The result of macroherbivore grazing 

 within the grass bed can be dramatic (Camp 

 et al . 1973). Of greater overall signifi- 

 cance, however, is the fragmentation of 

 living seagrass and production of particu- 

 late detritus coincident with feeding. 

 Further, the nature of urchin and parrot- 

 fish feeding results in the liberation of 

 living seagrass and its subsequent export 

 from the bed (Greenway 1976; Zieman et al . 

 1979). Zieman et al. (1979) observed that 

 manatee grass blades floated after detach- 

 ment, whereas turtle grass tended to sink; 

 the result was that turtle grass was the 

 primary component of the litter layer 

 available for subsequent utilization by 

 detritivores. 



Many of the macroconsumers, such as 

 Acanthurids, S^. rul^ripinne and S^. chrysop- 

 terum (Randall 1967) , fngesting living 

 seagrass take in only small amounts, the 

 majority of their diet consisting of epi- 

 phytic algae. Species primarily ingesting 

 seagrass (i.e., S^. radians ) typically pre- 

 fer the epiphytized portion of the sea- 

 grass blade. These observations suggest 

 that seagrass epiphytes &re important in 

 the flow of energy within the grass car- 

 pet. Many of the small, mobile epifaunal 

 species that are so abundant in the grass 

 bed and important as food for fishes feed 

 at least in part on epiphytes. Typically, 

 these animals do not feed on living sea- 

 grass, but often ingest significant quant- 

 ities of organic detritus with its asso- 

 ciated flora and fauna. Tozeuma carol in- 

 ense , a common caridean shrimp, feeds on 

 epiphytic algae attached to seagrass 

 blades but undoubtedly consumes coinciden- 

 tally other animals (Ewald 1969). Three 

 of the four seagrass-dwel 1 ing amphipods 

 common in south Florida use seagrass epi- 

 phytes, seagrass detritus, and drift algae 

 as food, in this order of importance (Zim- 

 merman et al . 1979). Epiphytic algae were 

 the most important plant food sources 

 tested since they were eaten at a high 

 rate by Cymad usa compta , Gammarus muc ro- 

 natus , and ^'el ita nitida . Epiphytic algae 

 were also assimilated more efficiently by 

 these amphipods (48?, 43?i and 75%, respec- 

 tively) than other food sources tested, 

 including macrophytic drift algae, live 

 seagrass, and seagrass detritus. Live 

 seagrass had little or no food value to 

 these amphipods. 



There is little doubt that the struc- 

 ture of many grass beds was profoundly 

 different in pre-Columbian times when tur- 

 tle populations were 100 to 1,000 times 

 greater than those now. Rather than ran- 

 domly cruising the vast submarine meadows, 

 grazing as submarine buffalo, turtles 

 apparently have evolved a distinct feeding 

 behavior. They are not resident in sea- 

 grass beds at night, but live in deep 

 holes or near fringing reefs and surface 

 about once an hour to breathe. During 

 morning or evening the turtles will swim 

 some unknown distance to the seagrass beds 

 to feed. What is most unioue is that they 

 return consistently to the same spot and 

 regraze the previously grazed patches, 

 maintaining blade lengths of only a few 

 centimeters (Bjorndal 1980). Thayer and 

 Engel ("S in preparation) calculated that 

 an intermediate-sized Chelonia (64 kg or 

 141 lb) consumes daily a dry v;eight of 

 blades equivalent to 0.5 m- of an average 

 turtle grass bed (500 g dw of leaves). 

 Since the regrazed areas do not contain as 

 heavy a standing crop as ungrazed grass 

 beds, it is obvious that their grazing 

 plots must be considerably larger. The 

 maximum length of grazing time on one dis- 

 tinct patch is not known, but J.C. Ogden 

 (personal communication) observed patches 

 that persisted for up to 9 months. 



The first time turtles graze an area 

 they do not consume the entire blade but 

 bite only the lower portion and allow the 

 epiphytized upper portion to float away. 

 This behavior was recently described in 

 some detail by Bjorndal (1980), but the 

 earliest description was from the Pry 

 Tortugas where John James Audubon observed 

 turtles feeding on seagrass, "which they 

 cut near the roots to procure the most 

 tender and succulent part" (Audubon 1834). 



It was previously thought that there 

 was an advantage for grazers to consume 

 the epiphyte complex at the tip of sea- 

 grass leaves, as this complex was of 

 higher food value than the plain seagrass 

 leaf. Although this seems logical, it 

 appears not to be so, at least not for 

 nitrogen compounds. While studying the 

 food of turtles, Mortimer (1976) found 

 that entire turtle grass leaves collected 

 at Seashore Key, Florida, averaged 1.7% fl 

 on an ash free basis, while turtle grass 



68 



