Variation in foot size and shape in some British land snails and its functional significance

Live weight, and the length, breadth and surface area of the extended foot sole were measured in 24 species of British terrestrial snails. Allometric relationships between these variables are described, concentrating on the relationship between foot surface area and weight. Intraspecific foot area/weight relationships deviate from isometric expectation for several species; this may be attributed to density changes, foot-shape variation or the physical constraint imposed by shell aperture size.
The rate of increase of foot sole area with live weight among species is greater than expected under isometry, indicating that larger species partially compensate for the increase in foot loading (weight per unit foot area). The rate of whorl expansion of the shell is related to the deviations from the interspecific foot area/weight relationship, reinforcing the possibility that shell aperture area may limit foot size in some species. Foot ratio (foot length:foot width) is negatively related to live weight and foot loading.
The findings are discussed in relation to the known behaviour and microdistribution patterns of the species.     

Phenotypic plasticity in reproductive traits: importance in the life history of Helix aspersa (Mollusca: Helicidae) in a recently colonized habitat

Reproductive traits of the land snail Helix aspersa Mtiller were investigated under artificial conditions from two samples, one collected from a population exposed to unpredictable human pressures in its natural environment, i.e. a recently created polders area with intensive agriculture, and the other from a snail farm in which animals were reared under constant conditions denned as 'optimal' for growth and reproduction. Results were compared with data collected from natural populations of the same region (Brittany) and from habitats spanning the environmental heterogeneity of the range of the species. A large part of the variation among populations could be explained by different phenotypic covariances between shell size, clutch size and egg size, but not by the number of clutches per snail. Thus, the higher egg production of snails from the polders was related to (i) a strong correlation between clutch size and shell size, shell size being in the upper limit of the overall range for the region concerned; (ii) an uncommonly low egg weight in comparison with the 'norm' of Helix aspersa , this trait seeming to be involved in a trade-off with clutch size. Second clutches were smaller than the first ones, but their eggs were significantly heavier. This difference may be linked to a size-dependent mortality of juveniles during winter which arises in all populations in which hibernation occurs as an adaptation to low temperatures. In addition to the selective regime usually involved for populations of helicid snails from Western Europe, several unpredictable mortality factors occurred in the polders area: herbicide and pesticide treatments (lethal for young snails), human predation (lethal for adults) and burning (letiial for all snails). Life-history patterns of Helix aspersa are discussed in relation to its ability to successfully colonize a large range of habitats modified by humans, to such an extent that it can become a pest.     

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.     

Appearance of morphological novelty in a hybrid zone between two species of land snail

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.     

Shell morphometry in barnacles: quantification of shape and shape change in Balanus

Geometric design in the barnacle genus Balanus has been studied in relation to variation in adult shell form, that includes differences among species, and size-related changes in shape. The genus comprises 40 Recent species, and as a group these display a more or less constant morphology over an extraordinary size range (10 to 200 mm in basal length).
Linear and volumetric measurements were collected from 232 adult individuals of 14 species representing the variation in size, shell form and shell design thought to occur in the genus. Specimens were chosen to represent the size ranges of the species. Only isolated individuals growing on planar surfaces were used; shells were complete, undamaged and undistorted.
Shell form differs among taxa, and no two species scale alike; intraspecific variation for five ratio variables shows strong allometry over the adult size range of each species. As size increases, there is a trend for the basis and orifice to maintain their shapes or to become slightly more elliptical, and for shells to become more conical and proportionately taller.
Throughout their size ranges, species can be described by these geometries: paraboloid (6 species), frustum of an ellipsoidal cone (5 species), frustum of a cone (2 species) and a cone (1 species). Shell geometry is not a function of species size, but there does appear to be a correlation between shell geometry and shell volume. Species with relatively small shell volumes are described by a frustum of an ellipsoidal cone, or by a cone, while those with a relatively large shell volume are described by a paraboloid, or by the frustum of a cone.     

