The phylum Rotifera comprises two disparate groups. Traditionally, rotifers were viewed as a small taxon of tiny, bilaterally symmetrical, unsegmented aquatic invertebrates. However, recent molecular analyses indicate that Acanthocephala, a group of obligatorily parasitic worms, are highly modified rotifers. (They are covered separately in the ELS.) Rotifers may be found anywhere water exists for a sufficient period allowing them to reproduce including lakes and ponds, ephemeral pools, water films coating terrestrial mosses and soil particles and brackish and coastal marine waters. Few are parasitic. In some habitats, rotifers may achieve high population densities (>1000 individuals per litre). Being consumers of bacteria, algae, protists and other rotifers, they are important in transferring energy to higher trophic levels. Because they serve as food for young fishes and are easily cultured, rotifers are grown in mass quantities for commercial aquaculture. They also serve as models for research on ageing, evolution of sex and ecotoxicology. Key Concepts Rotifers live in fresh and saline continental waters, in the moisture covering some plants such as mosses, in damp soils and in the interstitial spaces of aquatic sediments. Although not as abundant as in inland waters, rotifers often flourish in nearshore marine waters and estuaries. A few species live a parasitic life within the cytoplasm of certain large freshwater algae and the guts of freshwater, aquatic oligochaetes. Some rotifer populations crash when they become infected with parasitic fungi or microsporozoan parasites. Feeding mainly on other microscopic organisms, including bacteria, algae, protists and other rotifers, these small metazoans are important basal consumers in aquatic systems. They are a component of the microbial loop in which nutrients cycle and energy flows within the lower trophic levels. When consumed by higher trophic levels (e.g. aquatic arthropods and fishes) rotifers play an important role connecting the microbial loop to the classic food web. Rotifers possess several interesting features. The cell membranes between the cells of certain tissues in rotifers fuse, thereby forming a cytoplasmic syncytium. The body wall contains proteins that are only shared with one other group of invertebrates, the Acanthocephala. Many rotifers are capable of undergoing dormancy in a resting state that allows them to avoid harsh conditions. Rotifers are a first food for the larvae of many organisms; thus, they have been exploited by aquaculturists to maintain commercially important fishes, crabs and shrimps. Also, because rotifers are comparatively easy to culture and possess relatively short life spans, they are used in ageing studies and to assess the toxicity of heavy metals, pesticides, pharmaceuticals and other pollutants. Some species have specific ecological requirements making them useful as indicators of water quality. Being one of the few taxa capable of cyclical parthenogenesis, rotifers are also used as models for the study of the evolution of sex. The phylogenetic position of Rotifera has not been fully resolved, but they appear to be related to a group of small, jawed metazoans called the Gnathifera. Emerging molecular evidence indicates that the phylum Acanthocephala — a taxon of worm‐like parasites — should be considered to be highly modified rotifers (Witek et al ., 2009).