overenphasize the quantity of seagrass 

 naterial entering the grazing food chain 

 since urchins ^re not typically found at 

 densities of 20 urchins/n- as was the case 

 in Kingston Harbor (Ogden li^eO). In St. 

 Croix, it has been estimated that typi- 

 cally between 5% and 10% of daily produc- 

 tion of turtle grass is directly consuned, 

 primarily by Spar isoina radians and second- 

 arily by the urchins Oiadena antillarun 

 and Tripn eustes ven tricosus . Averaged over 

 the day, turtle 'grass production was 

 2.7 g dw/m /day of which only about 1% 

 was exported, while 60? to 100% of the 

 0.3 g dv//m /day production of manatee 

 grass was exported (Zienan et al. 1979). 

 From these figures it is conservatively 

 estimated that about 70« oF the daily 

 production of seagrasses was available to 

 the detrital system. 



Many of the small organisms in grass 

 beds use algal epiphytes and detritus as 

 their food sources. The gastropods are 

 the most prominent organisms feeding on 

 epiphytic algae in seagrass beds. Arphi- 

 pods, isopods, crabs, and other crusta- 

 ceans ingest a mixture of epiphytic and 

 benthic algae as well as detritus (Odum 

 and Heald 1972). As research continues, 

 it is becoming apparent that the utiliza- 

 tion of this combination of nicroalgae and 

 detritus represents one of the major 

 energy transfer pathways to hiaher oroan- 

 isms. 



Notable by their absence are the 

 large flocks of ducks and related water- 

 fowl found on temperate Zoste ra beds and 

 especially the freshwater Ruppia beds 

 (Jacobs et al . 1981). ricRoy and Helfferich 

 (19S0) list 43 bird species that consume 

 seagrass primarily in the temperate zone. 

 Relatively few species of birds ingest 

 seagrass species of the tropics or forage 

 for prey in the sediments of shallow grass 

 beds. 



Detritus undoubtedly serves as the 

 base of a major pathway of energy flow in 

 seagrass meadows. A significant proportion 

 of net production in the seagrass bed re- 

 sults in detritus either by dying in place 

 and being broken down over a period of 

 months by bacteria, funqi and other organ- 

 isms (Robertson and Hann 1980) or by being 

 consumed by large herbivores, fragmented, 

 and returned as feces (Ogden 1980). In 



Piscayne ^ay, turtle grass formed the most 

 important constituent of the detritus 

 present (37. 1?,). while other portions 

 included 2.1% other seagrasses, 4.6% 

 algae, 0.4% animal remains, 3.3% mangrove 

 leaves and 2.5% terrestrial material 

 (Fenchel 1970). The microbial community 

 living in the detritus collected consisted 

 mainly of bacteria, small zooflagellates, 

 diatoms, unicellular algae, and ciliates. 

 It is these types of organisms which form 

 the major source of nutrition for detrital 

 feeders. Bloom et al . (1972), Santos 

 and Simon (1974), and Young and Young 

 (1977) provided species lists annotated 

 witii feeding habits for molluscs and 

 polychaetes, many of which ingest detri- 

 tus. 



Typically penacid and caridean shrimp 

 are considered to be o.nnivores. The pink 

 shrimp (Penaeus duorarum ), in addition to 

 oroanic detritus and 



detritus 

 chaetes, nematodes, 

 mysids, copepods, 

 ostracods, molluscs 

 (Eldred 1958; Eldred 

 consumers strip the 



sand, ingests poly- 

 caridean shrimp, 

 isooods, amphipods, 



and foraminiferans 

 et al. 1061). These 



bacteria and other 



organisms from the detritus, and the fecal 

 pellets are subsequently roingested fol- 

 lowing recolonization (Fenchel 1970). 

 Some fishes, notably the mullet ( Mugil 

 cephalus ), are detrital feeders (Odum 

 1970). Several large invertebrates such 

 as the gastropod Str ombus giga s (Randall 

 1964) and the asteroid Oreaster reticula- 

 tus (Schoibling 1980) take detritus as a 

 part of their food. To emphasize the 

 importance of detritus to higher trophic 

 levels within the grass, the work of Carr 

 and Adams (1973) should be noted. They 

 found that detritus consumers were of 

 major importance in at least one feeding 

 stage of 15 out of 21 species of juvenile 

 marine fishes studied. 



It is well documented that fishes 

 feed while occupying grass beds (Carr and 

 Adams 1973; Adams 1976b; Brook 1975, 1977; 

 Robertson and Howard 1978), as opposed to 

 simply using them for shelter. Typically, 

 seagrass-associated fishes are small, gen- 

 eral ist feeders, tending to prey upon epi- 

 faunal organisms, primarily crustaceans. 

 Infaunal animals are under used in propor- 

 tion to their abundance as few fishes 

 resident in the grass beds feed on them or 

 on other fishes (Kikuchi 1980). 



58 



