BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



243 



in the animal life of the deep sea. The 

 abyssal community, with no plants other 

 than bacteria, occupies a domain of vastly 

 greater volume than that of the parent com- 

 munity in the lighted zone of the sea (Sver- 

 drup, Johnson, and Fleming, 1942). The 

 bizarre forms of deep sea creatures reflect 

 the necessities of their food-getting devices 

 as well as their diflSculties of locomotion and 

 of the finding of individuals of one sex by 

 those of the other, and are perhaps corre- 

 lated also with absence of predation pres- 

 sure such as we know in the denser popu- 

 lations of the lighted zone. 



SYMBIOSIS 



Within the loose bonds of the animal and 

 plant community and among the more 

 sharply defined associations of the compo- 

 nent biocoenoses there develop the remark- 

 able cooperative pairings of specific plant 

 with plant, plant with animal, and animal 

 with animal commonlv termed symbiosis. 

 Symbiosis is often defined to include only 

 mutually beneficial relations of such part- 

 ners. The concept of symbiosis is here 

 broadened in accordance with its literal 

 meaning to include the phenomena of com- 

 mensalism, in which the benefit relation is 

 one-sided, without injury to the host, and 

 parasitism, in which the relation is typically 

 detrimental to the host (Steinhaus, 1946). 

 This broad meaning of symbiosis is the orig- 

 inal one of De Bary (1879), and the use 

 of the term in this sense has the support of 

 the American Society of Parasitologists. The 

 term "mutualism" in our usage corresponds 

 exactly to the more limited concept of sym- 

 biosis that has been widelv current. Quito 

 evidently such relations pertain to the biotic 

 environment at an individual level. Anti- 

 biosis is the term applied to the opposite 

 relationship, of mutual antagonism (ZoBell, 

 1946), familiar, for example, in the Protista. 



COMMENSALISM 



Van Beneden (1876, p. 1) defined a 

 commensal organism as a messmate that 

 "requires from his neighbor a simple place 

 on board his vessel, and does not partake 

 of his provisions. The messmate does not 

 live at the expense of his host; all he desires 

 is a home or his friend's .superfluities." The 

 relation in commensalism is one of individ- 

 ual to individual, and the relation is essen- 

 tially unequal, active on the part of the 



commensal partner and passive on the part 

 of the host. 



So defined, the concept of commensalism 

 already diflFers considerably from the sim 

 plest implications of being messmates that 

 is, from those collections of diverse sorts of 

 animals about a common food supply. This 

 is a common, well-known type of aggrega 

 tion (see Aggregation, p. 393). In present 

 usage, commensalism has been expanded to 

 include all those ecological unions in which, 

 although both parties do not benefit, as in 

 mutualism, neither one is harmed, as in 

 parasitism, by the association. Space, sub- 

 strate, shelter, and transport relations may 

 be involved, as well as food. 



The attachment of one animal to another 

 for shelter, support, locomotion, or a food 

 supply (exclusive of feeding on the living 

 tissues of the host) may be either facultative 

 or obhgate. "Obhgate" commensahsm refers 

 on the one hand to the essentiahty of the re- 

 lation for the guest, and on the other to re- 

 lations with a taxonomically defined special 

 host. In either facultative or obligate com- 

 mensalism, one of the animals (or plants) is 

 the host, and the animal guest may be ex 

 pected to be somewhat or considerably 

 smaller. The four main ties of shelter, sup 

 port, locomotion, and food supply that re- 

 late guest to host may be single or variously 

 combined, and loose or specific. Dispersal 

 may obviously be added to this list, as an 

 extension of the usefulness to the guest of 

 the locomotion of the host. The relation 

 may be without taxonomic specialization 

 as in algae of the same species growing on 

 a turtle shell and on driftwood, or speciali- 

 zation may have developed, as is illustrated 

 by algae found only on turtle shells (Rhizo 

 clonium on Chrysemys) ; the extreme is 

 reached in the special barnacles that live 

 only on other barnacles that live only on 

 whales. 



A commensal may be quite unattached to 

 its host, living in close and direct association 

 with it; it may live upon the host's bod)! 

 or be sessile upon it; or it may live actually 

 within the body of the host, in the respira- 

 tory or alimentary tract or in any other 

 cavitv of the body open to the exterior (see 

 p. 254). Many of the organisms living in 

 the water held in pitcher plants are in com 

 mensal relations with their host (p. 232) 



The size relations of host and guest de- 

 pend somewhat on whether the host is ses- 



