Consequences of a flattened morphology: effects of flow on feeding rates of the scleractinian coral Meandrina meandrites

The relationships among flow, colony orientation, position of polyps, and capture of particles (hydrated brine shrimp cysts) were examined in the coral Meandrina meandrites using a recirculating flow tank in the 'Aquarius' underwater habitat at St. Croix, U.S. Virgin Islands. Per polyp feeding rate was independent of the horizontal planform area of colonies. At the lowest velocities, most particles were captured on the upstream edge or in the middle of colonies. However, all positional bias in capture rate disappeared at higher velocities. Particle capture and increasing flow speed were negatively associated and there were small, but measurable, differences in mean tentacle length between corals feeding at different velocities. These results suggest that velocity-dependent feeding rate at most velocities was related to changes in flow rather than to changes in feeding behavior. In fact, experiments in which corals were turned upside down revealed that the increased capture rate for rightside-up corals feeding at low velocity could be almost entirely accounted for by gravitational deposition of particles on the corals' tentacles. Examination of flow profiles above and within the tentacles of each coral revealed that the tentacles form a canopy within which water movement was slowed, possibly facilitating gravitational deposition of non-buoyant or sinking food particles. Thus, the orientation of suspension feeders and the velocity of flows they encounter alters the relative success of mechanisms by which they remove particles from flow.