- The diffusive boundary layers surrounding sessile marine organisms have been implicated in con- trolling an organism's metabolism and growth. We studied boundary layers surrounding hermatypic corals by moni- toring oxygen concentrations on a submillimetric scale. Oxygen concentration within the boundary layers varied from supersaturation during the day to anoxia at night, although the ambient water composition remained con- stant. Detailed mapping and oxygen measurements re- vealed die1 oxygen fluctuations from supersaturation (373% air saturation) in the light to complete oxygen de- pletion at darkness in the massive coral Favia favus. Ex- posure to a 5-cm/s current reduced the boundary layer thickness from 2.44 mm to 1.90 mm, allowing more rapid oxygen exchange across the diffusive boundary layer. Similar patterns were found in the branching coral Sty- lophora pistillata. In massive corals, the thickness of the diffusive boundary layer was negatively correlated with the size of the polyp. We suggest that the distribution of corals in areas of differential turbulence is related to the thickness of the diffusive boundary layers surrounding them.