Effects of water movement on prey capture and distribution of reef corals

Abstract

Listen

Translate article

The water flow regimes on a coral reef change with depth, distance from shore, and orientation to incoming waves. Low flow conditions may limit the ability of corals to capture prey, because the rate of encounter with particles suspended in the water column depends on the transport of water past the corals’ feeding structures. High flow speeds, however, cause deformation or collapse of feeding structures, and prey capture success is likely to decrease under such conditions. The specific relationship between flow speed and particle capture by passive suspension feeders like anthozoans may be determined by tentacle size and shape, tentacle and polyp stiffness and colony morphology. The importance of particle capture, as well as the types (zooplankton, detritus) and the size range of particles captured by corals, are almost unknown. The only published information for any species to date is Porter’s (1974) report on coelenteron contents of Montastrea cavernosa. The relationship between water flow and particle (hydrated Artemia cysts) capture has been studied for two octocoral (Alcyonium) species by Patterson (1984) and McFadden (1986) and for one scleractinian coral (Meandrina meandrites, Johnson & Sebens, unpubl.) in laboratory flume studies. The present study is an attempt to relate prey capture to water flow under field conditions for two species (Meandrina meandrites, Madracis decactis) in Salt River Canyon, St Croix, U.S.V.I., using the underwater habitat ‘Aquarius’ in July 1988. Concurrent measurements of water flow and wave height were made at several positions on the reef between 7 and 45 m using recording electromagnetic current meters (Interocean S4) mounted on rigid posts 0.5 m off the substrate. During the same two-week period, particle release (hydrated Artemia cysts) experiments were carried out over colonies of Meandrina and Madracis in the field. Particles were released upstream of the corals for 3 min, after which corals were caused to retract their tentacles, and flow above the coral was measured using macrovideo photography of moving cysts in a slit beam of light parallel to the flow. The corals were then collected and preserved under water for coelenteron content analysis, including both captured cysts and natural prey (Zooplankton). Particle concentrations were determined from samples collected by pumping water from directly over the coral during the experiment, using intake heads that allowed omnidirectional lateral sampling.