638 



COLLEGE ZOOLOGY 



which to retreat for protection. The number 

 and variety of these is indicated by the re- 

 sults of an examination of 12 hving bath 

 sponges reported by Pearse from the Dry 

 Tortugas Islands near Florida. These 12 

 sponges, which ranged from 2 to 8 inches 

 in diameter, contained 683 other animals 

 belonging to 15 or more species, including 

 annelids, brittle stars, crustaceans, and bi- 

 valves. A somewhat similar association is 

 that of various types of animals that live in 

 the nests of social insects. For example, 

 termite nests are often inhabited bv other 

 species of insects; these are called termito- 

 philes. Over 100 species of termitophiles 

 have been recorded, including beetles, 

 aphids, ants, millipedes, and isopod crusta- 

 ceans. Ants and bees likewise serve as hosts 

 for many other species of animals. Certain 

 sessile animals attach themselves to free- 

 swimming species; for example, barnacles to 

 whales and protozoans to tadpoles, planar- 

 ians, and snails. 



Commensals may be internal as well as 

 external. Most protozoans that live in the 

 human digestive tract are of the commensal 

 type. They do us neither harm nor good, but 

 use our digestive tracts as a residence and 

 may feed on waste fecal material as some 

 of the amoebas do. 



Mutualism 



Mutualism is an association in which both 

 species derive benefit; in other words, the 

 relationship is mutually helpful, hence the 

 descriptive term "mutualism." One of the 

 best examples is that of termites and their 

 intestinal flagellates (Fig. 447). The ter- 

 mites eat wood but cannot digest cellulose; 

 but the flagellates in the termite intestine 

 can, so food is available to both. 



Without the flagellates the termites would 

 starve to death in the midst of plenty, as has 

 been proved by depriving termites of their 

 flagellates. The flagellates receive the follow- 

 ing benefits from this mutualistic relation- 

 ship: (1) a constant food supply, (2) 

 protection from enemies, and (3) a stable 



environment. It has been demonstrated that 

 these flagellates are unable to exist anywhere 

 except in the termite's intestine. 



A number of protozoans and coelenterates 

 live in mutualistic symbiosis with algae, 

 which are known as zoochlorellae. Paramec- 

 ium bursaria, for example, is colored green 

 by algae that live in its endoplasm. The 

 zoochlorellae use the waste products of 

 metabolism, including carbon dioxide, ni- 

 trogenous and phosphorous substances, and 

 supply the paramecia with food and oxygen. 

 Both the paramecia and algae can live alone, 

 but they are mutually beneficial while living 

 together. Protozoa may also contain brown 

 or yellow unicellular organisms called 

 zooxanthellae with which they are mutual- 

 istic. The green hydra owes its color to 

 zoochlorellae that live in the endoderm. 

 Brown and yellow zooxanthellae also live 

 mutualistically in coelenterates. 



Less intimate cases of mutualism occur 

 between crabs, sponges, and coelenterates. 

 Crabs may place sponges on their shells with 

 the result that the sponge is transported 

 from place to place, which may be of ad- 

 vantage to it; the crab is concealed from its 

 enemies and prey, and if detected, is not at- 

 tacked because of the disagreeable qualities 

 of the sponge. Hermit crabs that live in snail 

 shells often place hydroid coelenterates or 

 sea anemones on top of the shell; the 

 nematocysts of the coelenterates drive away 

 enemies, and the coelenterates are carried 

 about and probably obtain some of the food 

 captured by the crab. The shore crab of the 

 Indian Ocean is said to carry a sea anemone 

 in each pincer, which it thrusts into the 

 face of an approaching enemy and drives it 

 away with the nematocysts. 



Other interesting examples of mutualism 

 occur among insects and vertebrates. For 

 example, ants protect aphids, and in return 

 "milk" honeydew from them, which they use 

 as food. A fish lives among the tentacles of 

 a jellyfish (Fig. 59) which give protection 

 by their stinging cells. On the other hand 

 the jellyfish benefits, for the small fish acts 

 as a lure, attracting larger fish within the 



