CONSEQUENCES OF A FLATTENED MORPHOLOGY - EFFECTS OF FLOW ON FEEDING RATES OF THE SCLERACTINIAN CORAL MEANDRINA-MEANDRITES

The relatonships among flow, colony orientation, position of polyps, a nd 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 th e upstream edge or in the middle of colonies. However, all positional bias in capture rate disappeared at higher velocites. Particle capture and increasing flow speed were negatively associated and there were s mall, but measurable, differences in mean tentacle length between cora ls feeding at different velocities. These results suggest that velocit y-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 ca pture rate for rightside-up corals feeding at low velocity could be al most entirely accounted for by gravitational deposition of particles o n 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 gravita tional deposition of non-buoyant or sinking food particles. Thus, the orientaton of suspension feeders and the velocity of flows they encoun ter alters the relative success of mechanisms by which they remove par ticles from flow.