Diet of Cypraea caputdraconis (Mollusca: Gastropoda) As It Relates to Food Availability in Easter Island

Diet and food preferences of Easter Island's endemic cowrie, Cypraea caputdraconis (Melvill), are reported. Gut content analyses ofspecimens from different rocky intertidal localities around the island revealed that C. caputdraconis is primarily an herbivore. Algae composed > 90% of the cowrie's diet in all cases. Five algal genera, Cladophora, Sphacelaria, Ceramium, Galaxaura, and Pterocladia, were the most frequent and abundant items in the diet of C. caputdraconis. The feeding habits of C. caputdraconis are most similar to those of C. caputserpentis L. from Hawaii in that both have clearly herbivorous diets. Both species share the R-l type oftaenioglossan radula, which also supports their close phylogenetic relationship. Indo-West Pacific populations of C. caputserpentis have been suggested as ancestral to C. caputdraconis from Easter Island. PREVIOUS OBSERVATIONS on trophic relationships of intertidal and shallow subtidal communities at Easter Island have centered primarily on carnivorous invertebrates such as gastropod mollusks and starfish (Kohn 1978a,b). Knowledge of herbivore-plant interactions for the benthic fauna of Easter Island is scant, however. The endemic cowrie Cypraea caputdraconis (Melvill) (locally known as pure) inhabits rocky intertidal habitats around the island (Osorio and Cantuarias 1989). Their shells are widely exploited for making ornaments by local craftsmen, and the cowrie constitutes a valuable economic resource for the islanders. Information about the feeding habits of cowries is available for some Cypraea spp. from the North Pacific (Kay 1960b, Renaud 1976, Morris et al. 1980, Hayes 1983). These re orts show that cowries, as a group, are 1 Funds for this work were provided by the Departamento Tecnico de Investigaci6n, Universidad de Chile. Proyecto No. 3046·9012. Manuscript accepted 15 March 1992. 2 Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile. 3 Facultad de Pesquerias y Oceanografia, Universidad Austral de Chile, Casilla 1327, Puerto MonU, Chile. 4 Laboratorio de Botanica, Museo Nacional de Historia Natural, Casilla 787, Santiago, Chile. capable of feeding on a wide spectrum of resources. Thus, the group includes herbivores, carnivores, and scavengers (Renaud 1976, Hayes 1983). However, different species achieve some degree of specialization on particular foods. One factor that may influence the local distribution of cowries is food availability (Hayes 1983). Because intertidal gastropods may strongly determine the structure and organization of littoral communities (Lubchenco 1978, Underwood 1980, Jara and Moreno 1984), knowledge of their diet can greatly contribute to understanding their ecological role. In this study we documented the diet of C. caputdraconis based on gut content analyses of cowries from different rocky intertidal localities around Easter Island and assessed its food preferences with respect to food availability in the field. MATERIALS AND METHODS Observations were carried out at various rocky intertidal localities around Easter Island (27° 07' S, 109° 22' W) in two consecutive summer field seasons (January 1989 and January 1990). We also analyzed a sample obtained from one locality in the fall (May 1989). Diet of Cypraea caputdraconis at Easter Island-OsoRIo ET AL. 35 Cowries were collected manually during low tides from the intertidal in the manner described by Osorio and Cantuarias (1989). A total of 123 adult C. caputdraconis were used for gut content analyses. Upon collection in the field, cowries were immediately preserved in 7% formaldehyde. The specimens were later dissected in the laboratory to obtain the guts and their contents. Except for the degree of digestion, we found no differences between the contents of the anterior, middle, and posterior gut; therefore, we examined only the stomach contents using a stereomicroscope. Algae were determined to the generic or lowest possible taxonomic level. To obtain an initial identification of the components, we first examined qualitatively the contents of samples from Mauku Roa and Hanga Nui in 1989. For the remaining samples, we utilized both occurrence and numerical methods (Windell 1971). Given the small gastric volume, all pieces and structures found were identified and counted. Initially, the abundance of blue-green algae and diatoms was estimated qualitatively as rare, common, and abundant. Later, counts of the number offragments were also made for these two groups in the manner described above. To obtain a measure of food preferences, we gathered data on food availability in areas associated with cowries collected at two sites in January 1990. Two specimens from Anakena and three specimens from Hanga Vare Vare were examined. Using a metallic frame (5 by 5 cm), we collected all the material scraped from the substratum with a sharp knife at distances 0, 10, and 25 cm from each of the snails. The 5-cm samples of scrapings were preserved in 7% formaldehyde and later examined in the laboratory in a manner similar to that used in analyzing the stomach contents. The results from the different quadrats around each snail were later pooled to obtain the total abundance of each item. The percentage relative abundance of items available from the substrate was calculated from the totals obtained from all the quadrats sampled. The percentage relative abundance of items in the diet was calculated from the totals obtained from all five snails examined. To determine food preferences, the relative abundances of items in the field and in the diet were used to calculate Ivlev's (1961) electivity index. This index ranges from -1 to + 1; zero indicates no preference. Because the confidence limits for this index are unknown, we arbitrarily assigned electivity values to three categories: Neg. , Pos. , and None. Values ranging between ±0.35 were assigned to the None selectivity category. Electivity values greater than +0.35 (Pos.) were interpreted as positive selection (e.g., item was taken in a proportion much greater than its abundance in the environment). Conversely, electivity values lower than -0.35 (Neg.) were interpreted as negative selection.