This finding contradicts the conventional theory regarding mass transfer between corals and their environs: until recently, the assumption had been that, once released substances left the tissue in question, they simply moved from regions with higher concentrations to those with lower concentrations via diffusion. But if that were true, the researchers should have found the highest oxygen concentrations where the most oxygen was produced. The only explanation for a different pattern is if the corals actively transport the element elsewhere. And thanks to cutting-edge surveillance technologies, they now know exactly how it’s done.
“The trick is that the tiny hairs, or cilia, on the corals’ surface, when moved in unison, create small eddies,” Ahmerkamp explains. In this way, the polyps can shape local currents in order to specifically ventilate those areas that are rich in algae. To do so, they direct oxygen-poor water from above to those areas with the highest algal densities, where it becomes charged with oxygen. In turn, the upward portion of the eddy produced flows away from the corals and releases its load higher up in the water column. Using a computer model, the researchers simulated the interplay between diffusion and ciliary action on the corals’ surface. As the simulation shows, by producing these local eddies near the algae, stony corals can cut the area of their surface exposed to critical oxygen concentrations in half.
“Accordingly, these sessile corals aren’t completely at the mercy of their marine environment, as was previously believed,” summarises Moritz Holtappels. Influencing the mass transfer with their surroundings in a targeted manner, and fanning away surplus oxygen, can be vital for these organisms – especially those growing in waters with little or no current. However, most likely not all corals have such a refined ventilation system. This could explain why some undergo more extreme bleaching than others in response to adverse conditions.