Decomposers are organisms that break down dead or decaying organisms, and in doing so, they carry out the natural process of decomposition. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. While the terms decomposer and detritivore are often interchangeably used, however, detritivores must digest dead matter via internal processes while decomposers can break down cells of other organisms using biochemical reactions without need for internal digestion. Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are detritivores, not decomposers, in the technical sense, since they must ingest nutrients and are unable to absorb them externally.
The primary decomposers of litter in many ecosystems are fungi. Unlike bacteria, which are unicellular organisms, most saprotrophic fungi grow as a branching network of hyphae. While bacteria are restricted to growing and feeding on the exposed surfaces of organic matter, fungi can use their hyphae to penetrate larger pieces of organic matter.

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