Among the small fauna, rotifera are often found only when a significant proportion of water films exists in soils. They are usually considered to be aquatic organisms and may not be listed in major compendia of soil biota (Dindal, 1990); they are a genuine, albeit secondary, component of the soil fauna (Wallwork, 1976). While sampling for nematodes in the surface layers of agricultural fields near La Selva, in the Atlantic coastal forest of Costa Rica, one of the authors (Coleman) found virtually no nematodes, but large numbers of rotifers (tens of thousands per square meter), despite the soil being far from water saturation. The field was being maintained in a "bare fallow" regime with frequent weeding or denudation of vegetation, to deliberately reduce organic inputs. However, there seemed to be ample Cyanobacteria and perhaps other unicellular primary producers, which would have provided food for the rotifers. Some rotifers have been found in bagged leaf litter on forest floors in the southern Appalachian Mountains.
More than 90% of soil rotifers are in the order Bdelloidea, or wormlike rotifers. In these creeping forms, the suctorial rostral cilia and the adhesive disc are employed for locomotion (Donner, 1966). Rotifers also form cysts to endure times of stress or lack of resources. Additional life history features of interest include the construction of shells from a body secretion, which may have particles of debris and/or fecal material adhering to it. Some rotifers will use the empty shells of Testacea, the thecate amoebae. The Bdelloidea are vortex feeders, creating currents of water that conduct food particles to the mouth for ingestion
(Wallwork, 1970). The importance of these organisms is largely unknown, although they may reach numbers exceeding 105 per square meter in moist, organic soils (Wallwork, 1970).
Rotifers are extracted from soil samples and enumerated using methods similar to those used for nematodes (see next section).
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