The crab's anatomy provides a passive, but effective, defense against some fish. 

 The extreme length and spininess of the appendages apparently make them diffi- 

 cult to eat. When releasing live arrow crabs in the entry well of the TEKTITE 

 habitat, I noticed them attracting the small coneys ( Cephalopholis sp.) and 

 snappers would often dart in, grab the crab by a leg, and then release it, 

 allowing it to settle to the bottom without further harassment. When a crab 

 without all or most of its legs was dropped, however, it was quickly taken and 

 eaten. It appeared that the crab itself is a desirable food item, but the legs 

 provide a deterrent to potential predators. 



The arrow crabs displayed two forms of active defense, fending and retreat, dur- 

 ing my attempts to collect individuals from the reef. The way in which each 

 form was employed depended upon the stimulus to which a crab was reacting. When 

 I would reach slowly for a crab, it would typically raise one or both walking 

 legs of the first pair and direct them at my hand in an apparent effort to fend 

 it off. At the same time, the crab would slowly retreat, usually toward a more 

 protected position. The reaction of a crab to a rapid approach by my hand was 

 to retreat quickly by a series of erratic, darting movements, and, secondarily, 

 to display the fending behavior. At times the crab would retreat among the 

 spines of one or more urchins ( Diadema sp.). Whether this reaction was by 

 design or chance is unknown, but the net result was extremely effective. Crabs 

 would often react to closely approaching fishes by displaying fending behavior. 

 Larger or rapidly approaching fish would elicit a fending response from a greater 

 distance than a smaller or slowly approaching fish. 



ROLE OF ARROW CRABS IN REEF ECOLOGY 



Arrow crabs appear to fit into the overall ecology of the coral reef environment 

 principally as scavengers. They apparently do not prey upon any of the other 

 living organisms, nor do they serve as important food to the predators. The 

 crabs utilize the abundant dead organic matter present and then probably are 

 themselves consumed by other scavenging organisms of the reef. 



LITERATURE CITED 



Hartnoll, Ro G. 1969. Mating in the Brachyura. Crustaceana 16: 161-181. 



Watson, J. 1969. Biological investigations on the spider crab Chionoecetes 

 opilio . Industro Dir. Serv. , Dept. Fish. Can., Ottawa. Meeting on 

 Atlantic Crab Fishery Development, March 4-5, 1969. [Processed.] 



Williams, Austin B. 1965. Marine decapod crustaceans of the Carolinas. U.S. 

 Fish Wildl. Serv., Fish. Bull. 65: 1-298. 



VI-220 



