Darters (Family Percidae)
Ph.D. Candidate, Department of Biology (Evolution, Ecology, and Behavior), Indiana University, Bloomington, Indiana
When you look into an eastern North American stream in the springtime, you probably see little shapes flitting through the water here and there, and never give them a second thought. But if you look a little more closely, you might be astonished to see a parade of radiant colors and patterns displayed by feisty fish no larger than your finger. You might think that they were exotic tropical fish released into the water by irresponsible pet owners, but they are in fact native fish called Darters, who have inhabited those waters for millions of years.
Darters (Family Percidae) are a large group of tiny, colorful freshwater fishes found only in North America. There are more than 200 described species, some of which were discovered just recently (Kuehne & Barbour 1983; Page 1983, Page & Burr 2010). Their common name refers to the fact that they are built for acceleration and can rapidly “dart” backwards or forwards in bursts of speed. Darters fall into four to five genera (Ammocrypta, Crystallaria, Percina, Etheostoma, and- according to some researchers- Nothonotus) (Near et al. 2011), the largest of which is by far Etheostoma, with 157 species. They are broadly distributed in streams across the eastern United States and parts of Canada, as well as Mexico. The geographic ranges of individual species vary widely (Page & Burr 2010); while species such as the Greenside Darter (Etheostoma blennioides) are found as far south as Alabama and as far north as Canada (Page 1983), species such as the Bluemask Darter (Etheostoma akatulo) (Laymen & Mayden 2009) are restricted to a single river system, others even to a single stream.
Darters live primarily in stream habitats with clear, fast-flowing, oxygen rich water. However, microhabitat preferences can vary by species. For example, Rainbow Darters (Etheostoma caeruleum) prefer to inhabit riffles where water flows rapidly over cobble, while Johnny Darters (Etheostoma nigrum) prefer sandy substrate with some gravel, and can inhabit cloudy water. Darters primarily feed on stream invertebrates, and are prey items for larger fish, as well as some snakes and birds. Multiple species frequently co-occur in streams, although microhabitat preferences allow for effective niche partitioning between species (Kuehne & Barbour 1983). However, introgressive hybridization has been observed between some closely related species (Harrington et al. 2012).
Darter species vary dramatically in their secondary sexual characters and reproductive behaviors. One particularly striking example are male nuptial coloration differences found between species. During the breeding season (which in most species occurs in the spring), males of many species undergo dramatic transformations in which they exhibit brilliant colors and patterns (Kuehne & Barbour 1983). Although this has long been thought to attract mates and appeal to female preferences, recent research shows that male breeding coloration might actually serve as a signal that mediates male-male competition: males recognize their own species colors and patterns and can direct aggressive behavior towards males of their own species, while avoiding aggressive interactions with heterospecific males who are unlikely to pose competition for mates (Moran & Fuller, in press). The extent to which females prefer bright male coloration is still under investigation.
Darters also vary in their spawning behavior. In species where male breeding colors are more prominent, spawning typically occurs on the substrate or among aquatic plants, and the eggs are not tended by either parent. In species such as the Fantail Darter (Etheostoma flabellare), whose males are less colorful, however, eggs are attached to the undersides of rocks, and males provide all the parental care for the eggs. Males will fan the eggs with their tails and clean the eggs with their mouths until the fry hatch (Kuehne & Barbour 1983; Page 1983). Females are attracted to males who already possess egg clutches from other females, presumably because this suggests the males are capable caregivers (Knapp & Sargent, 1989). Males, meanwhile, will take over and “adopt” nests of genetically unrelated eggs to increase mating opportunities, since multiple females will contribute to a single existing nest. Males in breeding condition also exhibit small ornaments on their dorsal fins called “egg mimics” that closely resemble darter eggs. Several studies demonstrate that females prefer males who possess egg mimics (Strange, 2001; Knapp & Sargent, 1989).
Although Darters are too small to be of interest to most game fishermen, they serve an important purpose as biological indicators of stream health. Darters are highly sensitive to habitat quality and water cleanliness. Many species are threatened or endangered, largely due to poor water management practices and habitat destruction (Grabarkiewicz & Davis 2008).
Despite the many threats faced by these vulnerable fish, there is hope for their future. Organizations, such as Conservation Fisheries in Tennessee, are working to restore native fish habitats, while breeding threatened species in captivity to maintain “arks” against extinction. After successful habitat restoration, these fish will then be reintroduced to their natural habitats. Up to this point, Conservation Fisheries has successfully propagated more than 65 vulnerable native fish species, including many Darters (Conservationfisheries.org 2018). While captive breeding programs are a beneficial step to preserve native biodiversity, long-term habitat management solutions are needed to ensure the continued survival of Darters in the wild.
Given their diverse characteristics and behaviors, darters are wonderful models for multiple areas of scientific research spanning ecology, evolution, animal behavior, and conservation biology. The extreme diversity of the darter group makes them excellent for studies in phylogenetics and diversification (Near et al. 2011). Their life histories have been documented extensively by ichthyologists for decades (Page 1983). The variation in reproductive behaviors and social structures have made them appealing to researchers interested in mate choice and behavioral isolation between species (Mendelson et al. 2018, Williams & Mendelson 2013). One research group has even recorded male vocalizations (“singing”) in several darter species (Noel & Johnston 2015).
