EXPERIMENTAL-ANALYSIS OF GROWTH AND SURVIVORSHIP IN A MARINE GASTROPOD AGGREGATION - BALANCING GROWTH WITH SAFETY IN NUMBERS

The relative roles of habitat quality and predation were examined to test the hypothesis that survivorship in a marine gastropod, Strombus gigas, is enhanced by living in aggregations when young. Enclosure and tether experiments were conducted in and around a juvenile aggregation located in the middle of a tidal flow field in the Bahamas which occupied ca 21 ha. Eight stations were selected to represent a gradient from bare sand to high seagrass biomass and a transect through the aggregation (inside density = 0.40 to 0.56 juveniles M-2; outside density less-than-or-equal-to 0.08 juveniles m-2). Seagrass characteristics proved to be poor predictors of habitat suitability in terms of S. gigas growth and, compared with the presence of conspecifics, relatively unimportant predictors in terms of shelter from predation. Although growth was > 0.1 mm d-1 at all but 2 stations, juveniles (60 to 80 mm shell length) suffered higher proportional mortality when tethered outside the aggregation (48 to 100 %) than inside (18 to 20%). Mortality showed a strong negative correlation with wild S. gigas density, suggesting that small-scale distribution is greatly influenced by density-dependent phenomena. Presence of conspecifics proved to be an important factor affecting survivorship in seagrass meadows of the Bahamas.