The hypotrichs are a group of ciliate protozoa, included among the spirotrichs. Most are oval in shape, with a rigid pellicle, and have cirri distributed in isolated tufts on the ventral surface of the cell. Some also have dorsal cilia, which function as sensory bristles. Euplotes and Aspidisca are common genera.
The hypotrichs were first defined by Friedrich von Stein in 1859. Originally the stichotrichs, which also have cirri, were included here, but they were separated out by Small & Lynn, 1981, who placed the restricted hypotrichs among the Nassophorea because of various peculiarities in their infraciliature. More recent schemes reverse this move, and some molecular studies suggest they may be paraphyletic to the stichotrichs as currently defined.
Phylogenetic relationships among hypotrichous ciliates determined with the macronuclear gene encoding the large, catalytic subunit of DNA polymerase alpha.
Hypotrich phylogenies based on DNA polymerase alpha amino acid sequences are incongruent with morphological and other molecular phylogenies.
Based upon these data, we assert that, contrary to morphological data, O. nova and O. trifallax are different species, and we propose that the oligotrich Halteria grandinella be reclassified as a hypotrich.
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