the sperm, which coordinate their movements to produce anti-clockwise rotation, stationary twisting, or clockwise rotation, in a manner reminiscent of spirochaetes. Littorina sperm also exhibit slower methods of propulsion including swimming backwards (tail first) at 18 m, and gliding at about 30 m.The adaptive significance of the rapid propulsion is not obvious, because Littorina copulate and fertilize internally and at each stage in the transfer there are external aids to sperm transport, such as ciliary action (oviduct) and muscular expulsion (bursa and seminal receptacle). The filiform shape, however, is well-adapted for long-term storage in the female body. These points are discussed.     

Estimating the phylogeny in mollusc Littorina saxatilis (Olivi) from enzyme data: methodological considerations

The evolutionary history of 19 populations of Littorina saxatilis (Olivi) was estimated by four different approaches. Three of these operate upon a population by population matrix of genetic distances: average linkage clustering, and two versions of the Fitch-Margoliash method. The fourth method was a maximum likelihood estimate based on differences in allele frequencies between populations. The study aims to assess how well each method estimates the phylogeny by including seven populations of the closely related species L. arcana Hannaford Ellis. The rationale behind this is that a good estimation technique should be able to separate these two monophyletic taxa.The results show that, by our criteria, the maximum likelihood method yields the best estimate and the unconstrained Fitch-Margoliash technique gives reasonable estimates. Both average-linkage clustering and the Fitch-Margoliash method with evolutionary clock perform less well. We argue that this is expected since both these techniques are based on probably unrealistic assumptions such as the overall rate of evolutionary divergence being homogeneous over phyletic lines.     

Purine nucleoside phosphorylase polymorphism in the genus Littorina (Prosobranchia: Mollusca)

Examination of eight Atlantic species of the genus Littorina by starch gel electrophoresis of purine nucleoside phosphorylase revealed extensive polymorphism within the L. saxatilis complex. In this group, four alleles have been identified. Heterozygotes are four banded, and thus, as in vertebrates, the enzyme is likely to be a trimer. Breeding experiments confirmed the genetic interpretation of the phenotype patterns. Where species of the saxatilis complex [L. saxatilis (=L. rudis), L. arcana, L. nigrolineata, L. neglecta] are sympatric, there are sometimes significant allele frequency differences between them. A fifth allele was present at a high frequency in L. obtusata and L. mariae, and L. littorea and L. neritoides each possessed unique alleles. A total of eight alleles was identified. Densitometric scanning of heterozygote patterns pointed to activity differences between alleles and also showed that, while the heterotrimeric bands were never less intense than the homotrimeric bands, the heterotrimeric bands were sometimes less intense than expected. It is not clear whether this represents nonrandom association of subunits, decreased stability of heterotrimers, or simply an artifact of the staining and quantifying process.This work was supported by NERC Grant GR3/5319.     

Microgeographical variation in shell size of the land snail Chondrina clienta

Microgeographical variation in shell morphology of the rock-dwelling land snail Chondrina clienta , collected from 30 sites within an area of 0.5 km² on the island of Öland, Sweden, was examined in relation to its own population density and that of a potential competitor (the land snail Balea perversa ) and to environmental factors. Dispersal of marked individuals averaged 83 cm per year within a stone pile and 291 cm in an area of exposed bedrock. Local population density of active C. clienta ranged from 5 to 794 individuals per m². Shell characters were highly intercorrelated, both within and between populations. Principal component analysis revealed that most of the interpopulational variation could be expressed by the single character of shell height, which ranged from 5.54 to 6.94 mm. In all populations, snails of a given size had the same whorl number. Shell size was not influenced by habitat type (exposed rock surface, stone pile or stone wall) or proportion of calcareous stones within habitat. It was, however, negatively correlated with local population density, indicating intraspecific competition, and positively correlated with the degree of plant cover within the habitat. Analysis of variance revealed additionally a density effect of B. perversa on shell size in C. clienta , probably as a result of interspecific competition. Breeding experiments using C. clienta from different sites and carried out under unifrom conditions caused most of the phenotypic variation to disappear, demonstrating the high phenotypic plasticity of the species.     

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.     

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.     

Fine structural observations of sperm resorption in the seminal vesicle of a marine snail,Littorina scutulata (Gould, 1849)

Summary Waste sperm and spermatozeugmata in the seminal vesicle of Littorina scutulata are phagocytised either by cell buds (large vesicles given off from the epithelial cells) or by the epithelial cells themselves. Cell buds containing sperm, are in turn engulfed by epithelial cells. In both cases, heterophagic vacuoles are formed inside the cell and subsequently the vacuoles are fused with primary lysosomes or lysosomal derivatives to become secondary lysosomes. Throughout this process the sperm are being digested. The second lysosome transforms further to telolysosome and finally to residual body when the sperm is completely digested.     

