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Freshwater mussels of the family Unionidae, also known as naiads (or naiades), have inhabited fresh waters around the world for the last 400 million years (Smith 1976). They prospered in early streams and rivers long before dinosaurs and have continued to endure long after these giant reptiles passed into oblivion. Unfortunately, the continued survival of this group of unique mollusks is seriously in question. Early in the century, H.M. Smith of the U.S. Bureau of Fisheries (Smith 1919) wrote "Fresh-water Mussels: A Valuable National Resource Without Sufficient Protection" to highlight concerns about declining populations of unionids. Neves (1992) reported that among the nearly 300 North American species and subspecies, 6% are presumed extinct, 15% are currently federally listed as Threatened or Endangered, and another 23% are proposed to be listed. Similarly, Shannon et al. (1992) indicated besides the 18 species assumed extinct already, it is estimated that another 45 additional species will go the way of the passenger pigeon within the next 10 years. If unionid mussels were seal pups, pandas, whales, or song birds, the public and scientific communities alike would be outraged.
Unionids were actively harvested by Native Americans for food, tools, and jewelry, but with little apparent impact on mussel populations. Occasional "pearl rushes" prompted overharvest in some areas following discovery of gem-quality pearls, especially in the 19th and 20th centuries (Kunz 1897; Strecker 1931; Clemens 1981). Major damaging harvest occurred just before the turn of the century when mussel shells became the standard material for buttons; this industry continued into the mid-1900s and waned only after the development of plastics and depletion of many mussel beds (Coker et al. 1921). During this time, pollution, dam-building activity, and other human-related environmental alterations impacted freshwater mussel populations as well. Still, mussels survived. More recently, additional new problems threaten these animals even more. Environmental problems have, of course, continued, and in some cases worsened. Further, the development of the cultured pearl industry, which requires "beads" made of American freshwater mussel shell as a nucleus to produce cultured pearls, has also prompted massive harvest. The exotic Asian clam, Corbicula sp(p)., which competes with native mussels for food and space (Vidrine 1993), has spread throughout Texas in the last three decades. Perhaps most disconcerting of all is the invasion of the zebra mussel (Dreissena polymornhal), which entered the Great Lakes in the 1980s and is now spreading south along several major river systems including the Mississippi River. Zebra mussels can encrust native unionids and result in mortalities and shifts in population composition (Berg et al. 1992); upwards of 10,000 zebra mussels have been reported on a single native freshwater mussel (Schloesser 1992).
Despite the large number of species in the family, unionids have received relatively limited attention from the scientific community. Early taxonomists made efforts to list and classify American species. In the 1890s, the U.S. Fish Commission began scientific studies of mussels, but invariably with commercial harvest in mind (Neck 1982a). Major distributional, biological, and ecological studies have been limited. Texas in particular has been especially wanting in this area. A brief descriptive listing of species by Strecker (1931) and a number of subsequent journal articles were basically the extent of such efforts in Texas in the last half century. Indeed, when Read and Oliver (1953) reported briefly on freshwater mussels near Dallas, they specifically noted that little had been added to the understanding of unionids since Strecker's work. Among the 51 or so species of unionids found in Texas, larval stages have been described for only 55% (another 14% are described here) and fish hosts are known for only 47% (with several others added here) [RGH]. With the increased pearl-shell harvest and impending zebra mussel invasion (zebra mussels reached Mississippi and Arkansas in 1992), the need for baseline reference material was clear.
This publication provides an introduction to the unionid mussels of Texas with descriptions of each and a general overview of the family, as well as brief descriptions of other bivalves which may occur in the fresh waters of the state. Sadly, terms like "unreported" and "undescribed" appear far too many times herein; efforts are now being made to fill some of these voids.
Other Common Names
Paper heelsplitter, paper shell, cottonmouth.
(Murray and Leonard 1962; Oesch 1984).
