Morphological variation in coral aggregations: branch spacing and mass flux to coral tissues

Abstract

The scleractinian (reef-building) coral Agaricia tenuifolia (Dana) is one of the most common constituents of the barrier reef of Belize, Central America. This species grows almost exclusively in aggregations of clonemates and conspecifics, in which rows of thin, upright blades line up behind one another facing the dominant direction of flow. We quantified patterns in colony morphology, light levels and mainstream flow over a range of physical habitats (fore reef, patch reef and lagoon locations) near Carrie Bow Cay and in the Pelican Cays. 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. Aggregation size, branch spacing, height and width all varied with location. Mean branch spacing increased with depth on the fore reef by approximately 50%, but total branch height increased by only 20–25%, indicating that the shape of colonies did not remain constant. Colonies in the 1 m lagoon habitat (high light, low flow) were very similar to those at 1 m on the fore reef (high light, high flow). These results thus suggested that colony morphology was insensitive to the flow regime, despite previous studies that have linked flow-dependent mass flux to both coral respiration and symbiont (zooxanthellae) photosynthesis. Because of this discrepancy, we examined the effect of one aggregation parameter, branch spacing, to test the null hypothesis that mass flux to a coral's tissues is unaffected by colony morphology. We used two non-dimensional parameters, the Reynolds number ( Re) and the Sherwood number ( Sh), to examine the interaction between flow, morphology and mass transport. 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 how 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 turbulence levels in uniform flow showed that mass flux is significantly higher in solitary models compared to aggregations. Mass flux from branches within aggregations was highest at branch tips and decreased closer to the bottom. Measurements of boundary layer profiles overlying aggregations indicated higher boundary layer diffusivities to the surface of the tightly-spaced aggregation, per unit of substrate area. However, the increased amount of tissue surface area in these aggregations led to a lower flux per unit of coral tissue. Our results suggest that the coral A. tenuifolia displays different aggregation structures in response to light but not water flow, at least in shallow, high light environments. Nonetheless, our laboratory experiments show that branch spacing within an aggregation has significant effects on the flux of gases to the surface of corals. Because photosynthesis depends upon both mass flux and light, this apparent contradiction between field patterns and laboratory results suggests that A. tenuifolia and its symbionts may adapt physiologically rather than morphologically to variation in the local flow regime. The optimal branch spacing in any given environment 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 stresses and competition for space.