Gastropods As Predators and Prey at Easter Island!

Gastropods as Predators and Prey at Easter Island!

ALAN J. KOHN2

ABSTRACT: First observations are reported of predator-prey relationships among gastropod mollusks of the depauperate, Indo-West Pacific derivative, intertidal and shallow subtidal benthic fauna of Easter Island. Conus miliaris, which will be reported in detail in a separate paper, and Pisania decapitata englerli prey on polychaete annelids; Mitraflavocingulata preys on sipunculans; and Neothais nesiotes preys on barnacles intertidally and gastropods subtidally. Gastropods of at least three species are eaten by the most common Easter Island starfish, Astrostole paschae. EASTER ISLAND, THE MOST ISOLATED island Island from 15 April to 4 May 1977. Speci in the Pacific Ocean, may be viewed as a mens of Neothais nesiotes and Astrostole natural experiment in the assembly of biotic paschae were carefully examined in the field communities (Diamond and May 1976). For for feeding activity. In the case of N. nesiotes, this reason, its inshore marine biota has been this consisted of insertion of the proboscis studied increasingly in recent years; the into a prey organism or of drilling its shell. primarily tropical Indo-West Pacific affinity The cardiac stomach of A. paschae was ex- and limitednumbef of species 6nhe fishes -arruiied for'fhepresence of prey organisms and invertebrates have drawn particular at not visible externally. Specimens of Pisania tention (Devaney 1973, Fell 1974, Garth decapitata englerti and Mitra flavocingulata 1973, Kohn and Lloyd 1973, Randall 1976). were collected and retained 2 to 3 days indi However, the biological interactions of these vidually in 40-ml vials containing enough animals have remained unstudied. seawater to cover the shells. All fecal strings In the course of an ecological study of the produced were pipetted into I-ml vials con predatory gastropod Conus miliaris at Easter taining 70 percent alcohol with 5 percent Island, I examined individuals of three co glycerin, and smears of the feces were later occurring predatory gastropod species for prepared as permanent slides for examination evidence of the nature of their prey, and I under the compound microscope. examined specimens ofthe starfish Astrostole paschae (Clark) to determine whether its diet includes gastropods. The results ofthe Conus RESULTS study will be published separately; here, I report briefly on an initial effort to determine Although the results are based on small predator-prey relationships involving the samples (Table 1), they suggest that of the other gastropods. four species studied, only Pisania decapitata englerti (mean shell length 14.1 mm, range 12-15.5 mm) preys on polychaetes and over laps dietarily with the commonest predatory METHODS gastropod present, Conus miliaris. The latter All observations were made in the inter preys primarily on Onuphis sp. (31 percent), tidal and shallow subtidal region of Easter other eunicids (47 percent), and nereids (13 percent). Eunice (Nicidiiinrcariboea ana Lysidice collaris together constituted 30 per I This research was supported by National Science cent ofthe diet ofC. miliaris (Kohn, in press). Foundation grant no. BMS 75-03303. Manuscript re ceived 10 January 1978. The proportional similarity (Pielou 1977, p. 2 Department of Zoology, University of Washington, 337) between the diets of the two species is 24 Seattle, Washington 98195. percent. Populations of C. miliaris attain 35 36 PACIFIC SCIENCE, Volume 32, January 1978

TABLE I GASTROPOD MOLLUSKS AS PREDATORS AND PREY AT EASTER ISLAND

NUMBER OF PREDATORS EXAMINED

Pisania decapitata Mitra (Strigatel/a) Neothais Astrostole englerti flavocingulata nesiotes paschae (Hertlein) Lamy Dall (Clark) PREY ITEMS FOUND 12 26 28 13

Polychaeta Eunice (Nicidion) cariboea Grube 2 Lysidice col/aris Grube 7 Sipuncula Phascolosoma sp. IL Gastropoda Dendropoma sp. 3 Anlisabia imbricata (Gould) 2 3 Nerita morio Sowerby 2* Pisania decapilata englerli (Hertlein) I Crustacea Chthamalus belyaevi Zevina & Kurshakova 12 Pachygrapsus transversus (Gibbes) I Total prey items identified 9 II 17 7

