CONTROL OF CORAL REEFS BY BORING SPONGES 39 



tensive sedimentation due to transport of mixed reef detritus from 

 the shallow zones above. In localities where the slope is less than 

 the angle of repose, immense areas of the bottom are covered with 

 thick layers of fine muddy sand which is devoid of corals. Where 

 the angle of repose is exceeded, mass flow of sediments downslope 

 prevents accumulation and exposes outcrops of the underlying rub- 

 ble and rock strata. The latter provide a finii base for sessile organ- 

 isms including algae, Foraminifera, sponges, Gorgonacea, Anti- 

 patharia, Bryozoa, bivalve mollusks, tube worms, etc. Weak detrital 

 cementation is caused by encrusting Foraminifera and by litho- 

 thamnioid algae. 



Reef coral communities occur on the outer slope wherever bottom 

 conditions are favorable, but as the depth increases below 30 meters 

 the coral growth becomes patchier, with progressive reduction in 

 species number, colony size, and population density. Some typical 

 fore-reef slope communities are shown in Figs. 12 and 13. Primary 

 frame building does not take place here because of the lack of 

 large, massive coralla, the fast rate of erosion bv boring organisms, 

 and the paucity of encrusting calcareous algae, which are such im- 

 portant frame cementers in shallow water. 



Colony Form and the Attachment of Reef Corals 

 IN Shallow and Deep Water 



Before describing the effects of boring sponge erosion on deep 

 coral populations, it is necessary to point out that the corals of the 

 fore-reef slope differ stronglv from their shallow-water relatives in 

 colony shape, attachment to the substrate, and encrusting biota. 



Colony Shape 



The most striking property of corals on the fore-reef slope is their 

 flat shape; virtuallv absent are the massive hemispheroidal forms so 

 typical of the same species on the shallow reefs. The change from 

 a three- to a two-dimensional growth plan is correlated with re- 

 duced environmental light intensity, and has been observed only 

 in corals with zooxanthellae (Goreau, 1963), for example Sider- 

 astrea, Diploria, Stephanocoenia, Mussa, Mycetophijllia, Dicho- 



