Ecological Shift and Release in an Isolated Population: Conus miliaris at Easter Island

Abstract

With the exception of a recently discovered rare species, the gastropod genus Conus is represented only by Conus miliaris at Easter Island, and isolated outpost of the Indo—West Pacific (IWP) marine biota probably <2.5 million yr old. Throughout most of the IWP region. C. miliaris co—occurs with 4—23 congeners on marine benches and coral reefs. Compared with these central populations, Easter Island C. miliaris (1) is more abundant in favorable habitats, (2) utilizes different habitat and food resources, and (3) is ecologically released with respect to food and probably depth range, but not microhabitat. Most striking, the major prey species (Onuphis sp.; 31% of the diet) belongs to a family of polychaetes not known to be exploited by any Conus species elsewhere. In all, 6 species of polychaetes eaten rarely or not at all by C. miliaris elsewhere comprise 77% of its diet at Easter Island. Sixteen prey species are eaten at Easter Island (vs. 2—8 in central IWP localities) and prey species diversity measured by the H' statistic is 2.0 vs. 0.2—1.3. Shifts to larger prey species with increasing size of C. miliaris account for some of the high prey diversity, but analysis of multiple feedings revealed that 68% of such individuals had eaten polychaetes of 2 or more species. Several polychaete species that C. miliaris preys on the Easter Island but not elsewhere are important components of the diets of other Conus species in the central IWP. Some of the ecological expansion of C. miliaris at Easter Island is thus directed toward resources that absent congeners would be expected to utilize were they present, or if they should colonize the island in the future. It is suggested that the virtual absence of congeneric species and increased intraspecific competition in dense populations at Easter Island have selected for the observed ecological expansion.