Although the outer reefs are highly variable in their degree of develop- 

 ment, several distinctive features are held in common by reefs well advanced 

 in the successional sequence leading to the mature, climax serai stage. 

 These characteristics include: 



° the presence of the elkhorn coral ( Acropora palmata ) 

 at shallow depths. According to Shinn (1963) , the 

 spur and groove formations result from in situ growth 

 of elkhorn colonies. A significant proportion of 

 these formations is composed of encrusted rubble 

 and skeletal material, derived from this species, 

 which has been incorporated into the spur and 

 groove system; 



° a vertical coral zonation characterized in the deeper 

 zones of the reef by large, massive heads of brain 

 ( Diploria spp .) and star corals ( Montastraea spp .) and, 

 in the shallow, more turbulent areas, branching 

 colonies of Acropora (A. palmata and A. cervicornis ), 

 several types of fire coral , ( MTllepora spp .) anci 

 extensive colonies of the colonial zoanthids 

 Palythoa and Zoanthus; 



° a benthic macrobiota consisting of large populations 

 of the sea urchin ( Diadema antillarum , numerous species 

 of cryptic ophiuroids (brittle stars), a diverse 

 group of octocorals (sea fans and sea whips) and 

 sponges and the calcareous green alga Halimeda 

 opuntia ; 



° a highly diverse finfish fauna. Stark (1967) 



reported a total of 517 fish species from Alligator 

 Reef, of which 389 are coral reef forms. Many of 

 these fish populations are characteristic of 

 particular zones or specific habitats on the 

 reef while others have been found to be nonselective. 

 There is an apparent dependency relationship between 

 the abundant and diverse fish populations of the 

 Florida Reef Tract and the variety of available 

 habitat in the area, not the least of which is 

 the highly productive seagrass community in Hawk 

 Channel. 



Much of the reef's structure is derived from the mechanical and biogenic 

 breakdown of calcareous material. Kissling's analysis (1975) of reef sediments 

 indicate that coral rubble in cobble and boulder sizes represents the vast 

 bulk of reef sediment. Fine sediments result from further breakdown of the 

 coarse material and from contributions by foraminifera, echinoderms, molluscs 

 and calcareous algae. 



48 



