126 



COLIN 



% 



1>- 



Fig. 23 Upper left: Heavily grazed rock sub- 

 strata, seaward reefs. Right: Heavily grazed dead 

 coral skeleton on seaward reefs. Lower left: Sur- 

 geonfishes, probably Acanthurus mata, feeding on 

 algal films on sediment substrata near lagoon 

 margin patch reefs. 



Other herbivores are found on the sediment surface. 

 The gastropod, Strombus luhuanus, can occasionally occur 

 in localized high densities over open sediment in water 

 2 to 10 m deep. Densities more than 10 individuals per 

 m^ with distinct (advancing?) edges to the population were 

 often seen with adjacent areas lacking S. luhuanus. High 

 densities of S. luhuanus have been found at stations where 

 only a few weeks previously the species was absent. 



Numerous species of sea cucumbers (Holothuroidea) 

 are found on sediment — sometimes near reefs, but not 

 always. They process sediment through their gut and are 

 relatively immune to predation, probably because of their 

 toxin (holothurin). Lamberson (1978) found the relatively 

 large species (up to % m in length) Thelenota anax in 

 relative abundance at Encwetak at 5 to 30 m depth. This 

 species was found on lagoon pinnacles and patch reefs, on 

 sandy bottoms near reefs, and on the vertical slope off the 

 leeward side near Biken. 



Holothurians are important sand processors of reef 

 areas. Bakus (1963) reported that Holothuria dificilus in- 

 gested sediment particles up to 2 mm in size, but about 

 80% were less than 250 microns in diameter. Holothuria 

 atra fed on even larger rubble, up to 20 mm in size. Bakus 

 (1973) indicates that beyond selection of suitable size, 

 there is little specificity among tropical holothurians for 

 sediments ingested. Hammond (1981) found that among 

 West Indian holothurians and echinoids (irregular) that sig- 

 nificant carbonate dissolution and sediment grain-size 

 modification did not occur during passage of sediment 

 through the guts of five species of tropical deposit-feeding 

 cchinoderms. A sirr' r situation probably exists for 

 Enewetak species. 



Irregular urchins are important herbivores of open 

 sandy areas. Extremely high densities (more than 50 m^^) 

 of moderately large (more than 30 to 35 mm test length) 

 species have been observed over large areas. This implies 



