movement, as well as salinity, temperature, and sedimentation rates, may vary in 

 disparate ways. When all these factors act at once, individually or synergistic- 

 ally, on the physiology and morphology of the benthos, the outcome is not easily 

 foreseen. 



Even when considered in isolation, an ecological variable such as irradiance or 

 water movement does not have the expected biological effects. One might have anti- 

 cipated some linear relationship between irradiance or wave energy and relative 

 colony height, a relationship amenable to correlation or regression analysis. In 

 actuality, the roles of light and water movement are quite different: they do not 

 directly determine the shape of the colony, but rather set limits to the possible 

 range of morphological variation. 



A detailed discussion of the physiology of coral colony growth is beyond the 

 scope of this paper, but one question that should be addressed is whether the 

 observed environmental constraints on Porites colony morphology are environmentally- 

 induced or the result of natural selection at different depths for genotypes with 

 the requisite inherent shape characteristics. Examination of fine skeletal struc- 

 tures of the individual coral lites known to be unaffected by environmental condi- 

 tions (Brakel, 1977) showed that those colonies most flattened or constrained with 

 respect to colony height are not genetically distinct, but constitute a random 

 subsample of the population. This implies that their modified colony shape is the 

 result of a direct physiological response to environmental stimuli. The observed 

 morphological transition of £_. astreoides in relation to irradiance and water move- 

 ment is therefore principally due to phenotypic plasticity, not selection. 



The mechanism by which this morphological transition occurs is not under- 

 stood. Goreau (1963) and Barnes (1973) suggested that the form of the corallum 

 is the result of two separate processes; skeletal accretion (dependent on light) 

 and tissue growth (independent of light), so that at low light intensities cal- 

 cification cannot keep up with tissue growth, resulting in the lateral prolifera- 

 tion of excess tissue to form a flattened colony. The adaptive significance 

 of a flat colony profile in environments with high wave action has been discussed 

 by Graus, et aj. (1977), but again the mechanism by which water movement controls 

 colony height is not clear. Jokiel (1978) has suggested that water movement 

 influences corals by controlling the exchange of materials between the polyps 

 and the surrounding sea water. Whatever the nature of the physiological controls 

 on coral growth and form, it is evident that, with changes in depth, environmental 

 factors act on the colony in intricate ways and that the adaptive response of the 

 coral can be subtle and complex. 



ACKNOWLEDGMENTS 



This research was made possible with the support of the Biology Department, 

 Yale University, and with the assistance and facilities provided by the Discovery 

 Bay Marine Laboratory. 



LITERATURE CITED 



Barnes, D. J. 1973. Growth in colonial scleractinians. Bull. Mar. Sci . 23: 



280-298. 

 Brakel, W. H. 1976. The ecology of coral shape: microhabitat variation in the 



colony form and corallite structure of Porites on a Jamaican reef. Ph.D. Diss., 



Yale Univ. 246 p. 

 Brakel, W. H. 1977. Corallite variation in Porites and the species problem in 



corals. Proc. Third Int. Coral Reef Symp. 1: 457-462. 

 Brakel, W. H. 1979. Small-scale spatial variation in light available to coral 



reef benthos. Bull. Mar. Sci. 29: 406-413. 



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