244 



ANALYSIS OF THE ENVIRONMENT 



sile or mobile, colonial or individual. It may 

 be difllcult to distinguish commensalism at 

 its nonspecific level from many of the non- 

 predaceous relations within a biocoenosis. 

 Only when the host-guest relation is recog- 

 nizably specific, i.e., a particular species (or 

 group) as guest only of a particular species 

 (or group) as host, does commensalism be- 

 come easily definable. In the support rela- 

 tion, almost any sessile animal or plant with 

 a hard shell or exterior may serve as base 

 for encrusting or other sessile animals. Coral 

 reefs, for example, afiFord support for a vast 

 assemblage of associated plants and animals, 

 only a part of which is specifically limited 

 to the coral reef community, while still 

 fewer are demonstrably limited to coral it- 

 self. Nevertheless, the support relation of 

 the coral in the community is as evident as 

 is the shelter relation of its branched por- 

 tions (cf. the coral reef, p. 456). In this 

 case the host animal proper is smaller than 

 many of its supported or sheltered guests. 

 The opposite size relation is usual, as ex- 

 emplified by the inhabitants of worm tubes 

 or the nests of various animals (meadow 

 mouse nests), in the shelter relation. In the 

 support relation the supported guest like- 

 wise is usually the smaller, as in the en- 

 crusting bryozoa and hydroids of sargassum. 



When the supporting animal is active, 

 there is evident benefit to the passive guest 

 in the avoidance of stagnation in aquatic 

 habitats. The growth of algae on the iDacks 

 of the naiads of aquatic insects or on tur- 

 tles aflFords an example of facultative com- 

 mensalism. 



Representatives of many diflFerent phyla 

 grow as epicoles (epibionts) on the shells 

 or the skin of others without becoming 

 noticeably parasitic and without contribut- 

 ing anything to the well-being of the ani- 

 mals on which they perch. A basically simi- 

 lar, though more intimate, relationship 

 exists when one organism lives within the 

 body of another without otherwise becom- 

 ing a parasite. 



The shelter relation at the facultative 

 level is presumably exemplified by the 

 hosts of micro-organisms that live most 

 of their lives within the intestinal tracts 

 of animals, feeding upon the digesting 

 food or refuse, and necessarily dispersed 

 from animal to animal by an independent 

 stage of the life cycle. Many of these bac- 

 teria and protozoans, however, become 

 either obligate commensals or become in- 



volved in the process of digestion so as tc 

 enter the category of mutualists (p. 247). 

 A whole microcommunity of plants and ani- 

 mals lives in the canal system of sponges, 

 and the intestinal fauna and flora of rumi- 

 nants and other mammals are largely non- 

 parasitic. The Pinnotheres, that lives in the 

 mantle cavity of certain sea mussels, is 

 mainly a commensal; the crab steals food 

 collected by the host moUusk, but does 

 little if any other known injury. 



Specialization of the commensal relation 

 apparently begins at the behavioral level 

 —for example, in such beetles as are known 

 primarily or exclusively from the nests of 

 meadow mice, or in the commensal insects, 

 birds, and mammals that have attached 

 themselves to the society of man. These ex- 

 hibit an often completely obligate relation, 

 without distinctive structural adaptation 

 Commensal nest beetles are well exemplified 

 by Leptinus testaceus (Park, 1929). 



Structural specialization is notable in the 

 development of means of attachment to the 

 host by the guest, as of branchiobdellids on 

 crayfishes, or of the remoras on sharks and 

 other large marine animals. The differentia- 

 tion of species of barnacles found only on 

 whales and pelagic sea turtles suggests that 

 there must be some structural modification 

 of these forms to limit them to a living 

 substrate. Such extremely specialized forms 

 may, however, be obligate only in the 

 broadest sense; i.e., the same species of 

 remora may attach to various species of 

 sharks. A more strictly obligate relation 

 however, may readily develop in such 

 forms, as is illustrated by the small remora. 

 Pheirichthys lineatiis, that attaches to bar 

 racudas and spear fishes instead of to 

 sharks. Similar direct specialization of com- 

 mensals is seen in coral-inhabiting gastro- 

 pods and in the flattened inhabitants of 

 worm tubes. 



The commensal relation grades without 

 sharp distinction into external parasitism, 

 since mammals and birds both afford a food 

 supply of epidermal scurf to forms little 

 different from ectoparasites. Even internal 

 parasitism, if the parasitic inhabitants of the 

 alimentary tract of various animals be re- 

 garded as "internal," may have one of its 

 origins in the commensal inhabitants that 

 gain access to this tube from either the 

 mouth or anal opening. Gill and lung 

 cavities may have commensal inhabitants as 

 well as parasites. The hydrachnids found on 



