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Herpetological Conservation and Biology 7(2):258–264.
Submitted: 27 April 2012; Accepted: 3 August 2012; Published: 10 September 2012.
258
DIET OF RADIOTRACKED MUSK TURTLES, STERNOTHERUS
ODORATUS, IN A SMALL URBAN STREAM
CAITLIN E. WILHELM1 AND MICHAEL V. PLUMMER
Department of Biology, Harding University, Searcy, Arkansas 72143, USA, e-mail: plummer@harding.edu
1Current address: Department of Biology, Missouri State University, Springfield, Missouri 65897, USA,
e-mail: Wilhelm211@live.missouristate.edu
Abstract.—We used radiotelemetry to assess the diet of Sternotherus odoratus in Gin Creek, Arkansas, USA. Gin Creek
is a small, frequently disturbed, urban stream in which the invasive Asiatic Clam, Corbicula fluminea, has attained high
densities. Turtles foraged in small, well-defined home ranges within which we sampled the substrate for potential food
items. The diet of S. odoratus, as determined by analysis of fecal samples, compared favorably to prey availability in the
creek. The diet was similar to that found in previous dietary studies of typically omnivorous S. odoratus except that clams
were eaten much more frequently. An Index of Relative Importance (IRI) revealed the most important prey in both the
fecal samples and substrate was C. fluminea. We suggest the diet of S. odoratus in Gin Creek has shifted toward
molluscivory as the result of a probable 40-year presence of C. fluminea.
Key Words.– Corbicula; diet; invasive clams; Musk Turtle; radiotelemetry; Sternotherus odoratus; stream
INTRODUCTION
The diet of bottom-feeding freshwater turtles often
reflects the composition of the benthic macroinvertebrate
communities where they feed, which in turn may be
strongly affected by the presence of certain bivalve
mollusks (Newell 2004). For example, the recent
invasion of North American streams by the Asiatic Clam
(Corbicula fluminea) often results in the clam being a
dominant community influence (McMahon 1982;
Karatayev et al. 2003; Karatayev et al. 2005).
Corbicula is thought to affect ecosystem nutrient cycles
and energy flow (Sousa et al. 2008), community
composition (Werner and Rothhaupt 2008), and
competitive interactions of native macroinvertebrates
(Kraemer 1979). It also may affect dietary diversity of
turtles (Lindeman 2006a). For example, map turtles
(Graptemys spp.) respond to high densities of invasive
clams (Corbicula, Dreissena) by feeding heavily on the
clams instead of other food items typically found in the
diet (Shealy 1976; Moll 1980; Lindeman 2006a, 2006b;
Ennen et al. 2007).
The Common Musk Turtle (Sternotherus odoratus) is
a widely-distributed freshwater turtle, which prefers
relatively shallow waters with abundant submerged
vegetation and woody debris; it rarely leaves the water
even under drought conditions (Gibbons et al. 1983;
Ernst et al. 1994; Rowe et al. 2009). Generally
considered a bottom-feeding omnivore, S. odoratus has
been reported to feed on crayfish, insects, mollusks, fish,
amphipods, arachnids, algae, seeds, and other plant
material throughout its range (e.g., Berry 1975; Marion
et al. 1991; Ernst et al. 1994; Ford and Moll 2004).
Gastropod mollusks have occasionally been reported as
frequently eaten. For example, snails were found in 96%
of S. odoratus stomachs in Oklahoma (Mahmoud 1968)
and constituted 94% of the dietary animal biomass in
Florida (Bancroft et al. 1983). When given a choice
among five prey types in the laboratory, S. odoratus
preferred snails (Mahmoud 1968). In contrast, rarely
have bivalve mollusks been dominant in the diet
(Patterson and Lindeman 2009).
Our objective is to describe the diet of S. odoratus in a
small, frequently disturbed urban stream in which
invasive Corbicula fluminea has attained high densities.
We reasoned that because the diet of a generalist
omnivore should reflect availability of food items in the
habitat, S. odoratus would feed heavily on the
Corbicula. Our methods were distinctive among dietary
studies of turtles in that we assessed diet by resampling
individual radio-tracked turtles. This procedure
permitted us to compare dietary food items with food
items available in the turtles’ home ranges to determine
if the food eaten was actively selected or simply
reflected what was available to turtles.
