The abundances and distributions of molluscs in the southern Iberian Peninsula: A comparison of marine and terrestrial systems

75 Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 BOLETÍN. INSTITUTO ESPAÑOL DE OCEANOGRAFÍA ISSN: 0074-0195 © Instituto Español de Oceanografía, 2003 The abundances and distributions of molluscs in the southern Iberian Peninsula: A comparison of marine and terrestrial systems A. Menez1,2, D. A. Fa2, J. E. Sánchez-Moyano3, I. García-Asencio3, J. C. García-Gómez3and J. Fa4 1The University of Wales, Cardiff, UK 2The Gibraltar Museum, 18-20 Bomb House Lane, Gibraltar, UK. E-mail: [email protected] 3Laboratorio de Biología Marina, Departamento de Fisiología y Zoología, Facultad de Biología, Universidad de Sevilla, Avda. Reina Mercedes, 6, E-41012, Sevilla, Spain 4Jersey Wildlife Preservation Trust, Jersey, Channel Islands, UK Received January 2003. Accepted December 2003. ABSTRACT Molluscs are the second most diverse of all animal phyla, and occur in many habitat types. They are, therefore, a particularly good phylum with which to compare and contrast differences between ecosystems. Mollusc data from a number of sites along the southern coast of the Iberian Peninsula are analysed to study patterns of diversity and distribution using a range of multivariate techniques. Within each site, data are presented from three locations -fully terrestrial, rocky intertidal and soft bottom benthic (10 m and 20 m depths)- all in close proximity. The species are then classified in relation to morphology and size, and analysed at supraspecific levels to elucidate underlying patterns. The observed patterns are briefly discussed, with particular reference to the differential scope and importance of controlling factors in each ecosystem, such as dispersal processes. The results from the systems are compared and discussed in the context of ecological and evolutionary constraints in Mollusca. Keywords: Terrestrial, intertidal, benthic, system comparisons. RESUMEN Abundancia y distribución de moluscos en el sur de la península Ibérica. Una comparación entre los sistemas terrestre y marino Los moluscos constituyen el segundo filo animal más diverso y se encuentran en muchos tipos de hábitat, por lo que son idóneos para establecer comparaciones entre distintos ecosistemas. Se han analizado los datos de los moluscos obtenidos en una serie de emplazamientos que cubrían el sur de la península Ibérica para determinar, empleando distintas técnicas multivariantes, los patrones de diversidad y distribución de estos organismos. Los datos se tomaron de ejemplares capturados en lugares del medio terrestre próximos a la línea de costa, de la franja intermareal rocosa y de sedimentos de fondos marinos situados a 10 y 20 m de profundidad. Las especies fueron clasificadas atendiendo a la morfología y el tamaño, y se analizaron a nivel supraespecífico para elucidar los patrones generales, que se discuten aquí, brevemente, con especial énfasis en las diferencias según la importancia de los factores que controlan cada ecosistema, como, por ejemplo, los procesos de dispersión. Los resultados de los distintos sistemas se comparan y discuten en el contexto de las tendencias ecológicas y evolutivas de los moluscos. Palabras clave: Terrestre, intermareal, bentónico, comparación de sistemas. INTRODUCTION The Mollusca is a very old monophyletic lineage, dating from before the Cambrian (Barker, 2001), and is the second most diverse of all animal phyla (Morton, 1967; Russell-Hunter, 1983), occuring in many habitat types (Cain, 1983; Solem, 1984). A major constraint on land molluscs is humidity, and so the highest diversities generally occur in habitats with high humidity levels, although other factors are also important (Sacchi, 1965; Van Bruggen, 1969; Cameron and Redfern, 1976; Pfleger and Chatfield, 1988; Cameron, 1995; Emberton, 1995; Kerney, 1999). The shell, along with physiological and behavioural adaptations, contributes substantially to water conservation (Rollo et al., 1983; Cook, 2001). Shell-less species (e.g., slugs), which have arisen independently on several occasions (Hausdorf, 2001), are more dependent on humidity than shelled