The rainbow darter, or Etheostoma caeruleum, has a very unique reproduction method. For every one female, up to five males may choose to compete for rights to reproduce with her, and often times there is a correlation between sperm competition and sperm production in these male fish. This means that essentially, when competition is high, there is a reduction in sperm produced, simply because the male species can wait for another opportunity to come along, in which case an increase of sperm production would occur (Fuller, 1998).  
            During breeding season, male rainbow darters reside and swim in riffles, a stretch of rocky shoals or sandbars lying just below the surface of a waterway with rapidly moving currents. They wait here for the females (who inhabit the quiet waters underneath the riffles) to swim up towards the male-occupied areas of the water. Once this occurs, the male fish rapidly chase after the swimming female, fighting each other for their partner. Once this process is completed, reproduction can occur (Fuller, 1998).     

Rainbow Darter in gravel

            The Etheostoma caeruleum female has two techniques in order to complete her part in the spawning cycle: incomplete and complete nosedigs. In incomplete, the fish burrows her nose into the gravel on the water's floor and positions her body vertically. Once this is achieved, she quivers her body. However, after only this, it is impossible for the male rainbow darter to spawn with the female. In order for reproduction to occur, a complete nosedig must be performed by the female fish. This is done by again, digging the nose into the gravel and vertically quivering, eventually placing the ventral portion of the body into the sediment. With the female fish essentially buried, the male darter can mount and vibrate against the fish. This is when both the eggs and sperm are released (Fuller, 1998).
            It is possible, and probably likely, for problems to arise during this process. Often times, other male competitors continually try to interfere with reproduction. Some male darters attempt to sneak around the male and female while the spawning is in process and release their own sperm into the surrounding water. Because of this continual annoyance of intruders, the male in the process of reproducing frequently leaves to ward off other competitors, sometimes leaving the female buried in the gravel floor (Fuller, 1998).

            Overall though, the reproduction process of the Etheostoma caeruleum is a successful procedure. Once the rainbow darter is inseminated and the eggs are developed, new fish can be introduced into the population. After this point, no parental care occurs and the rainbow darters grow up on their own, eventually repeating this same process over again (Fuller, 1998).

            There are many studies that exist which analyze various impacts on the reproduction process of the rainbow darter. In Ontario, Canada at the Grand River watershed, biologists studied twenty-nine of these Municipal Wastewater Effluent (MMWE) discharges, including a mixture of both domestic and industrial wastes. They were looking for alterations in numerous species of fish and how they were affected by these waters. They found that the fish downstream were bigger in both size and length, regardless of sex. Additionally, the male darters that were exposed to the sewage had a greater difficultly producing testosterone and had decreased cellular development (Bennet et al., 2011).
            Another major issue that was discovered through this study in Ontario, Canada was the rates of intersex, or in other words, the abnormal condition of being intermediate between the male and female sex. As the Etheostoma caeruleum species were measured farther downstream, the level of intersex amongst the fish increased. This obviously can create setbacks in regards to the reproduction of the rainbow darter. The study proved that the effects of human disposal of sewage wastes may become more detrimental and pronounced if it is not monitored (Bennet et al., 2011).

            Although the rainbow darter isn't endangered, there are ways you can help preserve these fish, including smarter disposal of wastes. Visit Michigan's Department of Natural Resources to learn more about this and about the darter in general.

Next, read about interactions among the Etheostoma caeruleum species and others, or revisit the home page.