CHEMORECEPTION: LOCOMOTION AND ORIENTATION 289 



rheotaxis that would lead the organisms upstream and therefore to the 

 source of stimulation. They photographically recorded locomotor tracks 

 made by sharks kept in pens in a shallow lagoon near the Bahamian island of 

 Bimini, for periods of up to 15 min, as the animals responded to dissolved 

 chemical stimuli delivered through tubing below the water surface. Because 

 the water currents differed in various areas of the pens, the locomotor pat- 

 terns could be studied as a function of the direction and rate of water flow. 



When the chemical reached a resting nurse shark, the animal responded 

 with to-and-fro movements of the head and approached the stimulus source 

 along an S-shaped track. The authors concluded that this species locates the 

 source of the stimulus through true "gradient-searching" behavior. The re- 

 sponses of the lemon shark did not allow for as clear an interpretation of the 

 orientation mechanism involved. However, the authors felt that in this 

 species chemical stimulation merely triggers upstream orientation in the 

 strongest current, which, depending on the experimental situation, does not 

 necessarily lead the animal to the stimulus source. The authors pointed out 

 that "the use of chemical stimuli primarily to trigger a rheotaxis, as in the 

 lemon shark, makes for simpler demands upon the chemosensory receptor 

 than does any kind of gradient-searching mechanism." 



The rheotaxis-release mechanism had been proposed earlier by Kleere- 

 koper (1969), based on a laboratory analysis of locomotor behavior in 

 Mustelus, Scyliorhinus, and some teleosts (see below) and is similar to that 

 described for snails (Copeland 1918; Henschel 1932), insects (Fliigge 1934; 

 Steiner 1953; Murr-Danielczick 1930; Otto 1951), Planaria (Doflein 1926) 

 and Triton (Czeloth 1931), which respond either anemotactically or rheotac- 

 tically to attractant odors. Dijkgraaf (1975) recently reported that Scylio- 

 rhinus canicula responds to the odor of dead prey with alarm behavior 

 consisting of searching movements, the direction of which seemed de- 

 termined, in part, by the stimulus concentration. Although localization may 

 be rapid, snapping at the food source seemed dependent on tactile stimula- 

 tion and, possibly, electrolocation through the ampullae of Lorenzini, but 

 not on vision. Localization of living prey may be enhanced, at least at short 

 distances, by perception of water movement by means of the lateral-line 

 system. In one-directional water flow, rheotaxis may contribute to quick 

 orientation toward the odor source. 



STUDIES OF LOCOMOTOR BEHAVIOR AND ORIENTATION 

 MECHANISMS IN THE AUTHOR'S LABORATORY 



Even a summary review of the evidence on which analyses of orientation 

 mechanisms in fishes have rested reveals that a conspicuous characteristic of 

 general locomotor behavior in these (and probably most other) animals has 

 been neither recognized nor accounted for. This characteristic is variability 

 over time in an individual and among individuals of a species. The difficulty 

 that such variability creates in attempts to characterize and quantify loco- 

 motor responses to various experimental situations, though obvious, has 



