most abundant in seagrass meadows (primarily ... meadows (Wicklund et al., 1988), and habitats .... mately equal size, the dry weight contribution of each.
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J . Moll. Stud. (1991), 57,451-460
@ The Malacological Society of London 1991 Melody Ray-Culp
T R O P H I C BIOLOGY O F S T R O M B U S G I G A S IN NURSERY HABITATS: D I E T S A N D F O O D SOURCES I N SEAGRASS MEADOWS A L L A N W . S T O N E R and J A N I C E M . W A l T E Caribbean Marine Research Center, 805 46th Place East, Vero Beach, Florida 32963, U S A (Received 14 January.1991, accepted 17 April 1991)
ABSTRACT Ontogenetic, seasonal, and spatial vdriation in the diets of queen conch, Strombus gigas, were investigated in threc different nursery habitats in the Exumd Cays, Bdhamas Stable isotope ratios were used to examine the potential sources of nutrition Analysis of stomach contents for conch > 5 0 m m shell length showed little ontogenetic and seawndl variation in diets except that adult conch stomachs contained more algal material than did those of juveniles in the seagrass habitdt Spatial variation, rel&ed to the abundance of toods in particular habitats, was greater thdn ontogenetic or seasonal barlation 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 seagrds biomass, macroalgae comprised dpproximately 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 he based solely upon detritus at dny of the nursery habitats Macroalgae, particularly lLaurencra spp and Batophoru oerrtedr, were the most likely sources of food depleted in del”C, despite low standing crops Assessment of the abundance and/or productivity of the leys obvrous food items will be critical in evaluating habitat quality for juvenile queen conch
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INTRODUCTION The queen conch, Strombus gigas Linnacus, which ranges from Bermuda and south Florida in the United States to Brazil, is an important commercial gastropod in the Caribbean region. Queen conch arc found in a variety of habitat types including sand flats, algal beds, and rubble areas from a few centimetres depth to approximately 30 m (Brownell & Stevely, 1981). In important conch-producing areas of Venezuela (Weil & Laughlin, 1984) and the Bahamas (Stoner & Waite, 1990), juvenile conch are
most abundant in seagrass meadows (primarily Thalassia testudinum), where densities are commonly >I .O conch.m-’. Queen conch are now cultured successfully in hatcheries with the intent of field outplanting (Davis et a l . , 1987). Efforts to supplement dwindling wild stocks of queen conch with hatchery-reared individuals (Brownell et al., 1977, Appledoorn et al., 1987; Berg & Olsen, 1989), however, will depend upon identifying environmental factors needed for good nursery habitat. Despite large seagrass meadows covering thousands of square kilometres in the Bahamas, juvenile conch are consistently limited in distribution to localized areas within the meadows (Wicklund et al., 1988), and habitats similar in macrophyte, depth, and sediment characteristics can yield very different survivorship and growth rates (Stoner & Sandt, 1991). It is clear, therefore, that less obvious habitat features, including food types, need to be examined. Relatively little is known about the feeding biology of queen conch in the field, particularly with respect to the juvenile classes. Robertson (1961) and Randall (1964) reported that adult queen conch feed primarily on the dominant plant of a particular habitat; however, in the laboratory, conch were found to be highly selective in their diets (Crcswell, 1984). Juveniles preferred filamentous algae over commercial artificial diets, seagrasses, and coarse green macroalgae. Adult conch preferred certain red algae, such as Luurenciu pupillosu, Liugoru ceranoides and Spyridia aculeata, when various combinations of macroalgae were offered in the laboratory (Woon, 1983). In the field, juvenile queen conch appear to remove large quantities of detritus and macroalgae from seagrass beds (Stoner, 1989a; Stoner & Sandt, 1991). This study examines ontogenctic, seasonal, and spatial variation in the dietsof juvenile conch in nursery habitats. Because different food
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items are retained and assimilated differentially, stomach content analyses do not always provide the best information on actual sources of nutrition. To provide a second mode for determining nutritional sources, stable isotope ratios were measured for the tissues of juvenile conch and potential sources of food in each of the nurseries. Particular attention was given to the significance of benthic macroalgae. MATERIALS AND METHODS
Study Site Dietary analyses were conducted in the southern Exuma Cays, Bahamas, at two sites which consistently support high densities of juvenile queen conch. The first site (CBC), 1.5 km west of Children's Bay Cay (23"44.3'N, 76"04.5'W), has a consistent annual population of approximately 500,000 conch, studied for several years (Stoner et al., 1988; Wicklund et al., 1988; Stoner, 1989a, b; Stoner & Waite, 1990; Stoner & Sandt, 1991). The habitat is characterized by a gradual slope from a shallow sand bar into a turtlegrass bed (Thalassia testudinum) of moderate shoot density (700 sh0ots.m-') accompanied by an increase in seagrass detritus. Macroalgae arc found in low biomass. Depth ranges from 1 .0 m at mean low water on the sand bank to 3.5 m in the seagrass meadow. Tidal range is 1.0 m, and reversing tidal currents have velocities of approximately 50 cm/s at maximum flood and ebb. Although the conch population at this site is centred in moderate density seagrass (Stoner & Waite, 1Y90), conch were collected for stomach analysis in both sand and seagrass areas. The second site (NBC) was located off a small beach on North Bock Cay (23"48.8'N, 76"11.2'W). This site supports a small, discrete population of conch, present for at least three years, and with conch densities similar to those at CBC. The site consists of a seagrass bed of low shoot density (
