DECAPOD CRUSTACEANS OF THE WEST INDIES 85 



side of a rock in tlie current or, in greater numbers, clinging to rocks 

 in a sluggish ciurent or in a quiet backwater, often no more than a 

 few inches from a swiftly flowing part of the stream. The smallest 

 juveniles (carapace lengths 2.0-2.2 mm) were often abundant along 

 the shore in the lower reaches of the Layou. On February 12, 1964, 

 vast numbers of these juveniles were observed in shallow water near 

 the shore of a small, freshwater (sometimes brackish) embayment 

 near the mouth of the Layou. For yards along the shore there was a 

 continuous, densely packed school not more than one to one and 

 one-half feet wide and seldom more than six shrimps deep. 



Xiphocaris, as noted by other observers, is a very active shrimp. 

 When pursued wdth a net, individuals quickly jump above the surface 

 of the water and swim rapidly away from the point at which they land. 

 Apparently, their most effective retreat can be made by a sudden 

 flexion of the abdomen that carries them out of the water. It soon 

 became apparent that they could be collected most readily by thrust- 

 ing a net on top of them rather than by approaching in the con- 

 ventional manner from the side or below. They are such strong 

 swimmers that they can probably negotiate low cascades in their 

 presumed upstream migrations. Even when a sudden inopportune 

 jump lands them on dry sand or among rocks above the water line, 

 they rather startle an observer by rapidly, although not very grace- 

 fully, crawling back to the water. Several erratic jumps may precede 

 the methodical crawl, and frequently the first jump carries the shrimp 

 back into the stream. When being pursued, they seem to orient 

 themselves so that the first jump takes them into the current and 

 away. 



Even the juvenile specimens in the lower reaches of the river 

 have similar escape reactions. When the observer approached the 

 extensive school in the lower Layou, mentioned above, the shrimps 

 in the shallowest water jumped an inch or so above the surface in an 

 apparent attempt to reach deeper water as quickly as possible. In 

 this way the entire segment of the school moved offshore a foot or so. 

 When the observer ceased all motion, the school moved back inshore, 

 following the contour of the water line. The orientation of the individ- 

 uals in the school seemed to be controlled by the direction of the 

 local wind currents: when the water moved toward the mouth of 

 the river, the shrimps faced upstream and remained nearly stationary 

 relative to the shore; when the current was reversed, they responded 

 accordingly, facing the river mouth, but maintaining their original 

 relative positions. While the school was being watched, a 5- or 6-inch 

 fish attacked, breaching the ranks, but the school quickly reassembled 

 after the sudden attack. A group of "tii-tri" (larval fishes) swimming 

 slightly deeper and offshore from the shrimps seemed not to affect 