MATERIALS AND METHODS
We collected turtles by hand from 12 May - 23 June
2010 in Gin Creek, White County, Arkansas, USA. The
entire 6 km length of Gin Creek is included in the town
of Searcy and provides the major drainage for the
southern part of the city (Anonymous 1975; Muncy
1976). We caught turtles in a 600 m section of the creek
located between 15S UTM 3901000 N, 616850 E and
UTM 3901600 N, 616850 E. Gin Creek at our study
Copyright © 2012. Caitlin E. Wilhelm. All Rights Reserved.
Herpetological Conservation and Biology
259
area varies in width from approximately 5.0–9.5 m with
riffles and pools ranging in depth from approximately 2–
100 cm. Riffle substrate is mostly hard clay whereas
pools contain unconsolidated sediments (silt, gravel,
organic debris) up to ~25 cm in depth. Emergent
vegetation is limited to narrow bands along shoreline
banks and isolated small islands. Pockets of submerged
woody and leafy debris are common. Suspended and
attached algae are seasonally common throughout the
creek. Muskrat (Ondontra zibethicus) burrows are
common in the creek banks.
Upon hand capture, we recorded location (UTM
coordinates), habitat (pool, riffle), water depth (nearest
10 cm), and substrate type (hard clay, soft mud, debris,
rock, leaves). In the laboratory, we determined the sex
of turtles and measured their mass (g) and carapace
length (CL) in mm. Because the diet of adult
Sternotherus minor may differ from that of juveniles
(Tinkle 1958), we used only adult S. odoratus (CL ≥ 79
mm; Tinkle 1961). We numbered each turtle on the
second costal scute and pectoral scute with a Dremel©
rotary etching tool. We adhered radio transmitters
(Model LF-1; L.L. Electronics, Mahomet, Illinois, USA)
to the posterio-lateral edge of the carapace of 17 turtles
(10 males; seven females) with Plastic Welder® and
released the turtles back into the creek at the site of
capture 24 h after attachment. Transmitter mass was <
6% of each turtle’s body mass. We tracked turtles daily
until 6 August 2010; we determined position coordinates
with a Garmin eTrex® GPS.
We collected each turtle every 14–28 days to obtain
fecal samples. Each turtle was brought to the lab and
housed in a one gallon jar with 50 mm depth of water for
48 hours (Parmenter 1981) to obtain fecal samples. We
strained fecal samples and dried each in a drying oven
for 24 hours at 50 ºC. We sorted and identified samples
under a dissecting microscope and grouped items into
one of seven categories: algae, seeds, unidentifiable
plant material, clams, snails, insects, and other (primarily
detritus, rocks, sand, and other inorganic matter). For
each category, we counted the minimum number of
items if possible and measured mass (g) and volume (ml
by volumetric displacement). Small Corbicula were
often ingested whole, facilitating counting and
measuring minimum shell length. We estimated the
number and maximum shell length of larger Corbicula
based on shell fragments containing a hinge. Seeds were
mostly fragmented thus preventing counting.
To determine food availability in the creek, we
established nine substrate sampling stations spaced ~50
m apart throughout home ranges of turtles. At each
sampling station, we took three substrate samples every
two weeks, one near each bank and one in the middle of
the creek. We used a bottomless 5 gal bucket to
circumscribe a 530 sq. cm area at each sampling site.
We collected the top 5 cm of the substrate using a small
shovel and suspended material from the water column
with a small net. Samples were processed as above. We
determined the number of Corbicula by counting living
individuals and/or complete shells of dead individuals.
Because we did not know precisely where individual
turtles fed, we compared fecal contents to a pooled
sample of all sampling stations.
In cases where the dietary mass was < 0.001 g, we
assigned the value 0.0005 g for quantitative analysis. We
assigned volumes too small to measure accurately a
value of 0.01 ml. Following Lindeman (2006b), we
calculated an Index of Relative Importance (IRI) for
each prey category i as IRIi = 100 ViFi / Σ (ViFi), where
Vi = mean percentage of total volume and Fi =
percentage frequency for each prey category. We
calculated home range length for each turtle as the linear
section of the creek that extended from the extreme
upstream telemetry location to the extreme downstream
location for that individual. Because turtle movement
was limited to the water channel, we calculated home
range area for each turtle by multiplying the home range
length by the average creek width. Average creek width
was calculated by averaging the widths of the creek at
each substrate sampling station. Distance moved each
day was the difference between successive daily
relocations.
We used Systat® 13 (SYSTAT Software, Inc.,
Richmond, California, USA) for statistical analyses. We
checked for normality and homogeneity of variances
and, when necessary, log-transformed data to meet
assumptions of normality. We tested for sexual
differences in home range size and daily movement with
t-tests. We used Chi-square tests to compare habitat
usage on opposite creek banks and to compare sexual
differences in frequency of Corbicula in fecal samples.
