380 Relations between Species 



shelter within the cloacal cavity of a sea cucumber ( Holothuroidea ) . 

 The fish occasionally emerges to feed in the neighborhood. When it 

 wishes to reenter its strange retreat, it pokes its nose against the open- 

 ing of the cucumber's cloaca, then quickly reverses its position, and 

 allows itself to be drawn tail first inside its host. 



Some species appear to derive protection by acquiring a big brother, 

 that is, by associating themselves with other species recognized in the 

 community as being voracious or poisonous. Pilot fish {Naiicrates 

 ductor) follow along beneath sharks as closely as shadows, but can- 

 not attach to the surface of their hosts since they have no suction 

 discs as do the remoras. For some reason they are never eaten by 

 the sharks. In similar fashion particular kinds of fish find shelter 

 under the umbrella of poisonous jellyfish; one species somehow man- 

 ages to live unharmed among the tentacles of the Portuguese man- 

 of-war, whose stinging cells instantly paralyze other kinds of fish, 

 which are used for food. Another commensal fish has acquired a 

 type of swimming motion that allows it to move among the tentacles 

 of large sea anemones without causing the discharge of the poisonous 

 nematocysts. 



Other instances of the protection gained by the weak through as- 

 sociation with the strong are furnished by scavengers and mimics, al- 

 though their modes of life should perhaps not be termed commensal- 

 ism because of the absence of toleration or even awareness. In the 

 tropics the hyena lives as a scavenger on leavings from the lion's meal, 

 which consists of animals that the hyena could not possibly have 

 killed for itself. Similarly, in the far north during the winter the 

 arctic fox lives largely upon the remains of seals killed by polar bears. 

 Another type of protection is attained by weaker species through 

 mimicking in appearance and behavior certain species recognized as 

 voracious or poisonous by other members of the community, as de- 

 scribed in Chapter 6. The mimics, as well as the truly dangerous 

 models, are presumably avoided by the predators. 



The foregoing account of symbiosis is sufficient to indicate the 

 widely separated types of animals and plants that may be associated 

 in relations of advantage or of toleration. We have also reviewed 

 the great range that exists in the degree of dependence and of specific- 

 ity in these relationships. However, many fascinating questions 

 await future study as to the exact nature of the behavior patterns that 

 bring the symbionts together and of the physiological interdepend- 

 encies between them. The examples described show how these asso- 

 ciations may have arisen from casual contacts between species, which 

 then led to a recognizable commensalism. Subsequently the relation 



