The feeding activity of the arrow crabs varied strongly within each diel cycle. 

 Feeding activity was measured by recording the nvimber of chela-to-mouth move- 

 ments of an individual crab during a 2-minute period. Observation periods at 

 different times of the day included measurements for six to eight crabs each. 

 Summarized results of these observations (Table 3) show that the crabs feed 

 mostly during the daylight period, with a peak of feeding activity late in the 

 afternoon. Between dusk and dawn, feeding activity is at a very low level. 



Table 3. --Summary of feeding activity of arrow crabs at various times 

 throughout the diel cycle. 



Time 

 of day (hour) 



Number of crabs 

 observed 



Number of chela-to-mouth movements 



per crab per 2-minute period 

 Average Range 



Dark (0100) 

 Dawn (0600) 

 Daylight (0700) 

 Daylight (0815) 

 Daylight (1000) 

 Daylight (1345) 

 Daylight (1600) 

 Daylight (1645) 

 Daylight (1730) 

 Daylight (1800) 

 Dusk (1900) 



0.7 



2.3 



13.7 



9.2 



7.2 



9.0 



6.8 



19.7 



17.7 



12.4 



3.8 



1- 3 

 0-21 

 1-27 

 1-15 

 1-13 

 1-18 

 1-18 

 6-39 

 11-31 

 4-18 

 0-10 



The relative lack of feeding activity at night and the occurrence of a nightly 

 migration up onto the soft corals leads to speculation on the possible reason 

 for the diel movements. When the crabs do feed heavily during the day, they are 

 largely dependent upon the detritus clinging to their bodies. Perhaps, there- 

 fore, it is necessary for them to expose themselves to conditions in which they 

 can accumulate large amounts of detritus during the night. Possibly this is 

 accomplished by the crab's remaining poised high above the hard substrate 

 throughout the night. At that level water currents will be somewhat greater 

 than at the hard substrate level and the amount of drifting detritus passing 

 any given point may likewise be greater, allowing a crab to accumulate more 

 detritus than it could at a lower level. A food gathering sequence of this sort 

 could also explain the relatively high abundance of arrow crabs near the TEKTITE 

 habitat. The increased water turbulence and turbidity caused by concentrated 

 diver activities in the area may simply make more detrital food available to the 

 crabs during their nighttime collection periods. 



If this proposed mechanism of food gathering is correct, arrow crabs display a 

 unique feeding method among marine invertebrates. In effect, they would be 

 filter feeders, filtering drifting detritus from the water with the entire sur- 

 face of their bodies and later gathering and concentrating the detritis for 

 ingestion. 



DEFENSE MECHANISMS AND PREDATION 



No instance of natural predation on arrow crabs was observed during this study. 

 The crabs have several well developed passive and active defense mechanisms which 

 probably protect them from most potential predators of the reef environment. 



VI- 2 10 



