6 UNDERWATER GUIDE TO MARINE LIFE 



of our body is subject to 1 5 pounds of pressure at sea level. Yet we feel nothing. 

 The pressure outside us is balanced by pressure in our tissues. However, if 

 the pressure were exerted unevenly, there would be a startling difference. The 

 area of the palm of a hand is about 10 square inches. Very few of us could 

 move our hands if they were held down by 150 pounds of weight. At a depth 

 of 100 feet this weight would become 600 pounds. But, as many aqua-lung 

 divers know, there is little difference in swimming at 100 feet or near the surface 

 because the pressure is equal in all directions inside the body and out. This is 

 not to say, however, that dives to 100 feet are as simple as those at the surface. 



Pressure at any depth is due to the weight of water in a column above that 

 depth. As is well known, added weight on a body will tend to compress it, but 

 water is not very compressible. If water were incompressible, the surface of the 

 ocean would rise about 100 feet, but 100 feet in an average depth of two and 

 a half miles is a very small fraction. This comparative incompressibility of water 

 has the effect of keeping buoyancy about the same at all depths, so that incom- 

 pressible objects sink in the sea at an even rate all the way to the bottom. Com- 

 pressible objects, such as fishes with a swim bladder, would tend to sink faster 

 and faster with increased depth since added pressure reduces their volume 

 which, in turn, decreases buoyancy. To combat this, fishes increase the pressure 

 in their swim bladder to keep their body volume constant and to equalize 

 buoyancy at all levels where they swim. The depth to which any fish can safely 

 descend might depend in part on its ability to increase pressure in its swim 

 bladder to match water pressure. Hesse, Allee, and Schmidt (1951) state that 

 surface fishes with empty swim bladders can stand pressures of 100 atmospheres 

 without harm. 



Plankton is similar to fishes in being able to withstand rather drastic pressure 

 changes. By day, much plankton lives just at the edge of the lighted sea, at a 

 depth of 1,200 to 1,500 feet. By night, this plankton rises to the surface, a 

 change of 30 to 40 atmospheres of pressure, without harm. 



Though very little is known about pressure relations in the deep sea, one 

 thing seems fairly certain. The pressure itself plays a relatively minor part in 

 determining what forms of life live there. More important are temperature, 

 which is near the freezing point in the deep sea, lack of light, and relative 

 scarcity of oxygen and food. But pressure does have special effects to which 

 animals must adapt. The swim bladder problem is one that deep-sea fishes 

 must solve. Pressure also causes colloid gels (such as protoplasm) to take on 

 more water, and for this reason deep-sea animals are more gelatinous in texture 

 than shallow-water ones. 



Light 



Light is important to life in several ways. First and foremost, it supplies the 

 energy necessary for plant photosynthesis. Second, it is necessary for sight, 

 without which the behavior of many forms of life would be radically changed. 

 Third, it is responsible for color itself and for the adaptive coloration of animals 

 and plants. 



The nature of light and color are vitallv important concepts to grasp if we 