Predation and the geography of opercular thickness in turbinid gastropods

The heavily calcified opercula of turban snails (Turbinidae)function as passive-defensive structures against predators thatbreak the shell at the outer lip or that enter the shell byway of the aperture. Passive armour generally is more commonand much better developed in the tropics than in colder regions.In line with this expectation, the relative opercular thicknessof tropical turbinids (mean 0.358 ± 0.045, 33 species)significantly exceeds that of temperate species (mean 0.300± 0.051, 21 species). Small differences in relative opercularthickness occur among tropical biogeographic regions, but noneis significant. Patterns observed among turbinids as a wholealso appear in individual clades, indicating that the patternsare not dictated by a phylogenetic signal. (Received 5 September 2006; accepted 21 November 2006)     

The effects of exposure and predation on the shell of two British winkles

Changes in shell size and shell shape of the two British winkles Littorina nigrolineata and L. rudis were studied in relation to exposure and to crab-size. In both species, shells from exposed shores are smaller and more globose than those from sheltered shores. Also, in rudis of exposed shores the mouth is relatively wider. In shores of equally sheltered conditions, shells are bigger at those localities where crabs are large than at those localities where they are small. The largest shells are found in those localities where it is extremely sheltered, and the crabs are very large.
It is argued that on exposed shores, small shells are favoured because they have more possibilities than large ones to shelter in crevices and in barnacle interspaces, from the impact of winds and waves. A globose shell could accommodate more foot muscle and thus enable a stronger adherence to the rock; and an increased mouth diameter would increase the area of foot adherence to the rock. On sheltered shores, on the other hand, large, narrow-mouthed shells are favoured because they discourage crab predation, large crabs being abundant mainly on sheltered shores.
The possible significance of shell size and shape in relation to zonation is discussed, in view of the different predatory and physical conditions which prevail in different zones of the shore, and the different shell specializations which these conditions would require.     

Maintenance of the shore-level size gradient in the marine snail Tegula funebralis (A. Adams): Importance of behavioral responses to light and sea star predators

The intertidal gastropod, Tegula funebralis (A. Adams) exhibits a shore-level size gradient with mean shell size increasing in a down-shore direction. Snails transferred to zones where they do not usually occur migrated back towards their original zone, thus re-establishing a size gradient and implying differential movement among size classes. Both large (≥2.1 cm shell width) and small (≤ 1.77 cm) snails were photonegative on a horizontal surface and geonegative in the laboratory; there were no statistical differences between size classes. Light, however, inhibited upward, or caused downward, movement of large snails on vertical surfaces. Small snails were unaffected, ranging higher on illuminated vertical surfaces than large snails. Both sizes exhibited similar distributions in the dark. In an experimental chamber providing both emersed and immersed surfaces, T. funebralis established vertical size gradients when the chamber was illuminated from above. It is suggested that light is an important factor in the formation and maintenance of Tegula's shore-level size gradient.In response to water-borne chemicals derived from the sea star Pisaster ochraceus (Brandt), large snails moved up vertical surfaces in greater proportion than small. In response to contact with the predator, large snails moved away faster than small and individuals collected from crevices in the field moved away slower than those collected from open rock faces. Although predation may select for a size gradient in Tegulafunebralis, it is unlikely that responses to predatory sea stars directly and proximally cause or maintain them over the short term.     

EFFECTS OF HERBIVORE SIZE AND HUNGER LEVEL ON PERIPHYTON COMMUNITIES1

The effects of herbivore size and hunger level were tested on lotic periphyton community structure and ash-free dry mass (AFDM). My hypotheses were 1) that small herbivores would remove more periphyton per unit biomass than large herbivores of the same species because of energetic demands and 2) that within the same size class, starved herbivores would remove more periphyton than nonstarved herbivores. The herbivore used was the prosobranch snail Elimia clavaeformis Lea. Seven treatments were employed: 1) no snails (control); 2) small “starved” snails; 3) large “starved” snails; 4) small and large “starved” snails; 5) small fed snails; 6) large fed snails; and 7) small and large fed snails. Although snails removed significant amounts of periphyton AFDM relative to controls, neither snail size nor degree of starvation had a significant effect on loss of total AFDM. Small snails removed significantly more erect forms of Stigeoclonium tenue (C. A. Ag.) Kütz. than large snails, but snail size had no other significant effect. Starved snails removed significantly more Cocconeis placentula Ehr. than fed snails, suggesting that after the more susceptible growth forms (e.g. erect Stigeoclonium) were removed, Cocconeis cells became more vulnerable to grazing by hungry snails. When small and large snails were combined in chambers, large snails gained weight over time, whereas small snails lost weight. These results suggest that intraspecific competition may occur within populations of Elimia.     

