Variation in Foot Size and Shape in Some British Land Snails and Its Functional Significance

Biological Journal of the Linnean Society (1989), 36: 365-376. With 3 figures

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

P. TATTERSFIELD 4 Cracken View, New Smithy, Chin@, Stockport SK12 6Dz

Received 6 Jub 1988, accepted for publication 5 October 1988

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 arr described, concentrating on the relationship between foot surface area and weight. Intraspecific foot area/wcight 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 increasc 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/weigbt relationship, reinforcing the possibility that shell aperture area may limit foot size in some species. Foot ratio (foot 1ength:foot width) is negatively related to live weight and foot loading. The findings are discussed in relation to the known behaviour and microdistribution pattcrns of thc species.

KEY WORDS: Gastropod - allometry - land snails.

CONTENTS

Introduction ...... 365 Materials and mcthods...... 366 Results ...... 366 Intraspecific relationships ...... 366 Interspecific relationships ...... 368 Discussion ...... 372 Acknowledgements ...... 375 Refcrcncrs...... 376

INTRODUCTION In terrestrial pulmonates locomotion and the maintenance of position in the environment while active are accomplished by the muscular ventral foot which bears the shell and visceral mass dorsally. The mechanics of locomotion have been investigated in several species, but there have been few comparative studies examining the functional aspects of foot morphology among gastropod species, except in marine prosobranchs (Miller, 1974). The deployment of shell shape in many land snail faunas is non-random, with an excess of high- and low-spired forms and a paucity of globular species (Cain, 1977; Heller, 1987). Cain (1977) tentatively suggested that shell shape may hold 365 0024-4066/89/040365 + 12 %03.00/0 c1989 Thc Linnean Society of London 366 P. TA'ITERSI'IELD adaptive significance related to mechanical problems encountered during activity. Large differences in the micro-sites of activity, in relation to both substrate type and angle, of coexisting species have been reported in some faunas (Cameron, 1978; Cain & Cowie, 1978), and can sometimes be related to shell form. These observations suggest that adaptations in foot morphology might also be expected and Heller (1987) has argued that shell form may reflect selective pressures on foot size. This paper reports on a study of intra- and inter-specific allometric relationships between foot dimensions and snail size in 24 British snail species. It concentrates on the relationship between live weight and the surface area of the extended foot sole, which represents the maximum area of attachment of the snail to the substrate when crawling. It also examines variations in foot shape and its relationships with shell form. Nomenclature follows Kerney & Cameron ( 1979).

MAIERIALS AND METHODS Most of the snails used were collected in spring 1981 from woodland and grassland habitats on the Cotswold hills near Cheltenham, England, but some came from sites near Bath in Avon and Tenby in Dyfed (South Wales). Twenty- four hours before measurement, a selection of individuals representing a wide size range, was taken from the boxes in which they were maintained and each individual placed in a container supplied with food and water. Live weight was measured to the nearest 0.1 mg after removal of any surface water from the shell. The size of the foot sole was measured from photographs. Snails were allowed to crawl on clean, dry, transparent plastic sheets, and the ventral side of the extended foot was photographed on 35 mm monochromatic emulsion. Lighting was arranged to produce a sharp and distinct silhouette of the foot. A macro-lens or bellows unit provided adequate magnification for the smaller species. Each species, on each roll of film was photographed at constant focus and magnification. A scale, marked in millimetres, was also photographed at each magnification. The negatives were projected onto paper, and the image of the foot traced around. Image area was measured with a planimeter and this, and maximum foot length and width were converted to mm or mm2 using the scale. A foot ratio (foot length/foot width) was also computed for each photograph. Many snails were photographed at least once and some were photographed up to six times. All image areas were measured at least twice, but differences among either photographs or area measurements were generally small. The mean of these replicates has been used in the analyses. Most measurements were taken from snails crawling on a vertical (90") surface, but three species were also measured on horizontal (0') and overhanging (180") surfaces. Allometric relationships have been examined by using regression methods (where at least six individuals were measured) following logarithmic (base 10) transformation.

