During daylight all crabs are in contact with the stony coral substrate of the 

 reef,- and most are located in crevices, holes, or under overhangs; i.e., in 

 relatively protected and concealed locations. In late afternoon (between 1730 hr 

 and 1900 hr) , however, most crabs emerge onto the more exposed surfaces of the 

 stony coral substrate. Shortly before dusk (judged as the time when it became 

 necessary to use artificial lights to make observations) at about 1900 hr, the 

 crabs begin to ascend various soft corals and gorgonians. The periods of tran- 

 sition is quite short and by the time it becomes dark (about 1930 hr) most have 

 left the stony coral substrate and are in contact only with the soft corals and 

 gorgonians. The crabs ascend to the upper parts of these corals, some reaching 

 a height of a meter or more above the stony coral substrate. Throughout the 

 period of darkness most crabs are found on the upper parts of the soft corals 

 and gorgonians. Promptly at dawn (0530 hr to 0600 hr) the crabs descend from 

 the soft corals and return to their daytime locations in the relatively con- 

 cealed spaces among the stony corals. 



Although individual crabs were not recognized and followed from particular day 

 to night locations, it appeared that individuals probably did not travel far 

 during their diel migrations. At night crabs were often found on soft corals 

 or gorgonians attached within one or two meters of crevices which contained 

 crabs during the daylight. It appeared likely that at least some individual 

 crabs returned to the same crevice each day and to the same soft coral each 

 night; observations at specific stony coral crevices and soft corals during 

 successive days and nights showed that locations occupied at one time were usu- 

 ally occupied at a similar time in following diel cycles. 



FOOD AND FEEDING 



Arrow crabs appear to be principally omnivorous, opportunistic scavengers. They 

 were observed feeding on bits of detrital filamentous algae, fish fecal pellets, 

 dead fish, soft edge of a tarpon scale, and the moult or remains of another 

 species of crab. When crowded together in an artificial situation, some canni- 

 balistic feeding occurs. Most of their food, however, appears to be the fila- 

 mentous algal detritus. 



To collect the detritus that forms the bulk of their diet, the crabs either 

 gather it from the substrate or pick it off their own bodies. In feeding from 

 the substrate, the crabs actively probe with their chelae. Potential food items 

 are apparently recognized by a tactile sense and are quickly grasped and trans- 

 ferred to the mouth. When gathering food from the surfaces of their own bodies, 

 crabs methodically and continually perform grooming-like movements with the 

 chelae, picking detritus from wherever it has lodged on the pubescence or spin- 

 ules that cover much of the external surface and transferring the detritis to 

 the mouth. Even the chelae themselves are cleaned, one by the other. One crab 

 that was lacking a left cheliped removed detritus from the surface of the right 

 chela with the dactyl of the first left walking leg, then removed the detritus 

 from that appendage with the chela and carried it to the mouth. 



The self -cleaning method of food gathering was the one used most by the crabs. 

 Sixty-two feeding crabs (37 females and 25 males) were observed for 2 minutes 

 each and the methods of food gathering were noted. Among the females, 12 were 

 self-cleaning only, 19 were self-cleaning and bottom-searching, and 6 were bottom- 

 searching only. Among the males, 15 were self-cleaning only, 5 were self-cleaning 

 and bottom-searching, and 5 were bottom-searching only. 



VI-218 