不同壳色福寿螺形态性状与体质量的关系

【背景】福寿螺因其食性杂、抗逆性和繁殖力强以及自然天敌少等不断扩散,侵害农作物,被列为我国首批外来入侵物种。国内外学者一直致力于研究对其的防治与监控。自然界中福寿螺存在2种壳色——黄色和黑色,壳色受遗传因素和环境因素的双重影响。广东省福寿螺多以黑色为主,福寿螺倾向于与不同壳色的螺交配。壳色在一定程度上影响其交配的选择性,但2种壳色的福寿螺繁殖力指标差异不显著。而关于这2种壳色的螺在形态学上的差异鲜有报道。【方法】利用生物统计软件和分析方法进行相关性分析、通径分析及多元回归分析,计算相关系数、通径系数和决定系数,研究2种壳色福寿螺形态性状与体质量的关系。【结果】2种壳色福寿螺的体质量、层高的变异系数较大,且黄色比黑色变异系数大。对黄色福寿螺体质量影响较大的依次为壳高、口宽;对黑色福寿螺体质量影响较大的依次为口宽、层高。【结论与意义】2种壳色福寿螺在形态性状方面差异显著,可以将壳色作为特征标记,为福寿螺的监测与灾害评估提供参考。     

Population structure of Placostylus hongii (Gastropoda: Paryphantidae) on the Poor Knights Islands

Abstract

Populations of the native land snail Placostylus hongii (Lesson, 1830) on the Poor Knights Islands were investigated during visits in 1976, 1977, and 1978. Marking and recapture of individual snails showed that linear growth of the shell ceased when a lip was formed around the outer margin of the aperture. Measurements of snails suggested that increase in outer lip width was a good indication of age, but that the rate of lip thickening varied between populations. Large Placostylus showed a restricted range of movement. Both tagging studies and random sampling revealed that mortality was size specific. Most small snails (<60 mm shell length) located were dead, whereas most larger snails were alive. Population densities of living snails averaged 2.2 individuals per 0.25 m² in the study area. The local distribution was patchy owing to the tendency of snails to be associated with specific features of the vegetation.     

Shell shape and land-snail habitat in a Mediterranean and desert fauna

The bimodal distribution of shell shape (height: diameter), that is found in various geographically widely separate and taxonomically distinct land snail faunas of many different regions of the world, occurs also in a Mediterranean fauna and in a desert fauna that is derived from it. The desert fauna is, however, closer to the bisector than the Mediterranean one. High-spired snails are mainly rock-dwellers, and equidimensional to low-spired snails are bush-dwellers or soil-diggers, with a few rock-dwellers; litter-dwellers are small-sized species that may have either high- or low-spired shells. These results are discussed in adaptive terms. Litter is probably the more primitive of these micro-environments. Many of the small, litter-dwelling snails are ovo-viviparous rather than oviparous, perhaps so as to avoid attacks on the eggs by saprophytic fungi. The shift away from the litter environment is accompanied by a trend to abandon the ovo-viviparous strategy, in favour of oviparity, the snail using its foot to dig into the soil and lay eggs. The conchometric differences between bush-, ground- and rock-dwelling snails may perhaps reflect selective pressure to increase the size of the foot; and constraints of a habitat that consists of narrow interspaces between rocky boulders. Snails that habitually dig into the ground during periods of inactivity, and roam over the ground when active, requires a very large foot and, consequently, a very large-mouthed shell to accommodate it; the result is an equidimensional shell, globose or turbiniform in shape. Snails that climb up vertical vegetation would also require a large foot, and consequently a large-mouthed shell to contain it. A fully globose shell would however be disadvantageous, since it might cause undesired torque. Hence, bush-dwellers tend to be flatter than soil-diggers. Snails that habitually live in rock crevices, and on hard substrata, would not require a very large foot; they would need a narrow shell, both to enable easy manoeuvring through crevices and to reduce torque, the result being a small-mouthed, usually high-spired shell. The classification of land snails into bush-, soil- or rock-dwellers closely follows the taxonomic classification. In those species that depart from the habitat that is typical of their taxonomic group towards another habitat, the shell alters its shape accordingly.     

Population dynamics of two snail species,Planaxis sulcatus and Cerithium moniliferum,and their trematode species at Heron Island,Great Barrier Reef

