BIOLOGY OF THE SEA-HOW THEY LIVE 51 



THE ROLE OF SENSE RECEPTORS IN THE BEHAVIOR OF FISHES 



When the environment of fishes is compared with that of land animals 

 it is found that the sea is much less variable than land. The chemical content 

 and temperature of the sea vary little, food is easier to get, and there is no 

 weather as we know it on land (except near shores). This stability of environ- 

 ment requires a smaller repertory of adjustable activities, which accounts for 

 the lack in sea animals (except for the re-entrants) of the advanced psychological 

 organization that is found in the higher land animals. 



In spite of the fact that fishes live in a comparatively constant environment, 

 there are a great many types of habitats in the sea— coral reefs, seashores, open 

 sea, and the great depths. Physiological factors such as weak vision, a good 

 olfactory sense, possession of a swim bladder, and well-developed action equip- 

 ment determine in which of these environments a species will live and a 

 species' environment determines to a great extent its type of behavior. 



In the environment of a particular species, the pattern of behavior is to a 

 certain degree determined by the sense receptors it possesses. This close relation- 

 ship between sense receptors and characteristic behavior is clearly evident in 

 the feeding habits of the rays. These animals have weak eyesight and a well- 

 developed sense of smell and touch; they are for the most part bottom-feeding 

 animals (except the devil fish and the manta), which use their smell and touch 

 organs to locate crustaceans and molluscs in the sand. The jacks are an example 

 of fishes with excellent eyesight, which obtain their food by darting at or 

 hunting down small schooling fishes at the surface. 



The important place of sensory equipment in the activities of fishes is pointed 

 out by Jarmer (1928), whose studies show that when a fish is exposed to a visual 

 or olfactory stimulus (depending upon which is its main operating sense), it 

 automatically bites. This is characteristic behavior of fishes in general, resulting 

 from the possession of a primitive type of vertebrate nervous system. In the 

 higher vertebrates, as in the mammals, when one sensory system makes contact 

 with a prey, the animal becomes alerted and other sensory systems are brought 

 into play for a better orientation before striking. These animals have the ability 

 to delay their responses to a stimulus because of their advanced nervous system, 

 whereas in fishes the stimulus brings on a direct and automatic response. An 

 example of this type of behavior is given in the case of the diver who dove into 

 the water within a few feet of a barracuda, a visual feeder. The fish turned 

 instantly and struck the swimmer at the moment of the splash. Approached 

 underwater, the same barracuda would probably do no more than show 

 curiosity. The diver's splash and movement resembled the splash and movement 

 of the prey of this fish, that is, he resembled the characteristic visual feeding 

 stimulus, the splashing at the surface of a wounded fish or of schooling fish. 



The stereotyped visual feeding behavior of the barracuda was demonstrated 

 when the authors placed freshly killed fish on the bottom near a barracuda, 

 resulting in no feeding response. But moments later when a moving, wounded 

 fish was presented, the barracuda swiftly struck. A shark with weak eyes and a 

 strong sense of smell, on the other hand, would have reacted readily to the smell 

 of the dead fish on the bottom and also to the wounded, moving fish. Conse- 

 quently, an excellent method for understanding fishes' behavior is to group 

 them according to the way they secure their food. They generally fall into two 



