CONTROL OF CORAL REEFS BY BORING SPONGES 51 



boring acti\'it\ are more obvious than in shallow water because 

 biological calcification is slow and wave surge is negligible. 



2. Boring sponges burrow into anv substrate composed of arago- 

 nite or calcite, i.e. shells, corals, and limestone. In corals, the 

 sponges erode only, the nonliving skeleton; the living polyps are un- 

 affected. The older parts of the colonies are most damaged, es- 

 peciallv the holdfast, which is thin and fragile in deep-water reef 

 corals. The larger colonies often break off under their own weight 

 and fall into deep water, where thev are killed hx burial in soft 

 sediments. 



3. Most deep-water reef corals have a thin and flattened colony 

 shape. On account of their light weight and broad area of contact 

 with the substrate, such platelike colonies often remain in place 

 after their holdfasts ha\'e been eroded awav, whereas heavy rounded 

 coral heads will roll downslope and be lost. It is suggested that the 

 flattening response, believed to be an effect of low light intensity on 

 skeletogenesis, has considerable adaptive significance for reef corals 

 growing in deep water under conditions where heavv destruction 

 by boring sponges and precipitous bottom topography occur to- 



, gether. 



4. The widespread boring out of corals and the concomitant 

 softening of the limestone substrate weaken the entire reef and 

 make it more susceptible to the destructive action of storm waves, 

 earthquakes, and so on. Mass transport of loosened corals and coarse 

 detritus from the upper reef to the deep fore-reef slope is due to 

 (a) talus fall, (b) slides and avalanches, (c) reef subsidence and 

 slump. 



5. The deep fore-reef slope is normally covered by thick layers 

 of bare muddy sand devoid of corals. However, when large amounts 

 of coarse reef talus are deposited over the soft sediments of the 

 outer slope, a stable, well drained substrate is provided on which 

 dense coral populations subsequently become established. The depo- 

 sition of reef block talus in deep water thus results in an extension 

 of environmental conditions advantageous for the growth of coral 

 communities into regions where they cannot otherwise exist. 



6. It is suggested that the very fine calcareous flakes ejected by 

 active boring sponges contribute to the large volumes of mud pro- 



