from the analysis. Those few colonies with diameters greater than 30 cm were also 

 omitted. 



The above procedures yielded data on 27 P_. astreoides colonies. For each 

 colony the relative colony height was measured and the irradiance and water move- 

 ment in the immediate vicinity of the colony (based on its depth, location, expos- 

 ure, and substrate slope) were estimated. 



RESULTS 



The mean relative colony height of the corals sampled was 0.24 with a 

 range of 0.08 to 0.69. 



A graph of colony height as a function of depth is shown in Fig. 2. Depth is 

 plotted on a logarithmic scale to emphasize differences among the shallower sites 

 where physical gradients are most pronounced. Surprisingly, the expected monotonic 

 relationship between depth and colony morphology is not seen. 



When irradiance, L, is plotted against colony height (Fig. 3), a linear or 

 curvilinear relationship is still not apparent. Instead, it appears that irrad- 

 iance acts only to set an upper limit to colony height: under high illumination all 

 colony heights are possible, but as available light diminishes only increasingly 

 flattened colonies are seen to occur. The line drawn on the graph marks the limit 

 to colony height imposed by light regime. 



The graph of colony height as a function of water movement, W, (Fig. 4) shows 

 that this parameter also exercises a constraining effect on morphological variation 

 without directly determining the height of the colony. At low wave energies the 

 full range of coral 1 urn variability is seen; in more turbulent micro-environments 

 only the more flattened forms are found. 



DISCUSSION 



The data on Porites astreoides illustrate that the response of organisms such 

 as corals to changes in depth can be complex. As depth increases, different bio- 

 logically-relevant physical and chemical variables such as illumination and water 



2 3 



IRRADIANCE 



Fig. 3. Plot of relative colony 

 height as a function of irradiance. 



x 



(3 



LU 

 I 



> 



o 



_l 



o 

 o 



4 8 12 



WATER MOVEMENT 



Fig. 4. Graph of relative colony 

 height against water movement. 



16 



24 