We used ANCOVA with CL as the covariate to correct
for body size to test for sexual differences in fecal
composition of Corbicula by number, volume, and mass.
To assure independence, the data used in ANCOVA
consisted of a single mean value calculated over all
sampling periods for each turtle. Descriptive statistics
reported from ANCOVA analyses are least squares
means reported as mean ± SE. For all tests, α = 0.05.
RESULTS
Movement.—Two of the 17 turtles fitted with
radiotransmitters moved out of the study area 100–800
m upstream within four days of release and remained
there for the duration of the study; they were not used in
further analyses. Of the 544 telemetry relocations on the
remaining 15 turtles (36 ± 3 relocations ea.), 95% were
located within the central 392 m section of the 600 m
study area (Fig. 1). These turtles established broadly
overlapping home ranges (Fig. 1), which averaged 176 ±
23 m in length and 0.15 ± 0.02 ha in area. Home range
Wilhelm and Plummer.—Diet of Sternotherus odoratus.
260
size did not differ between males and females (t = 2.06,
df = 13, P > 0.05). Turtles moved an average of 23 ± 2.5
m (max 279 m) per day, but frequently (41%) did not
move from one day to the next. Daily movement did not
differ between males and females (t = 0.21, df = 13, P >
0.80). All turtles moved within home ranges, consisting
mostly of shallow pools with mud, gravel, and detritus
substrates. Most (86%) relocations were within 1 m of
the creek bank, most often (72%) the west bank (χ2 =
156.6, df = 1, P < 0.001), which had denser vegetation,
more overhanging limbs and roots, more submerged
woody and leafy debris, and more muskrat burrows
compared to the southeast bank. In 30% of all
relocations, turtles were inside muskrat burrows. Of the
15 tracked turtles, 13 were located inside muskrat
burrows at least once. We did not observe turtles on
land. Movement, and presumably foraging, of each
turtle occurred in areas where the substrate was sampled
for food availability.
Diet.—The dominant food types in fecal samples were
mollusks and seeds, together constituting 89.2% by
volume and 97.8% by mass of the diet of S. odoratus
(Table 1). Mollusks, of which 72% were Corbicula and
28% were snails, were clearly the most important food
type in terms of frequency, volume, mass, and IRI
(Table 1). The mean minimum length of Corbicula in
fecal samples was 2.7 ± 0.30 mm (range 1–7 mm) and
FIGURE 1. Extent and overlap of home ranges in 15 radiotracked Sternotherus odoratus in Gin Creek, Arkansas, USA using UTM coordinates.
Black bars indicate males; gray bars indicate females. Vertical line on each bar represents the initial capture location for that individual.
TABLE 1. Dietary composition of fecal samples of Sternotherus odoratus and substrate samples in Gin Creek, Arkansas, USA. IRI = Index of
Relative Importance. Frequency is the percentage of turtles containing a particular food item.
Fecal samples Substrate samples
Food type IRI
No.
%
Frequency
%
volume
%
mass
No.
%
Frequency
%
volume
%
mass
Mollusks 71.4 2136 80.0 61.6 90.3 884 100 93.7 94.2
Corbicula 58.9 1535 56.7 58.3 -- 881 100 93.4 94.1
Snails 12.5 601 55.0 3.3 -- 3 2.7 0.3 0.1
Seeds 22.9 -- 48.3 27.6 7.5 -- 15.7 < 0.01 < 0.01
Insect parts 4.4 -- 53.0 4.8 0.8 -- 16.7 0.2 < 0.001
Plant parts 1.2 -- 31.7 1.9 0.5 -- 45.4 2.1 1.8
Algae 0.1 -- 8.3 1.0 0.4 -- 33.3 4.0 4.0
Other < 0.1 -- 48.3 3.1 0.5 -- 0.9 < 0.01 < 0.001
Herpetological Conservation and Biology
261
the mean estimated maximum length was 7.9 ± 0.49 mm
(range 3–14 mm). The frequency of fecal samples
containing Corbicula was similar in females (83%) and
males (75%; χ2 = 3.35, df = 1, P > 0.60). The number of
Corbicula in fecal samples ranged from 0–86, except for
female no. 26 who defecated 864 very small (1–2 mm)
clamshells. For comparisons of Corbicula feeding
between the smaller (85 ± 2.4 mm CL) males and larger
(104 ± 2.5 mm CL) females, we treated the no. 26
sample as an outlier based on the extreme number and
small size of the ingested clams. ANCOVA revealed
that CL affected the mass (F1, 13 = 5.49, P < 0.05) but not
the number (F1, 13 = 0.46, P > 0.50) or volume (F1, 12 =
3.19, P > 0.10) of Corbicula in fecal samples. Sex
affected the volume (F1, 12 = 4.87, P < 0.05) and mass
(F1, 13 = 7.77, P < 0.05) of Corbicula in feces, but not the
number (F1, 13 = 0.48, P > 0.50). Female feces contained
34× the volume (females 0.68 ± 0.155 ml, males 0.02 ±
0.210 ml) and 2.8× the mass (females 22.3 ± 2.64 g,
males 8.1 ± 3.67 g) of Corbicula compared to males.
