CONTROL OF CORAL REEFS BY BORING SPONGES 45 



ner skeletons, slower growth rates, and large dead areas around the 

 base, are more riddled with sponge burrows than are shallow-water 

 colonies of the same size. The infestation starts in the older parts of 

 the exposed skeleton around the base and may spread to form a net- 

 work of cavities ramifying throughout the colony. 



There is no conclusive evidence that excavation of the nonliving 

 interior of corals by boring sponges has any injurious effect on the 

 polyps per se. The survival of the colony as a whole is not seriously 

 jeopardized until the attachment of the coral is weakened to the 

 breaking point. The fate of corals after they are dislodged depends 

 on the species and on the environment: on level bottom the colonies 

 may remain in place but will suffer injury if they are tilted on their 

 sides or turned over; in steeplv sloping areas the loose colonies 

 slide or roll into deep water and are killed by burial in the sedi- 

 ments. Corals encumbered by heavy growths of attached organ- 

 isms mav be at a considerable disadvantage because of the extra 

 load on the alreadv weakened base, but our observations also show 

 that large encrusting sponges often support and hold such corals in 

 place long after their original holdfasts have been eroded away by 

 boring organisms. 



The loosening of corals through destruction of their basal parts 

 is so marked at depths below 50 meters that large living colonies 

 can be collected easilv by hand without the use of tools. Such indi- 

 viduals are only able to maintain their position on steep slopes by 

 virtue of their light weight, and because the extreme flattening 

 provides a broad area of contact that becomes snagged on surface 

 irregularities of the substrate. The flattening also prevents the corals 

 from turning over. Under similar conditions, the heavier rounded 

 types of coral are unable to remain in place, since they roll down- 

 slope into deep water as soon as they come adrift. It is therefore 

 clear that the flattening response in reef corals has considerable 

 adaptive value in environments where heavy loss to the coral popu- 

 lation is caused by the combined effects of boring sponge activity 

 and precipitous bottom topography. 