RESULTS Intraspecajic relationships Table 1 gives logarithmic regression equations of extended foot sole area on live weight for eleven species crawling on a vertical surface, and for Trichia striolata, Discus rotundatus and Cochlodina laminata on horizontal and overhanging surfaces FOOT SIZE AND SHAPE IN LAND SNAILS 367 TABLE1. Logarithmic regression equations of foot area (mm2)on live weight (mg) for 11 species of snail

~ ~~~~ Angle Correlation category n Slope (b) S.E. (b) Intercept coefficient

Discus rotundatus Vertical 18 0.641 0.064 - 0.20 I 0.928 Horizontal 16 0.665 0.100 -0.292 0.872 Overhang 16 0.719 0.064 - 0.307 0.948 Joint 50 0.655 0.045 -0.259 0.902 Claudia bidentata Vertical 14 0.566* 0.046 -0.163 0.962 Oxjchilus alliarius Vertical 6 0.857 0.157 - 0.432 0.939 Ena obscura Vertical 13 0.424 0.1 19 0.193 0.732 Cochlodina laminata Vertical 19 0.539 0.073 0.042 0.874 Horizontal 16 0.680 0.049 - 0.257 0.965 Overhang 16 0.578 0.099 - 0.043 0.834 Joint 51 0.596 0.043 - 0.078 0.894 Oxychilus cellarius Vertical 10 0.686 0.096 -0.174 0.930 Trichia striolata Vertical 17 0.770* 0.047 - 0.304 0.973 Horizontal 16 0.724 0.060 -0.255 0.955 Overhang 15 0.783 0.059 - 0.358 0.965 Joint 48 0.764** 0.031 -0.315 0.964 Cepaea hortensis Vertical 14 0.681 0.046 - 0.064 0.974 Cepaea nemoralis Vertical 26 0.931 *** 0.058 - 0.835 0.956 Arianta arbustorum Vertical 11 0.593 0.054 0.402 0.965 Helix aspersa Vertical 17 0.752** 0.030 - 0.241 0.989

Probability levels are given where the null hypothesis that b = 0.667 is rejected. *P < 0.05; **P < 0.02; ***P< 0.001. All others P > 0.05. also. The regression coefficients for the latter three species do not differ significantly (P>O.l), among surfaces, so the joint regressions based on all individuals (Table 1) have been used in the following analyses. The small sample sizes for the other 13 species precluded regression analysis. Foot area for the heaviest individuals of these species are given in Table 2. The complete data set, including shell dimensions, is tabulated in Tattersfield (1981). With the exception of Ena obscura, all regressions give good fits, and account for

TABLE2. Maximum weight and foot area for 13 specics of snail while crawling on a vcrtical surfacc

Live weight Foot area (md (mm’)

Vitrea ctytallina 10.0 2.23 Halea peroersa 31.0 2.75 Vitrina pellucida 39.7 12.47 Macrogastra rolphii 74.9 9.37 Candidula interseeta 76.3 13.32 Trichia hispida 122.3 16.06 Aeppinella nitidula 133.9 20.55 Cochlicella acuta 120.9 16.00 Ena montana 218.5 31.15 Helicella itala 377.6 36.67 Helicigona lapicida 780.0 104.59 Theha pimia 925.8 91.54 Helix pomatia 24,727.3 1,697.62 368 P. TA'I'TEKSFIELD TABLE3. Logarithmic regression equations offoot length (mm) on foot width (mm) for 11 species of snail