species (Runham and Hunter, 1970), but are more mobile and faster (Wiktor, 1984). The majority of marine species do not have the same humidity constraints that land species have. An exception to this are some intertidal species which are influenced by dessication and temperature, but have adaptated to these factors (Lewis, 1964; Underwood, 1985; Fretter and Graham, 1994). Benthic molluscs are not influenced by dessication, although other factors, such as substratum type, availability for attachment, and water currents are important (Hartnoll, 1983; Hiscock, 1983; Russell-Hunter, 1983; Stanley, 1988). Groups that are present in more than one major ecological system provide an opportunity for the comparison of underlying physiological and functional adaptations to environmental constraints. Some groups are extremely diverse within one system, whilst being almost absent in others. An example of this are the Insecta, with the highest diversity of all groups, yet with few truly marine species (Barnes, 1987). Additionally, general trends in ecological data can be examined using taxon groups higher than species (Sale and Guy, 1992; Brown, 1995). In the present paper we examine molluscs’ abundances and distributions in four systems: 1) terrestrial; 2) marine intertidal; 3) marine soft bottom benthic, 10 m; 4) marine soft bottom benthic, 20 m. Data from 20 sites around the southern Iberian Peninsula are analysed. The species are classified by morphology and size, and analysed at the supraspecific level. The results are compared and briefly discussed in relation to ecological and evolutionary constraints in the Mollusca. MATERIALS AND METHODS Twenty sites were sampled around the southern Iberian Peninsula (table I and figure 1) for terrestrial molluscs (following the methodology in Menez, 2001), marine intertidal molluscs (following the methodology in Fa, 1998 and Fa and Fa, 2002), and marine soft bottom benthic molluscs, following the methodology in Fa et al., 2003. Familylevel analyses were carried out for 13 sites, and species level analyses were carried out for 20 sites (indicated in table I). Terrestrial and intertidal specimens (20 sites) were identified to the species level; benthic specimens (13 sites) were identified to family level. Abundances were obtained in the field (intertidal) or in the laboratory after specimen collection (terrestrial and benthic). Diversity is expressed as the Shannon index (H’) (Magurran, 1988) and Pielou’s evenness is calculated as: J’H’/ln (s). The Shannon index (an in- A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 76 Table I. The sites sampled in the study, showing abbreviations used throughout the text and figures. The table indicates sites sampled for species analyses (all sites) and those used for family analyses (13 sites) Site Abbreviation Species Families analyses analyses Punta del Estació EST * Águilas AGU * * Cabo de Gata GAT * * El Lance LAN * * Punta de la Mona MON * * Malaga MAL * * Punta de Calaburras CBR * * Puerto Banús BAN * * Puerto de la Duquesa DUQ * * Europa Point EUR * * Punta del Carnero CAR * * Tarifa TAR * Punta Camarinal CMR * Cabo de Trafalgar TRF * Cabo Roche ROC * * Rota ROT * Punta Umbría UMB * * Vila Moura VIL * * Cabo Sao Vicente VIN * Milfontes MIL * formation theory index) was chosen over a dominance index (such as Simpson’s index) because dominance measures are weighted towards the abundances of the most common species. Base 2 logs were used for calculating the Shannon index. Anova was carried out on the variables from the different systems to compare their means. Fisher’s least significance difference test was used as a post hoc test (at p0.05) to check for groups that were not significantly different. Hierarchical agglomerative clustering and nonmetric multidimensional scaling (Digby and Kempton, 1987; Clarke and Warwick, 1994; Van Tongeren, 1995) were done using the software package Primer for Windows (Clarke and Warwick, 1994). The data were 4th-root transformed, which retains quantitative information but downplays the species dominants. Cluster groups were assigned dependent on similarity levels on the cluster analyses, and groups on multidimensional