50 UNDERWATER GUIDE TO MARINE LIFE 



head and each eye is able to move independently. The eyes are able to focus 

 only on near objects. Distance focusing is limited. Fishes have a wide-angle field 

 of vision. Careful experiments have shown fishes to have color vision, but it 

 seems weak. Verrier (1928) found that the structure of the retina does not 

 permit good visual acuity. Acuity seems to be similar to the peripheral regions 

 of the human eye. It allows good vision of movements of objects, but perception 

 of form seems defective. The eyes are used by all fishes to some degree except 

 for a few nocturnal species. 



OLFACTORY 



Ciliated cells in sacs, which have an opening to the exterior nostrils are able 

 to detect the presence of weak concentrations of chemicals in the water 

 Cfig- 13). These receptors are used for locating food at a distance as well as for 

 detecting chemical gradients in water. Smell is used to some degree bv about 

 half of all fishes. 



TASTE 



There are many grouped cells with hairlike projections in and around the 

 mouth, on barbels (when present), on growths which hang from the lips, or 

 often covering the body surface. These receptors detect chemicals emanating 

 from food close to the body. The contact taste receptors in the mouth cavitv 

 are used for the selection of food and rejection of foreign objects. Some fishes, 

 mainly those that feed by sight, swallow objects without much attention to 

 selection, and often strange objects are found in their stomachs. Taste seems 

 to function in all fishes. 



THE LATERAL LINE 



Lateral line receptors are composed of ciliated cells clustered at intervals 

 along a lateral line on the sides of the body from head to tail and on some 

 fishes branching on the head (fig. 13^. These receptors are sensitive to the 

 intermediate vibrations between those perceived by the ear and those perceived 

 by the body surface. While the auditory sense can detect vibrations of high 

 frequency, lateral line receptors can detect only low frequency water movements. 

 Skin contact receptors are sensitive only to solids, water pressure, and very 

 slow vibrations. 



Lateral line receptors are used to locate prey, to avoid predators, and to 

 localize obstructions by detecting direct and reflected water movements. In some 

 fishes at night, these organs become the dominant sensory receptors. They are 

 also sensitive to relative water pressure as well as to direction and rate of flow 

 of water. 



ALIDITORY 



The auditory sense in fishes is poorly developed. However, there is evidence 

 that some fishes can hear sound frequencies of up to 8,000 cycles per second 

 or more. Some fishes are not able to hear at all. The ears of fishes are not visible 

 on the body surface and are used mainly for equilibrium and orientation. It is 

 possible that some fishes use vibrations reflected from the surface, the shore, 

 and the bottom as aids to orientation. 



