Etheostoma caeruleum, known by its common name the Rainbow Darter, is a small but brightly colored fish belonging to the subfamily of darters, Etheostoma (Harding1998; Ray et al. 2006). They are typically found around two and a half inches, while some males have been identified up to three inches in length (Reeves 1907). All darters are characterized by their two separate dorsal fins. The anterior dorsal fin has spines while the posterior dorsal fin has only soft fin rays (Bosanko 2019). Rainbow Darters have one to two anal fin spines, a characteristic that they share with their family, Percidae (Bosanko 2019). A male Rainbow Darter will have eight to thirteen blue bars along its side, which may all vary in opacity (Reeves 1907). During the breeding season, the color in these stripes intensifies for mate attraction (Zhou 2014). The most commonly mistaken species similar to the Rainbow Darter is the Orangethroat Darter (Etheostoma spectabile), which also has a blue-green and orange coloration (Zhou 2014). However, the anal fin on a male Rainbow Darter is identifiable by an orange spot that is surrounded by blue, which is absent in the Orangethroat Darter (Zhou 2014).

Systematics:  

The family of darters, Etheostomatinae, is one of the most diverse families of freshwater fish in North America, making the estimation of phylogenies particularly difficult (Near et al. 2011). An accurate phylogenetic tree has been difficult to estimate for Etheostomatinae because the fossil record is so dispersed and incomplete (Near et al. 2011). Genetic studies have been used in place of a fossil record to accurately relate each darter species based on a single sequence of mitochondrial DNA (Ray et al. 2006). Rainbow Darters belong to a subgenus called Oligocephalus, which contains approximately 37 other species of darters (Ray et al. 2006). Their closest relative both genetically and morphologically is the Orangethroat Darter (Ray et al. 2006). Within  Etheostoma caeruleum, there have been four clades identified by phylogeographical distribution of cytochrome b genetic variation (Ray et al. 2006). These clades are geographically identified as the Midwest clade, Eastern clade, Mississippi corridor clade, and the White River-Little Red River clade (Ray et al. 2006). It is believed that their speciation was allopatric and mainly due to the separation of glacial basins (Ray et al. 2006). 

Rainbow Darters are known as benthic fish that inhabit rapid streams and rivers (Harding 1998). Rocky bottomed rivers are the preferred habitat for many species of darters (Harding 1998). Within these fast-moving streams, Rainbow Darters have been observed seeking out localized areas of slower water velocity, termed “microhabitats” (Harding 1998). These microhabitats provide shelter from the intense surrounding environment (Harding 1998). Seasonally, when the water velocity becomes too intense and the competition for microhabitats increases, E. caerulem has been observed migrating to nearby pools to escape the high water velocity (Harding 1998). Rainbow Darters often inhabit areas that overlap with the habitat of Fantail and Greenside Darters, making them direct competitors for space and resources (Hlohowskyj 1986). However, unlike the Fantail and Greenside Darters, Rainbow Darters show no particular preference when it comes to substrate size which allows for intra-species coexistence. (Hlohowskyj 1986). Genetic analysis of four clades of Etheostoma including Rainbow Darters shows that their range extends both east and west of the Mississippi, but is predominantly east of the Mississippi (Ray et al. 2006).

Food: Rainbow Darters are insect-eating fish whose diet is fairly consistent across its regions (Stewart 1988). Results from multiple studies have shown that the main portion of their diet consists of chironomid larvae, which are small nematoceran flies (Stewart 1988). Other fly larvae that have been found in the Rainbow Darter’s diet are simuliid larvae, trichopteran larvae, and coleopteran larvae (Anderson . All of these larvae are water-dwelling or semi-aquatic, making them ideal prey for a darter. Their diet can also change seasonally depending on the availability of different larvae species, and feeding behavior of the fish (Stewart 1988). But the feeding behavior of the Rainbow Darters is described as opportunistic rather than selective based on the appearance of any abundant larvae species in different areas of the same stream (Stewart 1988).

