RELATIONS BETWEEN ANIMAL ORGANISMS 



523 



though 



the relationship means nothing to either 



become epizoic, 

 animal. 



564. Commensalism. — Epizoic associations merge into a type of 

 association in which one organism benefits and the other is not injured. 

 If, for instance, the accumulation of colonial hydroids upon the surface 

 of a crustacean forms a covering sufficient to conceal the crustacean, 

 which thereby secures benefit from the presence of the hydroids, the 

 relationship may be considered one of commensalism. The word com- 

 mensalism, however, means, literally, eating at the same table and was 

 originally applied with the idea that one of the organisms secured food 

 by utihzing the bits which the other dropped. It refers to other relation- 

 ships than those concerned with food. The remora, or sucking fish 

 (Fig. 364), fastens itself to the body of a shark and thus secures trans- 

 portation. Certain small fish hide among the tentacles or within the 

 bodies of coelenterates and gain security from their enemies. 



Fig. 364. — A remora, Remora remora (Linnaeus), cosmopolitan in warm seas. 



preserved specimen. X ^. 



From a 



565. Mutualism. — Such an association, however, as has been indi- 

 cated above under the name of commensalism merges into a third type 

 which may be termed mutualism and which involves association between 

 two animals of different species with benefits to each. Under this head- 

 ing come many associations which have often been called commensalism. 

 Such cases are, for instance, the association of a hermit crab and a sea 

 anemone (Fig. 365) or a sponge, either of the latter two being attached to 

 the shell which contains the former. In such a case the hermit crab 

 profits by being protected either by the nematocysts of the sea anemone 

 or by the inedibility of the sponge, and being a rather slovenly feeder it 

 allows bits of food to escape which are utilized by the associated animal. 

 Another similar association, described by Herodotus in the fifth century 

 B. c, is that between the crocodile of the Nile and a small plover-like 

 bird which enters the mouth of the reptile to pick leeches and insects of 

 different kinds from crevices in the skin and morsels of food from the 

 teeth. Mutualism not only merges into commensaHsm on the one hand, 

 but it also is rather arbitrarily distinguished from symbiosis on the other. 



