The relationship between position on shore and shell ornamentation in two size-dependent morphotypes of Littorina striata,with an estimate of evaporative water loss in these morphotypes and in Melarhaphe neritoides

Shells of small (< 7 mm) Littorina striata are frequently nodulose, but shells of larger individuals are striate. Nodulose L. striata dominated the littoral fringe of a black basalt Azorean shore where daytime rock temperatures rose significantly higher than nearby shores of different rock composition or colour. There was no evidence of intraspecific size-partitioning on the latter shores, where the numbers of striate and nodulose L. striata were approximately equal between high eulittoral (low-shore) and high littoral fringe (high-shore) localities. The prevalence of small L. striata in the littoral fringe is opposite to that usually characteristic of the Littorinidae, where the largest individuals usually occupy the higher positions on the shore. It is hypothesized that small L. striata attain a resting posture better able to minimize heat absorption from the substratum than attained by larger individuals. Smaller individuals also take advantage of both posture and a nodulose shell surface to re-radiate absorbed incident radiant thermal energy more effectively to the atmosphere by convection. Thus, small, nodulose L. striata are especially well adapted to occupy geologically young basaltic rocks commonly found fringing islands of the mid-Atlantic. The rate of evaporative water loss was determined for Melarhaphe neritoides and striate and nodulose L. striata for approximately 11 days emersion. All three groups are exceptionally capable of controlling evaporative water loss. Total percent evaporative water loss by nodulose L. striata (17.9%) was significantly greater than that lost by either striate L. striata (14.1%) or M. neritoides (13.5%) but, among 15 species for which evaporative water loss has been determined by similar methodology, M. neritoides and striate L. striata are the most capable of conserving body water during 11 days of emersion.     

The effect of coil phenotypes and genotypes on the fecundity and viability of Partula suturalis and Lymnaea stagnalis: implications for the evolution of sinistral snails

Why are sinistral snails so rare? Two main hypotheses are that selection acts against the establishment of new coiling morphs, because dextral and sinistral snails have trouble mating, or else a developmental constraint prevents the establishment of sinistrals. We therefore used an isolate of the snail Lymnaea stagnalis, in which sinistrals are rare, and populations of Partula suturalis, in which sinistrals are common, as well as a mathematical model, to understand the circumstances by which new morphs evolve. The main finding is that the sinistral genotype is associated with reduced egg viability in L. stagnalis, but in P. suturalis individuals of sinistral and dextral genotype appear equally fecund, implying a lack of a constraint. As positive frequency‐dependent selection against the rare chiral morph in P. suturalis also operates over a narrow range (< 3%), the results suggest a model for chiral evolution in snails in which weak positive frequency‐dependent selection may be overcome by a negative frequency‐dependent selection, such as reproductive character displacement. In snails, there is not always a developmental constraint. As the direction of cleavage, and thus the directional asymmetry of the entire body, does not generally vary in other Spiralia (annelids, echiurans, vestimentiferans, sipunculids and nemerteans), it remains an open question as to whether this is because of a constraint and/or because most taxa do not have a conspicuous external asymmetry (like a shell) upon which selection can act.     

Growth rate differences between upper and lower shore ecotypes of the marine snail Littorina saxatilis (Olivi) (Gastropoda)

Shell polymorphisms are widespread among those intertidal gastropods that lack a pelagic spreading stage. These polymorphisms may indicate diversifying selection in a heterogeneous habitat, but to do this the variation must be at least pardy inherited. Galician populations of Littorina saxatilis (Olivi) living in exposed rocky shores are highly polymorphic in several shell traits, e.g. ornamentation, banding and size. Mature snails of the upper-shore ridged and banded (RB) morph is, for example, often twice as large as mature individuals of the lower-shore smooth and unbanded (SU) morph of the same shore.
We investigated the hypothesis that lower-shore snails grow more slowly and that differences in growth rate were at least partly inherited and could be explained by diversifying selection. We released snails of different origin (upper, mid- and lower shore) and morph (RB, SU and hybrids) at different shore levels and compared their shell increment after one month of growth. We found that despite considerable variation among individuals and among replicate samples (together about 53% of the total variation), average rates of growth differed between morphs. RB snails both from the upper and mid-shores grew at a high rate at all shore levels, SU snails grew considerably less, and hybrids grew at intermediate rates, at all levels. Inherited difference among morphs explained about 34% of the total variation while effects of shore levels and the interaction morph x shore level explained only 5 and 7%, respectively. Thus a large part of the difference in growth rate leading to different adult sizes of the two morphs has probably evolved due to spatially varying selection favouring large sizes in upper-shore and small sizes in lower-shore environments.     

