44 U.S. NATIONAL AIUSEUM BULLETIN 292 



40 mm in length; two atyid shrimps, Atya innocous and Xiphocaris 

 elongata; two palaemonid shrimps, Macrohrachium carcinus and M. 

 crenulatum; and a crab, Guinotia dentata. No other animals were 

 observed. There were 1 crab, 2 M. carcinus, 12-15 A. innocous, about 

 the same number of M. crenulatum, 20-25 Xiphocaris elongata, and 

 approximately the same number of fish. 



When the pool was first approached by observers, the fish and 

 X. elongata were "resting" on the stones at the sides and bottom 

 of the pool, only swimming occasionally for a centimeter or so to take 

 another position on a rock. The crab was not in sight, and only the 

 chelae of the smaller of the two M. carcinus could be seen protruding 

 from beneath one of the larger rocks. In contrast, the individuals of 

 A. innocous and the smaller M. crenulatum were scurrying back and 

 forth across the bottom of the pool, disappearing beneath a stone 

 and reappearing shortly thereafter. Atya innocous was by far the most 

 active. Of the three or four large individuals of M. crenulatum, two 

 were in view but were not moving about, and the others were concealed 

 somewhere among the stones. Except for the almost incessant wander- 

 ings of A. innocous and the smaller M. crenulatum, there was httle 

 activity. 



When an earthworm, suspended on a string, was gently lowered 

 into the water, however, a chain reaction was initiated that set the 

 entire population of the pool in motion. The smaller individuals of 

 M. crenulatum were the first to show an awareness of the presence 

 of the worm. Although apparently they did not see it, the rate of 

 their random walking increased tremendously, perhaps best described 

 as "frantic," causing them to collide with one another and with the 

 other inhabitants; A. innocous jomed them, the larger M. crenulatum 

 began moving back and forth, and X. elongata left the stones on 

 which they had been comparatively still and swam about the pool. 

 The frenzied motion disturbed the fish and they, too, began to shift 

 their positions on the rocks. When one of the smaller M. crenulatum 

 finally located the position of the worm, the shrimp left the bottom, 

 swam to the worm, grasped it with its chelae, and attempted to swim 

 away with the worm. When this attempt failed, the shrimp used its 

 abdomen to give a rapid series of strong tugs. This motion apparently 

 attracted the attention of almost all of the shrimps in the pool. One 

 of the larger M. crenulatum swam toward the worm as the smaller 

 ones backed away, and when it had stripped the worm from the 

 string, the shrimp sank to the bottom of the pool and scurried for 

 the nearest cover. By this time, both individuals of M. carcinus had 

 moved into the open water of the pool, and as the larger one moved 

 about, all of the other shrimps retreated from its path, remaining 

 beyond reach of the large chelipeds. While the commotion was going 

 on, the crab slowly crawled from under the largest rock at the side 



