INTRODUCTION

In Texas, the current status and life history of rare freshwater mussel (Bivalvia: Unionidae) species are poorly understood. False Spike, Quadrula mitchelli (Simpson, 1895), is a rare species of mussel endemic to Central Texas and the Rio Grande drainage. Strecker (1931) described the species as being common wherever it was found. Over the last 30 years, considerable effort has been spent trying to locate live individuals of this species in stream segments where it historically occurred, but to date contemporary accounts are largely based on weathered shell material (Howells, 2010). The only exceptions are the discovery of several live individuals of Q. mitchelli in the Guadalupe River and a fresh dead individual in the San Saba River (Randklev et al., 2012; Randklev et al., in press). These accounts indicate that Q. mitchelli is still extant, but the absence of this species in other parts of its range suggest Q. mitchelli is no longer common, an observation reported as early as the 1970s (Stansbery, 1971). The decline of this species in stream segments where it once occurred has been attributed to anthropogenic impacts to streams and rivers coupled with record droughts and floods in the late 1970s and early 1980s in Texas (Howells, 2003). As a result, Q. mitchelli was designated as state threatened in 2009 (Texas Register 35 2010) and is currently being reviewed for listing under the Endangered Species Act (ESA; Federal Register 76 2011).

Historically, the range of Q. mitchelli included the Rio Grande, San Antonio, Guadalupe, Colorado, and Brazos river basins (Howells et al., 1996; Howells, 2010). The species is likely extirpated from the Rio Grande drainage (Howells, 2003). In central Texas, a single subfossil valve was collected from Salado Creek (Howells, 2002), representing the only record of the species in the San Antonio River drainage. Within the Guadalupe River drainage, Strecker (1931) and Wurtz (1950) collected live individuals from the Guadalupe River in the early decades of the twentieth century. Valves of a recently dead individual (shells in good condition, but soft tissue absent; Howells, 2003) were collected in the San Marcos River, a major tributary of the Guadalupe River, in 2000 (Howells, 2001). Quadrula mitchelli has been historically collected from Pecan Bayou (R.G. Howells database), Johnson Fork Creek (R.G. Howells database), Pedernales (Howells, 1994), San Saba (Strecker, 1931; Howells, 1995), and Llano (Strecker 1931; Howells 1996) rivers within the Colorado River drainage and from the Brazos (R.G. Howells database), Lampasas (R.G. Howells database), and Leon rivers (Strecker, 1931; R.G. Howells database) within the Brazos River drainage. Until recently, however, the only evidence to suggest the species still exists in Texas was the discovery of a valve of a fresh-dead individual (soft tissue present) from the San Saba River (Randklev et al., in press) and seven live individuals collected in the Guadalupe River in 2011 (Randklev et al., 2012).

METHODS

In July 2012, we conducted multiple-passdepletion surveys for state-threatened mussel species in the lower San Saba River as part of a larger, ongoing study in the river. While conducting timed searches, we collected one live Q. mitchelli (Fig. 1) at a site located 11.3 km east of San Saba, San Saba Co., Texas, approximately 200 m upstream from a fresh-dead specimen reported by Randklev et al. (in press). Gonadal fluid was extracted to determine sex and reproductive viability (Saha & Layzer, 2008).

FIGURE 1

Live individual of Quadrula mitchelli collected from the San Saba River, San Saba Co., Texas.

To improve our understanding of Q. mitchelli's habitat, we recorded physical measurements of habitat at the site. Six equidistant cross-section transects along the length of the site (76.5 m) were used to determine site-specific habitat characteristics. We measured depth (m) and velocity (ms-1) at 0.5 m increments along each transect to determine mean discharge (m3s-1). Pebble counts (Wolman, 1954) were conducted along each transect to determine median substrate particle size (Gordon et al., 2004). Additionally, three 0.25-m2 quadrats, placed on and directly adjacent to where the Q. mitchelli individual was collected, were used to determine microhabitat characteristics. We measured depth, velocity, and substrate characteristics (dominant, subdominant, and percentage fine sediment) for each of the quadrats mentioned above. We measured shear stress with FST hemispheres (Statzner & Müller, 1989) and visually determined percentage of benthic algae within each quadrat. Canopy cover (%) was visually estimated by three observers standing over the middle quadrat.

