252 CHEMICAL SENSES 



intensified its biting attempts, persisting for 40 s before swimming out of 

 the olfactory corridor. 



Open Sea Tests 



Further evidence of the efficacy of tertiary amine and amino acid in mix- 

 tures for eliciting feeding behavior came from open sea tests carried out 

 at the Lerner Marine Laboratory, Bimini. An underwater television camera, 

 at a depth of 65 ft (20 m), near the edge of the Gulf Stream, allowed the 

 behavior of sharks to be studied without any intrusion of observers. All 

 the sharks were wild and had never been confined. 



In these experiments, chemical stimuli flowed slowly from a perforated 

 source bottle. The fish and sharks that appeared before the camera, in the 

 vicinity of the chemical stimulus, could be compared with the fauna ob- 

 served in the same area during extended periods of study in the absence of 

 any added chemical stimulation, both before and after these experiments. 



When mixtures of tertiary amines and amino acids were first released at 

 the television study site, a sequence of fish species swam to the area, usu- 

 ally culminating in some of the larger species, such as the Nassau grouper 

 (Epinephelus striatus). Typically, all teleost fishes withdrew from the area 

 30-90 s before any sharks appeared, thus providing a convenient signal for 

 the observer monitoring the TV screen in the laboratory (Figure 10). 



Lemon sharks, nurse sharks, and sharp-nosed sharks (Rhizoprionodon 

 terraenovae) were the species most commonly attracted to the amine-amino 

 acid mixtures. Lemon sharks invariably approached by moving upcurrent, 

 whatever the direction of flow at the time of the test. Curiously, they were 

 never observed to bite at the stimulus source bottle; nor did they linger in 

 the area more than a few seconds, but always continued their swimming 

 against the current. 



Both nurse sharks and sharp-nosed sharks swam into the area from various 

 directions, but in most cases (12 out of 17 positive identifications) arrived 

 from a direction that was downcurrent at the time. Consequently, it was 

 concluded that some rheotaxic cues were used by these species, too, in 

 orienting toward a chemical stimulus. On arriving near the source bottle, 

 nurse and sharp-nosed sharks circled the bottle, occasionally settling down 

 to lie immobile with their snouts pressed against the concrete block sup- 

 porting it. There was no doubt that these species made a very precise local- 

 ization of the source of chemical stimuli. However, it was not certain, be- 

 cause of the size and the lighting of the TV image, whether jaw movements 

 occurred in these sharks when they were near the bottle. 



In an entirely different type of open sea test, mixtures of TMAO and 

 glycine were allowed to seep from a perforated source bottle that was 

 tethered in a narrow gully, cutting through a small fringing reef. The gully 

 carried currents up to 0.7 knot during tidal flow between the shallow lagoon 

 and an area seaward of the reef. The area just outside this fringing reef was 

 known to be a particularly good site for shark collecting near Bimini. Con- 

 sequently, the chemical stimuli were moving out from a well-defined olfac- 



