Tubifex tubifex, also called the sludge worm, or sewage worm, is a species of tubificid segmented worm that inhabits the sediments of lakes and rivers on several continents. Tubifex probably includes several species, but distinguishing between them is difficult because the reproductive organs, commonly used in species identification, are resorbed after mating, and because the external characteristics of the worm vary with changes in salinity. These worms ingest sediments, selectively digest bacteria, and absorb molecules through their body walls.
The worms can survive with little oxygen by waving hemoglobin-rich tail ends to exploit all available oxygen, and can exchange carbon dioxide and oxygen through their thin skins, in a manner similar to frogs. They can also survive in areas heavily polluted with organic matter that almost no other species can endure. By forming a protective cyst and lowering its metabolic rate, T. tubifex can survive drought and food shortage. Encystment may also function in the dispersal of the worm. They usually inhabit the bottom sediments of lakes, rivers, and occasionally sewer lines and outlets.