CHAPTER 14 



ASSOCIATIONS 



The shell-fish called Nacre (the Pinna) lives in . . . fellowship with the 

 pinna-guardian, a little creature of the crab family that acts as porter 

 and doorkeeper, sitting at the mouth of the shell, which it continually 

 keeps half open. 



Montaigne {trans. Trechmann) 



COMMENSALISM, PARASITISM AND SYMBIOSIS 



Associations between different species of marine organisms are infinite 

 in variety and degree of complexity, and are testimony of the pressure which 

 has driven animals into mutual or unilateral dependence. It is customary to 

 divide animal associations into the three broad categories of commensal- 

 ism, symbiosis and parasitism. Commensalism, broadly speaking, is an 

 external relationship between two species which live together in some degree 

 of harmony. The salient feature of this relationship is that neither partner 

 preys or adversely lives upon the other. Moreover, the associates are not 

 greatly modified structurally and the association is frequently facultative. 

 In symbiosis there is a close physiological association between two species, 

 often for mutual benefit. Parasitism, of course, is a unilateral association, 

 in which one member preys insidiously or lives upon the substance and 

 digested aliments of the other, without immediately destroying it. Usually 

 the parasite is profoundly modified for its specialized existence in a direc- 

 tion which can be recognized as morphological degeneration. 



Although many varieties of animal associations can be fitted into these 

 categories, there are innumerable transitional cases, associations in which 

 the balance is weighted more on one side than the other, and commensal 

 and symbiotic relationships that verge more towards parasitism than 

 mutual benefit. Where there is such an infinite variety of material to study 

 and unravel, it is perhaps only natural to encounter seriated examples 

 which display all degrees of evolution in the field under consideration. 

 Indeed, Caullery has emphasized that these various categories are merely 

 useful labels for modes of relationships, centres of scatter arbitrarily 

 chosen for our convenience. Various aspects of symbiosis in marine 

 organisms have been reviewed by Buchner (12) and Yonge (109, 110). 

 General accounts dealing with interspecific associations have been pre- 

 pared by Flattely and Walton (26), Borradaile (9) and Caullery (15). 



Plant-Animal Associations 



There are many examples of relationships between plants and animals 

 which range from mere fortuitous coexistence to deliberately selected 



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