46 t. f. goreau and w. d. hartman 



The Role of the Boring Sponges in the Erosional 

 Remodeling of Coral Reefs 



On the steep outer slopes where normal wave action is negligible, 

 the displacement and drainage of loose skeletal detritus out of the 

 reef communities occurs mostlv under the impetus of gravity. Our 

 observations on sediment distribution in these habitats, to be de- 

 scribed elsewhere in greater detail, show that the mass transport of 

 material eroded from reefs takes place in at least four different 

 ways, all influenced in varying degree by the activities of the boring 

 sponges. 



Talus Formation 



Reef associations continuallv produce large quantities of skeletal 

 debris ranging from big corals to mud-sized aragonite needles. This 

 material gravitates downward and distributes itself on the outer 

 slope according to size, the heavv coralla remaining where they fall 

 to build up a steeplv inclined scree zone flanking the lower edge of 

 the living coral belt, the lighter fine sediments drifting farther down 

 to accumulate in thick beds of muddv sand spread oxer the lower 

 reaches of the fore-reef slope. Talus fallout occurs at all times, even 

 in calm weather. Corals have been seen to tumble down, apparently 

 coming adrift through spontaneous fracture of the base. Such 

 freshlv fallen corals as we have inspected, however, were invariably 

 riddled with boring sponges, and it is probable that this was the 

 main cause of their accidental detachment. The forces initiating 

 coral fallout need not be large; indeed, the weak turbulence pro- 

 duced by the swimming movements of the divers is sometimes 

 enough to loosen the colonies and start them sliding down. 



Coral Slides 



In contrast to simple fallout, which involves both fine and coarse 

 detritus, large quantities of boulder-sized talus are transported by 

 underwater landslides, especiallv in areas where rich coral associ- 

 ations have become established on high submarine cliffs. Such slides 

 leave characteristic traces; the clifi^ face shows localized areas of 

 denudation (Fig. 16) and the fore-reef slope below is covered by 