Reproduction:  

During the breeding season, males undergo morphological changes while the females retain their normal features. The males that are prepared for breeding metamorphose into brightly colored fishes with areas of orange, blue, green, and red all over their bodies (Reeves 1907). Once breeding season arrives, the individuals will forgo their hiding places and microhabitats, and migrate to gravel flats which exists at the head of rapids (Reeves 1907). These areas are common breeding grounds for many other small fish species as well (Reeves 1907). Rainbow Darters are a promiscuous species with the males mating with up to five different females in a given breeding period (Reeves 1907). Males will guard a particular area as the females travel around, choosing a mate. Once she has chosen, a female Rainbow Darter will bury her head and pectoral fins into the substrate of a male’s territory and the two will spawn (Reeves 1907).

Other Interesting Facts:  

Rainbow Darters exhibit a fear response in reaction to chemical alarm cues that are released by other darters in the area (Crane et al. 2009). In a study done on Rainbow Darters in captivity, individuals that were presented with skin extracts from another individual darter and their behavior was observed (Crane et al. 2009). The darters that were exposed to the skin sample performed anti-predator behaviors such as seeking shelter or freezing altogether (Crane et al. 2009). This study shows a capability of Rainbow Darters to respond to their environmental stimuli and exhibit a fear response.

Rainbow Darters are not currently an endangered species and they have shown resilience in their ability to colonize distant river tributaries (Cessna et al. 2014). Their current range is not believed to native and it is suspected that human usage of Rainbow Darters as bait is the cause of this interbasin transfer (Cessna et al. 2014). The human-induced spread of Rainbow Darters has the potential to interfere with native species and create competition although their effects on surrounding fish species and flora has not been fully studied (Cessna et al. 2014). Although Rainbow Darters are not of specific concern, they are a host species for Gasconade River ellipse mussels, making their presence essential to the mussel’s survival (Bigham 2002).

Bigham, S.E. 2002. Host specificity of freshwater mussels: a critical factor in conservation. Southwest Missouri State University.

Bosanko, D. 2019. Fish of Minnesota Field Guide. Adventure Publications, Cambridge MN.

Cessna, J.F., Raelsy, R.L., Kilian, J.V., Cincotta, D.A., Hilderbrand, R.H. 2014. Rapid Colonization of the Potomac River Drainage by the Rainbow Darter (Etheostoma Caeruleum) Following Introduction. Northeastern Naturalist 21(1): 1-11.

Crane, A.L., Woods, D., Mathis, A. 2009. Behavioural responses to alarm cues by free-ranging Rainbow Darters (Etheostoma caeruleum). Behavior 146(11):1565-1572.

Harding, J.M., Burky, A.J., Way, C.M. 1998. Habitat Preferences of the Rainbow Darter, Etheostoma caeruleum, with Regard to Microhabitat Velocity Shelter. Copeia 1998(4): 988-997.

Hlohowskyj, I., Wissing, T.E. 1986. Substrate Selection by Fantail (Etheostoma flabellare), Greenside (E.blennioides), and Rainbow (E. caeruleum) Darters. Ohio Journal of Science 86(3): 124-129.

Near, T.J., Bossu, C.M., Bradburd, G.S., Carlson, R.L., Harrington, R.C., Hollingsworth Jr., P.R., Keck, B.P., Etnier, D.A.  2011. Phylogeny and Temporal Diversification of Darters (Percidae: Etheostomatinae). Systematic Biology 60(5):565–595.

Ray, J.M., Wood, R.M., Simons, A.M. 2006. Phylogeography and post-glacial colonization patterns of the rainbow darter, Etheostoma caeruleum (Teleostei: Percidae). Journal of Biogeography 33: 1550–1558.

Reeves, C.D. 1907. The Breeding Habits of the Rainbow Darter (Etheostoma caeruleum Storer), a Study in Sexual Selection. Biological Bulletin 14(1): 35-59.

Stewart, C.A. 1988. Diet of the Rainbow Darter (Etheostoma caeruleum) in Rock Run, Clark County Ohio. Ohio Journal of Science 88(5): 198-200.

Zhou, M., Johnston, A. M., Fuller, R.C. 2014. Patterns of Male Breeding Color Variation Differ across Species, Populations, and Body Size in Rainbow and Orangethroat Darters. Copeia 2014(2): 297-308