Boundary layer thickness is a potentially important component of the diffusive pathway for gas exchange in aquatic organisms. The soft coral Akyonium siderium (Octocorallia) and sea anemone Metridium senile (Actiniaria) exhibit significant increases in respiration with water flow over a range of Reynolds numbers encountered subtidally. A nondimensional mass transfer analysis of the effect of forced convection demonstrates the importance of the state of the organism's boundary layer in regulating metabolism in these invertebrates. Flow-modulated gas exchange may limit secondary productivity in subtidal environments.The condition of the boundary layer is the most pervasive physical influence on the biology of many aquatic organisms (1, 2). Through the action of fluid viscosity, boundary layers form over surfaces immersed in moving fluids. Boundary layer thickness is the distance over which fluid speed or dissolved species concentration changes from the value found immediately adjacent to the surface to some constant "mainstream" value found some distance above the surface. All mass transport with the environment occurs through this layer of relatively stagnant water overlying the organism. Although some aquatic invertebrates (3, 4) and vertebrates (5) depend on environmental flows to ventilate their exchange surfaces, the effect of boundary layer thickness has not been examined closely in studies of marine invertebrate respiration. The thickness of the diffusive boundary layer determines the concentration gradient of dissolved species. For example, this layer has been found to modulate bicarbonate diffusion into aquatic plants and, hence, to determine the maximum rate of photosynthesis (6). Similarly, diffusive boundary layer thickness affects gas exchange in symbiotic scleractinian corals (7,8) and, thus, their growth rates (9).The bottom momentum boundary layer is usually smoothturbulent to rough-turbulent in natural flows over benthic organisms (10). The character of the flow around an organism is set by the size and distribution of neighboring roughness elements (other organisms, topographic elements-such as boulders and sand), and the overall shape and the surface roughness or structure of the organism. In a smoothturbulent flow, a laminar sublayer exists immediately adjacent to the organism surface with a linear profile offlow speed vs. height. In the lower part of this section of the boundary layer, flux of material to an organism occurs by simple diffusion. In a rough-turbulent boundary layer, by definition, the roughness elements are of sufficient height to poke through and disrupt the laminary sublayer (11). Mass transfer increases greatly through the action of turbulent eddies. A final possibility is that the boundary layer over the organism may be transitional between these extremes and laminar sublayers can exist discontinuously in time and space.The flow past an organism attached to the substrate in a benthic boundary layer can be quite complex, with the thickness of slower moving water chang...