ASSOCIATIONS 583 



associations involving some degree of mutual benefit. In shallow coastal 

 waters algal spores settle everywhere and attempt to colonize all available 

 surfaces. Consequently, it is not surprising to find plants growing on the 

 backs of many sluggish littoral and sublittoral animals such as limpets, 

 periwinkles and crabs. Such chance associations are seasonal or are termi- 

 nated when the animal moults. The algae may occasionally have concealing 

 value to their possessor, and certain spider crabs such as Inachus and Hyas 

 have adopted the habit of masking themselves by cementing pieces of sea- 

 weed, hydroids and sponges to the spines distributed over their bodies. 

 The decorator crab Podochela hemphilli not only covers itself with bits of 

 weed, but also performs bowing movements to simulate the waving motion 

 of seaweed in moving water. Other forms of plant-animal associations are 

 encountered in gall-formation among fucoids. The nematode Halenchus 

 fucicola invades the tissues of the seaweed Ascophyllum nodosum, and 

 gives rise to warty patches or galls (166, 35). 



COMMENSALISM 



Commensalism between animals involves extremely diverse relationships 

 which are very difficult to classify in any logical system. A large part of this 

 difficulty is simply due to our ignorance of the exact relations existing 

 between the associated animals. There are many instances of animals, 

 termed epizoites, living attached to other species. This kind of association 

 may be rather fortuitous, or may assume a stable character which passes 

 into true commensalism. The commensal relationship may be entirely an 

 external one. In addition, many examples are known of another fairly well 

 demarcated category in which one animal, the commensal proper, has 

 taken up its abode in the tube or dwelling of another species, the host. 

 This relationship is termed endoecism. There is a third category, desig- 

 nated inquilinism, which includes those commensals accustomed to living 

 in the body cavities or internal chambers of their hosts. In some of these 

 associations one partner benefits by obtaining access to new food supplies, 

 by sharing its host's refuge, by taking advantage of repellent properties 

 of the host, etc. Reciprocal benefit, termed mutualism, is realized in certain 

 of these partnerships; frequently, however, they are one-sided, with benefit 

 to only one participant. From unilateral relationships and inquilinism 

 there are various transitions to parasitism. 



Epizoites 



Fortuitous associations between animals are often encountered in the 

 littoral zone, where so many sedentary species settle on all free surfaces. 

 For example, whelks, lobsters and crabs are often seen bearing epizoitic 

 populations of serpulids, barnacles and other sedentary species. Epizoites 

 sometimes display definite preference for particular species, e.g. the bryo- 

 zoan Loxosomella phascolosomata which forms small colonies on Golfingia 

 vulgaris. Many instances have been recorded of hydroids living attached 

 to the skin of fish. Simple commensal species merely use the fish as a sub- 



