||Water flow past corals may perform a number of functions during and following episodes of coral reef bleaching. Previous work has demonstrated that flow-modulated metabolism creates asymmetric bleaching patterns within a coral colony, but direct measurement of metabolic costs associated with bleaching have not been made at the level of the polyp. We examined the effects of flow on constitutive and regulated stress protein expression at the level of the polyp. We tested whether corals would exhibit a spatially asymmetric distribution of heat shock proteins 70 and 90 (hsp70 and hsp90) and the constitutive stress protein 70 (hsc70) related to velocity gradients (degree of mixing) across the coral colony. Flow manipulations were conducted from the NOAA underwater habitat Aquarius (FL, USA) on colonies of Montastrea annularis (Ellis and Solander, 1786), with controlled exposures to increased flow (ca. 40 cm s(-1)) and increased temperature (ca. 1.5-2 degrees C above ambient) using in situ flow chambers. Single coral polyps were sampled, processed and analyzed for heat shock proteins using western blotting methods. The short term (daily) and medium term (9 days) response within discrete locations of the coral colonies were examined. We probed for three different stress proteins, with only one exhibiting asymmetrical patterns of synthesis across a colony. Montastrea annularis colonies developed and sustained significant spatially asymmetric patterns of stress protein synthesis across the entire coral surface, with upstream sectors expressing more hsp70, at the same time that these upstream sectors developed and sustained a reduced photosynthetic efficiency or Quantum Yield (QY). The mechanism producing this pattern is unclear; we speculate that increased flow may lead to an initial up-regulation within the synthesis of heat shock protein (hsp70) by the entire colony, followed by a down-regulation in discrete areas through increased hydraulic stress or biochemical energy requirements and limitations. (C) 2010 Elsevier B.V. All rights reserved.