Page 15 - The Freshwater Mussels of Oklahoma

numerous river systems that drain into the Gulf of Mexico. Through much of the
Mississippi River system, Q. quadrula is the widespread Quadrula species. In the upper

Map 1. Shaded areas showing distribution of one form of Quadrula apiculata in
Oklahoma and Texas (upper Red River and Colorado River drainages).

Red River system the form of Quadrula differs considerably from that in the lower Red
River and its tributaries. The Washita River (from the Texas Panhandle through western
Oklahoma) joins the Red River at Lake Texoma has a form of Quadrula that appears to
be Q. apiculata. In fact, it is most similar to the form of Q. apiculata that occurs in the
Colorado River system! Is this a case of convergent evolution? Or, is this a case of
dispersal from the headwaters of one system to the headwaters of another? Or, perhaps
this is the result of stream capture! Was the Washita River drainage once a part of the
Colorado River system but was captured by the Red River system? Geologic literature
does give some credence to some of the more eastern Red River tributaries having been
once part of some of the more eastern Texas rivers. Electrophoretic studies of these two
populations may shed some light on this zoogeographic problem or at least narrow the
possibilities. There are several other mussels that show similar distribution problems in
this region (Truncilla and Potamilus). In addition, there may be similar distribution
patterns present in crayfish (Rollin Reimer, pers. comm.) and possibly other aquatic
invertebrates and vertebrates. This is just an example of a zoogeographical problem and
some of its possible explanations for one small region. Every area has numerous
examples of their own problems to solve with their own unique set of drainage patterns,
geology, etc.
Based on faunal affinities, Roback, Bereza and Vidrine (1980) illustrated their
concept of unionid faunal zones and provinces (see Map 2).

14