COMMENSALISM

In nature, the individuals of one species are often physically attached to those of an other. For example, birds nest in trees, and epiphytes are plants that grow on tht branches of other plants. In general, the host plant is unharmed, whereas the organism that grows or nests on it benefits. Similarly, various marine animals, such as bar nacles, grow on other, often actively moving sea animals and thus are carried passively from place to place. These "passengers" presumably gain more protection from predation than they would if they were fixed in one place, and they also reach new sources of food. The increased water circulation that such animals receive as their host moves around may be of great importance, particularly if the pas sengers are filter feeders. The gametes of the passenger are also more widely dispersed than would be the case otherwise.
The best-known examples of commensalism involve the relationships between certain small tropical fishes and sea anemones, which are marine animals that hare stinging tentacles. These fishes have evolved the ability to live among the tentacles of the sea anemones, even though these tentacles would quickly pardira other fishes that touched them. The anemone fishes feed on the dan-tus left from the meals of the host anemone, remaining uninjured under remarkabk circumstances.
The best-known examples of commensalism involve the relationships between certain small tropical fishes and sea anemones, which are marine animals that hare stinging tentacles. These fishes have evolved the ability to live among the tentacles of the sea anemones, even though these tentacles would quickly pardira other fishes that touched them .The anemone fishes feed on the dan-tus left from the meals of the host anemone, remaining uninjured under remarkabk circumstances.
On land, an analogous relationship exists between certain birds and gracing mi-| mats, such as cattle or the rhinoceros, which may benefit by having their parasits removed by birds. The birds spend most of their time clinging to the animals, picking off insects and other small bits of food, and carry out their entire life cycles in close association with the host animal. Cattle egrets, which have extended their range greatly during the past few decades, provide an example of a loosely coupled relationship of this kind.
In each of these instances, it is difficult to be certain whether the second partner receives a benefit or not, and there is no clear-cut boundary between commensalism and mutualism. For instance, it may be advantageous to the sea anemone to have particles of food removed from its tentacles; it may then be better able to catch other prey. The association of the grazing mammals and the birds, on the other hand, is quite clearly an example of mutualism. The mammal benefits by having parasites and other insects removed from its body, and the birds benefit by having a dependable source of food.