Angle S.E. Correlation category n Slope (b) slope Intercept coefficient __ Discus rotundatus Vertical 18 0.925 0.182 0.767 0.786 Horizontal 16 0.691 0.212 0.727 0.658 Overhang 16 1.131 0.151 0.780 0.895 Joint 50 0.897 0.108 0.756 0.767 Clausilia bidentata Vertical 14 1.065 0.187 0.617 0.855 Oxychilus alliariuJ Vertical 6 0.626 0.268 0.988 0.760 Ena obscura Vertical 13 0.022** 0.234 0.806 0.028 Cocklodina laminnta Vertical 19 0.650* 0.152 0.755 0.721 Horizontal 16 0.742 0.110 0.681 0.875 Overhang 16 0.657** 0.102 0.770 0.865 Joint 51 0.634* * * 0.075 0.748 0.771 Oxychilus cellarius Vertical 10 0.773 0.216 0.950 0.784 Trichia striolata Vertical 17 0.832 0.140 0.749 0.837 Horizontal 16 0.740** 0.080 0.757 0.926 Overhang 15 0.981 0.117 0.707 0.918 Joint 48 0.842* 0.071 0.740 0.868 Cepaea kortensis Vertical 14 0.748* 0.091 0.823 0.921 Cepaea nemoralis Vertical 26 0.650*** 0.058 0.937 0.916 Arianta arbustorum Vertical 11 0.528* 0.212 1.002 0.638 Helix aspersa Vertical 17 0.803** 0.065 0.801 0.954

~ ~~ Probability levels are given where the null hypothesis that b = 1.000 is rejected. *P < 0.05; **P < 0.01; ***P< 0.001. All others P > 0.1. at least 75% of the variation in foot area. The values of the regression coefficients have a considerable scatter around the isometric expectation of 0.667 for logarithmic plots of area on weight (volume) quantities (Thompson, 1942; Calow, 1975) but overall have a mean value of 0.682. Six species conform to the isometric expectation of area cc volume0~6"7,but three, 'Trichia striolata, Helix aspersa and Cepaea nemoralis, have slopes significantly exceeding 0.667, and Clausilia bidentala lies significantly below it. Variations in foot shape have been investigated by two methods. 1. Regressions of foot ratio on live weight, in general, give poor fits, with only the correlations for Helix aspersa and C'epaea nemoralis being significant (P<0.05) and negative. 2. Logarithmic plots of foot length on foot width (Table 3) typically have slopes which lie below the isometric expectation of unity, although only those for seven species do so significantly. Foot shape in these species therefore becomes disproportionately wider with size. The correlation for Ena obscura does not differ significantly from zero.

Interspec$% relationships Overall, the regression coefficients given in Table 1 differ significantly among species. If the slopes of these logarithmic plots lay parallel, interspecific comparisons could be made by examination of foot area at a fixed weight. Where the slopes differ, as here, such a comparison is not informative. Interspecific comparisons are therefore restricted to comparisons of foot area at adult weight (Table 4). FOOT SIZE AND SHAPE IN LAND SNAILS 369

Figure 1. Plot oflog,,,foot area at adult weight (mm') on log,,,adult weight (mg) for 20 species ofsnail. The points are labelled with the initial letters of the spccies they represent (e.g. Em= Ena montana). Regression equation: slope=0.832 (S.E. =0.025), intercept= -0.501, correlation roefficient=0.992.

TABLE4. Adult weight (mg), foot area at adult weight (mm') and estimates of W for 20 species of snail

~~ ~ Adult Foot area at weight (rng) adult weight (mm') W

Vitrea crystallina 10 2.23 1.55 Vitrina pellucida 38 12.10 2.58 Discus rotundatus 45 6.68 1.43 Clausilia bidentata 50 6.29 1.05 Oxychilus alliarius 55 11.47 1.70 Ena obscura 75 9.73 1.30 Macrogastra rolphii 75 9.40 1.30 Cochlicella acuta 140 16.90 1.20 Aegopinella nitidula 150 2 1.oo 1.78 Cochlodina laminata 160 17.14 1.15 Oxychilus cellarius 250 29.58 1.65 Candidula intersecta 250 34.32 1.54 Ena montana 400 37.50 1.44 Trichia striolata 5 70 61.60 1.53 Helicigona lapicida 800 105.00 1.74 Cepaea hortensis 2600 182.64 1.70 Arianta arbustorum 3000 291.02 1.64 Cepaea nemoralis 3000 252.46 1.72 Helix aspersa 12000 670.73 2.09 Helix pomatia 24700 1698.00 2.16 370 P. TATlERSFIELU