RESULTS

The individual collected was consistent with taxonomic descriptions provided by Howells (2010) and measured 68.4 mm in shell length, representing a large adult. The presence of eggs in gonadal fluid extracted from the visceral mass clearly indicated that the individual was a viable female capable of reproducing, but the individual appeared not to be gravid at the time of sampling. Timed searches were conducted for a total of 70.6 person-hours (p-h) at the site, with an overall catch-per-unit-effort of 13.1 mussels collected per p-h of search effort (Table 1). Eight species were collected throughout the site during our survey, including four species listed as state threatened (Table 1; Texas Register 35 2010). Of these, Q. houstonensis, Q. petrina, and Truncilla macrodon are listed as candidates for protection under the Endangered Species Act (Federal Register 76 2011).

TABLE 1

Freshwater mussel species collected from one site on the San Saba River, San Saba Co., Texas where one live individual of Quadrula mitchelli was observed. Total individuals collected and catch-per-unit-effort (CPUE) are provided.

The site inhabited by the Q. mitchelli individual was characterized by steep banks with extensive riparian vegetation and adjacent land uses comprised of pecan orchards and rangeland. The channel was relatively wide and shallow with gravel and cobble substrates and moderate to high water velocities (Table 2). The macrohabitat of the site consisted of a run-riffle-pool sequence. We collected the individual in a run, immediately upstream from where flow transitioned into a riffle, and Q. mitchelli was observed burrowed in very coarse gravel. Benthic algae were relatively abundant on gravel and cobbles and on the shells of live mussels.

DISCUSSION

Limited information regarding habitat preferences for Q. mitchelli exists. Wurtz (1950) and Randklev et al. (2012) collected live specimens in the Guadalupe River at sites with relatively shallow depths (< 0.7 m) with gravel and cobble substrates, similar to habitat observed in the present study. However, Wurtz (1950) noted water lilies at a site where Q. mitchelli was present but no other study reported macrophytic vegetation where the species was collected.

While only one individual was collected during our survey, the physical habitat measurements we recorded might be of use in locating similar habitat for Q. mitchelli in streams within the Colorado River basin. Also, if this species becomes listed under the ESA, our observations of habitat might assist resource managers with mapping of critical habitat. Finally, there is little information available on the effort needed to collect this species by using the timed-search method. Therefore, the amount of effort we invested to locate a live individual of Q. mitchelli can be used to guide surveys targeting this species, especially in streams where it is suspected to occur in low densities.

The status of Quadrula mitchelli in Texas, based on historical and contemporary surveys, is tenuous. The species appears to have been extirpated from much of its range and until our discovery in the San Saba River has only been recently collected alive from the Guadalupe River (Randklev et al., 2012). Given the amount of time expended to collect this individual (70.6 p-h) and the fact that we observed this species at only one site, despite surveying other locations with similar effort, does not bode well for Q. mitchelli in the San Saba River. Thus, it appears that Q. mitchelli is on the brink of local extinction in this river, which is problematic because it is only known to persist at one other locality. Currently, there are no substantive plans to mitigate the decline of this species which indicates to us that the likelihood of recovering Q. mitchelli is low unless prompt conservation action is taken by Texas Parks and Wildlife Department and U.S. Fish & Wildlife Service.

TABLE 2

Site-specific habitat and microhabitat estimates collected from San Saba River, San Saba Co., Texas where one live individual of Quadrula mitchelli was collected. Microhabitat refers to habitat measured from quadrats on and adjacent to the area where Q. mitchelli was collected.