TROPHIC BIOLOGY OF STROMBUS GIGAS IN NURSERY HABITATS: DIETS AND FOOD SOURCES IN SEAGRASS MEADOWS

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Abstract
Ontogenetic, seasonal, and spatial variation in the diets of queen conch, Strombus gigas, were investigated in three different nursery habitats in the Exuma Cays, Bahamas. Stable isotope ratios were used to examine the potential sources of nutrition. Analysis of stomach contents for conch >50 mm shell length showed little ontogenetic and seasonal variation in diets except that adult conch stomachs contained more algal material than did those of juveniles in the seagrass habitat. Spatial variation, related to the abundance of foods in particular habitats, was greater than ontogenetic or seasonal variation. At two of the nursery sites (one with high seagrass biomass and one with bare sand), detritus made up more than 90% of the stomach contents. At the site with low seagrass biomass, macroalgae comprised approximately 50% of the stomach contents, with detritus and small fauna making up the other 50%. Despite large amounts of seagrass detritus in the stomachs of conch from all size classes, stable istope ratios of carbon and nitrogen indicated that the nutrition of conch cannot be based solely upon detritus at any of the nursery habitats. Macroalgae, particularly Laurencia spp. and Batophora oerstedi, were the most likely sources of food depleted in del I3C, despite low standing crops. Assessment of the abundance and/or productivity of the less obvious food items will be critical in evaluating habitat quality for juvenile queen conch.

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