Distribution of Corbicula (Bivalvia: Corbiculidae) in the Brazos River system, Texas, 25 August-12 November 1980
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Corbicula was probably introduced into the Brazos River system in 1972-1973 and it is now the most common bivalve wherever it is found. Densities as high as 2500-3500 clams/m are reported. It was found at 34 of 66 sites. Those sites not containing Corbicula include: Lake Kirby (Taylor Co.), Buffalo Springs Lake (Lubbock Co.), the Navasota River and all of its lakes (except perhaps near its confluence with the Brazos River), Salt Fork and Double Mountain Fork of the Brazos River, White River (exclusive of White River Lake), all intermittent or shallow feeder streams, and the area below Road 1462 near Rosharon (Fort Bend/Brazoria counties) to the Gulf of Mexico. At least two forms of Corbicula are reported: the generally northern type has white nacre with purple highlights, a yellow or brown/black exterior, and an angular posterior shell shape in the adult; the southern type has deep purple nacre and exterior, is smaller in size, and generally has a thinner, more equilateral shell. Some Corbicula shells in the middle and southern areas have anterior-posterior brown pigment markings. Both forms may be found together in some southern locales but the purple form becomes predominant further downstream. Southern river and stream shells of both types tended to be smaller than northern forms and may indicate a more recent introduction, genetic variation, or environmental stress. Hand collecting net dredging and skin diving methods were used to collect the samples. Considering the short period of time that Corbicula is thought to have been in the Brazos River system, expansion of its range is probably not complete. Increased use of Lake Limestone may encourage its spread to the Navasota River.