Another example is the relationship between most mycorrhizae and certain plants. Mycorrhizae are fungal growths on the roots of such plants as heaths, orchids, and many conifers. The fungi penetrate the roots of the plants and make soil nutriments such as nitrogen available to the plants, receiving carbohydrates in return. There are also parasitic plants like mistletoe that draw their nourishment from the branches of other plants. The type of symbiosis known as commensalism is an association between two different kinds of nonparasitic animals, called commensals, that is harmless to both and in which one of the organism benefits.
Many commensals are free to separate.
Other commensals function together so completely that they cannot separate. They are not considered parasitic, however, because they do not harm each other. An example is a polyp found in deep water off the coast of Newfoundland. It attaches itself to the shell of a certain species of hermit crab and, by budding, covers the entire shell with a colony that dissolves the original shell. Because the colony grows at the same rate as the crab, it furnishes continuous protection, and the crab does not shed its shell at periodic intervals as it normally would.