scaling were compared to cluster groups (Clarke and Warwick, 1994). The distances between individuals and their positions in the habitat, as well as size, colour and shell shape, were obtained from field observations and specimen analyses in the laboratory. These data are used to give an indication of species’ tendencies for these variables. In some species, there may be more variation in values than indicated here, where the most frequent observations for the species are used. Slugs are excluded from these analyses (except for position in the habitat). Species with abundances of less than five from any site are excluded from the distance between individuals analyses, and bivalves are excluded from the shape analyses. For each of these variables, a simple scale is used to score values; these are shown in table II. A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 77 Table II. The scales used for scoring distance, position, size, colour and shape. For size, colour and shape, an example of a terrestrial species is given for each score value Scale for distance between individuals (distance) Score Distance (mm) 10.1 -.50 2 5.1 - 10 3 10.1 - 15 4 15.1 - 20 520 Scale for position in habitat (position) Score Position 1 Under substratum 2 On ground surface 3 On substratum at25 cm height from ground 4 On substratum at25 cm height from ground 5 On substratum both at25 cm and25 cm height from ground 6 On plants at25 cm height from ground 7 On plants at25 cm height from ground 8 On plants both at25 cm and25 cm height from ground Scale for shell size (size) Score Size 1 Minute (e.g. Truncatellina cylindrica) 2 Small (e.g. Ferussacia follicula) 3 Medium (e.g. Theba pisana) 4 Large (e.g. Otala punctata) Scale for shell colour (colour) Score Colour 1 Colourless (e.g. Cecilioides jani) 2 White (e.g. Helicella stiparum) 3 Brown (e.g. Cantareus aspersus) 4 White-brown (e.g. Otala lactea) 5 Red-brown 6 Green-brown 7 Red-white 8 Blue-black Scale for shell shape (shape) Score Shape 1 Cylindrical (e.g. Truncatellina cylindrica) 2 Conical (e.g. Cochlicella acuta) 3 Discoidal (e.g. Caracollina lenticula) 4 Spherical (e.g. Cantareus aspersus) 5 Fusiform Figure 1. Iberian Peninsula (above) showing extent of 38° latitude (box) and positions of sites along the coast of the southern Iberian Peninsula (below) (see table I for site abbreviations) RESULTS The first part of this section deals with family level data from 13 sites (table I). Terrestrial, intertidal, benthic 10 m and benthic 20 m data are analysed. In the second part, species data from 20 sites (terrestrial and intertidal) are analysed. Family data from 13 sites (terrestrial, intertidal, benthic 10 m, benthic 20 m) Figure 2 shows the number of families, abundances, Shannon diversity (log2), and Pielou’s evenness for all sites. Significant differences were found between the systems for number of families, Shannon diversity (log2), and Pielou’s evenness at p 0.001 or less. Table III shows the results of anova and post-hoc testing. Abundance does not achieve statistical significance for differences between the systems (although it almost does at p 0.06) and is not considered further here. Results of Fisher’s least significance difference tests, which identifies which groups in the anova analyses differ, show that the terrestrial and intertidal systems are not significantly different for number of families, Shannon diversity (log2) and Pielou’s evenness. Other system groups are all significantly different from each other. Families present at more sites have higher abundance than families present at fewer sites, even when we correct for the number of sites at which they occur (by dividing total abundance by number of sites). This applies to all systems (terrestrial: ρ0.645, p0.001; intertidal: ρ0.660, p 0.014; benthic 10 m: ρ0.642, p0.001; benthic 20 m: ρ0.768, p0.001), see figure 3 for graphs. Most families are represented by low numbers of individuals, although a few families have high numbers of individuals (figure 4). Figure 5 shows the percentages of abundance for each of the families out of the total abundance, in each of the systems. The number of families in the systems ranges