Sludge Worms

Nomenclature

Tubifex tubifex is the binominal name of the sludge worm, also known as the sewage worm, or lime snake. Sludge worms are a segmented benthic worm inhabiting lacustrine and riverine sediment on several continents and occasionally in sewers. The genus Tubifex is thought to include multiple species, but the reproductive organs used to identify species are resorbed after mating, and external characteristics vary with salinity, making the species virtually indistinguishable. The 13 known species include Tubifex tubifex. The Tubifex genus is part of the Tubificinae subfamily of the Naididae (Haplotaxida order, Oligochaeta subclass of Clitella, Annelida phylum). The Naididae were formerly known as Tubificidae. After DNA analysis indicated the family was probably not monophyletic, the name was changed under International Code of Zoological Nomenclature (ICZN) rules in 2008.

Characteristics

A healthy sludge worm is bright pink and usually 19–32 millimeters long, but capable of reaching lengths of 200 millimeters. The worms live in clusters and burrow into mud to ingest sediment while extending their bodies in the water above to collect oxygen and suspended organic matter, which they excel at. The bottom sediment has little oxygen content and may be further deoxygenated by pollution. Animals in this environment rely on hemoglobin to combine with oxygen and vastly increase the oxygen-carrying capacity of their blood. As the sludge worm lives head-down in the tube it has burrowed, it uses its tail as a gill; the less oxygen available, the more of the hemoglobin-containing tail is extended up into the water. Able to absorb molecules through their body wall, the worms exchange carbon dioxide and oxygen through their skin and can selectively digest bacteria. Encystment and lowering metabolic rate protect against drought and food shortage.

Indicators of Pollution

Well known for tolerance to organic pollution beyond that of virtually every other species, sludge worm populations increase exponentially in watercourses polluted with decomposable organics. Organics are a source of food when in suspension and are easy to tunnel into. Undisturbed sludge worm colonies can cover large areas. R. E. Richardson reported sludge worm density in the Upper Peoria Lake as 31 per square meter in 1915, and 20,400 per square meter in 1922. In some tests of contaminated U.S. lake sediment, sludge worms and pollution-resistant midge larva made up 90 precent of the total number of species present.

The 1969 Cuyahoga River Fire, though minor, was an iconic event in the history of environmental protection. This was the last occasion where the river caught fire because of the level of pollution, the first fire having been recorded in 1868. Time magazine noted that the heavily polluted river had been found to be bereft even of sludge worms and leeches. Sludge worms are routinely used in scientific studies to test toxicity and bioavailability of sediment, using an American Society for Testing and Materials (ASTM) methodology. Their tolerance to inorganic pollutants was being tested by 2010. Tests indicate that the high defecation and metabolic rate of the sludge worm stops heavy metal accumulation, excepting cadmium (one of the most dangerous heavy metals). This means that [Page 822]they are unsuitable bioindicators of heavy-metal pollution, and the accumulated cadmium presents a risk to predators.

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