CHAPTER 2 



COMMENSALISM 



And thou shalt eat bread at my table continually. 



II Samuel 9 : 7 



IF TWO animals live in close and harmless association with one another, 

 from which circumstances only one partner derives benefit, the 

 relationship is known as commensalism. Although widespread through- 

 out the animal kingdom, this somewhat mysterious phenomenon is 

 rarer than parasitism and not so easily recognised. If both associated 

 partners receive an advantage the relationship is known as symbiosis. 

 The most usual form of commensaHsm is one in which the smaller 

 and weaker of the two animals steals a Httle food from the larger 

 animal without causing it any appreciable loss or inconvenience. Thus, 

 a small species of muscid fly accompanies large blood-sucking clegs 

 (Tabanidae) and laps up a httle of the blood which may be left oozing 

 from the wound on the victim's skin. Although strictly speaking the 

 term commensal should be apphed to mess-mates only, in many such 

 associations the benefit received is one of shelter and indirect protection 

 from enemies rather than nourishment, while in others both these 

 advantages are conferred simultaneously. A minute worm, for example, 

 lives relatively safely in the groove on the arms of certain starfish and 

 when the host is feeding it wriggles up to the vicinity of its mouth and 

 surreptitiously catches a few stray morsels. In this way it obtains free 

 board and lodging. A more famous association exists between the 

 remora, or sucking-fish and whales and sharks. The remoras attach 

 themselves to the undersides of their giant companions by means of an 

 adhesive disc on their heads and thus obtain transport, protection and 

 any superfluous food accidentally dropped or discarded in their 

 vicinity. They also enter the gill cavities and mouths of large bony 



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