0*2510.20

.-0 'c-x I OAa / *Hp

c .-x "O5I

.- 0".-c -0.05~cb/00c

0 Ha OCh OMr .Em -0.10 /.C'

I I I 1.5 2 .o 2.5 W Figure 2. Regression of the residuals from the interspecific adult foot area/weight relationship (Fig. I) on the shell parameter W. Regression equation: slope = 0.197 (S.E. = 0.039), intercept = - 0.31 7: correlation coefficient =0.757 (P

Figure 1 shows a logarithmic plot of foot area at adult weight on adult weight for 20 species. Adult weight has been estimated from the largest individuals of each species, or taken from Cameron (1981), or by a combination of both methods. Adult foot area has either been computed from the intraspecific regressions for the 1 1 species in Table 1, taken from the largest individuals of Helix pomatia, Macrogastra rolphii, Vitrea crystallina and Vitrina pellucida or extrapolated from regressions of small samples (three or four individuals) ofjuvenile and sub- adult snails for Candidula intersecta, Aegopinella nitidula, Cochlicella acuta, Ena montana and Helicigona lapicida. Only one or two juveniles of Balea perversa, Trichia hispida, Helicella itala and Theba pisana were measured so adult foot area cannot be estimated; these species have been omitted. The interspecific regression gives a very good fit and accounts for c. 98% of the variation in foot area. The slope of 0.833 significantly (P< 0.001) exceeds the isometric expectation, and therefore large species partially compensate for increased loading (weight per unit foot area). Table 4 also gives estimates of Raup's (1966) shell parameter W, which describes the rate of whorl expansion, and is related to shell mouth (aperture) area for species whose geometries conform to a simple logarithmic spiral. Most of the values of W have been taken directly from Cameron (1981), but a few have been estimated from the illustrations of shells in Kerney & Cameron (1979) and Adam (1960), using Cameron's (1981) method. Figure 2 shows a plot of the residuals from the interspecific adult foot area/weight regression (Fig. 1) on W. FOOT SIZE AND SHAPE IN LAND SNAILS 37 1 The regression coefficient is highly significant and suggests that about 57O, of the scatter around the regression in Fig. 1 may be accounted for by variation in this parameter of shell geometry. Interspecific comparisons of foot shape are also complicated because some species have weight-related variation in foot ratio. However, foot ratio at adult weight has been estimated from intraspecific regressions of foot ratio on live weight for eleven species, and taken from the largest, nearly adult individuals for a further six species (Table 5A). Adult foot ratio cannot be estimated for Trichia hispida, Helicella itala, Theba pisana and Balea peruersa which were only measured as juveniles. The very small sample sizes and poor correlations between foot ratio and live weight also mean that adult foot ratio cannot be estimated for Candidula intersecta, Cochlicella acuta and Ena montana. However, the foot ratio of the largest individual of these seven species is also given in Table 5B. There is considerable variation among species in adult foot ratio, which ranges from 2.90 for Helix pomatia to 9.92 for Oxychilus alliarius. Foot ratio correlates negatively with adult weight (r= -0.497; P<0.05) and foot loading (r= -0.714; P

‘TABLE 5. Estimates offoot ratio and foot loading in (A) adults of 17 species and (B) juveniles of 7 species (see tcxt)

Foot loading Foot (mg mm-2) ratio

(A) Vitrea crystallina 4.48 6.88 Vitrina pellucida 3.14 7.17 Discus rotundatus 6.74 6.20 Claudia bidentala 7.95 4.44 Oxychilus alliarius 4.80 9.92 Ena obscura 7.71 5.10 Macrogastra rolphii 7.99 4.89 Aegopinella nitidula 6.52 4.53 Cochlodina laminata 9.33 5.00 Oqxhilus cellarius 8.45 7.93 Trichia striolata 9.25 4.52 Helicigona lapicida 7.62 4.66 Cepaea hortensis 14.24 3.96 Cepaea nemoralis 11.88 3.95 Arianta arbustorum 10.31 3.82 Helix aspersa 17.89 3.40 Helix pomatia 14.55 2.90 (B) Balea fierversa 11.27 3.45 Cochlicella acuta 7.56 3.03 Candidula intersecta 5.73 4.56 Ena montana 7.02 3.28 Trichia hispida 7.62 4.11 Helicella itala 10.30 3.60 Theba pisana 10.11 3.52 372 1'. 1'ArI'EKSvIELD