from 13 (intertidal) to 46 (benthic, 20 m), and the number of families representing greater than 1% of the total abundance ranges from 3 (intertidal) to 12 (benthic, 20 m). When the number of families representing greater than 1% of the total abundance are considered as a percentage of the total number of families, for each system, there is no significant difference in number (Kolmogorov-Smirnov Z 0.500, p 0.964). The numbers of families in each class are shown in table IV. Molluscs are represented in the terrestrial A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 78 Table III. Anova results for terrestrial (TER); intertidal (INT); benthic, 10 m (10 m); and benthic, 20 m (20 m) systems. Degrees of freedom (df), mean square, F ratio and significance for the variables, both between groups and within groups, are also shown. Results of post-hoc testing are shown in the lower part of the table. Fisher’s least significant difference test is used (at p0.05) to test for groups that are not significantly different. These groups are shown underlined. Groups that are significantly different are shown in bold, and are not underlined. In each case, the groups are arranged into ascending order of the mean Parameter Df Mean square F ratio Significance N. families (between groups) 3 255.763 36.929 0.001 N. families (within groups) 48 7.119 Abundance (between groups) 3 25630589.59 2.600 0.063 Abundance (within groups) 48 9857560.19 Shannon (log2) (between groups) 3 8.537 21.134 0.001 Shannon (log2) (within groups) 48 0.404 Pielou’s evenness (between groups) 3 0.258 6.688 0.001 Pielou’s evenness (within groups) 48 0.039 Number of families Abundance Shannon (log2) Pielou’s evenness TER INT TER INT TER INT INT TER TER 10 m TER 10 m TER 10 m TER 10 m TER 20 m TER 20 m TER 20 m TER 20 m INT 10 m INT 10 m INT 10 m INT 10 m INT 20 m INT 20 m INT 20 m INT 20 m 10 m 20 m 10 m 20 m 10 m 20 m 20 m 10 m A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 79 Terrestrial: number of families Terrestrial: abundance Number of families Log abundance 9 8 7 6 5 4 3 2 1 2 000 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 1 000 800 600 400 200 100 40 80 60 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL Terrestrial: Shannon (log2) Terrestrial: Pielou’s evenness Shannon log2 Pielou’s 1.8 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 1.0 0.8 0.6 0.4 0.2 0.0 Intertidal: number of families Intertidal: abundance Number of families Log abundance 10 9 8 7 6 5 4 3 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 30 000 10 000 5 000 2 000 1 000 500 200 400 300 20 000 3 000 4 000 Intertidal: Shannon (log2) Intertidal: Pielou’s evenness Shannon log2 Pielou’s 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 0.6 0.5 0.3 0.2 0.1 0.0 Figure 2. The number of families, total abundances of all families, Shannon diversity and Pielou’s evenness, for all systems at all sites (see table I for site abbreviations) A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 80 Benthic 10 m: number of families Benthic 10 m: abundance Number of families Log abundance 18 16 14 12 10 8 6 4 8 000 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 4 000 2 000 1 000 800 600 400 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL Benthic 10 m: Shannon (log2) Benthic 10 m:Pielou’s evenness Shannon log2 Pielou’s 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 1.0 0.8 0.6 0.4 0.2 0.0 Benthic 20 m: number of families Benthic 20 m: abundance Number of families Log abundance 22 20 18 16 14 12 10 8 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 4 000 1 000 800 200 100 2 000 400 600 Benthic 20 m: Shannon (log2) Benthic 20 m: Pielou’s evenness Shannon log2 Pielou’s 4.0 3.5 3.0 2.5 2.0 1.5 1.0 AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL AGU GAT LAN MON MAL CBR BAN DUQ EUR CAR ROC UMB VIL 1.0 0.8 0.6 0.4 0.2 6 000 6 Figure 2 (continuation) system by gastropods only. The intertidal system has more gastropod families than bivalve families, whereas the opposite is true for both benthic systems; this is clearly indicated by the bivalve:gastropod ratio. The results of hierarchical clustering and MDS ordination compare well. Figure 6 shows