This is reflected in the occurrence 

 of grass communities at much greater 

 depths than observed in Hawk Chan- 

 nel. 



The grasses include the turtle 

 grass ( Thalassia testudinum ) and 

 manatee grass ( Syrinqodium filifot — 

 me) , dominated by the former. Both 

 act, as they did on the mud substra- 

 tes, to stabilize skeletal and other 

 organic debris with their blade and 

 root systems. Common algal species 

 include the green algae Halimeda 

 opuntia , H. incrassata , Penicillus 

 capitatus , P. pyriformis , and Udotea 

 sp., and occassionally the red 

 algae, Melobesia membranacea . A 

 variety of mollusks, echinoids, 

 forams, sponges, arthropods, and a 

 few scleractinian corals, including 

 the rose coral Manicina arcolata and 

 the finger coral Porites divaricata , 

 make up the majority of the habi- 

 tat's fauna. The most common fauna 

 in terms of size and/or abundance 

 include the foraminifera Archais 

 anqulatus , the gastropods Modulus 

 modulus , Cerithium sp., and preda- 

 tor, Pleuroploca qiqantea and the 

 pelecypods Laevicardium laeviqatum 

 and Tellinia sp. (Enos 1977). Many 

 of the faunal species observed in 

 the grass covered mud bottom habitat 

 are common here as well. 



Calcareous Sand/ Bare Bottom 

 Habitat 



As stated previously, on the 

 sand shoals where wave action is the 

 most vigorous, the grasses, e.g. 

 Thalassia testudinum are absent. 

 The "White Bank" seaward of the up- 

 per keys is an example of the barren 

 calcareous sand blanket. The Hali- 

 meda-rich, medium to coarse grain 

 clean sand has been brought in from 

 the adjacent outer reef zone by 

 strong current action, forming 

 rippled sand bottoms and migrating 

 dunes. The habitat communities 



range from an "underwater desert" to 



a pioneer seagrass community. The 

 few consistently occurring inhabi- 

 tants include several echinoids 

 ( Meoma ventricosa , Clypeaster rosa- 

 ceus , Plaqiobrissus qrandis ) , and 

 the sand dollars Encope michelini , 

 Clypeaster subdepressus , and Leodia 

 sexiesperforata which burrow in 

 clean, grassless sand regions (Kier 

 and Grant 1965). Occasional conches 

 are observed and a population of 

 shrimps, crabs, flounders, star- 

 gazers, lizard fishes, and various 

 eels may also reside in the area, 

 emerging only at night (Multer 

 1977). In the less turbulent areas 

 but still devoid of grasses, calca- 

 reous green algae such as Halimeda 

 incrassata , H. tuna , Udotea sp. , and 

 Penicillus capitatus may form a 

 pioneer community. The accompanying 

 fauna include a few foraminifera, 

 the scleractinian corals Manicina 

 areolata and Porites divaricata , 

 several burrowing gastropods, pelec- 

 ypods, and annelids ( Arenicola sp.) , 

 the tunneling shrimp Calianassa sp., 

 the starfish Oreaster reticulatus , 

 and the previously mentioned echi- 

 noids (Kier and Grant 1965, Enos 

 1977, Multer 1977). 



Patch Reefs 



Over 6,000 patch reefs are 

 found, generally landward of the 

 shelf-edge slope break in areas of 

 sand, mud, or rock substrate, be- 

 tween Miami and the Marquesas Keys. 

 Most are located in a band that lies 

 3 to 7 km (2 to 4 mi) from the 

 islands between Hawk Channel and the 

 outer bank reefs (Marszalek et al. 

 1977). They typically cluster in 

 broad belts, individually surrounded 

 by a narrow band of bare sandy sedi- 

 ment. These bands occur even in 

 areas of dense seagrass cover or 

 bare mud bottoms. Randall (1965) 

 and Zieman (1981) have shown that 

 the bare sediment halos are a result 

 of overgrazing of grass by fish that 

 reside in the reefs by day and feed 



193 



