PATTERNS OF SHELL UTILIZATION IN TWO SYMPATRIC SPECIES OF HERMIT CRABS FROM THE NATAL COAST (DECAPODA, ANOMURA, DIOGENIDAE) BY T. REDDY and R. BISESWAR Department of Zoology, University of Durban-Westville, Private Bag X54001, Durban 4000, Republic of South Africa ABSTRACT Shell selection and utilization in two sympatric species of hermit crabs from the Natal coast, namely Clibanariusvirescens (Krauss, 1843) and Calcinuslaevimanus (Randall, 1840) were investi- gated. Shells of 23 gastropod species were occupied by C. virescenswhile 13 species were utilized by C. laevimanus.Shells of 11gastropod species were inhabited by both crabs but, of these only two shell species were used in significant numbers. The results showed a close correlation between the morphometric data of the crabs and the utilized shells. Clibanariusvirescens preferred high spired shells whilst C. laevimanusshowed prefer- ence for shells with low spires. As C. laevimanuspossesses a large cheliped it was restricted in the choice of shells in the natural habitat as it had to select shells with large apertures. Differential shell utilization has been advanced as the most important factor permitting coexistence of the two species. Clibanariusvirescens inhabited a succession of shell species as the crabs grew in hard carapace or shield length whereas C. laevimanusshowed preference mainly for two shell species namely Nerita albicilla Linnaeus, 1758 (carapace length less than 5 mm) and Turbo coronatusGmelin, 1791 (carapace length more than 5 mm). Calcinuslaevimanus may not be at a disadvantage by occupying Nerita shells as the large cheliped may be effectively used in sealing the aperture of the shell, and thereby minimizing water loss. Females of both species occupying lighter shells with large internal volumes were found to produce larger clutches as compared to females of similar carapace lengths occupying the heavier shell species. RÉSUMÉ Etude de la sélection et de l'utilisation de la coquille chez deux espèces de pagures sympatriques de la côte du Natal, Clibanariusvirescens (Kraus) et Calcinuslaevimanus (Randall). Clibanariusvirescens occupait les coquilles de 23 espèces de gastropodes, alors que Calcinuslaevimanus utilisait celles de 13 espèces. Les coquilles de 11 espèces de gastropodes étaient occupées par l'un et l'autre des pagures, mais parmi ces coquilles deux espèces seulement étaient utilisées en nombres significatifs. Les résultats ont montré une étroite corrélation entre les données morphométriques sur les pagures et les coquilles utilisées. Clibanariusvirescens préférait les coquilles à spires hautes et Calcinus laevimanuscelles à spires basses. Comme cette dernière espèce possède un chélipède de grande taille, elle était restreinte dans le choix des coquilles dans l'habitat naturel, puisque le choix devait porter sur des coquilles à grande ouverture. L'utilisation différentielle des coquilles a été avancé comme le facteur le plus important permettant la coéxistence des deux espèces. Clibanariusvirescens habitait successivement différentes espèces de coquilles au fur et à mesure de l'accroissement de sa carapace, alors que C. laevimanusmanifestait surtout sa préférence pour deux espèces de coquilles, à savoir Nerita albicilla Linné, 1758 (carapace mesurant moins de 5 mm) et Turbocoronatus Gmelin, 1791 (carapace de plus de 5 mm). Calcinuslaevimanus peut ne pas avoir de 14 désavantage à occuper des coquilles de Nerita car son grand chélipède lui permet d'obturer l'ouverture et de minimiser de ce fait la perte d'eau. On a constaté que les femelles des deux espèces qui habitaient des coquilles plus légères, à grand volume interne, produisaient des pontes plus importantes que celles, de même taille, occupant des coquilles plus lourdes. INTRODUCTION It has been well documented that shell inhabitation offers protection to hermit crabs from predation, physical stresses and competition (Bertness, 1981 a). The type of shell inhabited by a hermit crab can also influence its growth and in females this may result in a reduction in clutch size through the use of suboptimal shells (Fotheringham, 1976a,b). It has been shown that females occupying light, high volume shells have larger clutches than similarly sized females in heavier, low volume shells (Childress, 1972; Bertness, 1981b). According to Conover (1978) they do not occupy shells randomly but engage in a selection process. The actual criteria by which hermit crabs select shells are not well understood but it has been shown that shell configuration, aperture size, weight index (shell weight/crab weight) and volume index (shell volume/ shell weight) all provide important stimuli for selection (Markham, 1968; Kuris & Brody, 1976; Mitchell, 1976; Young, 1979; Hazlett, 1981; Gilchrist, 1984). Several studies have been undertaken on shell utilization patterns in coexist- ing species of hermit crabs with the purpose of determining the factors respons- ible for alleviating interspecific competition (Bollay, 1964; Vance, 1972b; Grant & Ulmer, 1974; Mitchell, 1975; Kellogg, 1977). Habitat differences and shell size partitioning seem to be important factors permitting coexistence of species (Kellogg, 1977). At present, there is very little information on southern African species of hermit crabs. The only significant works are those of Emmerson & Alexander (1986) and Walters & Griffiths (1987) who studied shell utilization patterns in Diogenes brevirostris Stimpson, 1859, a species that is fairly common along the Cape coast. This study was undertaken in order to examine the structure and patterns of shell utilization in two sympatric species of hermit crabs, namely, Clibanarius virescens (Krauss) and Calcinus laevimanus (Randall) that are equally abundant in the intertidal regions of some rocky shore localities of the Natal coast. Another objective of this study is to determine if competition for shells exists between the two species and which factors influence resource partitioning of shells. Clibanarius virescens, the smaller of the two species, has chelipeds that are subequal. This is in contrast to Calcinus laevimanus where the left cheliped is vastly larger than the right one. Field observations have revealed that there is no niche separation and both species coexist in the same intertidal zones. Shells must be a limiting factor in the areas studied as no empty gastropod shells could be found. .