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A small fish, the rainbow darter only grows to be 3 inches or 5 to 7 centimeters long (Williams and Gilbert, 2002). It can be a very brightly colored fish depending on the sex and whether it is breeding season. The base color is olive green and it is mottled with 6-10 brown saddles down the length of the body (Kuehne and Barbour, 1983). It also has up to 14 vertical stripes down the body, which are perhaps more clearly visible than the saddles. Females have brown stripes, while males usually have blue stripes that are separated by orange coloring. The first dorsal fins usually have red coloring close to the body with a blue fringe (Page, 1983). However, in female rainbow darters, this coloring is not very well developed and may simply appear as thin lines (Kuehne and Barbour, 1983). In fact, many of the fins on the rainbow darter are colored differently depending on the gender of the fish. For example, the second dorsal fins on male rainbow darters are usually blue with a red stripe running laterally down the middle, while females have thin black lines running laterally across the second dorsal fins. In addition, the pelvic fins of males are usually blue while female pelvic fins are usually clear. Finally, males may also have a red spot on the center of their blue anal fins. All other fins of the rainbow darter are usually clear with no coloring (Page, 1983). Rainbow darters have pointed snouts, and the greatest depth of their body usually occurs at the origin of the first dorsal fin (Williams and Gilbert, 2002).
The rainbow darter is found in North America, throughout the Great Lakes region and the Ohio River valley extending into northern Alabama and as far west as Missouri and Arkansas (Kuehne and Barbour, 1983). Distinct populations of rainbow darters have also been discovered in the tributaries of the lower Mississippi river in southwest Mississippi and eastern Louisiana (Page, 1983). They are now also extending beyond their native range, invading areas like the Genesee river system of New York State; the means of this introduction remains unknown (Fuller, 1999).
Rainbow darters prefer the fast-moving currents of shallow riffles in creeks and small rivers. They also have a preference for gravel or rocky-bottom streams (Williams and Gilbert, 2002). Typically, adult fish are found in faster and deeper running waters while younger rainbow darters are more common in slower, more shallow areas and pools (Page, 1983).
The eggs of rainbow darters are usually 1.6-1.9 mm in diameter, and typically hatch between 10-12 days after fertilization (Page, 1983).
Rainbow darters prefer to breed in water temperatures between 17-18°C (Kuehne and Barbour, 1983). Thus, depending on their regional location, these ideal-breeding conditions will occur at different times in the year (Page, 1983). Male fish are more brightly colored during the breeding season (Kuehne and Barbour, 1983). In addition, males exhibit territorial behavior in shallow riffles (25-55 cm deep) during the breeding season, scaring off other males through various intimidation tactics (Kuehne and Barbour, 1983). The larger the male, the more successful he is at intimidating his counterparts (Page, 1983). Females swim into a male territory from pools downstream (Kuehne and Barbour, 1983). Once in the riffle, the female buries the ventral half of her body into the gravel substrate and the male fish promptly mounts her. The two fish vibrate together; the male deposits his sperm and the female deposits 3-7 eggs in the gravel. The two fish then swim upstream a short distance and repeat the process over and over again for several days until the female lays about 800 eggs (Kuehne and Barbour, 1983).
Rainbow darters can live for up to four years (Williams and Gilbert, 2002).
The different coloration of males and females and changes in coloration during the reproductive season may serve as visual signals to other darters and likely play a role in sexual selection. Recent studies have suggested that rainbow darters show a decrease in activity levels when exposed to macerated skin from either other rainbow darters or a conspecific such as the yoke darter, Etheostoma juliae. The decrease in activity is a behavioral response to the threat of predation and most likely results from an alarm pheromone released through the skin of the macerated darter (Commens and Mathis, 1999).
Rainbow darters feed on a variety of aquatic insect larvae, small snails, and crayfish. They will also feed on various fish eggs, typically either minnow or lamprey eggs (Kuehne and Barbour, 1983). Rainbow darters are known to have a special preference for caddis fly larvae. However, it is important to note that the feeding habits of rainbow darters differ according to the time of day and also the time of year (Kuehne and Barbour, 1983).
The primay predators of the rainbow darter are larger freshwater fish (Paulson and Hatch, 2002).
Due to their low tolerance for poor quality water, the rainbow darter is used as an indicator species of stream health. (Paulson and Hatch, 2002). They act as a link in the food chain between low and higher trophic levels in stream ecosystems.
The rainbow darter does not appear to have any adverse impact on humans.
Although many darter species are endangered or threatened, the rainbow darter is one of the most abundant of all the darter species (IUCN, 2002; Kuehne and Barbour, 1983).
Katie Marko, University of Michigan
William Fink, University of Michigan
Commens, A., A. Mathis. 1999. Alarm pheromones of rainbow darters: responses to skin extracts of conspecifics and cogeners. Journal of Fish Biology, 55: 1359-1362.
Fuller, P. 8/23/1999. "USGS Nonindigenous Aquatic Species Website" (On-line). Accessed 11/2/02 at http://nas.er.usgs.gov/fishes/accounts/percidae/et_caeru.html.
Kuehne, R., R. Barbour. 1983. The American Darters. Lexington, Kentucky: The University Press of Kentucky.
Page, L. 1983. Handbook of Darters. Neptune City, NJ: TFH Publications.
Paulson, N., J. Hatch. 10/30/02. "Minnesota Pollution Control Agency" (On-line). Accessed 11/12/02 at http://www.pca.state.mn.us/kids/fish/rainbowdarter.html.
Williams, J., C. Gilbert. 2002. National Audubon Society: Field Guide to Fishes (North America). New York: Alfred A. Knopf.