interface. In the areas of extensive reefs, the crabs were almost invariably 

 in direct contact with either the stony corals (Order Scleractinia) or the soft 

 corals (Order Alcyonacea) and gorgonians (Order Gorgonacea) of the reef. In a 

 few instances crabs were found on the sand substrate immediately over the over- 

 hanging corals at the reef edge. Even in these instances, however, the crabs 

 usually maintained contact with the coral substrate with their posterior walking 

 legs. Crabs associated with the small patch- reefs were generally on the sand 

 substrate under the overhanging corals, but also were in direct contact with the 

 coral . 



Arrow crabs were also found on the extensive areas of sand substrate. Here, 

 however, they were almost invariably associated with relatively prominent, 

 stable objects such as sponges, anemones, and small "boulders" of dead coral. 

 Among 19 crabs observed in the sand substrate area, 8 were associated with 

 sponges, 7 with small "boulders" of dead coral, and 3 with anemones. Only one 

 crab found on the sand was not associated with any prominent object. Most crabs 

 seen on the sand areas were equivalent in size to the smaller crabs found in the 

 coral reef areas (carapace lengths of approximately 3 mm to 6 mm) . The only 

 medium and large individuals (carapace lengths estimated greater than 6 mm) seen 

 in the sand area were associated with the largest object--a "boulder" of dead 

 coral approximately 1.0 m by 0.3 m in size. 



The TEKTITE habitat was placed on an area of sand substrate and crabs had col- 

 onized the crevice between the base of the habitat and the sand during the 5 

 months the habitat was in place. 



POPULATION DENSITY 



The population density of arrow crabs was measured along two transects within 

 approximately 10 meters of the TEKTITE habitat. Both transects were located 

 along the edge of large reef areas; transect A was along the reef face immedi- 

 ately west of the TEKTITE structure, transect B was along the reef face south- 

 east of the structure. Each transect was 10 meters long and 1 meter wide from 

 the edge of the reef inward on the reef. Total counts of crabs within each of 

 these two areas were made 9 times, including 4 counts during daylight, 2 counts 

 during darkness, 2 counts at dawn, and 1 count at dusk (Table 1). Counts during 

 night and twilight periods were made with a hand-held diver's light. 



Table 1. --Numbers of arrow crabs counted along two transects 

 at various times of day. 



Number of crabs 



Count Time of 

 Date number day (hour) 



August 1 1 Daylight (1800) 



August 1 2 Dusk (1900) 



August 1 . 3 Night (2200) 



August 2 4 Night (0500) 



August 2 5 Dawn (0600) 



August 2 6 Daylight (0700) 



August 2 7 Daylight (1200) 



August 2 8 Daylight (1400) 



August 3 9 Dawn (0530) 



VI-21/i 



