RELATIONSHIPS BETWEEN SHELL SHAPE, WATER RESERVES, SURVIVAL AND GROWTH OF HIGHSHORE LITTORINIDS UNDER EXPERIMENTAL CONDITIONS IN NEW SOUTH WALES, AUSTRALIA

The shell morphologies of the highshore littorinids, Littorinaunifasciata Gray and Nodilittorina pyramidalis (Quoy & Gaimard)have previously been shown to vary at a variety of spatial scales,including among replicate sites at the same height, from heightto height and from shore to shore. In this study, the relationshipsbetween morphology of the shell, the reserves of water heldwithin the shell, the size of the foot and survival on differentshores and rates of growth in different habitats were examinedfor L. unifasciata and, to a lesser extent, N. pyramidalis.Reserves of water were not consistently related to size or shapeof the shell, but did increase as relative weight of shell increased.This may be due to the relatively smaller body providing moreinternal volume for extra-corporeal water. Water reserves andthe amount of free water held in the shell were also not relatedto loss of water or survival during extended periods of emersion.Although the shape of the shell on sheltered and exposed shoreswas correlated with size of the foot, with the snails on anexposed shore having larger apertures and feet than those ona sheltered shore, transplant experiments did not show differentialmortality between morphs from the different shores. All translocatedand transplanted snails disappeared from the exposed shore ata greater rate than from the sheltered shore, but this was probablydue to the snails dispersing out of the experimental areas ratherthan due to mortality. Therefore, many of the large-scale modelsthat have previously been used to describe patterns of shellshape in intertidal gastropods do not appear to be importantin these highshore littorinids. Finally, field experiments ongrowth of juvenile L. unifasciata indicated that rate of growth,largely governed by opportunity to feed rather than type andquantity of food, is the most likely explanation for the small-and large-scale patterns of shell shape that have been previouslydescribed in this species. (Received 22 October 1996; accepted 19 February 1997)     

ADAPTATION FROM RESTRICTED GEOMETRIES: THE SHELL INCLINATION OF TERRESTRIAL GASTROPODS

The adaptations that occur for support and protection can be studied with regard to the optimal structure that balances these objectives with any imposed constraints. The shell inclination of terrestrial gastropods is an appropriate model to address this problem. In this study, we examined how gastropods improve shell angles to well‐balanced ones from geometrically constrained shapes. Our geometric analysis and physical analysis showed that constantly coiled shells are constrained from adopting a well‐balanced angle; the shell angle of such basic shells tends to increase as the spire index (shell height/width) increases, although the optimum angle for stability is 90° for flat shells and 0° for tall shells. Furthermore, we estimated the influences of the geometric rule and the functional demands on actual shells by measuring the shell angles of both resting and active snails. We found that terrestrial gastropods have shell angles that are suited for balance. The growth lines of the shells indicated that this adaptation depends on the deflection of the last whorl: the apertures of flat shells are deflected downward, whereas those of tall shells are deflected upward. Our observations of active snails demonstrated that the animals hold their shells at better balanced angles than inactive snails.     

Small- and broad-scale patterns of distribution of the upper-shore littorinid Nodilittorina pyramidalis in New South Wales

Abstract mall-scale (within-shore) and broad-scale (among-shore) patterns of distribution, abundance and size structure of Nodilittorina pyramidalis were measured on a number of shores in New South Wales. This species showed significant patchiness in distribution and size structure. The mean density and mean shell length of snails differed among patches of shore only a few metres apart, among different heights on the shore and from shore to shore. Differences in densities appeared to be strongly correlated to the distribution of particular microhabitats, especially cracks, crevices and pits within the rock surface. This relationship and the presence or absence of these microhabitats appeared to determine the patterns of distribution of this species within and among different shores. The sizes of snails were not correlated with densities of Nodilittorina nor with the coexisting species, Littorina unifasciata, in the populations examined. Processes influencing the distribution and abundance of Nodilittorina were not investigated, but the patterns of distribution found on these shores suggest that recruitment was limited to areas of topographic complexity, ft also seems likely that limited dispersal by the adult snails retain them within restricted patches on the shore.     