Summary The population dynamics of the prosobranch snail Planaxis sulcatus and its trematode parasites on 600 m of beachrock on the southern side of Heron Island, Great Barrier Reef, were studied. Populations of Planaxis show little mixing, due to lack of long-distance movements. From March 1973 to June 1975, an increase in the numbers of small and medium-sized snails occurred, but the biomass (dry tissue weight) of the snail population did not change. Snails were infected with one species of Aspidogastrea and six species of cerariae. Infections with cercariae did not significantly affect the relative weight of the snails. Multiple infections were random, i.e. neither negative nor positive interactions between trematode species could be demonstrated. Proportions of uninfected snails and snails infected with various trematode species remained more or less constant from July 1973 to February 1975.The population dynamics of the prosobranch snail Cerithium moniliferum and its 11 species of cercariae and one species of Aspidogastrea in a small area at Heron Island were studied. Numbers of large snails decreased and numbers of small snails increased from August 1973 to June 1975. The biomass of the Cerithium population increased only slightly. Prevalence of infection changed significantly only in large snails.Infections of Cerithium moniliferum and Peristernia australiensis with an aspidogastrid species decreased strongly from January 1971 to March/April 1972 and had not recovered by mid 1975.Seasonal fluctuations could not be demonstrated for any of the snails or parasites.The data for Planaxis suggest equilibrium conditions and saturation of the habitat.     

Studies on the prolonged storage of Metarhizium anisopliae conidia: effect of temperature and relative humidity on conidial viability and virulence against mosquitoes

The shell lengths, dried shell weights, soft tissue wet weights, and soft tissue dry weights were ascertained for noninfected Ilyanassa obsoleta and for the same snail species naturally infected with the following digeneans: Himasthla quissetensis, Zoogonus lasius, Cercaria dipterocerca, Lepocreadium setiferoides, Microphalloides nassicola, Stephanostomum tenue, and Microbilharzia variglandis. Analyses of the data obtained indicate that infection with all of the trematodes listed above causes neither enhanced growth of soft tissues nor accelerated lengthening of the shell. However, snails infected with sporocysts of Z. lasius have significantly heavier shells than do noninfected snails. These data suggest that infection with Z. lasius may have caused enhanced growth in the form of increased calcium deposition in the shell of I. obsoleta, possibly as a result of parasitic castration. In addition, the mean dry weight of the soft tissues of snails infected with M. variglandis, when normalized for shell length, is significantly lower than that of noninfected snails, possibly as a result of parasite-induced pathology. Finally, comparisons among infected snails indicate a trend toward increased soft tissue dry weight in snails infected with L. setiferoides, although the mean dry weight of these snails does not differ significantly from the mean dry weight of noninfected snails.     

The origin and geometry of radial ribbing patterns in articulate brachiopods

Ackerly, S. C. 1992 07 15: The origin and geometry of radial ribbing patterns in articulate brachiopods.
Geometric models for simple. radial ribbing in articulate brachiopods include (1) ribs radiating isometrically from the shell umbo. (2) divergence of thc ribs from some 'point' within the shell, and (3) reorientation of the ribs at right angles to the shell margin. Analyses of the Orthida, the ancestral taxon of articulate brachiopods, indicate that rib geometries are isometric in Early Cambrian taxa (model 1). but that by the Early Ordovician rib orientations are generally perpendicular to the shell margin (model 3). A combination of functional and morphogenetic Factors explains the ribbing geometries observed in orthide brachiopods.     

Model assessments of the optimal design of nature reserves for maximizing species longevity

Using a computational model for the population growth and dispersal of a model species in a fluctuating environment, we test three nature reserve geometries (one large, many small, and a self-similar distribution of reserve sizes) to determine which geometry maximizes species longevity. The self-similar distribution is a close approximation to the distribution of managed areas in the conterminous United States. We consider models with and without migration from or between reserve fragments and both short- and long-range dispersal mechanisms. The optimal geometry depends on the type of dispersal and on the relative probability of survival in protected and non-protected areas. When no migration is allowed from or between reserve fragments of the three geometries, many small equally sized reserves are the optimal geometry. When migration is allowed, the optimal geometry is a single large reserve when the survivability in non-protected areas is low and a self-similar distribution when the survivability is high.     

Extreme,continuous variation in an island snail: local diversification and association of shell form with the current environment

On Rosemary Island, a small continental island (11 km²) in the Dampier Archipelago, Western Australia, snails of the genus Rhagada have extremely diverse morphologies. Their shells vary remarkably in size and shape, with the latter ranging from globose to keeled‐flat, spanning the range of variation in the entire genus. Based primarily on variation in shell morphology, five distinct species are currently recognized. However, a study of 103 populations has revealed continuity of shell form within a very closely‐related group. A phylogenetic analysis of specimens from Rosemary Island, and other islands in the Dampier Archipelago, indicates that much of the morphological variation has evolved on the island, from within a monophyletic group. Within the island, snails with distinct shell morphologies could not be distinguished based on variation in mitochondrial DNA or their reproductive anatomy. The shell variation is geographically structured over a very fine scale, with clines linking the extreme forms over distances less than 200 m. Although there is no evidence that the different forms have evolved in isolation or as a consequence of drift, there is a strong association between keeled‐flat shells and rocky habitats, suggesting that shell shape may be of adaptive significance. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 756–769.     

