Notes on habitat and burrowing behavior of obovaria jacksoniana (bivalvia: unionidae) in the upper neches river of east texas.
North American freshwater mussels in the family Unionidae represent a diverse and ecologically important group (Parmalee & Bogan 1998). They often occur in dense beds where biomass may be an order of magnitude greater than all other benthic organisms (Strayer et al. 1994). Filter feeding and burrowing behavior results in a variety of water column and sediment related ecosystem functions (Vaughn et al. 2001). Unionids are also sensitive to environmental degradation, making them important indicators of ecological integrity (Parmalee & Bogan 1998). A large number of species in the United States hold endangered or threatened status as a result of flow modification of rivers, pollution, and invasive species introductions (Lydcard et al. 2004). However, a lack of fundamental knowledge on life history, behavior and habitat associations of freshwater mussels hinders conservation efforts (Bogan 1993).
The southern hickory nut (Obovaria jacksoniana Fricrson 1912) occurs from Alabama, west to eastern Texas and in the Mississippi River drainage north to southern Missouri (Williams et al. 2008). In Texas, historically, the species occurred in the Neches, Sabine, and Red River drainages (Howells et al. 1996). Little information exists on the ecology and behavior of the southern hickorynut. Hoggarth & Gaunt (1988) reported glochidia in specimens in October; however, fish host preference has not been established (Williams et al. 2008). Habitat association data for the species has been very general. Oesch (1995) reported that southern hickorynuts showed a preference for creeks and rivers with moderate flow and gravel substrate. Williams et al. (2008) gave similar habitat preference for the species in Alabama. The few specimens that were collected in Tennessee occurred in slow water in silt and fine gravel (Manning 1989; Kesler et al. 2001). Data on southern hickorynut behavior are absent.
In November 2009, Texas Parks and Wildlife Department listed the southern hickorynut as threatened in Texas and has proposed listing this species as legally endangered. In Tennessee, fewer than six have been recorded (Parmalee & Bogan 1998), and in both Arkansas and Alabama, it is designated of special concern (Harris et al. 1997; Garner et al. 2004). The Natural Heritage Database lists it as critically imperiled in Missouri, Tennessee and Louisiana; and imperiled in Oklahoma, Arkansas and Mississippi (mdc.mo.gov/nathis 2009). In Texas, it has rarely been recorded and recent statewide surveys have not found them (Howells, pers. comm.). A small population known in Village Creek, Hardin County (Bordelon & Harrel 2004) has subsequently disappeared (Howclls et al. 2007). Improving knowledge of life history of rare species, including the southern hickorynut, serves as a key step toward their conservation. This report includes (1) detailed habitat conditions at two collection sites in the upper Ncches River, Cherokee/Anderson County, Texas, and (2) burrowing behavior in sand and gravel substrates.
In August 2009, 12 live southern hickorynuts were collected using timed surveys in wadeable (i.e., <lm deep) reaches of the upper Ncchcs River (Fig. 1). Tactile and visual perception were used to locate living mussels on the surface of the sediment and extending eight cm into the sediment. Six live mussels were collected 1.5 km upstream of the U.S. Highway 84 bridge (U.S. 84, Latitude 31.78763 N, Longitude 95.39433 W) and six live mussels 1.3 km downstream of U.S. 84 (Latitude 31.77128 N, Longitude 95.39632 W) between Cherokee and Anderson counties in east Texas. In addition to the living mussels, the valves of two deceased individuals were collected during the timed surveys. From GIS maps, it was determined that the river at these sites drains 3234 km" of primarily grazed and forested land. A wide, low lying floodplain characterizes this stream section (-20 km) and the river is not as entrenched as stream sections upstream (i.e., immediately below Lake Palestine Dam) and downstream (i.e., near State 294) but is more sinuous. Standard habitat assessment protocols were employed to assess physical/chemical conditions at the reach scale (i.e., 400m) at four equally spaced transects on 27 July 2009 (Texas Commission on Environmental Quality 2007).