Are the opposing selection pressures on exposed and protected shores sufficient to maintain genetic differentiation between gastropod populations with high intermigration rates?

Throughout the world intertidal gastropods living on exposed rocky shores differ strikingly in a number of morphological and life history traits from those on protected shores. Where surf is heavy gastropods tend to be smaller and to have thinner and smoother shells with larger apertures than do those from sheltered areas where crab predation is more intense. These morphological differences can occur within a species and there is evidence that they can be partially genetic and partially environmental. In addition the convergence of shell features in each habitat suggests that there are consistent differences between the selective pressures on exposed shores and the selective pressures on protected shores. I constructed a simulation model for a polygenic trait that experiences different selective pressures on exposed and sheltered shores. The results show that genetic differences can be maintained between the two populations despite high intermigration rates. Replacement of a portion of the random environmental variance with adaptive environmental variance reduces the effect of selection and thus the size of the difference maintained between the two populations. Genetic differentiation between exposed and protected populations can persist for significant periods of time and may have sometimes been the first step in speciation.     

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.     

Influence of shell morphometry,microstructure, and thermal conductivity on thermoregulation in two tropical intertidal snails

Tropical intertidal organisms tolerate large fluctuations in temperature and high desiccation rates when exposed during low tide. In order to withstand the short‐term heat stress, intertidal organisms adopt behavioral responses to maximize their survival. Our previous research showed that tropical littorinids found at the upper and lower intertidal shores in Singapore exhibited different behavioral adaptations during low tide. Most of the upper‐shore Echinolittorina malaccana kept a flat orientation, with the aperture against the substrate and the long axis of the shell towards the sun, whereas a majority of the lower‐shore individuals of Echinolittorina vidua stood with the edge of the aperture perpendicular to the substrate on the rocky shore during low tide. This prompted analyses of the shells of these two species to determine whether the differences in the shell morphometry, microstructure, and thermal conductivity of shells of E. malaccana and E. vidua were associated with their respective behavioral responses to thermal stress. Analyses of shell morphometry and thermal conductivity showed that shells of E. malaccana were more likely to minimize heat gain, despite having a higher thermal conductivity on the outer surface, due to their light‐gray, elongated shell. By contrast, the dark‐colored, globose shells of E. vidua probably gain heat more readily through solar radiation. Scanning electron microscopy images of the shells of both littorinid species further revealed that they have cross‐lamellar structure; however, only individuals of E. vidua showed the presence of disjointed rod layers and a pigmented inner shell surface. Individuals of E. malaccana had a rough outer shell surface with holes that inter‐connect to form water‐trapping channels that probably aid cooling. Individuals of E. vidua, however, had a smooth outer surface with rows of kidney‐shaped depressions as microsculptures which probably help to stabilize shell shape. In both Echinolittorina species, behavioral responses were used to overcome thermal stress during low tide that was associated with shell morphometry and shell thermal conductivity. Such combined adaptations increase survivability of the littorinids at their respective tidal levels.     

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.     

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.     

Water-borne cues from a shell-crushing predator induce a more massive shell in experimental populations of an intertidal snail

Inducible defenses are important in the life strategies of many taxa. In some species of marine gastropods, water-borne chemical cues from potential predators induce defensive changes in shell form and differences in growth rate. We examined such phenotypic plasticity in the direct-developing snail, Littorina subrotundata (Carpenter, 1864). Among experimental field populations of L. subrotundata exposed to differing intensities of predation by the purple shore crab, Hemigrapsus nudus (Dana, 1851), snails collected from predation-intense environments often had more massive shells than closely related snails from adjacent environments where predation was negligible. Snails collected from both environments were raised in tanks containing cages of H. nudus that were feeding on conspecific snails and compared to a control group raised in the absence of this stimulus. Most snails developed significantly more massive shells in the presence of the crabs suggesting that adaptive phenotypic plasticity may account for some of the variation we observed in the field. In one case, snails from a predation-intense environment did not exhibit a statistically significant amount of plasticity, but instead grew a more massive shell irrespective of the laboratory stimulus. We interpret this as evidence for a genetic difference in the plasticity of shell form among experimental populations, caused by intense selection by H. nudus. There was no statistical difference in the growth rates of snails among treatments.     

