Some nematodes are able to suck or injest other nematodes or other small animals. Predatory nematodes represent approximately 5% of the overall soil nematode community. In soils, predatory nematodes vary in physical size wheares, in contrast, in the marine environment predatores are frequently the largest nematodes.
In the Netherlands many nematode species occur which feed (as adults) on the smaller soil organisms, including nematodes. As juveniles they are microbivores. In a petri-dish mononchs are easily recognizable at low magnification by their mobility and bucket shaped mouth cavity.
In members of the Mononchidae, like Prionchulus, the mouth cavity is provided with a large tooth which they use to damage the cuticle of their prey; subsequently they suck the liquid content out of the prey body. In the intestine of these mononchs, remainings of the prey can be visble. Members of the Anatonchidae, Anatonchus at the lower photo, have three posterior directed teeth in the mouth cavity which prevent escaping the preys, they swallow the whole undamaged nematode which often can be observed in the intestine. In a petri-dish, mononchs appear to be very active and give the impression that mononchs offer interesting possibilities for the control of plant-parasitic nematodes. In the soil, however, they have to invest much more energy in finding their prey. The possibilities to use mononchs for biological control of plant parasites are not yet convincing.
(Source: European Atlas of Soil Biodiversity|Section2: Organisms of the Soil)