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. The polyp, in turn, benefits by moving about with the crab, thereby obtaining a greater food supply than it would if attached to a stationary object.
Bacteria feed on dying material and convert it back into basic substances.
This process of decomposition is as significant as photosynthesis, for without it food chains would cease, and fallen trees, leaves, and other refuse would simply pile up. Bacteria also strongly influence the movement of key elements, such as sulfur, iron, phosphorus, and carbon, around the globe. The weathering of rocks, which releases elements back into life systems for use, is substantially enhanced by the breakdown processes of bacteria. The main cleansing agents in sewage treatment are a variety of specialized bacteria that convert, mostly through fermentation, the organic materials of sewage into carbon dioxide, methane, and hydrogen gases. There is a bacterial species involved with the production of nearly every familiar product.