DEPTH-RELATED CHANGES IN THE COLONY FORM OF THE REEF CORAL 



PORITES ASTREOIDES 



Will em H. Brakel 

 Department of Biology 

 Loyola College 

 Baltimore, MD 21210 



ABSTRACT 



The depth-related morphological transition of Porites astreoides colonies from 

 near-hemispherical to flattened forms is well known, but requires closer exam- 

 ination. For this reason, colonies of _P. astreoides were surveyed at 7 sites 

 spanning a depth range of 27 m at Discovery Bay, Jamaica. The depth, substrate 

 slope, and degree of exposure of each colony were assessed prior to measurements 

 of colony shape and corallite structure. Regression analysis of quantum irradiance 

 measurements taken throughout the study area provided equations to estimate the 

 light available to each colony as a function of its depth, substrate slope, and 

 exposure. Predicted wave heights and periods from wind statistics were used in 

 conjunction with theoretical equations for the attenuation of wave motion with 

 depth to estimate the relative water movement about each colony. 



It was found that availability of light does not directly determine the shape of 

 the colony, but only sets an upper limit to colony height, a limit that decreases 

 with decreasing illumination. Similarly, water movement places only an upper 

 constraint on morphological variation, with only very flat colonies possible in 

 high energy environments. Consequently, flattened colonies were found at all 

 depths, but were exclusively present in very shallow water (due to high turbulence) 

 and in very deep water (due to low light). These morphological constraints were 

 shown to be primarily the result of a phenotypic response of the colony to its 

 environment. 



INTRODUCTION 



Coral reefs typically exhibit sharp gradients in environmental factors such as 

 illumination and water movement over relatively short vertical or horizontal 

 distances. Physical gradients in relation to depth and their effects on reef 

 organisms are of particular interest at this workshop on deep and shallow reefs. 

 One approach to investigating the impact of depth and depth-related variables on 

 reefs is to examine those organisms that have somehow managed to overcome the 

 physiological stresses imposed by the depth gradient and that have, as a con- 

 sequence, achieved a relatively wide depth range. By studying the adaptations 

 of these eurytopic reef organisms we can better understand the effects of depth 

 on the entire biota. 



Among the reef-building scleractinian corals of the Caribbean region, several 

 species that are primarily massive in their colonial growth form exhibit broad 

 depth ranges for light-dependent organisms. They include Montastrea annularis 

 and Porites astreoides , both of which are common from shallow lagoons to depths of 

 27 m or more (Roos, 1964; Goreau & Goreau, 1973). These species, along with 

 others such as Meandrina meandrites , Stephanocoenia michel inii , Colpophyllia natans, 

 C_. breviserialis , Dichocoenia stokesi , and Montastrea cavernosa , show a morpho- 

 logical transition from roughly hemispherical colonies in shallow water to 

 flattened plates at greater depth. The consensus has been that this change in 

 colony shape is a phenotypic response to maximize light interception by the polyps 

 (Goreau, 1959, 1963; Roos, 1964; Macintyre & Smith, 1974). This view was rein- 

 forced by Roos (1967), who was able to relate morphological change in Porites 



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