[FIGURE 1 OMITTED]
All physical/chemical variables appeared comparable in both upstream and downstream reaches with the exception of temperature, turbidity and mean current velocity (Table 1). Although there was a significant difference in temperature between the two reaches, the difference of less than one degree Celsius is less than diel and seasonal variation and, likely is not biologically relevant. On a stream section scale (e.g., 5-20 km), channel width and bank angle were substantially less when compared to stream sections upstream (i.e., immediately below Lake Palestine Dam) and downstream (i.e., near State 294). The remaining physical/ chemical variables (Table 1) did not differ significantly from stream sections upstream and downstream of the two collection sites.
Table 1. Mean physical/chemical conditions (n=4) for two reaches on the upper Neches River near U.S. 84 between Anderson and Cherokee counties, Texas. Values within parentheses indicate standard error of the mean. Asterisks indicate level of significant differences between reaches (Kruskal-Wallis test, * P<0.05, ** P<0.01) Variable Above U.S. 84 Below U.S. 84 Temperature ([degrees]C) 28.64 (0.025) ** 28.47 (<0.00) ** pH 7.72 (0.02) 7.69 (0.03) Conductivity (mS/cm) 0.21 (0.00) 0.21 (0.00) Turbidity (FAU) 27.55 (0.29) * 29.98 (0.63) * Dissolved oxygen (mg/L) 6.3 (0.35) 6.25 (0.05) Wetted channel width (m) 12.88(0.77) 13.13(0.31) West bank angle ([degrees]) 3.25(1.18) 1.25(0.75) East bank angle ([degrees]) 9.75 (5.09) 5 (5.00) Mean depth (m) 0.91 (0.08) 1.13(0.23) Mean current velocity (m/s 0.22 (0.03) * 0.12(0.02) * Canopy cover (%) 28.75(18.86) 33.75 (2.39)
On the micro-habitat scale, southern hickorynuts occurred in a variety of physical conditions. In the reach downstream of U.S. 84, southern hickorynuts occurred in areas with depths of 0.4 to 1.0 m, moderate to fast current velocities, and gravel substrates (mean particle diameter 10 mm). In the reach upstream of U.S. 84, southern hickorynuts occurred in areas with 0.4 to 1.0 m depths with slow to moderate current velocities. Fine sand and silt (mean diameter <0.13 mm) containing a substantial amount of detritus comprised mostly of twigs and broken down pieces of large wood composed the substrate in this reach. Generally, wood and detritus in or on the substrate indicate poor mussel habitat, but this stream section is highly connected to its floodplain and it is likely that this section supports these mussels because channel scouring during floods is reduced (Zigler et al. 2008; Galbraith & Vaughn 2010). These findings agree with those in Tennessee, where the southern hickorynut also occurred in silty and sandy substrate (Manning 1989). Because the specimens appeared to be both light-sensitive and tactilely sensitive, they might be able to burrow quickly in such substrate in response to rising waters as suggested for species without pimples and pustules (Watters 1994; Perles et al. 2003; Allen & Vaughn 2009). Therefore, a laboratory behavior experiment was prepared to test their burrowing behavior.
Mkthods and Materials
To investigate burrowing behavior, mussels were observed in sand (mean diameter <0.5 mm) and gravel (mean diameter 8 mm) in laboratory aquaria. Eight individuals from the upper Neehes River (near U.S. 84 on the Cherokee and Anderson County border) were collected on 18 August 2009, 72 hours prior to the beginning of the experiment. Prior to the experiment, mussels were housed in a 38 liter aquarium lined with sand substrate. Each mussel was placed flat and in the center of a 38 L aquarium (25 cm by 50 cm by 30 cm) lined with either 10 cm of sand (n = 4) or gravel (n = 4) for 72 h. After 72 h, mussels were switched to an aquarium with the other substrate and the experiment was repeated. Sand and gravel were obtained from a home improvement store and washed prior to the experiment. These substrates were mined from naturally occurring deposits rather than being manufactured by crushing, so surfaces were similar in shape to the substrates that occur in the Neehes River. Prior to the experiment, observations of captive mussels indicated that most movement occurred within approximately one hour of being placed on the surface of a substrate. Once these initial movements occurred, minimal movement occurred over the next 24 to 72 h. Accordingly for the experiment, mussel movements were recorded every 0.25 h for the first 1.25 h and every 24 h after. Horizontal movement was measured as the distance between starting location and final location (after 72 h) using a ruler to measure distance moved on an X-Y axis. Vertical movements were measured as percent buried. Orientation (e.g., siphon up, angled posteriorly, etc.) was also noted. Kruskal-Wallis tests were used to test for significant differences in burrowing behavior among sand and gravel treatments.
