Effects of water flow and branch spacing on particle capture by the reef coral Madracis mirabilis (Duchassaing and Michelotti)

The scleractinian coral Madracis mirabilis forms colonies composed of many narrow branches whose spacing varies across habitats; this is especially evident along a depth gradient. Environmental factors such as irradiance and water movement co-vary along this gradient and both factors could have effects on branch spacing. We examined the effects of water flow on particle capture by Madracis mirabilis in a laboratory flume at Discovery Bay, Jamaica, using hydrated Artemia cysts as experimental particles. Isolated branches of Madracis showed highest particle capture rates in the 10–15 cm s −1 range of flow speeds, although capture was still occurring at about one fourth the maximum rate even at 40–50 cm s −1. The ability to capture particles at these higher flow speeds results from polyps on downstream sides of branches capturing particles from turbulent eddies in the wake of the branch. At high flows, these polyps are not deformed (flattened) as are the upstream polyps. Two aggregation densities were tested at three flow speeds and both flow and particle capture were measured at each branch. Low density aggregations, comparable to those in low flow and deep reef habitats, captured particles best at the lowest flow speeds tested and capture was relatively uniform through the aggregation. High density aggregations captured particles best at high flow speeds, especially near the downstream end of the aggregation. At low flow speeds, the highest capture rates occurred at the upstream end of the aggregation. Flow speed decreased downstream within aggregations at both low and high densities, especially at higher flow speeds. Turbulence intensity also changed within aggregations, increasing behind the first row of branches at all flow speeds and in both aggregation densities. Total capture rate per polyp was highest at intermediate flow speeds (10–15 cm s −1) for single branches and for aggregations due to high encounter rates, while capture efficiency (flux adjusted capture rate) was greatest at low flow speeds. Patterns of flow and particle capture within aggregations suggest that high density aggregations function better in high flow environments. Low density aggregations were able to capture only one fourth as many particles as high density aggregations at the higher speeds used in these experiments. Conversely, high density aggregations captured only about half as many particles at the low flow speed, compared to low density aggregations. Factors other than fl