Use of adjacent grass and sand flats 

 by coral reef creatures is not strictly a 

 nocturnal phenomenon, but seems to be the 

 dominant pattern. Only large herbivores 

 (e.g., Chelonia nydas , Scarus quacamaia ) 

 venture far into the grass bed away from 

 the shelter of the reef. Mid-sized herbi- 

 vores are apparently excluded by predators 

 and feed only near the reef (Ogen and Zie- 

 man 1977). Randall (1965) reported parrot- 

 fishes ( Scarus and Spar i soma ) and surgeon- 

 fishes ( Acanthurus ) feeding on seagrasses 

 ( Thalassia and manatee grass) closely 

 adjacent to patch reefs in the Vircin 

 Islands during the day. He attributed the 

 formation of halos around patch reefs in 

 St. John to this grazing. 



7.3 CONTINENTAL SHELF 



Recently interest has been sparked in 

 estuarine-Continental Shelf interactions 

 (Darnell and Sbniat 1979). The seaarass 

 meadow represents a highly productive, 

 faunally rich habitat within south Flor- 

 ida's estuaries and coastal lagoons. Many 

 species are dependent on the seagrass bed 

 and estuary. The pink shrimp' Penaeus 

 duorarum , the lobster Panulirus arqus . 

 and the grey snapper Lutjanus griseus 

 may serve as examples of estuarine or 

 lagocnal dependent fauna which at one life 

 stage or another are found in seagrass 

 meadows. 



In south Florida, pink shrimp spawn 

 in the vicinity of the Tortugas Bank, the 

 pelagic larvae returning to the estuary 

 and perhaps the seagrass bed (Yokel 

 1975a). Eventually mature individuals re- 

 turn to the spawning grounds. Similarly, 

 the lobster natures in inshore seacrass 

 nursery grounds and as a sub-adult resides 

 on inshore reefs while continuing to feed 

 within the grass bed at night. As'^sexually 

 mature adults, female lobsters move to 

 deep offshore reefs and spawn. The grey 

 snapper initially recruits into grassland 

 with growth moves into mangrove^ habitat 

 and eventually on to coral reefs and deep- 

 er shelf waters. Coming or going, these 

 organisms and others like then serve to 

 transfer energy from the seagrass bed to 

 offshore -waters (see section 7.5), as has 

 been shov/n by Fry (19C1) for brown shrimp 

 (f.- iztecus) in Texas waters. 



7.4 EXPORT OF SEAGRASS 



The most recently recognized function 

 of seagrass beds is their ability to 

 export "large quantities of organic matter 

 from the seagrass meadows for utilization 

 at some distant location (Zieman et al . 

 1979; Wolff 1980). This exported material 

 is both a carbon and nitrogen source for 

 benthic, mid-water, and surface-feeding 

 organisms at considerable distances from 

 the original source of its formation. The 

 abundance of drifting seagrass off the 

 west Florida shelf is illustrated in 

 Figure 25 (Zieman et al., in preparation). 

 This material originates on the shallow 

 grass flats and is transported westward by 

 the prevailing winds and tides. 



Leaves and fragments of turtle grass 

 were collected by Menzies et al . (1967) 

 off the North Carolina coast in 3,160 m 

 (10,368 ft) of water. Although the near- 

 est source of turtle grass was probably 

 1,000 km (625 mi) away, blades were found 

 at densities up to 48 blades per photo- 

 graph. Roper and Brundage (1972) surveyed 

 the Virgin Islands basin photographically 

 and found seagrass blades in most of some 

 5,000 photographs taken at depths averag- 

 ing 3,500 m (11, 48^ ft). Most were clearly 

 recognizable as turtle grass or manatee 

 grass. Seagrasses v/ere collected by trav;l- 

 ing in three Caribbean trenches and sea- 

 grass material v/as found in all the 

 trenches sampled (Wolff 1976). Most of 

 the material collected was turtle grass, 

 and there was evidence of consumption by 

 deep-water organisms. Interestingly, 

 some grass blades collected from 6,740 m 

 (22,113 ft) in the Cayman Trench showed 

 the distinctive bite marks of parrot- 

 fish which are found only in shallow 

 waters. 



The primary causes of detachment are 

 grazing by herbivores, mortality on shal- 

 low banks caused by low tides, and wave- 

 induced severing of leaves that are becom- 

 ing senescent. In addition, major storms 

 will tear out living leaves and rhizomes 

 (Thomas et al . 1961). Which mode of 

 detachment will be most important in a 

 particular area will be largely deter- 

 mined by physical conditions such as 

 depth and wave exposure. Peduced salin- 

 ity or extreme temperature variation will 



78 



