The hcrbivory of parrotfish and sea ur- 

 chins may be important in the back reef 

 areas and in Hawk. Channel; but, with the 

 exception of sporadic grazing by passing 

 turtles, herbivory is low or non-existent 

 in the areas to the west of the Florida 

 Keys (J.C. Zieman, personal observation). 



Parrotfish typically move ofT the 

 reef and feed during the day (Randall 

 1965). Spar isopa radians , S^. rubripinnc, 

 and S^. chry sopterun are known to feed on 

 seagrass and associated algae (Randall 

 1967). The bucktooth parrotfish {S_. rad i - 

 ans ) feeds almost exclusively on turtle 

 grass. Other fishes that dre important 

 seagrass consumers are surqeonfishes 

 ( Acant huridae) (Randall 1967;" Clavijo 

 1974),"" the porcies ( Sparidae ) (Pandall 

 1967; Adams 1976b), and the halfbeaks 

 ( Heiniramphidae ) , 



Fishes in the Caribbean seagrass beds 

 tend to be general ist herbivores, select- 

 ing plants in approximate relation to 

 their abundance in the field (Ogden 1976; 

 Ogden and Lobel 1978). Some degree of 

 selectivity is evident, however. Sparisoma 

 chrysopterum and S^. radians , when gi"ven a 

 choice, will select seagrass with epiphy- 

 tes (Lobel and Ogden, personal communica- 

 tion). Seagrasses (turtle grass, manatee 

 grass, and shoal grass) ranked highest in 

 preference over common algal seagrass 

 associates. 



Urchins that feed on seagrass include 

 Eucidaris tribuloides , Lytechinus varieaa- 

 tus , Diadema antil la run 

 yentricosus (llcPherson 



_ and Tripneustes 



1964, 1968; Randall 



and Grant 1965; Moore 



Prim 1973; Abbott 



et al. 1973; Moore 



The 



et al. 1964; Kier 

 and r'cPherson 1964; 

 et al. 1974; Ooden 

 et al. 1963a, lS63b; Greenway 1976), 

 latter two urchins feed in approximate 

 proportion to food abundance in the area. 

 Where present in seagrass beds, J. ventri- 

 cosus and D. antillarum feed on seagrasses 

 with epiphytes exclusively (Ogden 1980). 

 L ytechinus variegatus is largely a detri- 

 tal feeder (Ogden 1980), but has denuded 

 large areas in west Florida (Camp et al . 

 1973). 



The queen conch ( Strombus giqas ), 

 once a common inhabitant of Caribbean sea- 

 grass beds, has been dramatically reduced 



in many areas because of its high food 

 value and ease of capture by man. Conchs 

 are found in a variety of grass beds, from 

 dense turtle grass to sparse manatee grass 

 and Halophila . V.'hen in turtle grass beds 

 conchs primarily feed by rasping the epi- 

 phytes from the leaves as opposed to eat- 

 ing the turtle grass. In sparse grass 

 beds, however, conchs consumed large quan- 

 tities of manatee orass and Halophila 

 (Randall 1964). A maximum of 207. of the 

 stomach contents of conchs at St. John, 

 U.S. Virgin Islands, was comprised of tur- 

 tle grass. In manatee grass ( Cymodocea ) 

 beds, conchs consumed mostly this seagrass 

 along with some algae. The maximum quan- 

 tity of seagrass found v;as 80% Halophila 

 from the gut of four conchs from Puerto 

 Pico. 



The emerald nerite ( Smaragdia v i r i - 

 d i s ) , a small gastropod, commonly ~5 to 

 8 mm long, can be numerous in turtle grass 

 beds although it is difficult to see be- 

 cause its bright green color matches that 

 of the lower portion of the turtle grass 

 blades. It is a direct consumer of turtle 

 grass where it roams about the lower half 

 of the green blades; the snail removes a 

 furrow about 1 mm wide and half the thick- 

 ness of the blade with its radula (J.C. 

 Zieman and P.T. Zieman, personal observa- 

 tion). 



^'ost studies (for review, see Law- 

 rence 1975) indicate that the majority of 

 seagrass consumers have no enzymes to di- 

 gest structural carbohydrates and that, 

 with the exception of turtles and possibly 

 manatees, they do not have a gut flora 

 capable of such digestion. Thus, most 

 macroconsumers of seagrasses depend on the 

 cell contents of seagrasses and the at- 

 tached epiphytes for food and must have a 

 mechanism for the efficient maceration of 

 the material. The recent work of Weinstein 

 et al . (in press), however, demonstrated 

 that the pinfish was capable of digesting 

 the structural cellulose of detrital mat- 

 ter or green seagrasses. Feeding rates 

 are high for urchins and parrotfishes, 

 while absorption efficiency is around 50* 

 (Moore and McPherson 1965; Lowe 1974; 

 Ogden and Lobel 1978). Assimilation effi- 

 ciencies for ]_. ventricosus and \^. varie- 

 gatus are relatively low, 3.8% and 3.0% 

 respectively (Moore et al . 1963a, 1963b). 



67 



