316 



CHEMICAL SENSES 



suggested a role for mechanoreception, probably through the lateral-line 

 system. 



It is of particular interest that the nurse shark's locomotor velocity is not 

 only affected by the proximity of a wall but that this effect is different in 

 flowing and stagnant conditions. In flowing water, the ability to perceive the 

 walls seems enhanced (Gruber 1976). This supports the assumption that the 

 lateral-line system mediates the remote perception of barriers, in this case 

 the tank's walls. In three nurse sharks, flow conditions affected also the step 

 length, which increased significantly in stagnant water, based on data col- 

 lected during experimental periods of longer than 20 h each. As to the angle 

 size of turns, individual sharks displayed considerable variations with respect 

 to this variable, and no generalization could be made. With respect to other 

 locomotor variables, the effects of flow are summarized for both species of 

 sharks in Table 2. 



Maynard (1976) monitored the locomotor patterns of single lemon sharks 

 first in uniformly flowing water (1.74 cm/s), in monitor II (square matrix of 

 photocells), for 24 h. Next the flow pattern was arranged to have flow only 

 in the central one-third "channel" of the tank. In three successive experi- 

 ments the flow rate in that channel was adjusted to 1.75, 0.80, and 0.40 

 cm/s (Figure 32). The experiments were separated by periods during which 

 there was no flow at all. 



Typical responses to these conditions are illustrated in the tracings of 

 Figures 33, 34, and 35. The animal restricted its locomotor pattern to the 

 "channel," even at the lowest flow rate tested (0.40 cm/s), but the effect 

 was maximal at the highest rate of flow. It required a few minutes to 



I. UNIFORM FLOW 



2. NO FLOW 



3. CHANNEL- 1.75 cm/s 



4. NO FLOW 



5. CHANNEL- 0.80 cm/s 



6. NO FLOW 



7. CHANNEL-0.40 cm/s 



Figure 32 Schematic of experimental flow 

 conditions in monitor tank II. From Maynard 

 (1976). 



