BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



245 



the gills of fresh-water bivalves do not seem 

 to feed upon the tissues of their host; at 

 most they derive nourishment from the mu- 

 cous secretions (Welsh, 1930). 



The transition to obligate commensalism 

 from facultative is illustrated in sessile ani- 

 mals that depend for support on the encrus- 

 tation of solid or hard objects and avoid 

 soft-bodied animals. This is notably evident 

 in the barnacles, which attach at the close 

 of their free-Hving larval stage primarily to 

 nonliving soUd objects, as well as to corals, 

 crustacean shells, and the like, and not to 

 other types of marine creatures, such as sea 

 anemones, echinoderms, or fishes. Whatever 

 the barrier to the attaching barnacle larva 

 may be, it has been overcome by the evolu- 

 tion of special types adjusted to special 

 hosts, like Coronula on whales, Chelonobia 

 on sea turtles, and Alepas on sharks and on 

 sea snakes. The step from such obligate 

 (perhaps we might WTite "doubly obligate") 

 commensalism to parasitism is evidently a 

 short one. It is illustrated by the isopods 

 that Uve as external commensals on fishes; 

 Ichthyoxenus, for example, calls forth a 

 gall-hke modification of the belly of the 

 host, and Cymothoa praegustator lives in 

 the mouth of the sardine-like menhaden 

 {Brevoortia tyrannus), stealing a httle food 

 as it passes along. Other isopods (Jordan, 

 1905; Smith, 1909) associated with fishes 

 may be attached to their hosts for only part 

 of their lives, and even perhaps discontinu- 

 ously. 



Commensalism in which the relation be- 

 tween host and guest is limited to the trans- 

 port of the guest by its larger host has been 

 distinguished as phoresy (Fr. phoresie). It 

 appears as a relatively widespread phe- 

 nomenon. Small diptera are transported by 

 dung beetles to suitable breeding sites for 

 both, larvae and adults of certain beetles 

 are transported to the nest of the host, or 

 from nest to nest, and pseudoscorpions and 

 mites are similarly transported by various 

 insect;;. Ants appear to be especially given 

 to the role of "porteur." The analogy to the 

 impressment of mammals as agents of dis- 

 persal by plants is evident, though only re- 

 motely a commensal relation. 



Notable obligate commensahsm is that 

 of the small fishes attendant upon siphono- 

 phores and sea anemones. Such fishes evi- 

 dently derive shelter and protection from 

 their hosts and may obtain part of their 

 food from the food of the hosts as well. In 



these, as in the holothurian-inhabiting 

 Fierasfer, the relation is extremely intimate, 

 but no benefit to the host is discernible. The 

 Httle pomacentrid fish, Amphiprion percula, 

 with an especially brilliant coloration, is so 

 regular an associate of the large sea anem- 

 one Discosoma of the East Indian coral 

 reef that some mutual advantage may be 

 suspected, and certainly far-reaching phys- 

 io-psychological adjustment has been at- 

 tained by the fish, since it swims freely 

 among the tentacles that paralyze other 

 fishes, and even enters and re-emerges 

 from the stomach cavity of its fish-eating 

 host. The relation between the fishes of the 

 genus Nomeus and Physalia is similar^ but 

 there is some possibility that Nomeus feeds 

 on the tentacles and zooids of its host 

 (Kato, 1933). 



MUTUALISM 



The often obscure relations of host and 

 uninvited guest crystalfize into the more 

 sharply defined mutually beneficial relations 

 of partner with partner summarized under 

 the concept of mutualism ("symbiosis" of 

 many authors). The origin and development 

 of mutuaUstic relations is of such great in- 

 terest in connection with evolution that this 

 subject will receive fuller treatment in 

 Chapter 35, page 710. In a sense, animals 

 as a whole are broadly symbiotic in their 

 relations with the plant kingdom. To some 

 extent herbivorous animals are the commen- 

 sal guests of plants, feeding on their surplus 

 without doing them vital harm;" the re- 

 ciprocal metabolic relations of the two king- 

 doms may be thought of as mutualistic; 

 when the animal partner gets out of hand 

 (so to speak), as in overgrazed lands, it 

 may correspond at this level of discussion to 

 a parasite. 



At the individual level, the relation of 

 metabolism benefit exchange between par- 

 ticular kinds of plants and special animals 

 may be distinctively mutuahstic. The plant 

 partner supplies synthesized carbohydrate 

 food, elaborated proteins, and oxygen by its 

 metabolic processes, while those of the ani- 

 mal produce nitrogenous wastes and car- 

 bon dioxide useful to the plant. When the 

 relation is between algae and larger animals, 

 the animal provides support and defense, 

 and a biotic niche in addition. The coloni- 



' Though they produce on them a selection 

 pressure made evident by its evolutionary 

 effects on many plant structures. 



