surface in the coral reef environment. 



Lateral movement of sand: To determine the rate at which or- 

 ganisms could move sand laterally at the sediment surface, a 

 plastic sheet, 35 x 80 cm in size, was staked upon the sand 

 surface in the channel mouth and left for 6 days. Sand accu- 

 mulated more or less evenly over the sheet; it was collected, 

 dried and weighed. Texturally this sand was poorly sorted, 

 the particles ranging in size from silt to granules. A total 

 fo 212 grams accumulated on the sheet during the 6-day period; 

 the daily rate of lateral movement to produce this accumulation 

 averages 126 grams per square meter. If this rate of lateral 

 transport obtains along all the reef front, the amount of sand 

 shifted by organisms is considerable. In a 10-m-wide strip 

 bordering the 500 m of reef front in the reefs near the habitat, 

 225,000 kg of sand is shifted yearly by organisms. 



EFFECTS OF SPECIFIC ORGANISMS: 



The time spent in the water during the Tektite program permitted 

 observation of the effect on the sediment of a number of specific 

 organisms. These organisms could be classed as exofauna if they 

 live in the water or above the sediment, endofauna if they 

 actually live within the sediment. 



The exofauna can be subdivided by their activity; some mix the 

 sediment, whereas others excavate holes in it. The mixing or- 

 ganisms include both vertebrates and invertebrates. Most conspi- 

 cuous among the fish that mix the sediment are the mullidae 



(goatfishes) , commonly seen in schools, stirring up clouds of 

 suspended material as they work the sediment with long chin barbels 

 in their search for food. Other fish, such as the synodontidae 



(lizard fishes) stir the sand by partly burying themselves within 

 it. Invertebrate exofauna also can greatly disturb the sand. 

 Large gastropods, in particular Strombus gigas , leave a prominent 

 track where they cross the sand. Hermit crabs occupying large 

 gastropod shells greatly stir the sediment to a depth of several 

 centimeters as they feed, backing across the sand and churning it 

 with their claws . 



Holes are excavated either in a search for food or in construction 

 of a burrow. The largest holes are dug by rays, which, by pumping 

 their wings, generate currents that create broad shallow excavations, 

 generally a 1 to 1.5 meters across and 10 to 15 cm deep. Within a 

 few minutes a ray less than a meter across dug with its nose a hole 

 about 40 cm across and 13 cm deep, the steepest measured. Commonly 

 the excavations made by rays show a slightly positive rim. 



VI-31 



