CONTROL OF CORAL REEFS BY BORING SPONGES 49 



being of the sessile communities: in shallow water, wave turbulence 

 is the prime mover; in deep water, it is mainly the force of gravity 

 that induces flow on sloping surfaces. The mobilitv of the sediments 

 is enhanced by the lubricating and fluidizing eflect of the mud 

 fraction, the origin of which is still very poorly understood. 



Preliminary studies near Runawav Ba)' have shown that the fore- 

 reef slope sediments immediately below coral communities contain 

 a much higher proportion of calcareous mud than those farther 

 away, and that the accumulation of mud is greatest where the 

 growth of corals, algae, and sponges in the overlying reefs is best. 

 Although there appears to be a general relationship between mud 

 formation and biological activity in coral reefs, it is not yet known 

 what specific mechanisms are involved. It is probable, however, that 

 the ability of boring sponges to reduce massive skeletal carbonates 

 to very small particles is of major importance in the production 

 of fine sediments in the reef habitat. 



So far, the greatest emphasis has been on the destructive aspects 

 of boring sponge activity, but the same processes which promote 

 the breakdown of reefs also make possible the establishment and 

 growth of large coral communities on the deep slope. The coarse, 

 sponge-riddled detritus transported onto the fore-reef slope pro- 

 vides an ideal foundation for highly specialized sessile populations 

 that could not under normal circumstances colonize this habitat. 

 The dominant coral of the deep talus belt is Agaricia iindata, a 

 species restricted almost wholly to localities where avalanche detri- 

 tus has been deposited over the soft and barren sediments of the 

 open fore-reef slope at depths between 35 and 75 meters. Here, 

 A. undata aggregates with other agariciid corals such as A. cucuUata, 

 A. fragilis, and an undescribed species to build up large compound 



Fig. 16. A submarine cliff recently stripped of corals by an avalanche. The 

 area on the left has been recolonized by sponges, algae, and Antipatharia, but 

 corals are still lacking though they are abundant on adjacent parts of the cliff. 

 Below the diver is the top of a slide mass of coral detritus. Unlike the older 

 talus heaps shown in Figs. 12 and 13, this detrital mass has not yet been 

 populated by corals. The treelike object in the foreground is a large anti- 

 patharian on which some sponges are also growing. This photograph was taken 

 at a depth of about 35 meters near Cardiff Hall, Jamaica. 