Additionally, darters are hosts to a variety of local parasites, including a genus of gill parasite (Aethycteron) that infects darters as their preferred host group. Many of the parasites in this genus have been found on only one darter species each, demonstrating a high degree of specialization on their hosts (Suriano & Beverly-Burton 1982). Thus, co-occurring darter species have presented with dramatically differing gill parasite burdens (Hanson & Stallsmith 2013, Million et al. unpublished data). However, we even find within-species parasite load variation: in Fantail Darters, peak gill parasite infection tends to occur during the hosts’ breeding season, and males are generally more heavily infected than females (Million et al. 2017). My current research focuses on the intersection of parasitism, mate choice, and immunogenetic diversity in Darters. For this, I use Darters of Indiana and several of their local parasites (including Aethycteron) as models to evaluate hypotheses designed to explain the high diversity of MHC genes (genes that code for a component of the vertebrate immune system). The variation in host reproductive behaviors and characteristics and the presence of multiple wild pathogens that interact closely with their hosts make Darters an excellent system to study the mechanisms that maintain immunogenetic diversity in vertebrates.
About the author
Kara Million is a Ph.D. candidate in Ecology, Evolution, and Behavior at Indiana University. She is advised by Dr. Curtis Lively. Her research focuses on parasitism, mate choice, and immunogenetic diversity in Darters.
Grabarkiewicz, J.D. & W.D. Davis. 2008. An introduction to freshwater fishes as biological indicators. Washington, DC: Environmental Protection Agency. 96 p.
Hanson, R.G. & B.W. Stallsmith. 2013. Patterns of infection by monogenoideans in an assemblage of darters. J. Freshw. Ecol. Available at: http://dx.doi.org/10.1080/02705060.2013.769195
Harrington, R.C., E. Benavides & T. J. Near. 2012. Phylogenetic inference of nuptial trait evolution in the context of asymmetrical introgression in North American darters (Teleostei). Evolution 67(2): 388-402.
Knapp, R.A & R.C. Sargent. 1989. Egg-mimicry as a mating strategy in the fantail darter, Etheostoma flabellare: females prefer males with eggs. Behav. Ecol. Sociobiol. 1989(25) :321-326.
Kuehne, R.A. & R.W. Barbour. 1983. The American darters. Lexington, KY: University Press of Kentucky. 208 p.
Layman, S.R. & R.L. Mayden. 2009. A new species of the darter subgenus Doration (Percidae: Etheostoma) from the Caney Fork River System, Tennessee. Archived October 2, 2011, at the Wayback Machine. Copeia 2009(1): 157-170.
Mendelson, T.C., J.M. Gumm, M.D. Martin & P.J. Ciccotto. 2018. Preference for conspecifics evolves earlier in males than females in a sexually dimorphic radiation of fishes. Evolution 72(2): 337-347.
Million, K.M., C.L. Tarver & B.W. Stallsmith. 2017. Does Infection by the Monogenoidean Gill Parasite Aethycteron moorei Affect Reproductive Ecology of the Darter Etheostoma flabellare in Mill Creek, Tennessee? Copeia 105(1):75-81.
Moran, R.L. & R.C. Fuller. In press. Agonistic character displacement of genetically based male color patterns across darters. Proc. R. Soc. London, B. doi:10.1098/rspb.2018.1248 (bioRxiv preprint)
Near, T.J., C.M. Bossu, G.S. Bradburd, R.L. Carlson, R.C. Harrington, P.R. Hollingsworth, B.P. Keck & D.A. Etnier. 2011. Phylogeny and temporal diversification of darters (Percidae: Etheostomatinae). Syst. Biol. 60 (5): 565-595.
Noel, P.S. & C. Johnston. 2015. Geographic variation in acoustic signaling in the guardian darter Etheostoma oophylax: effects of contemporary versus historical isolation. Environ. Biol. Fishes 98:1355–1363.
Page, L.M. 1983. Handbook of Darters. Neptune, NJ: TFH Publications. 272 p.
Page, L.M. & B.M. Burr. 2010. A field guide to freshwater fishes of North America north of Mexico. Boston, MA: Houghton Mifflin Company. 432 pp.
Strange, R.M. 2001. Female preference and the maintenance of male fin ornamentation in three egg-mimic darters (Pisces: Percidae). J Freshw Ecol 16(2):267-271.
Suriano, D.M. & M. Beverly-Burton. 1982. Aethycteron n.g. (Monogenea: Ancyrocephalinae) from darters (Percidae: Etheostomatini) in Ontario, Canada with descriptions of A. caerulei n.sp., A. micropercae n.sp., and A. nigrei n.sp. from Etheostoma spp. Can. J. Zool. 60:1397-1407.
Williams, T.H. & T.C. Mendelson. 2013. Male and female responses to species-specific coloration in darters (Percidae: Etheostoma). Anim Behav. 85:1251-1259.