Mollusks had the highest IRI in the diet with a value >
3× that of seeds, the next highest category (Table 1). For
all mollusks (clams and snails), summed IRI values were
females = 67.5 (Corbicula 59.2, snails 8.3) and males =
61.7 (Corbicula 36.6, snails 25.1).
Substrate samples.—As in the fecal samples,
mollusks were the dominant food type in substrate
samples, constituting > 90% by volume and mass (Table
1). All substrate samples contained mollusks, of which
99.8% were Corbicula. Other dietary components were
each < 5% by volume and mass. The mean minimum
length of Corbicula in substrate samples was 10.5 ± 0.62
mm (range 4–20 mm) and the mean maximum length
was 23.8 ± 0.55 mm (range 19–32 mm). Density of
individual Corbicula (living individuals + complete
shells) in 36 substrate samples ranged 0–3,453 m-2
(mean 452.8 ± 95.8 m-2). Except for Corbicula, all
substrate dietary components constituted < 5% of
substrate samples by volume and mass; some
components, such as snails, seeds, and insects, occurred
more frequently in feces than in the environment (Table
1).
DISCUSSION
Our results are largely consistent with previous
movement and dietary studies on S. odoratus. Data
from mark-recapture (Holinka et al. 2003; Smar and
Chambers 2005; Andres and Chambers 2006) and
radiotelemetry (Belleau 2008; Rowe et al. 2009) studies
of S. odoratus have shown that individuals are relatively
sedentary and move within discrete aquatic home ranges
to which they return if displaced. Our radiotelemetry
data indicated less daily movement and smaller home
ranges than radio-tracked S. odoratus in larger habitats
(Belleau 2008; Rowe et al. 2009). Small home ranges in
a small creek also characterized radio-tracked
Sternotherus depressus (Dodd et al. 1988). Having
small home ranges facilitated our sampling efforts as all
turtles foraged within well-defined areas of mostly
comparable habitat where we systematically sampled
potential food items.
Our dietary results are consistent with other dietary
studies on S. odoratus, with one notable exception. The
species has been described as an omnivore whose diet
includes crayfish, mollusks, insects, fish, amphipods,
arachnids, algae, seeds, and other plant material
(Mahmoud 1968, 1969; Berry 1975; Marion et al. 1991;
Ernst et al. 1994). A recent dietary analysis of a S.
odoratus population in a lake located within 250 km of
Gin Creek is generally typical of the species with a
Corbicula frequency of approximately 17% and a
proportion by volume of < 2% (Ford and Moll 2004).
The major dietary difference we found was a
pronounced frequency and proportion by volume of
Corbicula compared to previous studies, which indicates
a more molluscivorous diet. Our results suggest females
eat more bivalves than do males and males eat more
snails than females, as indicated by the IRI. This pattern
of sexual dimorphism in diet, as well as greater body
size and relative head width in females, has been
reported previously in Graptemys spp. (Lindeman
2006a, 2006b) and other Sternotherus spp. (Berry 1975),
but not in Sternotherus odoratus.
The methods of diet determination we used by
examining feces may have limited our data compared to
dissection or stomach flushing techniques. For example,
it may be difficult to detect soft-bodied organisms and
measure the original volume and mass ingested when
analyzing only feces (Marion et al. 1991; Lindeman
2006a). Despite these limitations, dietary results for S.
odoratus determined by fecal analysis (Marion et al.
1991), gut dissection (Berry 1975), and stomach flushing
(Ford and Moll 2004) have yielded similar qualitative
results.
