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 esp ecially evident along a depth gradient. Environmental factors such as irradiance and water movement co-vary along this gradient and both fac tors could have effects on branch spacing. We examined the effects of water flow on particle capture by Madracis mirabilis in a laboratory f lume at Discovery Bay, Jamaica, using hydrated Artenia cysts as experi mental particles. Isolated branches of Madracis showed highest particl e capture rates in the 10-15 cm s(-1) range of flow speeds, although c apture was still occurring at about one fourth the maximum rate even a t 40-50 cm s(-1). The ability to capture particles at these higher flo w speeds results from polyps on downstream sides of branches capturing particles from turbulent eddies in the wake of the branch. At high fl ows, these polyps are not deformed (flattened) as are the upstream pol yps. Two aggregation densities were tested at three flow speeds and bo th 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 aggregati ons captured particles best at high flow speeds, especially near the d ownstream end of the aggregation. At low flow speeds, the highest capt ure rates occurred at the upstream end of the aggregation. Flow speed decreased downstream within aggregations at both low and high densitie s, especially at higher flow speeds. Turbulence intensity also changed within aggregations, increasing behind the first row of branches at a ll flow speeds and in both aggregation densities. Total capture rate p er polyp was highest at intermediate flow speeds (10-15 cm s(-1)) for single branches and for aggregations due to high encounter rates, whil e 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 e nvironments. Low density aggregations were able to capture only one fo urth as many particles as high density aggregations at the higher spee ds used in these experiments. Conversely, high density aggregations ca ptured only about half as many particles al the low flow speed,compare d to low density aggregations. Factors other than flow, especially lig ht interception, are likely to affect branch spacing as well. Shallow reef habitats, with high irradiance and hi,oh flow conditions, may thu s favor tight branch spacing as a response to both environmental varia bles. (C) 1996 Elsevier Science B.V.