170 



MARSH 



Smith and Paulson (1974) studied four transit times 

 and gut pH's in two parrotfish species, Scarus jonesii and 

 S. gibbus. They found that both species begin feeding 

 around first light (reported to be about 07:30 hr) and con- 

 tinued feeding until about 19:00 hr, at which time indivi- 

 duals began aggregating in groups of 15 to 50 and then 

 dispersed at last light (about 19:50 hr). They were seen 

 feeding only on dead coral with its covering of filamentous 

 algae, contrary to reports of Hiatt and Strasburg (1960) 

 that these species feed on live coral. Smith and Paulson 

 concluded that material ingested by the parrotfish at dawn 

 is often evacuated within 4 hr and that food consumed at 

 dusk passes through the gut in 6 hr or less. Such calcu- 

 lated transit times of 4 to 6 hr correspond to filling the gut 

 at least twice a day. Feeding is intermittent. 



In feeding S. jonesii, the anterior three gut regions 

 (pyloric caecum, small intestine, large intestine) were found 

 to be more acidic than seawater, with values of 6.8 to 7.5; 

 the rectum (pH = 8.2) was not. In S. gibbus, all four 

 regions of the gut (pH ranging from 6.4 to 7.5) are more 

 acidic than seawater. Smith and Paulson emphasized that 

 these were feeding fishes rather than those with empty 

 guts. They concluded that CaCOs mai; dissolve in the par- 

 rotfish gut. 



Reese (1977) considered the coevolution of corals and 

 coral-feeding fishes of the family Chaetodontidae. He 

 placed these buttcrflyfishes into one of three feeding 

 categories: coral feeders, omnivores which feed on bcnthic 

 invertebrates other than corals, and plankton feeders. The 

 coral feeders may be obligative or facultative. At 

 Encwetak, 10 of the 17 species studied were coral feeders 

 (with four of these being obligate coral feeders), five were 

 omnivores, and two were planktivores. Laboratory studies 

 conducted in Hawaii showed that Chaetodon thfasciatus 

 and C ornatissimus preferred the coral Pocillopora dam- 

 icornis over Montipora verrucosa over Porites compressa. 



Nolan et al. (1975) examined the fish communities 

 inhabiting two small nuclear test craters at Enewetak. They 

 found the standing crops of herbivorous and carnivorous 

 fishes to be 35.7 and 61.3 g m~ , respectively, in 

 LaCrosse Crater and 5.7 and 16.8 g m~^, respectively, 

 in Cactus Crater. This was higher than the 10.3 and 

 4.6 g m^ , respectively, reported by Odum and Odum 

 (1955) for their ^one of large heads. In the two nuclear 

 test craters, carnivores constituted 74.7% and 63.2% of 

 the total biomass, but Odum and Odum reported 

 herbivorous fish biomass to be four to five times that of 

 carnivores. Nolan et al. estimated that 100 kg of goatfish 

 alone might be harvested from the two craters every 1 to 

 2 days because there is continuous immigration. 



The Role of Detritus 



The role of detritus as a link between the reef flat and 

 the lagoon was first noted by Odum and Odum (1955), 

 who observed the transport of algal fragments from the 

 back-reef zones into the lagoon. The next paper was by 

 Marshall (1965), who believed that particulate matter car- 

 ried off the windward reefs might constitute a substantial 



contribution to the trophic system within the lagoon. He 

 stated that most detritus on cleared filters, sampled from 

 water crossing the reef flat, appeared to be of plant origin, 

 but he also noted the presence of amorphous organic 

 aggregates. A lagoon sample from a coral knoll appeared 

 to be similar to that of the reef flat. He found more 

 detritus over the reefs and in the lagoon than in samples 

 from the deep pass or seaward of the reef front. His 

 values for combustible material trapped on glass filters 

 were at least an order of magnitude greater than those of 

 the earlier Odum and Odum study and sometimes almost 

 two orders of magnitude greater. Ash-free dry weights 

 from the lagoon averaged more than 0.1 g m . Chloro- 

 phyll a values ranged from 0.08 to 0.14 mg m""' in the 

 channels, 0.21 to 0.33 on the coral-algal ridge, 0.15 to 

 0.39 in waters crossing the reef flats, and 0.16 to 0.61 in 

 the lagoon. Ash-free dry weights were 0.04 to 0.15 g 

 m ■' in the channels, 0.10 to 0.99 on the coral-algal 

 ridge, 0.15 to 0.62 on the reef flats, and 0.06 to 0.22 in 

 the lagoon. 



Johannes (1967), in the first paper to focus on the role 

 of coral mucus, further considered the ecology of organic 

 aggregates and noted that these showed a markedly 

 increased concentration as oceanic water crossed the reef 

 and entered the lagoon. These aggregates consisted largely 

 of coral mucus. Johannes estimated the export of mucus 

 into the lagoon as 20 mg m"'' h~', or about 20% of the 

 total reef production and 40% of total coral respiration. A 

 few meters lagoonward of the drop-off at the back of the 

 reef flat, organic aggregates were usually the only identifi- 

 able suspended objects in the water column; most of the 

 algal fragments and sediment particles had settled out. In 

 laboratory experiments, Artemia nauplii survived longer 

 and grew faster in water with added mucus than in filtered 

 seawater. 



Coles and Strathman (1973) made further observations 

 on coral mucus "floes" and their potential trophic signifi- 

 cance. They found that visible mucus floes contain signifi- 

 cant quantities of organic matter compared to microscopic 

 suspended particle concentrations in surrounding water. 

 Carbon to nitrogen ratios suggested that suspended mucus 

 floes are enriched with nitrogen compared to more recently 

 secreted coral mucus or microscopic particulate organic 

 matter. Freshly collected mucus, after drying, had organic 

 contents comparable to other biological materials, 26% C 

 and 3% N; caloric values were 3.95 gcal mg~' (ash-free 

 dry weight) for mucus collected from Fungia scutaria. 

 Suspended mucus floes collected on the lagoon side of the 

 windward reef at Enewetak closely resembled mucus 

 obtained from Acropora in the laboratory and contained 

 algae, occasional protozoa, organic debris, and inorganics. 

 Mucus floes from different corals differed in C:N ratios and 

 in the total quantities of organic C and N. 



Gerber and Marshall (1974a, 1974b) considered the 

 role of reef pseudoplankton in trophic systems of the 

 lagoon. Gut analyses of Undinula vulgaris (a calanoid 

 copepod), Oikopleura longicaudata (a larvacean), and 

 several species of planktivorous fishes showec. that detritus 