Enantiomorphs differ in shape in opposite directions between populations

Abstract Development is left–right reversed between dextral and sinistral morphs of snails. In sympatry, they share the same gene pool, including polygenes for shell shape. Nevertheless, their shell shapes are not the mirror images of each other. This triggered a debate between hypotheses that argue either for a developmental constraint or for zygotic pleiotropic effects of the polarity gene. We found that dextrals can be wider or narrower than sinistrals depending on the population, contrary to the prediction of invariable deviation under a developmental constraint. If the pleiotropy is solely responsible instead, the mean shape of each morph should change, depending on the frequency of polarity genotype. Our simulations of this mean shape change under zygotic pleiotropy, however, show that the direction of interchiral difference remains the same regardless of genotype frequency. Our results suggest the presence of genetic variation among populations that changes the maternal or zygotic pleiotropic effect of the polarity gene.     

Morph‐dependent form of asymmetry in mandibles of the stag beetle Prosopocoilus inclinatus (Coleoptera: Lucanidae)

Abstract 1. The form of asymmetry in bilateral organs usually follows the same pattern within single populations. However, some exceptions may occur when a population consists of different phenotypes that are from different ontogenic backgrounds and under different selective pressures. We investigated the asymmetric patterns of mandibles of larvae, females, and males in the stag beetle Prosopocoilus inclinatus. 2. Larval mandibles exhibited directional asymmetry both in length and cross direction, whereas female mandibles showed directional asymmetry in cross direction. These asymmetric structures might be more effective in cutting wood fibres. 3. For the relation of male mandible length to body size, a model with a switch point showed a better fit to the data than a convex curve model. This shows that the males are dimorphic with two distinct morphs. 4. The form of asymmetry in male mandible length differed between the morphs. The smaller males exhibited left‐biased directional asymmetry in common with larvae, whereas the larger males exhibited fluctuating asymmetry. 5. This is a novel finding of a morph‐dependent asymmetry. The morph‐dependent asymmetry in males may be as a result of different selection on each morph or a developmental constraint from larval mandibles to adult ones.     

PHYSIOLOGICAL STRESS AND COLOR POLYMORPHISM IN THE INTERTIDAL SNAIL NUCELLA LAPILLUS

The intertidal snail Nucella lapillus exhibits considerable variation in shell color both within and between populations differentially exposed to wave action. Populations from high-wave-energy shores tended to be highly polymorphic and were dominated by pigmented morphs (especially brown), while those at more sheltered locations exhibited less polymorphism and were predominantly white. Field and laboratory experiments were conducted to determine the role of physiological stress and selective predation in maintaining the observed distribution of color morphs. The results demonstrated that 1) physiological stress from high temperature and desiccation during periods of tidal emersion was greater on protected shores, 2) under similar natural conditions, brown morphs heated up faster, attained higher temperatures, desiccated more rapidly, and suffered greater mortality than did white morphs, and 3) when pairs of brown and white morphs were tethered intertidally there was virtually no mortality of either morph on the exposed shore or in shaded microhabitats on the protected shore, but brown morphs suffered much greater mortality in sunny microhabitats on the protected shore. These findings demonstrate that the interpopulation variation in shell color of N. lapillus is in part a response to a selective gradient in physiological stress. Selection for crypsis by visually hunting predators did not appear to play a prominent role; however, only adults were considered, and the predation experiments were conducted in the fall before shorebirds that prey on whelks had arrived from their summer feeding grounds. Further experimentation to quantify the effects of visual predators such as birds and fish, particularly on juvenile snails, is necessary to assess adequately the importance of predation.     