Allozyme comparison of four littorinid species morphologically similar to Littorina sitkana

Eight species of the genus Littorina were hitherto recognised in the north-western region of the Pacific Ocean: L. sitkana, L. brevicula, L. mandshurica, L. squalida, L. aleutica, L. naticoides, L. kasatka and L. subrotundata. Using allozyme electrophoresis it has been demonstrated that, in the Kurile Islands, three of these species (L. sitkana, L. subrotundata and L. kasatka) co-occur, together with a fourth, still undescribed species (L. sp.). These four species were compared at 16 loci coding for 13 enzymes. All species were easily distinguished by diagnostic enzyme markers. The mean genetic distances and ranges between species pairs are: L. sitkana and L. sp. D = 0.622 (0.561–0.741), L. sitkana and L. subrotundata D=0.981 (0.821–1.110), L. subrotundata and L. sp. D=0.975 (0.955–0.995). The genetic distance between L. kasatka and each of the other three species was greater than 1 (range 1.123–2.087). These data suggest that L. sitkana, L. subrotundata and L. sp could be members of a species complex; according to current classifications these three belong to the subgenus Neritrema. However, the genetic distance between L. kasatka and L. sitkana is much greater than between L. sitkana and other Neritrema species, and thus supports the classification of L. kasatka in the subgenus Littorina.     

Lack of metabolic temperature compensation in the intertidal gastropods,Littorina saxatilis (Olivi) and L. obtusata (L.)

Two intertidal snails, Littorina saxatilis (Olivi, 1972) (upper eulittoral fringe/maritime zone) and Littorina obtusata (Linnaeus, 1758) (lower eulittoral) were collected from a boulder shore on Nobska Point, Cape Cod, Massachusetts, in July and acclimated for 15–20 days at 4 ° or 21 °C. Oxygen consumption rate (Vo₂) was determined for 11–15 subsamples of individuals at 4 °, 11 ° and 21 °C with silver/platinum oxygen electrodes. Multiple factor analysis of variance (MFANOVA) of lo₁₀ transformed values of whole animal Vo₂ with log₁₀ dry tissue weight (DTW) as a covariant revealed that increased test temperature induced a significant increase in Vo₂ in both species (P<0.00001). In contrast, MFANOVA revealed that temperature acclimation did not affect Vo₂ in either L. saxatilis (P= 0.35) or L. obtusata (P= 0.095). Thus, neither species displayed a capacity for the typical metabolic temperature compensation marked by an increase in Vo₂ at any one test temperature in individuals acclimated to a lower temperature that is characteristic of most ectothermic animals. Lack of capacity for metabolic temperature acclimation has also been reported in other littorinid snail species, and may be characteristic of the group as a whole. Lack of capacity for respiratory temperature acclimation in these two species and other littorinids may reflect the extensive semi-diurnal temperature variation that they are exposed to in their eulittoral and eulittoral fringe/maritime zone habitats. In these habitats, any metabolic benefits derived from longer-term temperature compensation of metabolic rates are negated by extreme daily temperature fluctuations. Instead, littorinid species appear to have evolved mechanisms for immediate metabolic regulation which, in L. saxatilis and L. obtusata and other littorinids, appear to centre on a unique ability for near instantaneous suppression of metabolic rate and entrance into short-term metabolic diapause at temperatures above 20–35 °C, making typical seasonal respiratory compensation mechanisms characteristic of most ectotherms of little adaptive value to littorinid species.     

Modeling variation in interaction strength between barnacles and fucoids

The strength by which species interact can vary throughout their ontogeny, as environments vary in space and time, and with the density of their populations. Characterizing strengths of interaction in situ for even a small number of species is logistically difficult and may apply only to those conditions under which the estimates were derived. We sought to combine data from field experiments estimating interaction strength of life stages of the barnacle, Semibalanus balanoides, on germlings of Ascophyllum nodosum, with a model that explored the consequences of variability at per capita and per population levels to the abundance of year-old algal recruits. We further simulated how this interaction affected fucoid germling abundance as the timing of their respective settlements varied relative to one another, as occurs regionally across the Gulf of Maine, USA. Juvenile S. balanoides have a weak estimated per capita effect on germlings. Germling populations are sensitive to variation in per capita effects of juvenile barnacles because of the typically large population sizes of the latter. However, high mortality of juvenile barnacles weakens the population interaction strength over time. Adult barnacles probably weakly facilitate fucoid germlings, but greater survival of adults sustains the strength of that interaction at the population level. Germling abundance is positively associated with densities of adult barnacles and negatively associated with that of juvenile barnacles. Metamorphosing cyprid larvae have the strongest per capita effect on germling abundance, but the interaction between the two stages is so short-lived that germling abundance is altered little. Variation in the timing of barnacle and A. nodosum settlement relative to one another had very little influence on the abundance of yearling germlings. Interactions between barnacles and germlings may influence the demographic structure of A. nodosum populations and the persistence of fucoid-dominated communities on sheltered rocky shores in New England. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.