the dendrograms and two-dimensional ordinations for all systems. The stress function for all MDS analyses was 0.2, indicating acceptable ordinations in two dimensions (Clarke and Warwick, 1994). For the present paper, a new system for illustrating group membership was used (table V) which enabled us to rapidly A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 81 Log x+1 (total abundance/number of sites) 7 Number of sites Number of sites 6 5 4 3 2 0 1 7 6 5 4 3 2 0 1 8 Terrestrial Intertidal Number of sites Number of sites 7 6 5 4 3 2 1 87 6 5 4 3 2 1 Benthic 10 m Benthic 20 m 02468101214 02468101214 02468101214 02468101214 Log x+1 (total abundance/number of sites) Log x+1 (total abundance/number of sites)Log x+1 (total abundance/number of sites) Figure 3. Families present at more sites have higher abundances than families present at fewer sites, even when the number of sites at which they occur is corrected for (terrestrial: ρ0.645, p0.001; intertidal: ρ0.660, p 0.014; benthic 10 m: ρ0.642, p0.001; benthic 20 m: ρ0.768, p0.001) Table IV. Numbers of families in each class, total number of families and Bivalve:Gastropod (B:G) ratio for each system Sites / system Polyplacophora Gastropoda Bivalvia Total B:G 20 Sites terrestrial (species) 0 18 0 18 0 20 Sites intertidal (species) 2 13 4 19 0.31 13 Sites terrestrial (families) 0 16 0 16 0 13 Sites intertidal (families) 2 8 3 13 0.38 13 Sites benthic 10 m (families) 0 16 22 38 1.38 13 Sites benthic 20 m (families) 3 15 28 46 1.87 visualise memberships. For the terrestrial system, there is a weak longitudinal effect, with most site groupings approximating actual site positions on the coast (figure 1). The intertidal system has a major group along most of the east coast between Malaga and Punta del Carnero. Groupings for the two benthic systems differ markedly from each other, suggesting differences in these systems. A large group in the benthic 10 m system approximates the extent of grouping on the east coast for the intertidal system. Species data from 20 sites (terrestrial and intertidal) Figure 7 shows the number of species, abundances, Shannon diversity (log2) and Pielou’s evenness for all sites. There were significant differences between the two systems for abundance, Shannon diversity (log2) and Pielou’s evenness, but not for number of species (table VI). Species present at more sites have higher abundances than species present at fewer sites, even when we correct for the number of sites at which they occur (by dividing total abundance by number of sites). This applies to both terrestrial and intertidal systems (terrestrial: ρ0.492, p0.001; intertidal: ρ0.598, p0.001), see figure 8 for graphs. Most species are represented by low numbers of individuals, while a few species have high numbers of individuals (figure 9). As in the previous section, the results of hierarchical clustering and MDS ordination compare well. A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 82 Number of families 1.2 Log x+1 abundance Log x+1 abundance 1.0 .8 Terrestrial Intertidal Log x+1 abundance Log x+1 abundance 10 8 6 4 2 0 12 14 10 8 6 4 2 0 Benthic 10 m Benthic 20 m 0246810 024681012 0234567 Number of families Number of familiesNumber of families 0246810 89 1.2 1.0 .8 12 Figure 4. The majority of families are represented by low numbers of individuals, whereas a few families have high numbers of individuals Figure 10 shows the dendrograms and two-dimen- sional ordinations for the two systems. The stress function for the MDS analyses was0.2, indicating acceptable ordinations in two dimensions (Clarke and Warwick, 1994). The weak longitudinal effect present for the terrestrial system with the family analyses at 13 sites (see previous section) is not present with the species data. One of the groups (group 3, see table VII) includes sites from the east and west coasts. However there is a marked longitudinal effect for the intertidal system, which is clearly apparent in table VII, where the close site fidelity of coastal positioning in relation to groups obtained from the analyses is represented by the diagonal arrangement of site groupings. Scoring criteria for distances between