Publication Date


Document Type


Degree Name

Master of Science in Biology (MS)

Committee Chair

Mott, Daniel J.


Overharvesting, environmental degradation, pollution, and the rapid spread of exotic mussel species have been instrumental in 15 native freshwater mussels being placed on the threatened/endangered species list by the Texas Parks and Wildlife Department. Nine species of freshwater mussels currently listed at a state level are being petitioned by the U.S. Fish and Wildlife Service to be placed as threatened or endangered under the Endangered Species Act of 1973. Truncilla cognata, commonly known as the Mexican Fawnsfoot, is considered to be endangered at both the federal and state level. Little is known regarding habitat preference, larval stage, fish host, and spawning seasons of T. cognata. The focus of this study was to examine various hydrological environments in order to assess current distribution, locate and evaluate existing populations, and determine the habitat requirements of T. cognata. Several sites were selected along an 88 kilometer stretch of the Rio Grande in Webb County near Laredo, TX. The riverbed for each site was searched for live T. cognata at multiple random points either by hand, the combination of 51 m2 plots (see Randklev et al., 2010), and/or single 1 m2 random plots. Exposed river banks were also searched for deceased specimens. Measurements of the length, width and height (thickness) of each specimen were recorded (Warren, 1958). Data collected during this research established the largest known population of T. congata with 35 live and 206 very recently dead found at a single site. Although T. cognata is not as uncommon as it was initially thought to be, optimal habitats are scarce and populations limited to substrates with a mixture of medium sand (0.25-0.5 mm) and medium (8-16 mm) to coarse (16-32 mm) gravel in continuously running shallow (< 1.0 m) waters.