52 UNDERWATER GUIDE TO MARINE LIFE 



groups— the visual feeders and the chemical feeders (mainly through sense of 

 smell)— with one system dominating consistently. 



1. Chemical Feeders. The chemical feeders, such as the mullets, rays, and 

 most sharks, among others, usually have poor eyesight, but they have a 

 well-developed olfactory sense supported by touch and taste organs, and 

 these are used to locate bottom life and carrion, and to sift organic matter 

 from the bottom. 



The olfactory system is a general "exciter," which increases searching 

 activity around the area where the scent is detected, and as the fish gets 

 nearer to the source of the scent, the paired olfactory organs direct the fish 

 to the food. Vision and the contact sense probably come into play in the 

 immediate vicinity of the food. 



2. Visual Feeders. Many species of fishes respond more to the visual stimula- 

 tion of a moving object than they do to chemical stimuli (barracuda, 

 bluefish, tuna, cod, blennies, etc.). These are the fishes that pursue their 

 prey, and vision is the dominant sense used in making contact with their 

 environment. 



These fishes, upon immediate sighting of a moving prey or preylike 

 object across their fields of vision, respond with a compulsive and direct 

 strike. This response appears only when the prey lies within the field 

 of visual acuity of a given species, an acuity which in fishes is generally 

 limited to short distances. The eyes and body move simultaneously, as if 

 sighting and striking were one movement, so powerful is the role of vision 

 in these species. The voracious bluefish is a good example and is well 

 known among fishermen for striking at any object that falls within its 

 visual field. The eyes have a rather powerful, direct and stereotyped 

 influence on behavior in the visual group, almost always producing the 

 biting response, whereas in the chemically dominated group, olfaction 

 plays the part of a general exciter and accounts for less stereotyped 

 behavior. 



The other sensory systems generally support the dominant systems, but they 

 may take the place of chemical or visual senses as the dominant sense under 

 some conditions; for instance, at night or in murky waters the lateral line 

 receptors often become the dominant sense. These exteroceptive systems in fishes 

 generally work independently with little intersensory integration. 



In the behavior of the higher vertebrates, such as some amphibians, reptiles, 

 birds, and mammals, no one sensory system dominates consistently in the way 

 that fishes' behavior is dominated by vision or olfaction. One sensory system, 

 when stimulated, brings another system into action with overall integration of 

 activities. Bringing other sensory systems into play requires more time, gives a 

 better orientation to the stimulus, and allows greater flexibility in responding, 

 enabling the animal to make a better and wider adjustment to its environment. 



