Abstract
In Galveston Bay, Texas, pier pilings within 10 m of the shore are dominated by the American oyster
Crassostrea virginica (Gmelin) while the ivory barnacle
Balanus eburneus (Gould) predominates on pilings beyond 10 m. This pattern of horizontal zonation reflects differential settlement: barnacles and oysters settle in areas of high and low water motion, respectively. High water motion promotes barnacle growth, but has no effect on oyster growth. The higher settlement of barnacles onto substrata in higher water motion appears to be a combination of arrival rates to these sites and a preference for these areas. Adult-larval interactions further enhance the degree of differential settlement. Differential settlement is a major determinant of the horizontal zonation observed between these two species and may serve as a mechanism for avoiding potentially detrimental post-settlement interactions.