582 



Fishery Bulletin 88(3). 1990 



the two sides of the enclosures. All treatments and size- 

 classes resulted in highly significant differences 

 (p<0.01), except for two cases (Table 7). Adults in the 

 Low/Sand treatment and adults in the High/Moderate 

 treatment showed no significant habitat preference 

 (p>0.05). In these two experiments adult conch were 

 highly motile, and more than 50% of the individuals 

 were found traveling around the walls of the en- 

 closures. Because only animals not touching the walls 

 were included in the analysis, the numbers for analysis 

 were small and highly variable (Table 7, Fig. 5). Where 

 a high-preference habitat (Moderate Density) was 

 paired with a low-preference habitat (Low Density or 

 Sand), adult conch were not found against the walls 

 of the enclosures in such high numbers (28%) and 

 habitat selection was highly significant (Table 7). 



Discussion 



Many marine organisms are known to prefer seagrass 

 beds over bare sand. For example, amphipods, tanaida- 

 ceans, decapods, and fishes have all been found to be 

 more abundant in seagrass beds than on bare sand 

 (Heck and Thoman 1981, Holt et al. 1983, Stoner 1983, 

 Heck et al. 1989). Also, abundance of animals within 

 seagrass beds appears to be influenced strongly by the 

 local amount of macrophjte structure. Numbers of both 

 decapod and peracarid crustaceans have been cor- 

 related with seagrass biomass (Heck and Orth 1980, 

 Stoner 1980a, Lewis 1984). A direct linear relationship 

 between seagrass biomass and harpacticoid abundance 

 and diversity was attributed to an increase in habitable 

 space, nutritional resources, and reduced levels of 

 predation (Hicks 1980). Communities of epibenthic fish 

 on the banks of Florida Bay were associated with areas 

 of high seagrass biomass and with accumulations of 

 seagrass detritus (Sogard et al. 1987). Detritus was 

 thought to provide a rich food source as well as a refuge 

 from predators. 



The general association of queen conch with turtle- 

 grass Thalassia testudinum is well known (Randall 

 1964, Hesse 1979, Weil and Laughlin 1984), but quan- 

 titative relationships between the mollusc and seagrass 

 beds are reported for the first time in this study. The 

 distributional patterns of queen conch near Lee Stock- 

 ing Island were similar to patterns found for other 

 taxa, with few individuals associated with bare sand 

 and low-density seagrass. As was true for other spe- 

 cies, juvenile conch densities increased with macro- 

 phyte biomass and shoot density over a wide range; 

 however, optimal levels of macrophyte cover were 

 found, beyond which only larger individuals were 

 associated. The observed association of juvenile queen 

 conch with moderate amounts of macrophyte cover 



(shoot density, biomass, etc.) probably results from a 

 combination of mechanisms: (1) a simple inability of the 

 animals to maneuver into or through heavy stands of 

 seagrass and detritus, (2) active habitat choice for 

 specific seagrass cover, and (3) differential survivor- 

 ship associated with different macrophyte densities, 

 especially in the smaller size-classes. 



The upper limit of seagrass density with which juve- 

 nile queen conch are associated is probably set by their 

 locomotory abilities. Randall (1964) speculated that 

 thick stands of seagrass obstruct the movements of 

 small conch. High shoot density is also responsible for 

 heavy accumulations of detritus and soft sediments 

 which may impede locomotion. Given that queen conch 

 propel their heavy shells with thtaists of a pointed oper- 

 culum on the end of a muscular foot, locomotion is most 

 efficient on a firm substratum. 



The direct relationship between conch density and 

 seagrass density below the optimal level is most likely 

 linked to more complex interactions. It is known that 

 abundance of food can be limiting for juvenile conch 

 in the field at natural density (Stoner 1989). The 

 primary sources of food for juvenile conch in nursery 

 habitats are macrodetritus and algal epiphytes (Stoner 

 and Waite, In review); both of these food items increase 

 with seagrass density. Blade and detritus productiv- 

 ity increase with shoot density, and seagrasses have 

 the effect of increasing the entrapment of fine sedi- 

 ments and detritus (den Hartog 1967, Orth 1977, this 

 study), useful only up to the point of impeded locomo- 

 tion. Algae consumed by conch in the seagrass nursery 

 areas are primarily those growing as epiphytes on 

 Thalasnia blades. Production of these epiphytes can be 

 as high as 50% of the seagrass production (den Har- 

 tog 1979), and increases with seagrass shoot density. 

 The increase in density of conch with seagrass shoot 

 density, therefore, may be a function of habitat choice 

 by individuals for areas with greater abundance of 

 foods and/or a function of food limitation in the 

 population. 



The relationship between conch density and seagrass 

 shoot density can also be a response to predation. This 

 response could be direct, whereby the survivorship of 

 conch increases with increasing seagrass structure, or 

 indirect, where habitat preference for high-structure 

 habitats has evolved as a response to predators. In 

 either case, the role of seagrass biomass in protecting 

 prey species is known for a wide variety of predator- 

 prey combinations (Coen et al. 1981, Stoner 1982, 

 Leber 1985, Heck and Wilson 1987), and experiments 

 with crustaceans have shown that prey species are pro- 

 ficient in choosing high-density seagrass (Stoner 1980b, 

 Coen et al. 1981, Bell and Westoby 1986). The fact that 

 small conch were more proficient in selecting habitats 

 of particular seagrass biomass or shoot density than 