Effects of trematode parasitism on the shell morphology of snails from flow and nonflow environments

The primary function of the gastropod shell is protection. However, shells that function well in one environment may be maladaptive in another. Upon infection, the snail shell protects internal parasites and it is to the parasite's advantage to optimize, or not interfere with, shell functionality. However, parasites, particularly trematodes, are often pathogenic and it is not clear if parasitism will induce environment‐dependent or ‐independent changes to gastropod shells. We conducted a field study and a complementary laboratory experiment to examine the effects of trematode parasitism on shell characteristics (shape, size, and crush resistance) of Physa acuta snails in flow and nonflow environments using geometric morphometrics and crush assays. Field results indicate wetland (nonflow) snails had large, crush resistant shells with narrow apertures and tall spires. In contrast, stream (flow) snails had small, weak shells with wide apertures and short spires. Parasitism had no apparent effect on the crush resistance of wetland snails but significantly reduced the crush resistance of stream snails. Parasitism had no significant effect on overall shell shape in stream or wetland snails. Similar to the results of our field study, nonflow tank snails had significantly more crush resistant shells than flow tank snails. Additionally, the shapes of flow and nonflow tank snails significantly differed where nonflow tank snails resembled wetland snails and flow tank snails resembled stream snails. For laboratory snails, parasitism reduced crush resistance regardless of flow/nonflow treatment. Our results demonstrate that habitat and/or flow treatment was the primary factor affecting P. acuta shell morphology and that trematode parasitism played a secondary role. J. Morphol. 277:316–325, 2016. © 2015 Wiley Periodicals, Inc.     

Dislodgement force and shell morphology vary according to wave exposure in a tropical gastropod (Cittarium pica)

Wave exposure has strong influences on population density, morphology and behaviour of intertidal species in temperate zones, but little is known about how intertidal organisms in tropical regions respond to gradients in wave exposure. We tested whether dislodgement force and shell shape of a tropical gastropod, Cittarium pica, differs among shores that vary in wave exposure. After adjusting for body size, we found that C. pica from exposed shores required greater dislodgement force to remove them from the shore, had slightly larger opercula (the closure to the shell aperture), and were slightly squatter in shape (reduced in shell height relative to shell width) than C. pica from sheltered shores. These morphological adjustments are consistent with those observed in temperate gastropods, which are argued to represent adaptive responses to the risk of mortality associated with dislodgement.     

TRANSFER EXPERIMENT SUGGESTS ENVIRONMENTAL EFFECTS ON THE RADULA OF LITTORARIA FLAVA (GASTROPODA: LITTORINIDAE)

Phenotypic variation in radulae has been studied in severallittorinid species because of this organ's intrinsic relationshipwith diet and, consequently, with the environment. In this work,we compared the radulae of the Brazilian species Littorariaflava found in mangroves and on rocky shores. Individuals ofL. flava showed marked differences in the shape of the cuspsamong samples from rocky shore and mangrove. In a transfer experiment,the shape of the radula changed within 40 days. A differentresponse was observed in individuals transferred to mangrove,where two different phenotypes were found, suggesting eitherintrapopulational variation in the responses to change of environmentalconditions, or that some snails showed a slow reaction to theenvironmental changes. The alterations could be attributed toecophenotypic plasticity. Analysis of variance showed that thelength of the radula in L. flava was strongly influenced bythe substrate (F_6,22=17.13, P<0.000), but apparently notby the transfer experiment. (Received 3 May 2005; accepted 18 July 2005)     

Adaptive responses of independent traits to the same environmental gradient in the intertidal snail Bembicium vittatum