Mate search and aggregation behaviour in the Galician hybrid zone of Littorina saxatilis

In Galician rocky shores two ecotypes of the snail L. saxatilis can be found in sympatry. A ridged and banded ecotype (RB-morph) and a smooth and unbanded ecotype (SU-morph) overlap in midshore with the production of some hybrids. The distinct morphs mate assortatively and there is evidence of a partial reproductive barrier between them. This sexual isolation is caused by a nonrandom microdistribution and mate choice behaviour. Mucus trail-following, movement rate and aggregation behaviour were studied to determine their roles in the mating behaviour and sexual isolation of this species. Morph-specific mucus trail-following could not, in our experiments, explain either of these two processes. The reasons for the aggregation of morphs were investigated by Monte Carlo simulations of data from natural populations, which showed that size aggregation (refuge sizes fit different sized morphs differently) could explain only about 36% of the morph aggregation in adult snails. In the laboratory, morph aggregation was still present, and simulations suggested that size aggregation was the possible explanation. Thus, morph aggregation in Galician L. saxatilis has to be explained also by other causes in addition to size aggregation. These may be a combination of contrasting preferences for barnacle and mussel patches in the two morphs, and possibly longer copulation and pair formation time with similar sized snails of the same morph. Thus aggregation behaviour, but not trail-following, contributes to incipient reproductive isolation and perhaps sympatric speciation in Galician L. saxatilis populations.     

Evolution of whole-body enantiomorphy in the tree snail genus Amphidromus

Diverse animals exhibit left-right asymmetry in development. However, no example of dimorphism for the left-right polarity of development (whole-body enantiomorphy) is known to persist within natural populations. In snails, whole-body enantiomorphs have repeatedly evolved as separate species. Within populations, however, snails are not expected to exhibit enantiomorphy, because of selection against the less common morph resulting from mating disadvantage. Here we present a unique example of evolutionarily stable whole-body enantiomorphy in snails. Our molecular phylogeny of South-east Asian tree snails in the genus Amphidromus indicates that enantiomorphy has likely persisted as the ancestral state over a million generations. Enantiomorphs have continuously coexisted in every population surveyed spanning a period of 10 years. Our results indicate that whole-body enantiomorphy is maintained within populations opposing the rule of directional asymmetry in animals. This study implicates the need for explicit approaches to disclosure of a maintenance mechanism and conservation of the genus.     

Effects of habitat on growth and shape of contrasting phenotypes of Bembicium vittatum Philippi in the Houtman Abrolhos Islands,Western Australia

Translocation experiments were used to test the effect of habitat on growth and shape of three contrasting phenotypes of Bembicium vittatum: dwarf, highly domed snails from an usually dry tidal pond; large, moderately domed snails from a sheltered, regularly inundated pond; and relatively flat snails from a vertical, exposed shore. Snails from both ponds grew nearly twice as fast in the wet pond as in the dry pond, indicating a high degree of plasticity of growth. Associated with these changes in growth rates was convergence of shape. Under conditions of rapid growth, the dwarf snails became relatively flatter, and hence more similar to the native snails at that site. These results indicate that the dwarf phenotype is largely a plastic stunting in response to conditions of little submersion time. The snails from the exposed shore also grew faster in the sheltered, wet pond than at their native site. However, they not only retained their flat shape, but actually became flatter (and hence divergent from the pond snails) when grown in the pond. Thus, variation in shell shape was due to interactions between source population and a common plastic association of flatter growth profile with more rapid growth. Previous experiments had demonstrated high heritability of the flat phenotype, while the present results show that the expression of the genetically different types is affected substantially by the conditions of growth, and that phenotypic differences among populations may either overestimate or underestimate the underlying genetic differences. This unpredictability of the relationship between variation in shell form and its underlying genetic basis complicates interpretations of geographical variation or palaeontological sequences based on shell form.     

Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size-frequency distributions indicate that a predator consuming snails within the 1.3-1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.     

Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations

Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation. On the shores sampled, male–female pairs were strongly assortative both with respect to morphs (mean Yule's V = 0.77) and size (mean Pearson's r = 0.47). In the laboratory, males of both morphs mounted other snails and mated other males and juveniles at random. However, mature females of equal sizes mated assortatively with respect to morph. The two morphs were nonrandomly distributed over barnacle and mussel patches in the hybridization zone. Monte Carlo simulations showed that this microdistribution could explain about half the morph and size relationships in male–female pairs, while a simple rejection mechanism, rejecting the first 1–3 mates if they were of contrasting morphs, accounted for the remaining part of the reproductive isolation, and for parts of the size relationships found between mates. A size discriminant mate choice mechanism may also, to a lesser extent, contribute to the sexual isolation. Sexual selection was observed for female size (larger ones being favoured) and among certain morphs, but distinct biological mechanisms may cause these processes.     

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.     