000 lot

Foot loading mg .mm-' Figure 3. Logarithmic plot of adult foot ratio on foot loading for 17 species of' snail. Regression ccluatioii: slope= -0.576 (S.E.=o.l 12), iritrrrept= 1.226, correlation coefficient= -0.799 (P< 0.00 1 ) .

DISCUSSION In general, the intraspecific relationships between foot area and live weight are adequately described by the logarithmic regression equations. There is considerable variation amongst the regression coefficients, and no single, simple factor obviously accounts for all the species which deviate from the pattern expected under conditions of isometric growth. Cameron (198 1) suggested that changes in density with growth, as the proportions of shell, air and living body tissue vary, might account for his uniformly sub-isometric shell aperture area/ weight regressions, but the scatter of slopes, on both sides of 0.667 indicates that other factors are also probably important here. The foot length/foot width relationships suggest that size-related foot shape changes may be involved for some species, or alternatively, the cross-sectional areas of shell whorls or shell aperture area might limit foot growth for some species. This latter hypothesis is to some extent supported because the high-spired and slowly expanding shells of Clausilia bidentata, Cochlodina laminata and Ena obscura, all have low weight/foot area coefficients, whereas the rapidly expanding shells of Cepaea nemoralis, Helix aspersa and Trichia striolata are accompanied by slopes which significantly exceed the isometric expectation. Examination of Cameron's ( 198 1) mouth area regressions indicate that foot area/mouth relationships for Clausilia bidentata and Cochlodina laminata follow an isometric pattern, with logarithmic regression slopes of 1.002 and 1 .OOO respectively, but that foot area increases disproportionately faster for all the large and globular species (Trichia striolata: 1.219; Cepaea nemoralis: 1.589; Helix aspersa: 1.277) studied except Arianta arbustorum (1.055). FOOT SIZE AND SHAPE IN LAND SNAILS 373 Denny (1981) reported a pattern of constant foot loading of about 0.95 g cm-* in the large American slug Ariolimax columbianus, in the weight range 5-22 g, and suggested shape changes as a possible mechanism. Only Helix pomatia and Helix aspersa attain weights of5 g in this study, at which the latter has a foot loading of 1.44 g cm-'. Loading at adult weight ranges from 0.3 (Vitrina pellucida) to 1.79 (Helix aspersa) g cm-'. The variation in intraspecific foot area/weight relationships makes interpretation of the differences between species difficult (Gould, 1966), but comparisons at adult weight indicate that larger species tend to have disproportionately larger foot areas, and therefore sustain lower levels of foot loading than would be expected under isometric allometry. If Discus rotundatus and Clausilia bidentata followed their intraspecific growth patterns, and grew to the size of Helix aspersa, their foot loadings would be 2.6 and 4.8 times greater respectively. An expectation of an isometric relationship is however unrealistic, as Cameron (1981) points out, because the species differ in shape. The relationship between the residuals from the interspecific regression and the shell geometry parameter W is similar to Cameron's (1981) observation, that the range of shell mouth areas at a standard weight (in a similar suite of species to those examined here), is accounted for by the product of two shell coiling parameters, W and 1-D, where D is the proportion of any shell radius which is umbilicus (Raup, 1966 gives a further definition of these shell coiling parameters). W(1-D) is proportional to mouth area in shells which follow a simple logarithmic spiral, and also correlates significantly with the foot area deviations, although less closely than W. Snails with rapidly expanding whorls and small or non-existent umbilicuses, such as the globular Cepaea nemoralis, Helix aspersa, Helix pomatia and Vi'itrina pellucida therefore have both larger shell mouth and foot areas, whereas the tightly coiled, high-spired Clausiliids and Ena obscura have smaller ones. These observations tend to reinforce the possibility that shell geometry, and hence shell mouth cross sectional area may limit foot area. If this is the case, and preferred sites of activity and the maintenance of position in the environment are related to foot size, then correlations between shell morphology and microdistribution might be