When does morphology matter? Biomechanical approaches to studying the physiological ecology of corals.

Brian Helmuth
University of South Carolina,
Department of Biological Sciences, Columbia, SC 29028

Kenneth P. Sebens
University of Maryland, Department of Zoology, College Park, MD 20742;

Biomechanical techniques can potentially serve as very powerful tools for examining the interactive effects of an organism’s morphology with the characteristics of its ambient environment. Shapes which increase rates of mixing at an organism&s surface can enhance the uptake of gases, nutrients, and particulates, but also tend to increase the risk of dislodgment due to hydrodynamic drag. Given measurements of environmental parameters such as water flow, engineering approaches can be used to quantify these rates of mass and momentum flux, and can thus serve as a means for predicting the physiological consequences of any given morphology. Because the effects of shape can in many cases be decoupled from those of organismal characteristics such as behavior, this approach can be used to examine extinct or hypothetical forms as well. In almost all cases, however, investigations of the role of morphology must include empirical measurements conducted under controlled conditions, and whenever possible should be tested using living analogs. To this end, we have been using a combination of laboratory and field measurements, biomechanical techniques and field experiments to examine the effects of morphology on the physiological performance of several common Caribbean scleractinian corals (Agaricia spp., Madracis mirabilis and Porites spp.). Our results have shown that rates of feeding, gas exchange and light capture are significantly affected by colony morphology, and can vary consistently within regions of a single colony. In some cases these processes result in conflicting selective pressures, so that a coral can never have a globally optimum shape. Furthermore, when corals exist in aggregations, the shape of the aggregation can supercede the effects of the shape of individual ramets which the aggregation comprises.

Selected references:

Helmuth B and KP Sebens. 1993. The influence of colony morphology and orientation to flow on particle capture by the scleractinian coral Agaricia agaricites (Linnaeus). J. Exp. Mar. Biol. Ecol. 165: 251-278.

Helmuth BST, KP Sebens and TL Daniel. 1997. Morphological variation in coral aggregations: branch spacing and mass flux to coral tissues. J. Exp. Mar. Biol. Ecol., 209: 233-259.

Helmuth BST, EF Stockwell and DR Brumbaugh. 1997. Morphological and environmental determinants of mass flux to corals, Proc. 8th Int. Coral Reef Symp., Panama, 2:1103-1108.

Helmuth BST, BEH Timmerman, and KP Sebens. 1997. Interplay of host morphology and symbiont microhabitat in coral aggregations. Mar. Biol., 130:1-10.

Sebens KP, SP Grace, B Helmuth, EA Maney, Jr. and JS Miles 1998. Water flow and prey capture by three scleractinian corals, Madracis mirabilis, Montastrea cavernosa and Porites porites in a field enclosure. Mar. Biol. 131:347-360.

Sebens KP, J Witting, and B Helmuth, 1997. Effects of water flow and branch spacing on particle capture by the reef coral Madracis mirabilis (Duchassaing and Michelotti). J. Exp. Mar. Biol. Ecol., 211:1-28.