The snail Bembicium vittatum occupies a wide range of intertidal habitats in the Houtman Abrolhos Islands, Western Australia. Allozyme variation reflects patterns of connectivity, which are independent of local habitat. In contrast, heritable differences in shell shape among 83 shore sites vary with habitat, indicating local adaptation. Here we examine dimorphisms of colour and spotting of the shell in the same populations, as a test of consistency and complexity of patterns of local adaptation. Within populations, the frequency of spotted shells is higher in dark shells. Despite this association, spatial variations of colour and spotting are only weakly correlated. As predicted for traits associated with local adaptation, subdivision is greater for colour, spotting and shape than for allozymes. Colour and shape are associated with local habitat, such that populations on vertical shores have higher frequencies of dark and relatively flatter shells than those on gently sloping shores. These associations are repeatable between three separate groups of islands. Spotting shows a weaker, but significant association with the same gradient. Although shape does not differ between colour morphs within populations, the proportion of dark shells is strongly associated with shape. Thus, the independent shell traits are apparently adapted to a common, biologically significant gradient, even though the adaptive mechanisms probably differ for colour and shape. The parallel variations of independent traits highlight both the complexity of local adaptation and the potential to reveal evolutionarily significant environmental contrasts by examining adaptively relevant traits.     

Distribution in relation to life history in the directdeveloping gastropod Batillaria cumingi (Batillariidae) on two shores of contrasting substrata

Population structure and spatial distribution with growth ofthe direct-developing gastropod, Batillaria cumingi, were investigatedon two shores of differing substrata. Sand-mud shore and rockyshore populations differed in size structure; first-year snailswere ca. 7 mm in shell length (SL) in both populations, whereassecond-year snails, merging with older cohorts, measured 15–25mm SL in the sand-mud shore population and ca. 15 mm SL in therocky shore population. Egg distribution matched adult distributionin the sand-mud shore population, but was more restricted thanthat of adults in the rocky shore population. The distributionof newly-hatched juveniles (0–1 mm SL) was restricted inboth populations, but the growth stage at which snails extendedtheir distribution differed between the two populations; 1–2.5mm SL on the sand-mud shore and 5 mm SL on the rocky shore. Floatingachieved by early juveniles (ca. 2 mm SL), was commonly observedin the sand-mud population, but rarely in the rocky shore population.The sudden expansion in distribution of the 1–2.5 mm SLgrowth stage in the sand-mud shore population is consideredto have been caused by floating, while expansion of the distributionof older growth stages (>5 mm SL) in the rocky shore populationprobably occurs by crawling. (Received 13 May 1998; accepted 25 August 1998)     

Differences in shell shape of naturally infected Lymnaea stagnalis (L.) individuals as the effect of the activity of digenetic trematode larvae

The shells of Lymnaea stagnalis show great morphological variability. This phenomenon has been described as the result of an environmental influence. The main object of the present study was to compare some biometric data from shells of naturally infected and uninfected snails from 25 different lakes in the central part of Poland. The height of the shell, the height of the spiral, and the width of the shell were measured. Some inter- and intrapopulation differences among individuals were found. Greater variability of shell shape was observed among snails parasitized with digenean larvae than in nonparasitized ones. Snails infected with Echinoparyphium aconiatum, Echinostoma revolutum, Diplostomum pseudospathaceum, and Opisthioglyphe ranae differed in shell shape compared with uninfected individuals. Snails infected with Plagiorchis elegans did not differ from uninfected individuals. The same was true of snails in which the commensal oligochaete, Chaetogaster limnei, was found. The results of the present study support the assumption that the deformation of shells of the snails under study was in some way influenced by the presence of certain species of digenetic trematodes.     

How does life adapt to a gravitational environment? The outline of the terrestrial gastropod shell

How do several characteristics adapt to gravity while mutually influencing each other? Our study addresses this issue by focusing on the terrestrial gastropod shell. The geometric relationship between the spire index (shell height/diameter) and outline (cylindricality) is theoretically estimated. When the shell grows isometrically, a high-spired shell becomes conical in shape and a low-spired shell becomes cylindrical in shape. A physical model shows that the lowest- and highest-spired shells are the most balanced. In addition, a cone shape is the most balanced for a low-spired shell, and a column shape is the most balanced for a high-spired shell. Spire index and cylindricality measured for freshwater gastropods follow the relationship estimated by the model, whereas those for terrestrial gastropods deviate from this relationship. This translates to a high shell being more cylindrical than a flat shell, except in the case of extremely high or low shells. This suggests that the shape of the most balanced shells (lowest and highest shell heights) is constrained by coiling geometry but that relatively unbalanced shells (intermediate shell heights) do not follow a coiling geometry, as a result of adaptation to enable the snail to carry its shell more effectively.     