NOTE: Numbers in the body of the table are numbers of prey organisms ofspedes at left observed eaten in nature by the predator at top. ·One of these had been captured-and was firmly held by tube feet-midway along an arm, but was not actually being.eaten. densities of 8 individua1s/m2 in favorable of T. ferruginea Latreille recovered from the habitats at Easter Island. No individuals of stomach of another asteroid, Leiaster leachii P. decapitata englerti were encountered in (Gray) at Easter Island. With that exception, 137 t-m2 quadrats censused for C. miliaris, to my knowledge the observations reported although the two species shared the same here provide the first information on preda habitat. torprey relationships in the benthic inverte Mitra (Strigatella) flavocingulata (mean brate community at Easter Island, the most shell length 23.1 mm, range 19.5-28.5 mm), isolated outpost of the Indo-West Pacific which occurred at a mean density of 0.12 marine realm. The trophic structure of this individual/m2 in the quadrats sampled for community is ofparticular interest because of C. miliaris, probably preys exclusively on this distance and because ofthe low diversity sipunculans, as do all other mitrids studied of its allopatric populations and species to date (Kohn 1970, Miller 1974, West 1978). derived from the rich central Indo-West Neothais nesiotes (mean shell length 17.8 Pacific inshore marine biota. Although the mm, range 14.5-21 mm), which occurs higher observations reported here are limited, they in the intertidal region than any of the other present initial knowledge necessary to pro gastropods studied, was observed eating only vide a basis for comparisons of trophic barnacles (Chthamalus belyaevi) intertidally, structure that will permit interpretation of and only gastropods (Antisabia imbricata, the results of natural experiments in the Dendropoma sp.) subtidally. assembly and organization of insular marine Five prey organisms of Astrostole paschae communities. (radius 75-90 mm) that had certainly been capnrrea allve Were gastropo-ds; only one protopodite of the crab was present (Table ACKNOWLEDGMENTS 1). Garth (1973) reported on two individuals I thank Benito Alarcon for assistance in ofthe crab Trapezia areolata Dana and three the field, and Gerardo Velasco and Carlos Gastropods as Predators and Prey-KoHN 37

Pisani for aid with logistics at Easter Island. KaHN, A. J. 1970. Food habits of the gas I also thank Harald A. Rehder for helpful tropod Mitra litterata Lamarck: relation discussion and assistance in identification of to trophic structure ofthe intertidal marine the gastropods; the names used in this paper bench community in Hawaii. Pac. Sci. 24: follow the usage in his forthcoming mono 483-486. graph of the Mollusca of Easter Island. I ---. in press. Ecological shift and release thank Dennis M. Devaney and David L. in an isolated population: Conus miliaris at Pawson for aid in identifying the asteroid, Easter Island. Ecology. Dora P. Henry for identifying the barnacle, KaHN, A. J., and M. C. LLOYD. 1973. Marine and John S. Garth for identifying the crab polychaete annelids of Easter Island. Int. and for helpful comments. Rev. Ges. Hydrobiol. 58:691-712. MILLER, A. C. 1974. A comparison of gas tropod species diversity and trophic struc ture in the rocky intertidal zone of the LITERATURE CITED temperate and tropical west Americas. DEVANEY, D. M. 1973. Zoogeography and Ph.D. Thesis. University ofOregon. 143pp. faunal composition of southeastern Poly PIELOU, E. C. 1977. Mathematical ecology. nesian asterozoan echinoderms. Pages 357 Wiley-Interscience, New York. 366 in R. Fraser, ed. Oceanography of the RANDALL, J. E. 1976. The endemic shore South Pacific. 1972. New Zealand National fishes of the Hawaiian Islands, Lord Howe Commission for UNESCO, Wellington. Island and Easter Island. Pages 49-73 in DIAMOND, J. M., and R. M. MAY. 1976. Biologie marine et exploitation des res Island biogeography and the design of sources de l'Ocean Indien Occidental. natural reserves. Pages 163-186 in R. M. Trav. Doc. l'O.R.S.T.O.M., 47. May, ed. Theoretical ecology: principles WEST, T. 1978. Structure, functional mor and applications. W. B. Saunders, Phi phology, and feeding in Mitra idae Melville ladelphia. and Mitra catalinae Dall (Mollusca: Gas FELL, F. J. 1974. The echinoids of Easter tropoda). M.S. Thesis. University of the Island (Rapa Nui). P~c. Sci. 28: 147-158. Pacific, Stockton, Calif. GARTH, J. S. 1973. The brachyuran crabs of Easter Island. Proc. Calif. Acad. Sci. 39: 311-336.