Microhabitats, Abundance and Food of Conus on Atoll Reefs in the Maldive and Chagos Islands

Abstract

Assemblages of up to 17 species and averaging 13 species of Conus were found on topographically complex subtidal coral reefs (Type III habitat) in the Maldive and Chagos Islands, Indian Ocean. Topographically simpler intertidal smooth limestone benches (Type II habitat) supported eight species. At a station with intermediate habitat, 12 species were present. Only two species were found in extensive areas of sand substrate (Type I habitat). Although most of the commoner species occurred in habitat Types II and III, their relative abundances differed strikingly. Species characteristic of the physically harsher, topographically simpler Type II environment are smaller, sometimes occur in dense populations of up to 1 individual/m2, and are quite specialized predators on herbivorous errant polychaetes. Species characteristic of the more equable but topographically diverse Type III environment are larger, have lower population densities of 0.03—0.15 individual/m2, and are somewhat more generalized predators on deposit—feeding sedentary polychaetes. Detailed analysis of diets supports the conclusion of prior studies that most species of Conus are primary carnivores. The 13 most abundant species of Conus in the Maldive and Chagos Islands feed almost exclusively on polychaetes, which comprised 96% of all food items identified. Species that eat gastropods and fishes were present but uncommon. The nature of the food of eight species was determined for the first time. Calculations of amount of food eaten, based on rather crude data for five species, gave a mean of about 10% of body weight eaten per day, but some of the estimates seemed unreasonably high. Species with more specialized diets are larger numbers of prey items than generalists, but the larger ratio of prey size to body size of the latter probably compensates, so that specialists are not more successful predators than generalists. In the habitats studied, Conus species are not more specialized predators where larger numbers of congeners co—occur. The topographic uniformity and less patchy nature of Type II habitat probably favor both increased density of the prey and mobility of Conus, thus making appropriate food more abundant and accessible than in Type III habitat. Conus diets are correspondingly more specialized in Type II than Type III habitat. These opposite feeding strategies foster increased feeding efficiency in the two topographically contrasting habitats. It is proposed that topographically simpler, more uniform, intertidal bench habitats favor lower species diversity, specialization, smaller body size, and higher population density, while topographically more complex, patchier, slightly subtidal coral reef habitats favor higher species diversity, somewhat more generalized habits, larger size, and lower population density.