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Deposit feeding is a feeding mode used by organisms to obtain food by sieving the soil. Examples include earthworms, fiddler crabs, and marine species. It only works in fertile areas with plenty of pre-existing life, and the top layer of soil is targeted. Earthworms have a threefold benefit to the earth and its plants by breaking debris into humus, breaking the soil down into small pieces, and aerating the soil by tunneling it. Marine deposit feeders are more poorly understood due to their remote location and fragility.
Deposit feeding is one of the five feeding modes used by organisms to obtain food, the others are fluid feeding, filter feeding, bulk feeding and phagocytosis. Deposit feeders obtain food particles by sieving the soil, loosely analogous to the way filter feeds obtain food by filtering water. Prominent examples are earthworms, other annelids such as polychaete worms and fiddler crabs. Insects and their larvae, which can burrow through live or dead plants and animals, or feces, are also considered deposit eaters.
Deposit feeding is a feeding strategy that only works in fertile areas with plenty of pre-existing life. The top layer of soil is targeted, typically within six inches of the surface, as this is the soil most likely to contain food particles that have not yet been completely broken down. Biologists call these food particles debris. After the debris is broken down to a chemically neutral state, it becomes known as humus. Humus has a black color due to its high carbon content.
Among deposit feeders, the precise strategies vary. Earthworms are unique animals in the world for feeding deposits and, in general, for the presence of an oral cavity that connects directly to their digestive system without any intermediary. Praised for their benefits to earthworms, Charles Darwin wrote: “It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.” By breaking debris into humus, breaking the soil down into small pieces that maximize available nitrogen and phosphates to plants, and aerating the soil by tunneling it, earthworms have a threefold benefit to the earth and its plants.
In addition to earthworms, feeding on terrestrial deposits is practiced by fiddler crabs. These crabs pick up small balls of dirt, take them to their mouths, and pick up any edible material, including colonies of microbes. Then, the orbs are discarded as quickly as they were collected. These little balls of dirt can be found wherever fiddler crabs inhabit.
There are marine species that also practice deposit feeding, which burrow through the slime on the ocean floor. These include polychaete worms, some bivalves, and giant protozoa called xenophophores. Marine deposit feeders are more poorly understood due to their remote location and fragility when brought to the surface.
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