Microgeographic evolution of snail shell shape and predator behavior

Genetic divergence in geographically isolated populations is a prerequisite for allopatric speciation, one of the most common modes of speciation. In ecologically equivalent populations existing within a small, environmentally homogeneous area, an important role for environmentally neutral divergence is often found or inferred. We studied a species complex of conspicuously shaped Opisthostoma land snails on scattered limestone outcrops within a small area of lowland rainforest in Borneo. We used shell morphometrics, mitochondrial and nuclear DNA sequences, and marks of predation to study the factors involved in allopatric divergence. We found that a striking geographic divergence exists in shell morphology, which is partly associated with neutral genetic divergence. We also found geographic differentiation in the behavior of the snails' invertebrate predator and evidence of an evolutionary interaction between aspects of shell shape and predator behavior. Our study shows that adaptation to biotic aspects of the environment may play a more important role in allopatric speciation than previously suspected, even on a geographically very small scale.     

Evidence of a reproductive barrier between two forms of the marine periwinkle Littorina fabalis (Gastropoda)

Studies of allozyme variation may reveal unexpected patterns of genetic variation which challenge earlier conclusions of species delimitations based on morphological data. However, allozyme variation alone may not be sufficient to resolve this kind of problem. For example, populations of the marine intertidal snail Littorina fabalis (=Littorina mariae) from wave exposed parts end from protected parts of the same shores are distinguished by different alleles of arginine kinase (Ark) while indifferent, or very nearly so, in another 29 loci. Intermediate populations have large deficiencies of exposed/sheltered heterozygote classes of Ark and we have earlier suggested habitat-related selection in this locus as the explanation. In this study we estimated growth rate of individual snails of different Ark-genotypes in three different habitats (exposed, sheltered and intermediate). In all habitats the snails homozygous for alleles of ‘exposed’ type grew faster and matured at a larger size than did snails homozygous for alleles of ‘sheltered’ types. This relationship was indirectly confirmed in three additional sites of intermediate exposure where exposed AA-genotypes dominated among large (>8 mm) snails while the sheltered genotypes dominated among small (<5 mm) snails of truly sympatric samples. We furthermore found small differences in allele frequencies of two other loci (Pgi and Pgm-2) and in shell colour frequencies, comparing sympatric snails of exposed and sheltered Ark-homozygotes. Although we found no signs of habitat-related selection among snails of different Ark-genotype, or selection against heterozygotes, we cannot reject selection in Ark, as our experiments only covered one island, one season and grown-up snails. The coupling between allozyme and phenotypic characters in strictly sympatric samples of snails suggests the presence of two gene pools. Perhaps the large and small forms of L. fabalis represent very closely related cryptic taxa. However, introgression between them seems a possible explanation for the striking similarities in the vast majority of morphological and allozyme characters.     

Shell size and shape in Madeiran land snails: do niches remain unfilled?

The distribution of shell heights and diameters in the mainly endemic Madeiran land snail fauna shows the bimodal pattern of high- and low-spired shells found in many other faunas. Field and laboratory studies show that shell shape is associated with the angle of substrate on which the snails crawl; as elsewhere, tall spired species use vertical surfaces or burrow in soft material. Flattened species predominate on horizontal surfaces, while globular species are less specific in their preferences. Detailed comparisons with the fauna of N.W. Europe show that the proportion of high-spired species in the Madeiran fauna is low, and large high-spired species associated with vertical surfaces are very few in number despite an apparent abundance of suitable habitats. Amongst low-spired species, one family, the Helicidae, dominates the Madeiran fauna. While the overall distribution of size in these species is much as in Europe, Madeiran helicids extend into smaller size classes than do those in Europe, and they appear to fill a gap in the scatter created by the absence of other families.
Non-endemic species, other than those strictly associated with man-made environments, are generally small in size. In the upper scatter, their size distribution parallels that of endemics, but in the lower scatter they constitute the whole of the smallest size classes.
The role of interspecific competition in determining these distributions is discussed. The range of helicid sizes is compatible with a relaxation of competition or predator pressure relative to other areas, but in the upper scatter there appear to be gaps in the range of size and shape expected despite a long period in which the fauna could evolve. This could indicate the existence of adaptive troughs blocking, or delaying, radiation over the full spectrum of size and shape.