138 UNDERWATER GUIDE TO MARINE LIFE 



Most sharks and rays possess a hole just behind the eye. This is the spiracle 

 and is exceedingly important to' the bottom-living species for the purpose of 

 breathing. In the normal breathing pattern of fishes, water enters the mouth, 

 passes the gills where oxygen is extracted, and is expelled through the gill slits. 

 In bottom-living forms, notably rays, it is not possible for water to enter the 

 mouth for the simple reason that in so doing the animal, with its underslung 

 mouth, would gulp more sand and mud than water. Therefore, water enters 

 the spiracle into the throat and gill cavity. Pelagic and mid-water species such 

 as mackerel sharks do not need the spiracle and may lose it completely. In man 

 a vestigial spiracle is seen in the Eustachian tube, which connects the ear and 

 throat cavities. 



We can understand something of the durabilitv of the sharks and ravs over 

 the centuries by examining their senses and body form. While it is true that 

 sharks and rays possess very limited brain power, some senses are remarkably 

 developed. The sense of smell is powerful and is the chief means of finding food. 

 The lateral line, also important in finding food, is sensitive to low frequency 

 water vibration and "hears" in water in the same way, perhaps, as our ears 

 perceive air vibrations. The ampullas of Lorenzini on the snout sense hydrostatic 

 pressure and perhaps also temperature change. Thousands of tiny sensory 

 crypts on the skin act like taste buds. The ear, entirely internal, gives the animal 

 a sense of position and balance. It is no wonder that, with such equipment for 

 smelling, "hearing," and "feeling" water, sharks and rays have little need of 

 eyesight. The eyes are quite capable of defining images, but their extreme 

 nearsightedness is well known. It has been experimentally shown that a 

 dogfish will not react by sight to an object until it is only one foot away. 

 Nevertheless, sight should by no means be underestimated until more is 

 known about it. One "sense" that is poorly developed, it seems, is that of 

 response to pain. 



In body form, sharks and rays are also well adapted. They are admirably 

 streamlined and beautiful to watch under water. Though none are as fast 

 or as maneuverable as the best among the bony fishes, the advance over 

 lampreys and hagfishes is great. One shark, Carcharinus, a ground shark, was 

 clocked at 40 mph over a very short distance (Budker, 1947). The fins of typical 

 sharks are beautifully designed hydrodynamically but fairly rigid in contrast 

 to those of bony fishes and are thus used mainly as planing devices like the 

 fins on a submarine. The vertical fins, dorsals and anal, prevent yawing and 

 rolling. The paired fins, chiefly the pectorals which are moderately movable, 

 give a dynamic or changing pitch equilibrium. In other words, they are used 

 to point the shark's body up or down. The tail fin propels the shark and also 

 turns it left or right. The upper lobe of the tail fin is longer, which tends to 

 drive the head down. To counteract this, the head is flattened and tends to point 

 the body upwards. If we add up all of these factors, we see that sharks have, 

 in spite of fin rigidity, remarkable stability in all dimensions, even though 

 maneuverability and some speed are sacrificed; sharks, for instance, cannot stop 

 suddenly but can only come to a gradual halt. Stability is desired because it 

 reduces to a minimum the amount of effort needed to keep the fish going 

 in any one direction. It makes all motion extremely smooth and graceful. 

 Certainly, sharks are as graceful as any animals that swim. Stability is also 



