Regulation of fish reproduction by a predatory gastropod: an experimental investigation of indirect effects in a seagrass community

The indirect effects of the horse conch, a predatory gastropod, on reproduction in a suite of fishes that use the shells of the gastropod's main prey (the pen shell Atrina rigida (Sol.)) as nest sites in St. Joseph Bay, Florida, was investigated by examining the two links (direct interactions) that create the indirect effect. The horse conch– A. rigida predator–prey interaction was quantified by measuring predator feeding rate and distribution across a range of prey density. To determine the effect of A. rigida mortality on fish reproduction, the rate of shell availability was experimentally manipulated and egg production measured in four fish species. Horse conch feeding rate and distribution were not functions of prey density, but A. rigida mortality was correlated with conch abundance, supporting previous evidence that horse conchs regulate new shell availability. Increased rates of shell availability caused increases in the number of clutches laid and average clutch size in one fish species, Chasmodes saburrae Jordan & Gilbert, probably because new shells provide a greater surface area for eggs compared to older shells. These results suggest that horse conchs indirectly affect C. saburrae reproduction both at the level of the individual (clutch size) and the population (egg production) by regulating the availability of new shells. Because prey density had little effect on the predator, variation in the indirect effect of conchs on fish reproduction will be primarily influenced by the distribution of the predator, not A. rigida.