expected. Observations on microdistribution suggest that coexisting species in some habitats have activity sites with markedly different physical characteristics. Similar associations occur repeatedly in a variety of habitats (Cameron, 1978; Tattersfield, 1981), at least in the British fauna, and laboratory investigations on both active and resting snails tend to confirm these under artificial conditions (Cook & Jaffar, 1984).The sites of activity of some species also correlate broadly with shell shape. The large, globular-shelled helicids tend to be associated with living and senescent vegetation, and less restricted to substrates of a particular angle (Cain & Cowie, 1978). The larger foot areas and lower loadings (compared to isometry) of Cepaea nemoralis, Helix aspersa and Trichia striolata (Table 1) may be important on such unstable microsites because they reduce the possibility of detachment. The globular species studied here tend to raise the shell from contact with the substrate when crawling by contraction of the columellar muscle. This reduces drag, but also increases the distance between the surface and the centre of gravity, thus reducing stability. The disproportionately broader feet of large Cepaea nemoralis, Cepaea hortensis, Arianta arbustorum, Trichia striolata and Helix nspersa (Table 3) might therefore be advantageous on an unstable surface (Cain, 1977) 374 P. ’I’ATI’EKSFIELD because they would be less liable to lateral displacement; a broad foot may also hold other mechanical implications (see below). Evidence for behavioural differences between the three species which deviate from an isometric foot area/weight pattern and the large and globular Cepaea hortensis and Arianta arbustorum which follow one is scanty; they all regularly climb vegetation (Cameron, 1970; Jaremovic & Rollo, 1979; Tattersfield, 1981), and this behaviour may be connected with thermo- or hygro-regulation, foraging or predator avoidance Cjaremovic & Rollo, 1979). Juvenile Cepaea horlensis and Arianta arbustorum are found more frequently on vegetation than adults in mixed natural populations, and the adults of the latter species are relatively more predominant on the ground (Cameron, 1970). Comparable information about Cepaea nemoralis is contradictory; juveniles tend to be active at a greater height than adults on beech (Fagus Vluatica L.) tree trunks (Tattersfield, 1981), but Jaremovic & Rollo (1979) found no association between distance climbed and size in Canadian bushes. In Britain, Arianta arbustorum tends to occur in cooler and more humid habitats, with longer vegetation than Cepaea nemoralis (Cameron, 1970). The necessity to ascend unstable vegetation, to avoid thermal or water stress, may therefore be less important for this species, which could adopt the tactic preferred by Cepaea nemoralis of resting on the soil surface in dcnse vegetation, or low on plant stems (Jaremovic & Rollo, 1979). Cepaea hortensis has an intermediate habitat preference in relation to humidity in some regions (Cameron, 1970), but comparable information about Helix aspersa and 7richia striolata is unknown. Personal observations of Helix aspersa indicates that when they do climb, it is often on hard and immobile substrates, but the larger size of this species, and greater foot loading, may mean that it encounters more severe mechanical constraints. Climbing behaviour related to foraging may be more plastic, and restricted to periods with little wind or vegetation movement. The two grassland and sand-dune species, Cochlicella acuta and Candidula intersecta, also ascend vegetation, but the high-spired Cochlicella acuta tend to be found active on substrates of a shallower angle, from which dislodgement is presumably less likely, than the more flattened Candidula intersecta, or indeed other coexisting helicellines including Helicella ilala and Cernuella virgata (Tattersfield, 198 1). In contrast, the very high-spired Clausilia bidentata and Cochlodina laminata tend to be strongly associated with hard, relatively stable vertical surfaces such as dead wood, rock faces and tree trunks (Cameron, 1978; Tattersfield, 1981) from which there will be less chance of dislodgement. Cameron (1978) reported a similar pattern for Clausilia dubia in the Yorkshire Dales, but Macrogastra rolphii, the only other clausiliid examined here does not climb (Kerney & Cameron, 1981). Ena obscura, which also has a relatively tall shell, also climbs tree trunks and other stable surfaces rather