Rotifers are found in aquatic and semi-aquatic habitats, but are predominantly freshwater inhabitants. Rotifers are very important in these systems because of their incredible reproductive rates. Population densities often reach greater than 1000 individuals per liter.
Rotifers contribute to both the microbial loop and to higher trophic levels.
In addition, species assemblages of rotifers are useful in characterizing lakes in relation to their trophic status. Rotifers may also be very abundant in the interstitial water of soils reaching densities up to 2 million per square meter. The corona is located anteriorly and functions in locomotion and food gathering. The corona is modified extensively in some species. The mastax is a muscular pharynx containing a complex set of hard jaws or trophi and is found in all rotifers.