Our study provides interesting dietary results in a
small, frequently disturbed, urban stream in which
maximum Corbicula density (3,453 clams m-2) compares
favorably with maximum densities reported from other
localities such as Virginia (2,990 clams m-2; Hornbach
1992), central Europe (3,520 clams m-2; Werner and
Rothhaupt 2008), and the Iberian Peninsula (2,152 clams
m-2; Sousa et al. 2008). Because the composition of our
fecal samples tracked that of the substrate samples in
Gin Creek, the molluscivorous diet likely resulted from
the abundance and availability of Corbicula rather than a
physiological molluscan preference.
The recent invasion of North American streams by
Corbicula (McMahon 1982; Karatayev et al. 2005)
appears to have altered the diet of some populations of S.
odoratus as has been suggested for Sternotherus
Wilhelm and Plummer.—Diet of Sternotherus odoratus.
262
depressus (Marion et al. 1991) and some map turtles
(Shealy 1976; Moll 1980; Shively and Vidrine 1984;
Porter 1990; Lindeman 2006a). Similar results have
been reported for map turtles and the invasive Zebra and
Quagga Mussels (Dreissena spp.; Lindeman 2006b).
Corbicula was first detected in the Arkansas River in
Arkansas in the mid-1960s and was considered
ubiquitous in the Arkansas River by 1979 (Kraemer
1976; 1979). Corbicula seems to reach its highest
densities in highly managed harsh environments
(Kraemer 1979) such as Gin Creek, which has been
channelized and repeatedly disturbed since the early
1970s to facilitate urban runoff (Plummer and Mills
2008). Assuming Corbicula invaded Gin Creek (~70 km
northeast of the Arkansas River) in the early 1970s and
that its steady increase in density was reflected in the
diet of S. odoratus, it is possible that the diet of Gin
Creek S. odoratus has been influenced by the presence
of Corbicula for 30–40 years.
Being able to exploit rapidly invading exotic mollusk
populations could have favorable population
consequences for the mostly widespread and common S.
odoratus throughout its North American range and
possibly also in Iowa, Maine, Vermont, Quebec, and
Ontario, where it is of conservation concern
(NatureServe. 2012. NatureServe Explorer: An online
encyclopedia of life. Version 7.1. NatureServe,
Arlington, Virginia. Available from http://www.nature
serve.org/explorer. [Accessed 14 July 2012]). A
possible mechanism for this scenario is that the trophic
apparatus of various freshwater turtles (e.g.,
Sternotherus minor, Graptemys spp.) is known to
respond morphologically to a durophagous diet of thick-
shelled bivalves by increasing head width and jaw
breadth (Berry 1975; Lindeman 2006a). The
biomechanical changes involved in developing bigger
heads are known to increase gape and bite force in
turtles (Herrel et al. 2002; Bulté et al. 2008), which
could free a gape-limited predator to feed on larger,
energetically unfavorable prey (such as mollusks) and
thus increase energy intake. In turn, greater fitness could
be achieved through better body condition and greater
reproductive success (Bulté et al. 2008). Whether the
head morphology of S. odoratus from Gin Creek is
responding similarly to eating the hard and thick-shelled
(Cloe et al. 1995; Zhou et al. 2011) Corbicula is
currently under investigation.
Acknowledgments.—We thank Pat Brown, Steve
Cooper, and Nathan Mills for assistance in the field. Pat
Brown took the substrate samples. Hagen Atkins, Steve
Cooper, Ron Doran, Joseph Goy, and Nathan Mills
assisted in sorting and identifying food items. Peter
Lindeman and Nathan Mills commented on the
manuscript. This research was approved by the Harding
University Animal Care Committee and followed the
ASIH “Guidelines for Use of Live Amphibians and
Reptiles in Field and Laboratory Research”
(http://www.asih.org/files/hacc-final.pdf). This research
was conducted under Scientific Collecting Permit
#020120101 from the Arkansas Game and Fish
Commission and was supported by a grant from the
Margaret M. Plummer Research Fund of Harding
University.
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CAITLIN WILHELM recently received her B.S. in biology from
Harding University. Since then, she has radio-tracked Ornate Box
Turtles (Terrapene ornata) with the Colorado Reptile Humane
Society and is currently pursuing an M.S. in ecology and
evolutionary biology from Missouri State University. Her research
interests lie broadly in herpetology and conservation biology.
(Photograph by Mike Plummer)
MIKE PLUMMER is Professor and Chairman of the Department of
Biology at Harding University where he teaches biostatistics,
herpetology, and seminar. He holds a Ph.D. from the University of
Kansas and a M.S. from Utah State University. His research focuses
on the ecology and physiology of snakes and turtles. His publications
span more than 37 years. (Photograph by Sara Pilgrim)
