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COLIN 



been seen to expel sand through the gills after feeding, a 

 process in eommon with many smaller fishes, and appears 

 to feed on fine algae detritus (Randall, 1980). 



The largest planktivorous bony fish at Enewetak is 

 probably the milkfish, Chanos chanos, which occurs occa- 

 sionally on outer reefs and in the lagoon. The largest 

 planktivore, at least within the lagoon, is the manta ray, 

 Manta alfredi. They frequent lagoon margin areas, often in 

 water 3 to 6 m deep, and the wide channel area. On one 

 occasion, a group of more than 100 M. alfredi with a 2 m 

 or more span were seen from the air in the deep ocean 

 just beyond the wide channel. 



Randall (1980) has summarized the food habits of 

 larger groupers (Serranidae), snappers (Lutjanidae), and 

 emperors (Lcthrinidae). All are benthic predators, although 

 some groupers and snappers will rise to a lure in mid- 

 waters. 



Large oceanic predatory fishes occur commonly around 

 Enewetak. Tunas, wahoo (Acanthoc\,;bium solanderi), 

 dolphin (Cor^/phaena hippurus), and billfishes are known to 

 frequent waters within a few kilometers of shore (Schultz 

 et al., 1952). Hiatt and Strasburg (1960) note that the 

 presence of larval fishes and crustaceans produced by reef- 

 and shore-dwelling adults, "supplementing the usual high 

 seas forage species, probably is significant in attracting 

 tunas (and other large pelagic fishes) to mid-ocean islands." 



There are numerous fishes that are highly specialized 

 in their food habits. For example, in the Chaetodontidae, 

 Reese (1975, 1977, 1981) found that at Enewetak, 10 of 

 17 species are coral-feeders, whereas two are planktivores 

 and five are "omnivores " Among coral feeders, four were 

 believed to be obligate coral predators, with fine comb-like 

 teeth for biting off coral polyps. One species, Chaetodon 

 ornatissimus, appears to eat coral mucus with its fleshy 

 lips rather than biting off the polyps like other species. 

 Other coral-feeding species at Enewetak ingested other ani- 

 mal matter as food. The other extreme is C unimacuhtus 

 which even ingests fragments of septa as it feeds on 

 polyps. 



Other coral-polyp feeders include Oxiimonocanthus lon- 

 girostris, Labnchth\js unilineata, and Labropsis spp. Some 

 damselfishes, such as Plectrogi;lphidodon johnstonianus and 

 P dickii, have been observed to feed on coral polyps (Ran- 

 dall, 1974). 



Few Enewetak fishes feed on sfjonges. Hiatt and Stras- 

 burg (1960) recorded only Arothron mappa, a puffer, as 

 having eaten sponges. They examined, however, only one 

 species of Pomacanthidae, a group shown to contain 

 sponge-feeding species in the West Indies (Randall and 

 Hartmann, 1967). 



Among fishes there are several "cleaners" at 

 Enewetak, those species which eat ectoparasites and con- 

 sume mucus from the bodies of other, usually larger, 

 fishes. Most important are members of the wrasse genus 

 Labroides, particularly L. dimidiatus. There are also various 

 invertebrate cleaners, usually shrimps, on Enewetak reefs. 



Some fishes associate with invertebrates that are 

 avoided by predators as one method of gaining protection. 



Anemonefishes associate with sea anemones; in spite of 

 this, they are occasionally eaten by other fishes. Hiatt and 

 Strasburg (1960) found a juvenile Amphiprion melanopus 

 in Apogon noLiem/asciafus. Allen (1972a) reported that 

 disoriented Amphiprion (due to "fin-clipping" manipula- 

 tions) were sometimes eaten by groupers, particularly 

 An[^perodon leucogrammicus. Allen (1972b) described a 

 cardinalfish, Siphamia fuscolineata, sheltering among the 

 venomous spines of the crown-of-thorns starfish, 

 Acanthaster planci. Between eight and 31 fish were found 

 with each of four A. planci; however, only a small percent- 

 age of starfish had the apogonid associated with it. Species 

 of Siphamia are more often found associated with diadema- 

 tid sea urchins. 



A small group of fishes shelter among branched corals, 

 some never leaving the coral. Hiatt and Strasburg (1960) 

 illustrate some which include the gobies of Gobiodon and 

 Paragobiodon , plus the scorpaenoid genus Caracanthus. 

 There are similar invertebrate associates, particularly crabs 

 of the genus Trapezia and some alpheid shrimps. A much 

 greater number of fishes temporarily shelter in branched 

 corals when danger threatens. The hundreds of Chromis 

 caerulea. C atripectoralis. Dasc^illus reticulatus, and 

 D aruanus stationed above small heads of Pocillopora 

 corals which can take nearly instant shelter on that head 

 (Hobson and Chess, 1978) are astounding. 



Hobson and Chess (1978) have examined the feeding 

 relationship between zooplankton and planktivorous fishes 

 of the lagoon margin. At their two study areas, one 

 northeast of Jedrol within the strong influence of currents 

 in the deep channel and the second in the lee of Bokandre- 

 tok where currents are weak, they found that current pat- 

 terns sharply affected trophic relationships. The plankters 

 ingested by diurnal and nocturnal planktivores were quite 

 different. There was an abundance of suitable zooplankton 

 in strong current areas, whereas areas of weak currents 

 were poor in zooplankton. These poor areas in the lee of 

 reefs and islands were, however, rich in debris from the 

 reefs and, among diurnal planktivores, many fishes here 

 were adapted to feeding on algal fragments. Some species, 

 common in both strong and weak current areas, showed a 

 shift in diet between areas reflecting the type of food items 

 in the water column. 



Nocturnally, fish planktivores were more abundant in 

 weak current areas feeding on larger zooplankton which is 

 absent from the water column during the day. Much of this 

 zooplankton shelters on or in the substrate during the day, 

 rising into the water column at night. During the day many 

 nocturnal planktivores shelter on reefs. Horch (1973) 

 found both M\jripristis violaceus and M. pralinius common 

 in shallow water during the day, coexisting in coral caves 

 of patch reefs and reefs fringing some islands. At night 

 they left their shelters and often fed in mid-water within a 

 meter of the water's surface. 



Hobson and Chess (1978) found a clear-cut differentia- 

 tion in the distance that various planktivorous reef fishes 

 move away from reef shelter to feed in the water column. 

 On windward lagoon margin patch reefs, they found that 



