First described by Anton Van Leeuwenoek in the late 1600s, Rotifera is a small phylum of about 2000 species of tiny, bilaterally symmetrical, unsegmented animals traditionally described as pseudocoelomate. Because they are among the smallest of freshwater metazoans – most are between 50 and 2000 μm – rotifers are often mistaken for protists. Even so, they exhibit diverse morphologies, possess varied life history strategies, and occupy a wide range of habitats. Their distribution includes marine, brackish, and fresh waters, as well as the thin films of moisture that cover terrestrial mosses and hydrate soils (limnoterrestrial). Rotifers fill important ecological roles in many inland waters, both fresh and saline. Because of their high reproductive rates, they can easily reach population densities over 1000 individuals per liter, occasionally dominating zooplankton communities. Rotifers may be so numerous that in spite of their small size they represent a significant portion of total zooplankton biomass; and they are an important link between the microbial loop and higher trophic levels. Because of their rapid reproduction and ready consumption by larval fishes, rotifers are grown in mass quantities for aquaculture. They also serve as invaluable bioindicators for ecotoxicogical studies.