- Unio fragilis, Rafinesque 1820
- Unio gracili, Barnes 1823
- Lasmonos fragilis, Rafinesque 1831
- Unio dolosus, Lea 1861
- Lampsilis gracilis, Simpson 1900
- Paraptera racilis, Ortmann 1912
- Legtodea cilis, Simpson 1914
- Lampsilis racilis (Barnes) = U. fragilis (Raf.) = Lasmonos fragilis (Raf.), Walker 1918
- Leptodea fragilis, Ortmann and Walker 1922
Occurs from the Colorado River basin into drainages to the north and east.
(Murray and Leonard 1962; Valentine and Stansbery 1971; Oesch 1984; Fuller 1985; RGH).
Size: To 133 mm shell length in Texas (Neck 1989a); also reported to 160 mm in Missouri (Oesch 1984), 152 mm in Kansas (Murray and Leonard 1962), and 178 mm in Illinois (Haas 1941).
Shell: Oblong, elliptical, or ovate; females expanded posteriorly; males often greatest in height near the middle of the valve, females posterior to the middle; thin, brittle, fragile, often cracks upon drying; large right-angled wings may be present or absent (eroded or broken) posteriorly, and a smaller anterior wing may also be present; dorsal margin may be rounded if wing is completely eroded away to strongly angular with wing present; rounded anteriorly and posteriorly (more bluntly truncated in females), ventrally straight or slightly rounded; compressed side to side; beaks low, beak cavities shallow; posterior ridge rounded and ill-defined; shell relatively smooth, with fine concentric growth lines.
Shell Teeth: Two left pseudocardinal teeth low thin, parallel to dorsal line, but posterior tooth may be absent; single right pseudocardinal tooth low, thin; left lateral teeth short, low, thin, slightly curved; right lateral tooth smooth, slightly curved, sometimes high.
External Color: Light yellowish-tan or yellow-green in young, to brown in older specimens; green rays may cover the entire shell or be completely absent.
Internal Color: Bluish-white to silvery-white; sometimes slightly pink-tinted dorsally; purple coloration may be most intense at muscle scars; often iridescent; nacre relatively thin and transparent in very young shells.
Soft Tissues: Whitish in color, gills dark tan but in gravid females become enlarged, gelatinous, and semitransparent (rather than milky-white as in other unionids); foot is very large and extensile.
It may occur in small streams to large rivers; on mud, mud and gravel, gravel, and occasionally on pure sand; and in clear or murky waters (Oesch 1984); it may occur on silt (Buchanan 1980), or rocky bottoms, in deep or shallow waters (Murray and Leonard 1962); in still to swiftly flowing waters (57.9 cm/second) (Buchanan 1980). The species has been reported as tolerant of environmental alteration, and as possibly exploiting impoundments (Fuller 1985).
The species is bradytictic, breeding August to mid-July (Ortmann, cited in Oesch 1984). Glochidia have been reported present April (Hoggarth 1988), and July and August (Baker 1928); Littleton (1979) reported gravid females in the Navasota River, Texas, in March, with a minimum shell length at maturity of 53 mm; females with glochidia were taken in central Texas June-September [RGH].
Shells very small, 0.085 x 0.075 mm, subelliptical, ventral margin rounded, gaping hookless, short hinge line undulate (Utterback 1915-16). Hoggarth (1988) described glochidia as subelliptical, very small, 0.072-0.073 mm shell length, 0.080-0.083 mm shell height, and 0.030-0.035 mm dorsally along the hinge line.
Freshwater drum (Aplodinotus grunniens) (Fuller 1978).
Fragile papershell may be confused with pink papershell Potamilus ohiensis which is often more boldly purple interiorly, sometimes nearly black exteriorly, and may have hinge teeth which are slightly larger and more clearly defined.
Ortmann (1919) described fragile papershell as an active species, often crawling with unusual speed. Strecker (1931) indicated the species would die if water depth dropped below 15.2 cm; however, specimens have been maintained at TPWD's Heart of the Hills Research Station for many months in flow-through raceways at water depths less than 15 cm.