The inter-relationships between living organisms have been described in general terms in this chapter. There are, however, between specific organisms some examples of special relationships which biologists classify as either commensalism, symbiosis, or parasitism, although it is not always easy to make a clear distinction between these categories.
Commensalism is the term which describes the relationship between two different organisms often Iiving closely together, in which one of the organisms derives some benefit from the association while the other is relatively unaffected. For example, a species of bristle Worm lives in the shell of a hermit crab. The worm is thus provided with shelter and is able to feed on the debris left from the crab's meals; the crab is unharmed by the worm's presence. Another example is the remora, a small fish with a sucker on its head. It attaches itself with the sucker to the underside of a shark. The shark is unaffected but the remora can feed on particles lett from the shark's prey; it has to use very little energy to obtain this food and it enjoys security from predators while attached to the shark.
Symbiosis. In this relationship, both organisms are thought to obtain some benefit. Certain molluscs, flatworms and coelenterates have living in their cells single-celled algae called zoochlorellae. In one species of Hydra these microscopic plants make the animal appear bright green. The algae benefit from the secure and uniform environment in the cells and are able to utilize the animal's excretory products such as carbon dioxide and nitrogenous wáste for photosynthesis and protein synthesis. The removal of excretory products from its cells is probably advantageous for the animals and they also receive Oxygen from the photosynthesis of the algae. When the algae die, the substances they have made by photosynthesis become available to the animal.
An intimate association between certain fungi and algae produces organisms called lichens which are able to colonize areas where neither the algae nor the fungi could survive on their own. The cells of the algae are enclosed in the fungal mycelium and the structure so produced forms either an encrustation over rocks or tree trunks or a branching filamentous form on dead wood or soil. The aigae make their food by photosynthesis; the fungi can digest the algae when they die and absorb the food into the hyphae. The algae probably benefit from the moisture retained by tbe fungal threads and the safe habitat they provide.
The protozoa and bacteria living in the gut of animals such as termites are symbionts. They are able to digest the cellulose in the termite's diet which the insect itself cannot digest. When the micro-organisms die they are digested and so form a source of food for the termite. The termite's gut, however, provides food and shelter for the micro-organisms while they live.
Parasitism is an association between two organisms in whicn one derives food at the expense of the other but without killing it. The organism which supports the parasite, the host, derives no benefit from the association and may, in fact, be harmed. If, however, the parasite is well-adapted to the host, the latter seems to suffer no ill-effects. Ectoparasites such as fleas, lice orr aphids remain outside the host and suck fluids from its body. Endoparasites such as tapeworms live in the body of the host.