CONTROL OF CORAL REEFS BY BORING SPONGES 37 



their erosional effects are compensated by rapid overgrowth of 

 corals and cementing coralhne algae. 



The Fore-Reef, 15 to 30 Meters 



This region interdigitates with the base of the reef buttress but 

 is distinguished from it by having little primary framework. The 

 depth is great enough so that wave surge is much reduced, yet 

 there is sufficient turbulence to prevent accumulation of fine reef 

 silt; it is a medium-energy environment with the ambient light in- 

 tensity averaging about 25 per cent of the surface value. The coral 

 fauna is still extremely varied as regards species number, but colony 

 size and population density are decreased, and much of the avail- 

 able space is occupied by sand-producing calcareous algae, sponges, 

 and large Gorgonacea. 



The Deep Fore-Reef Slope, 25 to 70 Meters 



This zone is the seaward extension of the fore-reef and is the 

 transitional region between the true shallow-water coral reef prov- 

 ince and the deep-water biotopes of the oceanic slope. An important 

 physical property of this habitat is its precipitous topography: the 

 gradient is seldom less than 45° and often more than 70°. The main 

 drop-ojBF faces north in this part of Jamaica, thus the area is badly 

 illuminated and the bottom light intensity at the 50-meter level is 

 estimated to be about 5 per cent of the surface value. There is in- 



FiG. 10. Topographic profiles from fathometer surveys on Jamaican reef 

 systems where extensive erosion of coral by boring sponges was observed. 

 Sections A and B are profiles of reefs off Mangrove Point and Maria Buena 

 Bay near Duncans; sections C and D were taken off Eaton Hall and Cardiff 

 Hall at Runaway Bay- The lower-case letters indicate major reef features re- 

 ferred to in the text: a, shallow coral framework of the reef crest; b, reef 

 buttress composed of spurs and chutes, c; d, fore-reef; e, fore-reef slope with 

 scattered large coriil aggregates; /, slump due to collapse of the basal reef 

 structure onto the outer slope; g, near-vertical submarine cliff with rich coral 

 and sponge biota; h, lagoon-type sand bottom sloping away from the shallow 

 inshore reef at right; i, gap in reef due to drowned river valley of Pleistocene 

 age; /, detrital cone of shallow-water reef talus transported onto forward slope 

 through gap at i. The arrows on profiles A and B indicate the approximate 

 location of the underwater scenes shown in Figs. 11, 12, and 13. To find the 

 distances in meters, multiply the scale distances by 0.3045. 