PREDATOR-INDUCED SHELL DIMORPHISM IN THE ACORN BARNACLE CHTHAMALUS ANISOPOMA

Field experiments were conducted in order to determine the nature of shell dimorphism in the acorn barnacle Chthamalus anisopoma and the adaptive significance of the atypical form. The typical morph has the conical shape which is characteristic of acorn barnacles, while the atypical morph appears bent over, with the rim of its aperture oriented perpendicular to its base. The experiments showed that: 1) the bent-over morphology is an environmentally-induced developmental response to the presence of a carnivorous gastropod (Acanthina angelica) and 2) that “bents” are more resistant than “conics” to specialized predation by this snail. The results also showed that predation by A. angelica is patchy and heaviest in the near vicinity of cracks and crevices, which it uses as refuges during periods of tidal inundation. Because predation is patchy and bents are less fecund and grow slower than conics, the conditional developmental strategy is likely to be favored over strict genetical control of shell morphology.     

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.     

SPATIAL PATTERNS OF SHELL SHAPE OF THREE SPECIES OF CO-EXISTING LITTORINID SNAILS IN NEW SOUTH WALES, AUSTRALIA

The shape and relative weight of the shell have been shown tovary intraspecifically and interspecifically in a number ofspecies of gastropods, including many different littorinids.These differences give rise to different shell forms in differenthabitats. In those species which have non-planktotrophic development,differences in shell form among shores have been usually explainedin terms of natural selection because exposure to waves supposedlyfavours light shells with large apertures, while predation bycrabs on sheltered shores favours elongated, thick shells withsmaller apertures. Differences in shell shape among speciesfound at different heights on the shore have been explainedin terms of resistance to desiccation and temperature. Suchvariables would tend to act on a relatively broad-scale, i.e.causing differences among heights on a shore or among shores.Rates of growth, which might vary at much smaller scales withina shore, have also been shown to affect the shapes of many shells. In this study, the shape and relative weight of shells of threespecies of co-existing littorinids (Littorina unifasciata, Bembiciumnanum and Nodilittorina pyramidalis) were measured. These speciesall haveplanktotrophic development and they are found on manyshores where there is no evidence that they are preyed uponby crabs. Before explanations of shell shape are proposed, itis necessary that patterns of variation, within different partsof ashore and among different shores are clearly documented.These patterns were measured at a number of different spatialscales within and among replicate shores with different amountsof wave exposure. Large and small specimens were included toallow intraspecific comparisons among snails of different sizesfound at different heights on the shore. The results showedsignificant differences among shores in shape and relative weightof shells, but these differences could not be explained by exposureto waves. In addition, snails of different sizes and differentspecies did not show the same patterns although they were collectedfrom the same sites. Importantly, the shell shape of Liuorinaunifasciata varied significantly among sites at approximatelythe same height within a shore. These differences could notbe clearly correlated with density, mean size nor exposure towaves. The only consistent pattern was a decrease in relativeaperture size in specimens living higher on the shore. Modelsthat have commonly been proposed to explain shape and relativeweight of shells in other species of gastropods are not adequateto explain the small- and large-scale variation of the measurementsdescribed here. It is proposed that any selective advantageof shell morphology and the effects of any variables on thedevelopment of shell morphology in these species can only beidentified after appropriately designed and replicated fieldexperiments. (Received 4 March 1994; accepted 13 September 1994)     

Shell colour polymorphism in a mangrove snail Littorina sp. (Prosobranchia: Littorinidae)

Shell colour polymorphism was examined in populations of a mangrove snail Littorina sp. in Queensland, Australia. Three morphs were recognized, yellow, red and brown, and morph frequencies varied both between widely spaced populations and between islands less than 1 km apart. Morph frequencies also varied with time of year. There was a relationship between shell colour and position on the tree, with yellow snails more often occurring amongst the foliage and brown snails more often on trunks and branches. In some populations yellow snails appeared to survive better than the other morphs, while in other populations there was no difference. The evidence for the maintenance of the polymorphism by natural selection is discussed.     

