378 Relations between Species 



only in abandoned holes made by the Gila woodpecker in the stems 

 of the large Sahuaro cactus found in Arizona and neighboring re- 

 gions. 



Temporary or intermittent contact between two animal commensals 

 is seen in such special associations as that of the remora fishes and the 

 sharks, whales, or sea turtles to which they attach. The dorsal fin of 

 this "shark sucker" has become modified in the course of evolution into 

 a most effective suction disc by means of which the fish attaches itself 

 to the under side of a shark or other large animal. The sucker can 

 release its grip at will, swim about gathering fragments of food re- 

 sulting from the shark's meal, and return to hook another ride on the 

 body of the host. Several suckers may be found attached to the 

 same shark, but they do not seem to hinder the powerful fish ap- 

 preciably. 



Commensalism involving close association between two species but 

 without the attachment of the guest to the host occurs in a wide 

 variety of animal groups. The guest may live in a burrow or other 

 retreat of the host species, and sometimes the relation is highly specific. 

 Certain beetles, for example, are known exclusively from the nests 

 of meadow mice. The burrowing owl which often nests in the bur- 

 row of a prairie dog is an accepted member of the prairie dog "village" 

 while the village is actively populated by these rodents. Many such 

 uninvited guests take up residence in the burrows or tubes of aquatic 

 animals. The decapod crustacean PoJyonx lives in the "back entry" of 

 the U-shaped tube of the marine annelid CJiaetoptertis, where it is 

 well hidden from enemies and where it can obtain particles of food as 

 well as a supply of oxygen from the water forced through the tube by 

 the pumping action of the worm's parapodia (Fig. 10.9). 



Other marine commensals live within a water cavity of their host. 

 In certain instances in which the relationship is highly specific the 

 commensal has been shown to be guided to the proper host by chem- 

 ical emanations or to be kept within the host by definite tactic re- 

 actions (Davenport, 1950). The oyster crab is a familiar example of 

 this type of commensalism and one among several of a similar nature 

 in which the commensal eventually becomes a prisoner. The oyster 

 crab is originally carried as a planktonic larva into the mantle cavity 

 of the oyster by incoming water currents produced by the feeding and 

 respiratory activity of the host. The tiny larva metamorphoses, and 

 eventually grows into an adult about a centimeter in length and too 

 large to escape through the narrow opening between the valves of 

 the oyster. The crab leads a completely sheltered life, stealing par- 

 ticles of food from the oyster, to be sure, but apparently doing its 



