Crystal darter

The crystal darter can grow to 12 cm (5 in) in length. It is olive-colored to tan with four dark saddles extending downward to its lateral line, a brownish stripe, and a whitish belly. Its maximum reported age is three years. ==Distribution and habitat==

The historical range of the crystal darter included the Mississippi River basin, from Wisconsin and Indiana, southwards to southeastern Oklahoma, northern Louisiana, southern Mississippi, the Mobile Basin, Pascagoula, Pearl River, Florida, Alabama, and Mississippi. It is now absent from much of this range and is rare in Wisconsin, Minnesota, Iowa, and Missouri. It is found in swift-flowing streams with clear or slightly turbid water and moderately swift riffles, on small or medium-sized rivers with beds of sand or gravel. It is not found on silty bottoms or areas with vegetation. Individual fish often hide under stones or bury themselves in sand with just their eyes showing. ==Reproductive biology==

One study focused on the Saline River, Arkansas, suggested that crystal darters spawn multiple times from January through mid-April. The explanation for such discrepancy in breeding season timing and length is unknown, however, Hubbs (1985) suggested a difference in latitudinal location could explain the variation. Juvenile crystal darters grow rapidly and reach sexual maturity before age one, but do not spawn until the following season. Clutch size ranges from 106 to 576 mature oocytes per female, and clutch size is positively correlated with fish length. The crystal darter is sexually dimorphic and mature males possess longer soft dorsal and anal fins than females. ==Burrowing behavior==

The crystal darter burrows in sandy substrates with only its eyes protruding. Results from Daniels' study may also explain burying behavior in the crystal darter. ==Threats==

The crystal darter is susceptible to a number of threats including overexploitation, habitat degradation through pollution and siltation, and habitat modification through stream flow alterations and artificial impoundment. This habitat requirement makes them sensitive to activities that cause pollution and siltation, such as mining, logging, natural gas exploration and extraction, and agriculture. Because crystal darters are geographically confined to a few freshwater systems with clear, fast-moving water they are particularly vulnerable to extinction when their limited habitat is degraded. High concentrations of fine sediment can also affect fish by reducing their rate of growth, decreasing their tolerance to disease or directly killing them by clogging gill rakers and filaments. Siltation also reduces the suitability of critical spawning habitat, and thus hinders the stability and growth of future populations. Siltation is particularly threatening to the crystal darter, because it relies on clean sand and gravel habitats to reproduce. In addition, natural fish migration patterns can be modified when certain areas of this route become uninhabitable. Finally, siltation negatively affects the efficiency of hunting in visual feeders, such as the crystal darter. ==Conservation==

The crystal darter is listed as vulnerable by the IUCN on its Red List because of "a decline in area of occupancy, extent of occurrence and/or quality of habitat". and the Arkansas Game and Fish Commission, as an endangered species by the Wisconsin Department of Natural Resources and the Missouri Department of Conservation, as an extirpated species by the Illinois Department of Natural Resources, and as a tier 1, or critically impaired, species by the Mississippi Department of Wildlife, Fisheries and Parks because of "extreme rarity (five or fewer occurrences or very few remaining individuals or acres) or because of some factor(s) making it vulnerable to extirpation." ==Taxonomy==

The crystal darter was first formally described as Pleurolepis asprellus by the American ichthyologist David Starr Jordan (1851–1931) with the type locality given as a small rocky tributary of the Mississippi River at Warsaw, Illinois. When Jordan and his colleague, Charles Henry Gilbert (1859–1928) subsequently assigned Pleurolepis asprellus to the new genus Crystallaria in 1885 P. asprellus was its type species by monotypy. ==References==