MORPHOLOGICAL VARIATION IN CORAL AGGREGATIONS - BRANCH SPACING AND MASS FLUX TO CORAL TISSUES

The scleractinian (reef-building) coral Agaricia tenuifolia (Dana) is one of the most common constituents of the barrier reef of Belize, Cen tral America. This species grows almost exclusively in aggregations of clonemates and conspecifics, in which rows of thin, upright blades li ne up behind one another facing the dominant direction of flow. We qua ntified patterns in colony morphology, light levels and mainstream flo w over a range of physical habitats (fore reef, patch reef and lagoon locations) near Carrie Bow Cay and in the Pelican Gays. Water flow and light levels both decreased with depth on the fore reef. Light levels in the lagoon environment (1 m depth) were comparable to those at the same depth on the fore reef, but flow speeds were markedly lower. Agg regation size, branch spacing, height and width all varied with locati on. Mean branch spacing increased with depth on the fore reef by appro ximately 50%, but total branch height increased by only 20-25%,indicat ing that the shape of colonies did not remain constant. Colonies in th e 1 m lagoon habitat (highlight, low flow) were very similar to those at 1 m on the fore reef (high light, high Bow). These results thus sug gested that colony morphology was insensitive to the flow regime, desp ite previous studies that have linked flow-dependent mass flux to both coral respiration and symbiont (zooxanthellae) photosynthesis. Becaus e of this discrepancy, we examined the effect of one aggregation param eter, branch spacing, to test the null hypothesis that mass flux to a coral's tissues is unaffected by colony morphology. We used two non-di mensional parameters, the Reynolds number (Re) and the Sherwood number (Sh), to examine the interaction between Bow, morphology and mass tra nsport. Using physical scaling arguments, we measured water loss rates from scale models in air as proxies for gas flux from corals in water . We created two types of solitary models, horizontal (unifacial) and upright (bifacial) plates and two types of aggregations, widely-spaced (5 cm between rows) and tightly-spaced (2.5 cm spacing), to examine h ow morphology affects mass flux to a branch's surface under conditions of uniform flow. Measurements at two Re (4 000 and 21 000) and two tu rbulence levels in uniform flow showed that mass flux is significantly higher in solitary models compared to aggregations. Mass flux from br anches within aggregations was highest at branch tips and decreased cl oser to the bottom. Measurements of boundary layer profiles overlying aggregations indicated higher boundary layer diffusivities to the surf ace of the tightly-spaced aggregation, per unit of substrate area. How ever, the increased amount of tissue surface area in these aggregation s led to a lower flux per unit of coral tissue. Our results suggest th at the coral A. tenuifolia displays different aggregation structures i n response to light but not water flow, at least in shallow, high ligh t environments. Nonetheless, our laboratory experiments show that bran ch spacing within an aggregation has significant effects on the flux o f gases to the surface of corals. Because photosynthesis depends upon both mass flux and light, this apparent contradiction between field pa tterns and laboratory results suggests that A. tenuifolia and its symb ionts may adapt physiologically rather than morphologically to variati on in the local flow regime. The optimal branch spacing in any given e nvironment is thus unlikely to result from a single selective pressure but rather from a suite of environmental parameters acting in concert , including light, water flow, sedimentation rate, hydromechanical str esses and competition for space. Copyright (C) 1997 Elsevier Science B .V.