QUANTITATIVE GENETICS OF SHELL FORM OF AN INTERTIDAL SNAIL: CONSTRAINTS ON SHORT-TERM RESPONSE TO SELECTION

We investigated the genetic and environmental determinants of shell form in an intertidal snail (Prosobranchia: Littorina sp.) to identify constraints on the short-term response to selection. Our quantitative genetic parameters were estimated from a half-sib experimental design using 288 broods of snails. Each brood was divided into two treatments differing in snail population density, and therefore in grazing area per snail. Differences in population density induced marked differences in shell form. Snails in the low density treatment grew faster and had lighter shells with narrower whorls and narrower apertures than their siblings at high density. Despite this environmental plasticity in shell shape we found significant additive genetic variance for components of shell shape. We discuss two mechanisms that may maintain additive genetic variance for shell shape in intertidal snail populations: migration between environments with different selective pressures and migration between environments with different mean growth rates. We also estimated a genetic variance-covariance matrix for shell form traits and used the matrix to identify constraints on the short-term response to selection. We predict the rate of response to selection for predator-resistant morphology such as would occur upon invasion of predatory crabs. The large negative genetic correlation between relative spire height and shell weight would facilitate simultaneous selection for a lower spire and a heavier shell, both of which would increase resistance to predatory crabs.     

SIZE, SHAPE AND SHELL MORPHOLOGY IN THE ANTARCTIC LIMPET NACELLA CONCINNA AT SIGNY ISLAND, SOUTH ORKNEY ISLANDS

Comparison of the size, shape and shell morphology in littoraland sub-littoral morphs of the Antarctic limpet Nacella concinnareveal differences in shell morphology which are enhanced bystructural anomalies within the shells of the two types. Infestationof sub-littoral shells by the conchocelis phase of an endolithicalga significantly affects shell density and total chlorophylllevels in the two shell morphs. The surface sculpture of sub-littoralshells is characterised by a series of grooves, the configurationof which closely resembles that of the radular teeth in N. concinna.Limpets utilise the available food supply within the shell matrixof other limpets by grazing the shell material. Epibiotic growthof calcareous algae prevent erosion and preserve underlyingshell layers. In severe cases, where protection is lacking,intraspecific shell grazing may remove parts of the shell exposingthe internal tissues. The Dominican Gull, Larus dominicanus, is a major shore predatorof both shell morphs. Gull middens contain both shell typesbut are dominated by the more accessible littoral shells. Comparisonof living populations and midden assemblages indicates thatsize and shape selection of prey occurs, with pear-shaped limpetsbetween 21 mm and 29 mm in length being taken preferentially. Apparent differences in shell form are induced by physical,biological and behavioural influences. Littoral animals arerobust in nature, resist avian pre-dation and are not extensivelygrazed whereas those of the sub-littoral are not subject tothe same degree of predatory attention but suffer a gradualdepletion of their shallower shell form through a combinationof algal infection and intraspecific shell grazing. (Received 21 February 1990; accepted 5 July 1990)     

Some observations on shell shape variation in Pacific Nucella

This paper considers the patterns of shell shape variation shown by Nucella canalicuata, N. emarginata and N. lamellosa from two areas of the Pacific Northwest: the shores near Friday Harbour on San Juan Island and near Bamfield on the west coast of Vancouver Island. No clear pattern of variation in association with changes in exposure was seen in either N. canaliculata or N. lamellosa . It appears that genetic influences are more important controls of shell shape than environmental selection in both these species. Nucella emarginata shows the nearest approximation to the pattern shown by the Atlantic species, N. lapillus , but only at the exposed end of the wave-action gradient. On those shores, enclaves from the most surf-washed open coast headlands have shells with proportionally larger apertures (and thus a shorter, squatter form) than their equivalents in local shelter. But, unlike in N. lapillus , the trend does not continue onto genuinely sheltered shores. Under these circumstances the species is generally rare and, where enclaves do occur, their shells are of much the same shape (although of a much larger size) as in more exposed situations.