Predator-induced morphological plasticity across local populations of a freshwater snail

The expression of anti-predator adaptations may vary on a spatial scale, favouring traits that are advantageous in a given predation regime. Besides, evolution of different developmental strategies depends to a large extent on the grain of the environment and may result in locally canalized adaptations or, alternatively, the evolution of phenotypic plasticity as different predation regimes may vary across habitats. We investigated the potential for predator-driven variability in shell morphology in a freshwater snail, Radix balthica, and whether found differences were a specialized ecotype adaptation or a result of phenotypic plasticity. Shell shape was quantified in snails from geographically separated pond populations with and without molluscivorous fish. Subsequently, in a common garden experiment we investigated reaction norms of snails from populations' with/without fish when exposed to chemical cues from tench (Tinca tinca), a molluscivorous fish. We found that snails from fish-free ponds had a narrow shell with a well developed spire, whereas snails that coexisted with fish had more rotund shells with a low spire, a shell morphology known to increase survival rate from shell-crushing predators. The common garden experiment mirrored the results from the field survey and showed that snails had similar reaction norms in response to chemical predator cues, i.e. the expression of shell shape was independent of population origin. Finally, we found significant differences for the trait means among populations, within each pond category (fish/fish free), suggesting a genetic component in the determination of shell morphology that has evolved independently across ponds.     

Polymorphism in relation to habitat in the snail Cepaea hortensis in Iceland.

Polymorphism for shell colour and banding pattern in Cepaea hortensis was studied in a confined area in south Iceland. Morph freqquencies can be related to habitat. Yellow unbanded snails are more frequent in grassland and herb meadows thatn in "darker" habitats such as in ddense Angelica . Fused banding is relatively more frequent in "daarker" habitats. Predation by birds is not known to occur and rodent predation in winter or genetic drift cannot explain the observed correlations. Habitats differ in their microclimate and it is suggested that climatic selection is important. Differences in morph frequencies between juvenile and adult snails support this view.     

Usefulness of the opercular nucleus for inferring early development in neritimorph gastropods

The early ontogeny of gastropods (i.e., planktotrophic vs. nonplanktotrophic) may be inferable from the morphology of the protoconch in adult shells. The protoconch consists of both embryonic and larval shells in species with planktotrophic development; the embryonic shell forms in the intracapsular period and the succeeding larval shell gradually develops during the larval period. In nonplanktotrophic species, on the other hand, there is no additional growth of the larval shell and the protoconch consists exclusively of a relatively large embryonic shell formed prior to hatching. This "shell apex theory" has been applied to many species of shell-bearing gastropods, but biotic and abiotic erosion of the apex often prevents detailed examination of the protoconch and subsequent inferences about ontogeny. I examined the gastropod operculum to test its utility for predicting developmental mode, drawing on the Neritimorpha as model taxa. Most aquatic members of Neritimorpha were found to bear an operculum with a clearly demarcated nucleus; SEM observations reveal four types of nuclei, which correspond to different types of protoconch morphologies and observed ontogenies for the study species. The nucleus is secreted before metamorphosis, fits into the shell aperture of the larva, and reflects early ontogeny as morphology, as does the protoconch. Moreover, the apparently organic (rather than calcareous) composition of the nucleus makes it nearly invulnerable to erosion and very advantageous, compared to the protoconch, in this ecologically diverse group, whose habitats range from freshwater streams and mangrove swamps to rocky shores and deep-sea hydrothermal vents. The measurements of the nucleus are also valuable for taxonomic purposes, especially in the species identification of veliger larvae and juvenile snails. On the other hand, the opercular nuclei of the Caenogastropoda and Heterobranchia are often eroded away in adult individuals; even if present, the morphology of the nuclei does not seem to clearly reflect early ontogeny in those groups.     

Survival,behaviour and spatial distribution of shell morphs in a population of the snail Brephulopsis bidens (Pulmonata)

Summary Microspatial variation of banded and unbanded shell morphs frequencies as well as number of individuals per m², mortality, migration and burrowing into the ground were examined in a population of snail Brephulopsis bidens found in South Crimea mountains (USSR). Differential values of relative survival of morphs were determined by their thermotolerance. The relative survival of the banded morph was lower at the west sites of population area (W=0.273), and increased gradually up to 1 at the east sites. Survival of the banded morph was dependent on its burrowing activity. Differences in relative survival of morphs decreased parallel with increasing general mortality of snails.Burrowing activity and intensity of migration of the banded morph were significantly higher than that in unbanded. In experiments with artificial shaded sections, the banded morph preferred shaded sections, whereas unbanded chose illuminated sites. All these differences in behaviour probably form the main factors for microspatial variation of morph frequencies.