In a new study made available last week via PNAS, MIT scientists (and list of multi-national, multi-disciplinary) collaborators have uncovered that corals can actually manipulate flow near their bodies and are not left to depend solely on the whims of ambient flow. Corals are sessile and depend on flow to exchange nutrients and dissolved gases with the water around them. They also depend on flow to remove waste and bring in potential food items (ie: zooplankton) for them to capture with their stinging tentacles. As it turns out, corals can actually utilize small cilia to stir up vortices in a space up to 2 mm away from their skeleton.
So why does this matter? Other than the fact that these images look really cool, this means that corals can manipulate flow at small scales around their bodies. The authors of the study tested oxygen transfer using microprobes and found that this process enhances mass transfer rates up to 400%. Corals are essentially stirring their boundary layer when they do this, which greatly enhances their ability to exchange gases and nutrients with the environment. This is a pretty significant finding.
Before jumping to criticize the paper, you should note that the authors tested this with small explants of coral tissue (to see cilia action, see videos 1, 2, and 3), but also using actual coral colonies. This effect was not seen only in a vacuum. You can see it occurring on an actual coral fragment above (figure B is a zoomed in version of figure A). This type of vortical motion was also seen in several genera of coral including: Pocilopora (A, B, and C above) Stylophora (A below), Acropora (B below), Montipora (C below), Seriatopora (D below), and Favia (E below).
These images, captured by light microscopy, SEM, and darkfield microscopy are really cool and very well done. The tracer lines that you see in the images are either the result of dark field microscopy, or the use of fluorescent beads (fig E above) in fluorescent beads in epiflouresence microscopy. These awesome high-def images won first place at the 2013 International Science & Engineering Visualization Challenge (photography category).
Remember that this ability to manipulate current motion occurs at very small scales. Corals are not able to move enough water toward them to bring in zooplankton from the water column, but if such a food item were to get near the boundary layer on the coral, it is certainly possible that it may be pulled in via current motion. It is also likely that this current motion helps the coral to jettison waste products and get them away from other polyps without causing harm (**Note that this paragraph is context and conjecture from myself, this is not included in the peer-reviewed article).
Not every coral has these external cilia, but the ones that do seem to have evolved an advantage that could prove invaluable in low or intermittent current areas. As currents are subject to change in the face of a changing climate, such an adaptation may really help some species of coral continue to survive in coming years.
If you interested I encourage you to read the paper and check out the supplementary information here.