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Three species commonly specified to be used in freshwater biomonitoring tests are described below.... 1. Fathead Minnow (Pimephales promelas) Fathead minnows are often used in both acute and chronic biomonitoring tests. This small freshwater minnow is widely distributed throughout North America. It is a popular bait fish, which has led to its widespread distribution. It lives in a variety of habitats, but is most abundant in muddy streams, ponds and small lakes. Fathead minnows are easily cultured in the laboratory. Newly hatched young are about 5 mm in length (less than 1/4th of an inch). Growth is rapid and adults average about 50 mm (about 2 inches) in length. Because of their rapid growth, larval fathead minnows are ideal for use in short-term (7-day) chronic tests, in which the effect of effluents on their growth is observed to determine sub-lethal toxicity. When used in chronic (7-day) tests, fathead minnows must be less than 24-hours old at the start of the test. However, fish used in acute toxicity tests can be up to 14-days old. 2. Ceriodaphnia dubia This species is occasionally used in acute tests. However, its use is more often required in chronic (7-day) biomonitoring tests, where its reproduction is observed to determine sub-lethal effects of effluents. The species is a member of the cladoceran family, which are sometimes referred to as water fleas. Ceriodaphnia dubia have a world-wide distribution, and have been reported from lakes, ponds and marshes in almost every country. They are very small, rotund-shaped organisms, typically only about 1 mm in diameter as adults. They swim with a jerky eratic motion. Populations of Ceriodaphnia normally consist almost entirely of females, which can reproduce by a process called parthenogenesis in which males are not required to fertilize the eggs. They only live about 30 days, and are able to begin reproduction within 4 to 5 days after birth. The eggs hatch in a brood chamber within the animal, and the young (which are called neonates) are released when the animal molts. Each brood typically contains 5 to 12 neonates, and a female will normally produce 3 broods within the first seven days of life. Because of its rapid reproduction, this species is very useful in chronic tests where sublethal effects of pollutants on the number of neonates produced can be observed over a 7-day period. 3. Daphnia pulex This species is usually used in acute biomonitoring tests. It is a member of the cladoceran (water fleas) family. Daphnia are adapted to live in algal blooms where they feed on algae and bacteria. Because Daphnia, in turn, provide food for small fish, they are an important link in aquatic food chains. Adult Daphnia pulex are about twice as large as Ceriodaphnia pulex, but are still very small (2 - 3 mm in diameter). During most of the year, populations of this species are almost entirely females. The females usually reproduce parthenogenetically without fertilization of the eggs by a male. The young which are produced in this way are genetically identical with the mother. These organisms become reproductively mature within about 7 or 8 days after birth, and usually live about 50 days.
Page last updated: 07/22/99
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