Diet

Tardigrades are either herbivorous or carnivorous (they feed primarily on bryophytes, for example, the eciniscus testudo). The herbivorous species of tardigrades feed on mosses and lichens through a adapted pair of stylets, as well as a muscular pharynx.  Carnivorous tardigrades consume protozoa, nematodes and rotifiers but can also consume bacteria and fungi.

Tardigrades help regulate the ecological food chain by being predators of nematodes. One species (P. richtersi) can eat as many as 61 nematodes in a single day, thus causing the decline of the nematode population. If the tardigrades were not present to balance this ecosystem, the nematodes would upset the food web. Contrary to popular belief, tardigrades are able to swiftly attack and capture their prey.

But just like almost all other animals, tardigrades have their own set of predators as well. Other animals that live among the moss, such as snails, have taken to dining on these microorganisms.  For example, Bulimulus guadalupensis was found to have traces of tardigrades in their feces. Also possible is the idea that the snail consumed the eggs, which later hatched after passing through the gut.

Source: http://www.bryoecol.mtu.edu/chapters_VOL2/5-2Tardigrades_Reproduction_and_Food.pdf