Commensalisrti without Continuous Contact 377 



the laboratory only if this substance is provided. To the extent that 

 the host receives benefit from such epiphytic algae, the relationship 

 approaches mutualism. 



Many kinds of microorganisms take up residence within tissues or 

 cavities of larger plants and animals without causing any trouble for 

 the owner but without paying any rent. Since no light is available 

 for photosynthesis these commensals are represented by saprophytic 

 fungi and bacteria and by Potozoa. Many such organisms are 

 found in the lower intestines of animals where they consume un- 

 digested food and secretions and complete their life cycles unnoticed 

 by their hosts. The bacteria in the human colon, notably Esclierichia 

 coli, are a familiar example. As already mentioned, these commen- 

 sals may represent a mode of life that is transitional between that of 

 the parasites and the beneficial symbionts also found in such habitats. 



Permanently fixed commensals in the animal kingdom are repre- 

 sented by sessile invertebrates that grow attached to plants or to other 

 animals. Sometimes a highly specific relation exists between the 

 epizoan and its host, although the reason for this is often hard to dis- 

 cern. The oyster-like bivalve Ostrea frons grows almost exclusively 

 on the roots of the red mangrove in the shallow waters off the coast 

 of Florida; special hooks develop on the lower shell of this animal by 

 means of which it clings tenaciously to its host and is not displaced 

 by surf. Certain barnacles are found only on the backs of whales 

 where they benefit by a free ride; since they do not feed upon the 

 whale's flesh, they are not parasites in the strict sense of the term. 

 The marine environment furnishes many other instances of attached 

 commensals (Wilson, 1951, Ch. 11). Anyone who has an opportunity 

 to catch an elderly horseshoe crab {Limuhis pohjphemus) in the 

 shallow water off the New England coast is likely to find several 

 species of mollusks, barnacles, and tube worms attached to the shell 

 and a number of more motile commensals living in the "book gills" 

 or other anatomical nooks of this strange animal. 



Commensalism without Continuous Contact. The category of 

 commensals that are in temporary contact with their hosts or that are 

 associated without being in actual contact is also a large one. Inter- 

 mittent contact between animal commensals and plant hosts is dis- 

 played on land by squirrels, monkeys, tree frogs, and snakes, and by 

 a great many birds, insects, and other animals that use trees or other 

 plants for substrata, for shelter, or for breeding sites without harming 

 the host plant significantly. Many examples of partial or complete 

 specificity in commensalism of this type will occur to the reader. An 

 intriguing three-cornered relationship is that of the elf owl that nests 



