A small fish, rainbow darters only grow to be 3 inches, or 5 to 7 centimeters long. They can be a very brightly-colored fish, depending on their sex and whether it is breeding season. Their base color is olive-green and it is mottled with 6 to 10 brown saddles down the length of the body. They also have up to 14 vertical stripes down their 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. However, in female rainbow darters, this coloring is not very well developed and may simply appear as thin lines. In fact, many of their fins are colored differently depending on their gender. For example, the second dorsal fins on male rainbow darters are usually blue with a red stripe running down the middle, while females have thin black lines running 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 are usually clear, with no coloring. Rainbow darters have pointed snouts, and the greatest depth of their body usually occurs at the origin of the first dorsal fin. (Kuehne and Barbour, 1983; Page, 1983; Williams and Gilbert, 2002)
Rainbow darters (Etheostoma caeruleum) are native to the Nearctic region. Year-round, this species inhabits small rivers and streams in eastern North America. Rainbow darters have been widely located in vast numbers in the Ohio River valley and the tributaries of the Great Lakes. They are also found throughout the Mississippi River, as far north as Minnesota and as far south as southeastern Louisiana. This species also is located in the Potomac River in Maryland and Virginia, the Little Miami River in Ohio, the Hudson Bay tributaries in Minnesota, the Missouri River in Missouri, the Kanawha River in West Virginia and Virginia, the Wabash River in Indiana, the Green river in Tennessee and Kentucky, and the Osage River in Missouri. (Ray, et al., 2006)
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. Typically, adult fish are found in faster and deeper running waters, while younger rainbow darters are more common in slower, shallower areas and pools. (Page, 1983; Williams and Gilbert, 2002)
Adult rainbow darters spawn between March and June. The eggs are clear with a yellow yolk and a small black drop in the middle of the yolk. The length of the eggs averages 1.7 to 2 mm. The larvae hatch after 10 to 12 days, in water temperatures of 17 to 19 degrees Celsius. During the days before hatching, larvae develop a heartbeat on day 2 and their skeleton and teeth develop on day 8. Hatching larvae are 6.0 to 6.2 mm long. From larva to juvenile, the process takes about 18 to 21 days, with juveniles reaching lengths of about 13.0 to 15.0 mm. Juveniles are known to eat aquatic water insects and small freshwater shrimp. Juveniles reach adulthood about 47 days after hatching. (Cooper, 1979; Paine and Balon, 1984)
During the breeding season, both male and female colors become brighter. When females are ready for spawning, they travel to pools where the males live. Females spawn multiple times over the breeding season. Males that are larger or more colorful have a higher chance of mating. They use their size and color advantages to scare off lesser males. Multiple males will follow one female until she picks one, usually the brighter and bigger male. Once the female picks the male, she then buries her fins and torso into the gravel or sand of the streambed, only her head and tail stay unburied as she faces upstream. She buries and unburies herself several times, until she signals to the male. The male proceeds behind the female and they begin the spawning process. The female releases her eggs, while the male releases his sperm, fertilizing the eggs. The male only has approximately 20 seconds to fertilize the eggs. The pair will repeat this process several times as they move upstream, a short distance at a time. They proceed until other males disturb them, which is very common. (Fuller, 1999; Reeves, 1907)
Rainbow darters prefer to breed in water temperatures between 17 to 18°C. So, depending on their location, their ideal-breeding conditions occur at different times in the year. Male fish are more brightly colored during the breeding season. In addition, males exhibit territorial behavior in shallow riffles (about 25 to 55 cm deep) during the breeding season, scaring off other males through various intimidation tactics. The larger the male, the better he is at intimidating other males. Females swim into male territories from pools downstream. Once in the riffle, the female buries the bottom half of her body into the gravel and the male fish mounts her. The two fish vibrate together; the male deposits his sperm and the female deposits 3 to 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; Page, 1983)
The average lifespan of wild rainbow darters is three years. Their maximum recorded wild lifespan is five years. Their captive lifespan has not been reported. (Beckman, 2002; Gilbert and Williams, 2002; Page and Burr, 1991)
Rainbow darters are considered “shy” and stay hidden for most the day between or along rocks unless they are looking for food or reproducing. During their breeding season, they are more social; at this time, males follow females in hopes of reproducing. Males communicate with their vibrant colors and size. If rainbow darters are threatened, they will hide and not move until the danger has passed. Darters are alerted of danger through chemicals released when another darter is injured and skin is torn. If they feel threatened by another species close to their own, males have been known to try and scare them away by flapping their gills. Rainbow darters are crepuscular, active during dusk and dawn. They are active swimmers and do not defend territories except when breeding or finding food. (Commens and Mathis, 1999; Crane, et al., 2009; Reeves, 1907)
Rainbow darters are not known to maintain a home range.
Rainbow darters have the ability to sense chemical cues and behaviors from one another. In a situation where a rainbow darter is attacked by a predator, it can release a chemical cue that alerts other rainbow darters to the danger once the skin has been torn. Other darters respond to this cue by decreasing activity, in an effort to hide from the predator. Rainbow darters respond aggressively to similar species, such as bumblebee gobies and yoke darters, by flapping their gills. They are seen as competitors and a threat to their food supplies and young. Males and females also use their vibrant colors for communication. During the breeding season, these bright colors attract possible mates. (Commens and Mathis, 1999; Crane, et al., 2009; Gibson and Mathis, 2006)
Rainbow darters feed on a variety of aquatic insect larvae, small snails, and crayfish. They also feed on various fish eggs, typically either minnow or lamprey eggs. Rainbow darters are also known to have a special preference for caddisfly larvae. However, the feeding habits of rainbow darters differ due to the time of day and time of year. (Kuehne and Barbour, 1983)
Rainbow darters have many predators in their habitats. They are fed upon by many larger fresh water fish including smallmouth bass, spotted bass, bluegills, longear sunfish, creek chubs, and crayfish. If rainbow darters are threatened, they will hide and not move until the danger is gone. These darters can also give off a chemical signal to other darters alerting them to the danger. Once a victim’s skin or tissue has been torn, the chemical is released, warning other rainbow darters of the danger. (Harding, et al., 1998; Kuehne and Barbour, 1983)
Due to their low tolerance for poor quality water, rainbow darters are used as an indicator of stream health. They act as a link in the food chain between low and higher trophic levels in stream ecosystems. (Paulson and Hatch, 2002)
There are no known negative impacts of rainbow darters on humans.
Rainbow darters may be sold as pets as an aquarium species, however, they do not have a major economic impact on humans. (Katula, 2005)
The IUCN Red List considers rainbow darters to be a species of “least concern.” Its population is listed as stable and no management action is required at this time. In fact, they are one of the most abundant darter species in their range. (Kuehne and Barbour, 1983; NatureServe, 2013)
Kayla McNeilly (author), Radford University, Karen Powers (editor), Radford University, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
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