8 UNDERWATER GUIDE TO MARINE LIFE 



ing on water clarity and the angle of incident light. The extent of this 

 zone is not as deep near the poles as near the equator. Animals of this 

 zone frequently have large eyes in order to be able to see in weak light. 

 Animals with bioluminescence, that is, possession of organs that can 

 emit light, become common. Herbivores, or plant-eating animals, become 

 scarce or absent. In clear water, the intensity of light at 3,000 feet is one 

 three-millionth of its intensity at 3 feet. 

 3. Aphotic or Lightless Zone. Below the dysphotic zone, no light at all exists. 

 Many, if not most, animals possess luminescent organs, either for food- 

 finding or defense. Eyes may be totally lacking or reduced in size. 

 Herbivores are completely lacking, and only detritus eaters (those who 

 eat debris) or carnivores are left. 



Quality of light is also affected by water, but there are no zone names to 

 describe this. In clear waters, the long waves or reds are absorbed first and the 

 short waves or violets are absorbed last (Chapter 3). Silty or plankton-filled 

 waters reflect red and yellow and therefore appear greenish. By the use of 

 colored photographic plates exposed for long periods, it has been determined 

 that all wave lengths are still present in clear water at 300 feet, but the reds 

 are exceedingly weak. At 1,500 feet red and green are gone completely. 

 William Beebe, in his dives in the bathysphere, observed that a strangely bright, 

 blue light could be perceived by his eye up to about 1,800 feet. After that he 

 saw no light at all. Blue light not visible to the human eye persists quite a 

 distance below this depth, however. 



The colors of animals and plants vary in response to these changes in light 

 quality. It is not nearly as difficult to correlate the color changes in plants with 

 depth (Chapter 6) as it is to correlate these changes in animals. For instance, 

 it is well known that many nocturnal animals are red (squirrel fish, big-eye, 

 etc.) and that animals of the dvsphotic zone where red is lacking tend also 

 to be red (especially fishes and crustaceans). At those depths these animals 

 must appear black since there is no red left to reflect. Red animals give way 

 to brown, black, or transparent animals in the aphotic zone. So it would appear 

 that red coloration is an adaptation to dim light or to waters where red wave 

 lengths are lacking and that it acts as protective coloring for these animals of 

 nocturnal habits or dimly lit zones. Many people have wondered why so 

 many animals are red at depths where red supposedly cannot be seen. The 

 assumption is made that animals see as humans do. But many animals that live 

 in waters of low light level have the capacity to see objects that humans 

 cannot see. To these animals, red objects are protectively colored. 



Temperature 



Temperature determines to a very important extent where and how animals 

 live. Sea water varies from 27° to 108° Fahrenheit depending on latitude and 

 depth. The temperature of surface water varies the most, but deep water of 

 over 1,000 feet in depth in all latitudes is extremely uniform at about 35° 

 Fahrenheit. Temperature variation is not nearly as great anywhere in the sea 



