Photosynthesis, growth, distribution, and persistence of macroalgae are determined in part by the physical environment in which they live. Therefore, discerning how macroalgae interact with their physical environment is necessary to better understand their physiological performance. The purpose of this study was to examine what photosynthetic and hydrodynamic costs and benefits the morphology of Pachydictyon coriaceum (Phaeophyta) confers on the thallus in a given environment. Principal components analysis of morphometric measurements of Pachydictyon coriaceum from different flow habitats and depths separated thalli into three distinct morphs: shallow wave-exposed, shallow wave-protected, and deep. To test the hypothesis that thallus morphology affects net photosynthesis (NP), thalli of three morphotypes of P. coriaceum were incubated in an enclosed recirculating flume under three simulated light/water flow environments representing conditions from which the three morphotypes were collected. The wave-protected and deep morphs had significantly higher rates of photosynthesis than the wave-exposed morph for all three simulated environments. The dense, compact shape of the wave-exposed morph readily streamlines with flow and in doing so, potentially shades many of its internal blades likely accounting for its lower biomass-specific NP. Drag coefficients (C-d) were estimated for the three morphotypes over a range of flow velocities between 0.08 and 0.47 m s(-1). At lower water flow velocities (0.08-0.21 m s(-1)), wave-exposed morphs had the lowest C-d among the three morphotypes. But drag coefficients of the three morphotypes converged with increasing flow velocities, and at velocities > 0.31 m s(-1) there were no differences in C-d among the three morphotypes. The results of this study indicate that the environmentally-shaped morphs influence photosynthesis and, to a lesser degree, hydrodynamic forces acting on P. coriaceum.