Crystallaria asprella (Jordan, 1878)
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Basis for Listing
The Crystal Darter (Crystallaria asprella), also known as the Rough Sand Darter, reaches the northern limit of its range in the St. Croix River drainage in Minnesota and Wisconsin. After the Bluntnose Darter, this species is probably the rarest and least known of Minnesota’s darters. It occurs in small numbers in the St. Croix River downstream of Taylors Falls; the Mississippi River from just north of Redwing (U.S. Lock and Dam 3) to the state’s southern border; the Zumbro River downstream of Mazeppa; and the Root River near Houston. Although difficult to collect, the Crystal Darter is still believed to be relatively rare in Minnesota. Localized populations are vulnerable to extirpation from single destructive events (Hatch 1997).
In 1984, when the Crystal Darter was initially designated a special concern species, information on distribution, abundance, preferred habitats, and life history was limited. In 2013, following the completion and analyses of targeted surveys, the status of the Crystal Darter was elevated to endangered.
Crystal Darters are pale yellow, slender, and have 4-8 dark side bars, often connected to 4 dark saddles across the back. Body depth is no more than one-seventh of the total length from the tip of the snout to the base of the caudal (tail) fin. Adults may reach a length of more than 16 cm (6.3 in.) (Page and Burr 1991). The largest Crystal Darter collected in Minnesota was from the St. Croix River in Chisago County on 25 September 2006 and measured 15.6 cm (6.1 in.) long (Hatch and Schmidt, in preparation).
The Crystal Darter is very similar to the Western Sand Darter (Ammocrypta clara) (Lyons et al. 2012). Both species overlap in distribution and are often associated species in the same habitats. However, the Crystal Darter differs from the Western Sand Darter having a frenum (tissue connecting snout to upper lip), a continuous stripe over snout, and closely positioned dorsal fins.
In Minnesota, this species occurs in medium to large rivers, usually with clean sand and gravel bottoms and moderate to swift currents of 16-22 cm/s (0.52-0.72 ft/s). In Navigation Pools 4, 5a, and 8 of the Mississippi River, they have been taken from main channel borders, side channel borders, island borders, and tailwater zones of dams. Substrates in some of these areas were a mixture of coarse sand and gravel in which cobble, boulder, and fine dark particles were embedded (Schmidt 1995; Hatch 1997). In Pools 4 and 8, Secchi readings (transparency) ranged 58-107 cm (1.9-3.5 ft.), depths were 0.4-10.0 m (1.3-33 ft.), and current velocities were 0.05-0.52 m/s (0.16-1.7 ft./s) (LTRMP 2016).
In the Red Cedar River of Wisconsin, Crystal Darters were most often found in flocculent patches of sand and gravel. These deeper loose and shifting substrates were located prior to sampling by walking wadeable reaches until blindly stumbling into the species’ habitat (Katula 2000).
Biology / Life History
Crystal Darters likely spawn in spring in Minnesota (late March or early April into June) and are nonguarding egg buriers (Hatch and Schmidt, in preparation). In the Sabine River of Arkansas, they reach sexual maturity at age 1 (males at 61 mm [2.4 in.] and females at 50 mm [2.0 in.]) standard length (George et al. 1996). In Minnesota and Wisconsin, this species grows very rapidly, reaching 85-95 mm (3.3-3.7 in.) total length at the end of their first summer. A few live to 3 years old (Hatch and Schmidt, in preparation).
The Crystal Darter prefers deeper waters during the day, 2-5 m (6.6-16.4 ft.), moving to the shallows at night. It buries itself in sand and gravel with only its eyes protruding above the silt and awaits passing prey. Small juveniles consume zooplankton, while adults primarily eat midge larvae, mayflies, caddisflies, water scavenger beetles, and nematodes (Becker 1983; Hatch 1997; Roberts et al. 2007).
Conservation / Management
Population declines have occurred across much of the Crystal Darter range from activities such as channelization, dredging, and impoundments. Dams have impacted this species by reducing the amount of suitable habitat and isolating populations. The species is particularly sensitive to siltation, requiring clear fast-flowing rivers. While it is difficult to detect using standard fish survey methods, long-term population monitoring is needed to assess trends and guide management decisions.
Iowa DNR Long Term Resource Monitoring Program (LTRMP) biologists reported Crystal Darters from several stations in Mississippi River Pool 13 near Bellevue in 2014 and 2015. During the program’s 25-year history of intensive sampling with multiple sampling equipment and techniques, only one other specimen was collected. The suspected source of these recent occurrences is a downstream dispersion from Minnesota and Wisconsin Mississippi tributaries due to an extreme and prolonged period of high stream flows in 2014 (Bowler and Schmidt 2016).
There have been attempts to culture Crystal Darters. The first was accidental, but pond culture with further research may be the best approach if future reintroductions to historical habitats are deemed necessary for the species’ recovery. In late winter 1981, the Arkansas Fish and Game Commission filled a nursery pond from the White River near Augusta and stocked walleye fry (Sander vitreus). In early May, the pond was drained revealing no walleyes but containing hundreds of young-of-year Crystal Darters (30-38 mm [1.2-1.5 in.] standard length), indicating winter breeding season at Arkansas’ latitude (Robison and Buchanan 1988).
Two attempts have been tried in aquariums. The first produced eggs in two consecutive years that never developed, suggesting the broodstock was comprised of females (Katula 2000). Conservation Fisheries of Knoxville, Tennessee, recently used Crystal Darters as a surrogate species to develop culture techniques for the federally endangered Diamond Darter (C. cincotta). However, this effort produced only a small number of fry, which did not survive (J.R. Shute, personal communication).
Conservation Efforts in Minnesota
Stream surveys in 1990 failed to locate new records from the Whitewater, Zumbro, Cannon, and St. Croix rivers (Schmidt 1991). However, surveys in 1994 in the Mississippi River located the species at seven locations (Schmidt 1995). Minnesota DNR Fisheries sampled the species in the Zumbro River at Millville twice in the 1990s, and sampling by the University of Minnesota collected one individual at Taylor’s Falls on the St. Croix River. Minnesota DNR and University of Minnesota mussel biologists have observed and photographed this species a number of times at night using scuba gear in the St. Croix River downstream of Taylors Falls (M. J. Davis, B. Sietman, and M. Hove, personal communication). The designation of the St. Croix National Scenic Riverway protects much of the species mainstream habitats. The recent inception of Minnesota’s Clean Water Legacy Program will eventually yield benefits to Crystal Darter habitats in the Root and Zumbro rivers through sediment load reductions from establishing buffer strips for riparian protection.