BIOLOGY OF THE SEA-HOW THEY LIVE 39 



crabs become clothed in a sponge, which gives the crab a home and the sponge 

 transport. These are conjunctive examples. Disjunctive examples are found 

 among butterfly fishes, which relieve large predators such as snappers of throat 

 and body parasites with satisfaction to both parties. 



Parasitism 



In parasitism, which will not concern us very much in this book because most 

 parasites live within the bodies of other animals and cannot be seen, the relation- 

 ship is either conjunctive or disjunctive and is always antagonistically nutritive. 

 One of the svmbionts is small and benefits by partaking of the tissues of a 

 larger host. The host is always harmed to a greater or lesser degree. Parasitism is 

 derived from a predator-prey relationship, in which the parasite has become 

 adapted to feed on only one host (see lampreys. Chapter 8). In any case, the 

 ideal parasitic relationship is not one in. which the parasite kills its host, for in 

 so doing it destrovs its own home and its source of food. 



Parasites include various groups of worms, crustaceans, and protozoans among 

 others. Manv of the phyla of the animal kingdom have parasitic members. 



Certain worms and the remarkable deep-sea angler fish, Photocoryniis, have 

 males which are parasitic on the females. The male angler, for instance, attaches 

 himself at an earlv stage of growth to the body of the female. He is a true parasite, 

 receiving all food from the female, and degenerating to a small animal con- 

 taining reproductive organs only. 



ADAPTIVE COLORATION 



Whether the colors of an animal are changeable or not, the colors it exhibits 

 at anv one time are verv often adaptive in nature, that is, are of some benefit 

 to the animal in its environment. Color adaptation can take several forms- 

 concealment or warning, for instance— but in every case it is important to 

 view the colors of animals with careful reference to the animal's environment, 

 its behavior there, and the characteristics of the visual receptors of other 

 members of its community. Otherwise, colors lose their meaning. Furthermore, 

 it is necessary to realize that not all colors and color patterns are adaptive. Some 

 colors have no adaptive significance. Thus, the tasks that are faced when 

 studying colors are, first, to determine if the coloration is adaptive at all and, 

 second, to determine what purpose the adaption serves. Sometimes the adaptions 

 are obscure, and the diver must be very careful in his interpretation. 



Animals change color with response either to environment or to stimuli 

 associated with courtship or fear. The colors of fighting sailfish become notice- 

 ably more intense; the octopus shows a nearly white color phase when it is 

 frightened or alarmed; blennies and gobies change color with sexual excitement 

 during breeding. 



Adaptive coloration has been reviewed by Cott (1940) in a brilliant book 

 on the subject. Cott begins his book by pointing out the very important fact 

 that vision rests upon a three-fold basis. First, the physics of light is responsible 

 for the intensity and quality of rays that emit from an object. Second, the 

 physiology of the eye and nervous system gives a picture of that object to the 



