Benthic Macroinvertebrate Index

Group III Taxa: Pollution-Insensitive Organisms

These organisms are rather insensitive to pollution. While they require basic resources for life, they can be found in fairly polluted water. Each group of these animals which is found is given a score of 1 toward the overall index value. When they are the dominant benthic macroinvertebrates at a site, you should conclude that the water quality is probably poor.
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[Order Diptera]

These larvae are found around the world, commonly in fast-moving water. They anchor themselves to bedrock or other substrate with a sucker on their posterior end and filter-feed materials from the moving water.



[Order Diptera]

The larvae are elongated, cylindrical, slender and range in length from 2 to 30 mm. There are two pairs of legs. One pair is near the head; the other pair is near the tail. Coloration varies from white, yellowish, greenish, bluish, pinkish or deep red. The red-colored midge larvae have hemoglobin in their blood and are capable of living under conditions of extreme oxygen deprivation. The deep red species are often called "bloodworms". They are found in all parts of the stream, including in submerged vegetation. Some families construct a house to cover their fragile bodies. Typically they are herbivores and feed on algae, other plants and detritus. They are a very important food source for young and adult fish.



[Order Gastropoda]

The pouch snail may be found in streams that are fairly clean. However, their dominance may indicate that dissolved oxygen levels are low. These snails are different from right-handed snails because they do not breath under water by use of gills but instead have a lung-tike sac called a pulmonary cavity which they fill with air at the surface of the water. When the snail takes in air from the surface, it makes a clicking sound. The air taken in can enable the snail to breath under water for long periods of time, sometimes hours.

The pouch snails have two characteristics that will help you identify it. First, it has no operculum, or hard cover over the opening to its body cavity. Second, snails are either "right-handed" or "left-handed," and the pouch snails are "left-handed." You can tell the difference by holding the shell so that its tip is upward and the opening toward you. If the opening is to the left of the axis of the shell, the snail is termed sinestral -- that is it is left-handed. If the opening is to the right of the axis of the shell, the snail is termed dextral -- that is it is right-handed, and it breathes with gills.

Snails are animals of the substrate and are often found creeping along on all types of submerged surfaces in waters from 10 cm to 2 m deep. Common pulmonate snail families are Physiidae, Lymnaeidae, Planorbidae, Ancylidae, Lancidae and Acroloxidae.



[Class Hirudinea]

There are many different families of leeches, but all have common characteristics. They are soft-bodied worm-like creatures that are flattened when extended. Their bodies are dull in color, ranging from black to brown and reddish to yellow, often with a brilliant pattern of stripes or diamonds on the upper body. Their size varies within species but generally ranges from 5mm to 45cm. when extended. Leeches can be very good swimmers, but they typically move in an inch-worm fashion. They are carnivorous and feed on other organisms ranging from snails to warm-blooded animals. Leeches are found in warm protected shallows under rocks and other debris.



[Class Oligochaeta, Family Tubificidae, Genus Tubifex]

These worms are unique in the fact that they build tubes. Sometimes there are as many as 8000 individuals per square meter. They attach themselves within the tube and wave their posterior end in the water to circulate the water and make more oxygen available to their body surface. These worms are commonly red, since their blood contains hemoglobin. Tubifex worms can survive in very low oxygen levels and can live with no oxygen at all for short periods of time. They are commonly found in polluted rivers and streams, and feed on sewage or other detritus.


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  • Group I Taxa: Pollution-Sensitive Organisms
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