Abstract-North American freshwater mussels are diverse and
ecologically important, but are highly imperiled as a result of
alterations to river habitat. The southern hickorynut (Ohovaria
jacksoniana) occurs in the southeastern United States and has recently
been listed as threatened in Texas; however, minimal information exists
on its habitat associations and behavior. Here we (1) describe, in
detail, habitat conditions at two collection sites in the upper Neches
River in east Texas and (2) present the results of a laboratory
experiment on burrowing behavior in gravel and sand substrates. We
collected a total of twelve live mussels at two sites in the upper
Neches River. Mussels occurred in reaches with highly connected
floodplain. Mussels occurred in shallow water (< 1.0 m) and substrate
ranging from silt to gravel (i 0 mm diameter). In the burrowing
experiment, horizontal movement was significantly greater in gravel
compared to sand substrates. Vertical movement did not differ
significantly between sand and gravel treatments. All movements occurred
within the first 24 h. These results suggest that horizontal movements
in southern hickorynuts can differ among substrates and horizontal and
vertical movement can occur relatively quickly. Rapid horizontal and
vertical movement may be important in avoiding displacement and
mortality during unpredictable Hoods of regulated rivers. Moreover,
sediment pollution has altered substrates which may influence burrowing
behavior of this species.
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
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.,
[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.
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
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
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
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
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: firstname.lastname@example.org
Matt J. Troia* and Neil B. Ford
Department of Biology, University of Texas at Tyler Tyler, Tecas
* Present address:
Division of Biology, Kansas State University Manhattan, Kansas
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)