38 UNDERWATER GUIDE TO MARINE LIFE 



an occasional meeting in a community to the exceedingly delicate adjustment 

 required by parasitism or mutualism. 



From the purely chance meetings of species to the most delicately adjusted 

 symbiotic relationship, all shades of gray exist. Therefore, in order to identify 

 the three main categories of symbiosis— commensalism, mutualism, and parasitism 

 —the diver will often find himself in the position of having to make value 

 judgments as to just what the advantages or disadvantages of a relationship 

 might be. 



The evolution of symbiosis is bound to progress from the disjunctive (no 

 constant bodily contact) to the conjunctive (constant bodily contact), but from 

 there the paths of evolution diverge. Perhaps the most ideal relationship is one 

 in which both symbionts are benefited (mutualism). Commensalism evolves 

 into parasitism, and parasitism often evolves into mutualism. 



Commensalism 



This is a relationship in which the symbionts share the benefits unequally, one 

 receiving advantage and the other not affected except, perhaps, in a minor way. 

 Therefore, the symbiont receiving advantage is active and the other is passive. 

 The active member may benefit by obtaining shelter, support, locomotion, or 

 food (exclusive of the tissues of the passive member). 



Commensalism is the least involved of the three special types of symbiosis 

 discussed here. It is most often disjunctive, but approaches being conjunctive 

 in some cases and is either social or nutritive. If nutritive, however, the relation- 

 ship is not antagonistic. 



Well-known examples are the pearlfish, which uses the cloaca of the sea 

 cucumber for retreat, the conchfish, which uses the conch for the same reason, 

 and the many bony fishes which live in seaweed or under the bells of jellyfishes. 

 In the case of the man-of-war fish, commensalism is very near to being con- 

 junctive since this fish is rarely seen out of the company of the Portuguese 

 man-of-war and even feeds on some of the food that the jellyfish captures. A 

 truly obligatory conjunctive commensalism is found in the whale barnacles 

 which can only live on whales, the benefit of the association being all theirs. 



Mutualism 



In this symbiosis, both symbionts share the benefits of the relationship. Both 

 species need not benefit in the same way; in fact, they usually do not. One 

 may receive shelter or transport and the other food. The fine adjustments 

 necessary for two animals to live and benefit together are sometimes very 

 extraordinary. 



Usually mutualism is conjunctive, though it may be disjunctive. It is either 

 social or nutritive and is always non-antagonistic. As in commensalism, examples 

 from the sea are common and striking. The zooxanthellae of corals is one. Crabs 

 show mutualistic relations with anemones in which the crab carries the anemone 

 about on its back; the crab is protected by the stinging tentacles of the anemone, 

 and the anemone receives transport and support. Some crabs even hold anemones 

 in their claws and use these as weapons or food-getting devices. Other hermit 