than herbaceous vegetation (personal observation). A narrow shell, in which the centre of gravity lies close to the substrate, and the shell itself hangs in a stable, vertical position may be of more importance than adhesion problems in these climbing species, because of the reduction in torque, and greater possibilities for entering narrow bark and rock interstices (Heller, 1987). However, such selection pressures may have resulted in physiological constraints on foot size, which restricts activity to more stable substrates. Most of the remaining species are ground- and litter-dwellers, and, with the exception of the globular Vilrina pellucida, have flattened, discoid shells. However, these species are not representative of the spectrum of shell shapes from this FOOT SIZE AND SHAPE IN LAND SNAILS 3 75 microhabitat, because the British fauna also includes the cylindrical Acicula and Cecilioides, conical Carychium, globular Spermodea and Acanthinula, and fusiform Azeca and Cochlicopa. Although these small species may be broadly classified as litter-dwellers, very little is known of the ways in which they partition this structually complex microhabitat, and, as Heller (1987) comments, nothing is known about their modes and ease of locomotion. The mechanical implications of foot shape are uncertain, but for species which move by waves of direct muscular compression (like most of the species considered here), a broad foot, which has a greater surface area (and hence adhesive resistance) in contact with the substratum behind the wave, will have a greater propulsive force for a given foot length and wave pattern. Such considerations could help account for the close correlation between foot ratio and loading, but variation in foot shape may also have implications in, for example, locomotory speed and stability. Foot and shell shape do not appear to be closely related, apart from the fortuitous connection that the larger species also tend to have globular shells; the only small equidimensional-shelled species, Vitrinapellucida, conforms to the general pattern of foot shape for its size. The two Oxychilus species, which have very high foot ratios are both carnivorous (Boycott, 1934) and eat other snails; a long, narrow foot might facilitate entry into the small shell aperture of prey. Vitrina pellucida and Vitrea crystallina also attack other snails in this manner, and have high, but not exceptional foot ratios. The other zonitid represented here, Aegopinella nitidula (which is of comparable size to Oxychilus cellarius), attacks other snails by boring through the shell (Mordan, 1977) rather than entering through the shell aperture and has a foot ratio comparable with other species of its size. The mechanics of gastropod adhesion, and the implications of shell shape on stability are poorly understood. Some small species such as Discus rotundatus may adopt ciliary locomotion (Elves, 1961), and the physical and chemical properties of foot sole mucus may vary among species. Despite these potential differences, the correlations between behaviour, shell shape and foot size reported here lend some support to Heller’s (1987) suggestion that shell form differences among species which occupy different spatial niches (Heller’s bush-, ground- and rock-dwellers) may reflect selection pressures on foot size. There are however exceptions amongst the species examined here, of which the ground-dwelling, high-spired Macrogastra rolphii and relatively flat-shelled, rock-dwelling Helicigona lapicida are the most striking. However, the classification of microhabitats into the relatively broad classes adopted by Cameron (1978), Tattersfield (198 1) and Heller (1987) may obscure more subtle distributional and behavioural patterns hinted at by the angle of activity distribution of Cochlicella acuta on sand-dune vegetation (Tattersfield, 198 1 (see above)). Alternatively, selection pressures on shell aperture area, perhaps related to desiccation rates or predation, might impose a constraint on foot size which in turn could restrict the use of some of the available activity microsites.

ACKNOWLEDGEMENTS I would like to thank Dr R. A. D. Cameron for his constructive criticism, discussion and advice throughout this project, and D. Tattersfield for helpful comments concerning mechanics. This work was undertaken while I was in receipt of a S.E.R.C. Studentship at Birmingham University. 376 P. TATTERSFIELD

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