individuals, position in habitat, shell size, colour and shape are shown in table II. Data for these variables, A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 83 Percent of total abundance Family Kolmogorov-Smirnov Z 3.429; p < 0.001 Family Terrestrial Intertidal Family Family Benthic 10 m Benthic 20 m Percent of total abundance Percent of total abundancePercent of total abundance 1 0 0.009 2 4 6 8 10 12 143 5 7 9 11 13 15 16 1 2 4 6 8 10 1235791113 20 40 60 80 100 0 0 1357 9 0 10 5 10 20 10 20 30 15 30 40 20 40 50 25 50 60 30 60 70 35 Kolmogorov-Smirnov Z 2.869; p < 0.001 Kolmogorov-Smirnov Z 5.035; p < 0.001 Kolmogorov-Smirnov Z 5.265; p < 0.001 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 4513579 1113151719212325272931333537 0.018 0.045 0.018 0.045 0.099 0.099 0.209 0.372 0.635 0.871 0.971 1.198 1.361 17.672 76.377 0.002 0.008 0.02 0.018 0.078 0.103 0.107 0.121 0.137 0.878 2.945 37.142 58.445 0.015 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.035 0.052 0.07 0.087 0.139 0.139 0.157 0.157 0.278 0.296 0.33 0.452 0.469 0.556 0.643 0.695 0.904 1.061 1.078 1.199 2.695 5.824 5.946 10.675 12.57 51.826 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.028 0.055 0.055 0.055 0.055 0.083 0.083 0.083 0.138 0.138 0.166 0.194 0.194 0.249 0.277 0.332 0.332 0.36 0.498 0.554 0.665 0.72 1.024 1.91 2.243 2.436 2.769 2.796 3.391 5.537 9.136 16.418 16.445 29.707 Figure 5. Percentages of abundance that each of the families comprises within the total abundance, in each of the systems (see text for details) A. Menez et al. Marine and terrestrial molluscs in the southern Iberian Peninsula Bol. Inst. Esp. Oceanogr. 19 (1-4). 2003: 75-92 90 Score Frequency Frequency 2 12345 4 6 8 10 12 14 16 18 20 DISTANCE Score DISTANCE Terrestrial Intertidal 2 4 6 8 10 12 14 16 18 12345 Score Frequency Frequency 0 12345678 10 20 30 POSITION Score POSITION Terrestrial Intertidal 0 10 20 30 40 1234 5678 Score Frequency Frequency 0 1234 10 20 30 SIZE Score SIZE Terrestrial Intertidal 0 10 20 30 1234 Score Frequency Frequency 0 12345678 10 20 30 COLOUR Score COLOUR Terrestrial Intertidal 0 10 20 30 12345678 Score Frequency Frequency 0 12345 10 20 30 SHAPE Score SHAPE Terrestrial Intertidal 0 10 20 30 12345 Figure 11. Frequencies for distance between individuals (distance), position in habitat (position), shell size (size), shell colour (colour) and shell shape (shape), for species from all sites (see table II for scale values and text for details) colour combinations are present in other intertidal taxa (Riedl, 1986). The diversity of shell shape in molluscs has been reviewed several times in the literature (Vermeij, 1971, 1993; Stanley, 1988), although shapes can be described with a small number of geometric variables (Vermeij, 1993). Morphology has been related to predation, abiotic variables and evolutionary constraints (Currey, 1988; Stanley, 1988; Vermeij, 1982, 1993; Faller-Fritsch and Emson, 1985; Barker, 2001). Cain (1977, 1981) and Cook (1984) considered shell size/shape in groups of taxa in relation to preferred surface angles. Data from the present study show that most terrestrial species are spherical, and that most intertidal species are conical. The spherical shape may represent an optimal shape in relation to apertural area/shell volume as well as surface area/volume ratio (heat exchange relations) in addition to providing increased stability on many surfaces during movement. In the intertidal, where wave action is a prime component of dislodgement of individuals, a conical shell may represent an optimal shape to minimise this effect (Branch, 1985; Stanley, 1988). It may be hypothesised that any shell shape increasing the probability of dislodgement from the substratum as a result of wave action (or other mechanism) would either be absent from the intertidal or minimally represented. Some evidence for this may be provided by the lack of both cylindrical and discoidal shapes from this system, both shapes being less resistant to removal from the substratum. In the terrestrial system, where dislodgement from the substratum is not as important, both of these shapes are present. 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