title: Vortical ciliary flows actively enhance mass transport in reef corals.
abstract: The exchange of nutrients and dissolved gasses between corals and their environment is a critical determinant of the growth of coral colonies and the productivity of coral reefs. To date, this exchange has been assumed to be limited by molecular diffusion through an unstirred boundary layer extending 1-2 mm from the coral surface, with corals relying solely on external flow to overcome this limitation. Here, we present direct microscopic evidence that, instead, corals can actively enhance mass transport through strong vortical flows driven by motile epidermal cilia covering their entire surface. Ciliary beating produces quasi-steady arrays of counterrotating vortices that vigorously stir a layer of water extending up to 2 mm from the coral surface. We show that, under low ambient flow velocities, these vortices, rather than molecular diffusion, control the exchange of nutrients and oxygen between the coral and its environment, enhancing mass transfer rates by up to 400%. This ability of corals to stir their boundary layer changes the way that we perceive the microenvironment of coral surfaces, revealing an active mechanism complementing the passive enhancement of transport by ambient flow. These findings extend our understanding of mass transport processes in reef corals and may shed new light on the evolutionary success of corals and coral reefs.
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Plant Sciences, Weizmann Institute of Science, Rehovot 76100, Israel; orrshapiro@gmail.com romans@mit.edu.
authors.forename: Orr H
authors.identifier: http://orcid.org/0000-0002-3222-9809
authors.initials: OH
authors.lastname: Shapiro
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
authors.forename: Vicente I
authors.initials: VI
authors.lastname: Fernandez
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
authors.forename: Melissa
authors.initials: M
authors.lastname: Garren
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Mechanical Engineering, Tufts University, Medford, MA 02155; and.
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Guasto
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Mechanics, École Polytechnique, 91128 Palaiseau Cedex, France.
authors.forename: François P
authors.initials: FP
authors.lastname: Debaillon-Vesque
authors.affiliation: Department of Plant Sciences, Weizmann Institute of Science, Rehovot 76100, Israel;
authors.forename: Esti
authors.initials: E
authors.lastname: Kramarsky-Winter
authors.affiliation: Department of Plant Sciences, Weizmann Institute of Science, Rehovot 76100, Israel;
authors.forename: Assaf
authors.initials: A
authors.lastname: Vardi
authors.affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; orrshapiro@gmail.com romans@mit.edu.
authors.forename: Roman
authors.initials: R
authors.lastname: Stocker
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000502:physiology*; D005075:Biological Evolution; D001692:Biological Transport; D002923:Cilia / Q000502:physiology*; D057894:Coral Reefs; D004058:Diffusion; D004817:Epidermis / Q000502:physiology; D010100:Oxygen / Q000378:metabolism; D012212:Rheology
chemical_list: D010100:Oxygen
keywords: biological fluid mechanics; coral microenvironment; coral reef evolution; diffusion boundary layer; microfluidics