A significant difference in horizontal movement between mussels in gravel and sand substrates was not observed (df=1, U=43.5, P=0.224). However, horizontal movement was significantly lower in the second round of experiments compared to the first round (df=1 U=57.5, P=0.007), indicating that horizontal movement may have been affected by the repeated experimental protocol. Consequently, the first experiment was analyzed separately. Following removal of the second round of experiments, a significant increase in horizontal movement was observed in gravel substrates (5.45 cm) relative to sand substrates (2.92 cm) (df=1, U=16.0, P=0.021). All horizontal movement occurred between 1 h and 48 h after the start of the experiment (Fig. 2a). Total horizontal movement always occurred in a single incident lasting less than 24 h with the exception of one individual in a gravel treatment that moved in two separate 24 h periods (Fig. 2a).
[FIGURE 2 OMITTED]
Percent buried was not significantly different between the first and second rounds of experiments (df=1, U=42, P=0.283), indicating that vertical movement was not influenced by this experimental design. Percent buried did not differ significantly between sand and gravel substrates with both rounds included (df=1, U=22.8, P=0.308) or with the second round removed (df=1, U=4.0, P=0.234). However, visually it did appear that mussels tended to bury themselves more thoroughly in sand compared to gravel (Fig. 2b). Within 48 h, six of the eight mussels buried themselves 100% in sand. By contrast, none of the mussels in gravel buried themselves completely (i.e., 100%).
Mussels oriented themselves differently in sand versus gravel substrates. With the exception of the two mussels that did not burrow, all mussels in sand oriented themselves with their siphon facing up. This probably occurred because they buried themselves completely (i.e., 100%). In gravel, mussels oriented their siphons upward but remained at an angle; not directly upward as was the case with the mussels in the sand.
Understanding burrowing behavior is important because mussels may burrow to avoid displacement during high flows and associated bed movement (Watters 1994; Allen & Vaughn 2009). This behavior may be particularly important immediately downstream of dams where sudden and high magnitude Hows cause bed scouring (Galbraith & Vaughn 2010). Thus, rapid burrowing may prevent downstream displacement. The results of this behavioral study suggest that this species of mussel is able to burrow relatively quickly. Results of this study also suggest that this species responded to movement with any disturbance such as a person touching the aquarium or even nearby movements. In the upper Neches River, 24 h or more are required to increase discharge from baseflow to bankfull flow. In this study, southern hickorynuts were able to bury themselves completely in less than 24 h, indicating that they could respond to rising water and subsequent dislodgement by burying deeper in the substrate.
Understanding variation in burrowing behavior among substrate types (e.g., sand vs. gravel) is relevant to conservation because lotic habitats are increasingly polluted with fine sediment, thus potentially altering a mussel's ability to burrow and avoid downstream displacement during high flows (Watters 1994). The results of the burrowing experiment indicate that these mussels exhibit differences in horizontal (and possibly vertical) movement between the two substrates. It cannot be concluded whether these differences are the result of a reduced ability to burrow in the gravel substrate or a behavioral decision; however, results of this study demonstrate that horizontal movement varies among substrate types and this has implications for potential displacement during high flows.
The U.S. Fish and Wildlife Service (2005) has recently established the North Neches River Wildlife Refuge upstream of the collection sites and has proposed a more extensive refuge that would encompass both collection sites. The occurrence of this state threatened species within this proposed wildlife refuge validates the suggestion of the USFWS that preservation of this river corridor is important for biological conservation.
We would like to thank reviewers R.G. Howells of Biostudies and M. May of Texas Parks and Wildlife. This research was supported by Texas Parks and Wildlife Department State Wildlife Grant T-56-1.
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MJT at: email@example.com
Matt J. Troia* and Neil B. Ford
Department of Biology, University of Texas at Tyler Tyler, Tecas 75701
* Present address:
Division of Biology, Kansas State University Manhattan, Kansas 66506
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|Author:||Troia, Matt J.; Ford, Neil B.|
|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 2010|
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