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2010 SOUTHEASTERN NATURALIST 9(2):359–372
Aggressive Interactions of the Endangered
Nashville Crayfish, Orconectes shoupi
Emily A. Bizwell1,* and Hayden T. Mattingly2
Abstract - Aggressive interactions, differences in chela size, and the effect of chela
size on outcomes of aggressive interactions were studied in a laboratory setting using
the federally protected (endangered) Orconectes shoupi (Nashville Crayfish), and two
sympatric species, O. placidus (Bigclaw Crayfish) and O. durelli (Saddle Crayfish). Orconectes
placidus and O. durelli are potential threats to O. shoupi through competitive
or aggressive interactions. Understanding such interactions could help explain species
distributions, provide insight on additional threats, and guide management decisions regarding
Nashville Crayfish translocations. Aggressive interactions were examined with
30-min videotaped trials between body-size-matched hetero- and conspecific pairs.
The predicted influence of chela size on outcomes of aggressive interactions was also
analyzed. Our results demonstrated that O. shoupi males and females were significantly
more aggressive than O. placidus. However, O. durelli females won more encounters
and were slightly more aggressive than O. shoupi females. Significant differences
in chela size were detected in some body-size-matched species and gender pairings:
O. shoupi males had narrower chelae than O. durelli males; and O. shoupi females had
longer and wider chelae than O. placidus females, and longer but narrower chelae than
O. durelli females. Although chela size appeared to play a role in dominance, it was not
the only factor influencing outcomes of aggressive interactions. Our laboratory results
did not identify displacement threats to O. shoupi from O. placidus, and therefore do
not preclude introduction of O. shoupi into habitat occupied by O. placidus to meet
recovery plan objectives. However, interspecific aggression in the presence of a vital
resource (e.g., food or shelter) was not tested here and should be investigated to provide
a more comprehensive evaluation of possible threats to O. shoupi.
Orconectes shoupi Hobbs (Nashville Crayfish; Family Cambaridae) is
the only federally protected crustacean in Tennessee and one of only four
federally protected crayfish in the United States (Taylor et al. 2007). It was
listed as endangered in 1986 due to its restricted range and continued water
quality threats posed by development (USFWS 1987: 51 FR 34410). The
species is endemic to Mill Creek and its tributaries in Davidson and Williamson
counties in Tennessee (Barrociere 1986). Historical records for
O. shoupi included South Harpeth Creek and Richland Creek in Davidson
County, and Big Creek in Giles County (Bouchard 1984b), but attempts to
recollect specimens from these streams have been unsuccessful (O’Bara
et al. 1985, USFWS 1987). The restricted range of O. shoupi leaves it
1US Fish and Wildlife Service, 1875 Century Boulevard, Suite 200, Atlanta, GA
30345. 2Department of Biology, Box 5063, Tennessee Technological University,
Cookeville, TN 38505. *Corresponding author - email@example.com.
360 Southeastern Naturalist Vol. 9, No. 2
susceptible to adverse impacts on its habitat. The lower portion of the Mill
Creek watershed lies within a highly urbanized area of Nashville, and the
upper portion is exposed to agricultural activity and increasing development
(Bouchard 1984a, O’Bara 1985, USFWS 1987). Bizwell (2006) reviewed
the life history and ecology of O. shoupi and provided a general description
of the Mill Creek watershed.
One potential threat to the survival of O. shoupi is competition with Orconectes
placidus Hagen (Bigclaw Crayfish), a species not originally known
from the Mill Creek system (USFWS 1987), but currently found along with
O. shoupi in the Owl Creek tributary to Mill Creek (Pennington 1999). Invading
crayfish species have frequently caused reduction or replacement of
local species (Lodge et al. 2000). For example, Orconectes rusticus Girard
(Rusty Crayfish) has displaced both native and nonnative crayfishes in many
systems in which it becomes established (Garvey et al. 1994; Hill and Lodge
1999; Olsen et al. 1991; Page 1985; Taylor et al. 1996, 2007). Orconectes
placidus may have been responsible for the extirpation of O. shoupi from
Richland Creek (Bouchard 1984a), a Cumberland River tributary 14 km
west of Mill Creek, and is known to have established populations in altered
riverine habitats (USFWS 1987). Further alteration of the Mill Creek system
could make conditions favorable for O. placidus to invade from surrounding
waters, potentially undermining the viability of O. shoupi in Mill Creek.
Orconectes durelli Bouchard and Bouchard (Saddle Crayfish) is another
crayfish species sympatric with O. shoupi and O. placidus in Nashville Basin
streams. Because O. durelli and O. shoupi occupy many of the same streams,
and O. placidus inhabits bedrock streams (Bouchard and Bouchard 1995)
characteristic of O. shoupi habitat (Bouchard 1984a), the potential for competition
and possible species replacement among O. shoupi and O. durelli
and O. placidus is a valid consideration.
One potential mechanism for successful establishment of invading crayfish species is competitive exclusion (Bovbjerg 1970, Capelli 1982, Capelli
and Magnuson 1983, Capelli and Munjal 1982). Competition for limited
resources may lead to aggressive interactions between animals and has direct
fitness consequences by determining access to essential resources, such as
food, shelter, and mates (Vorburger and Ribi 1999). Interspecific aggressive
interactions may restrict subordinate species to suboptimal habitats or diets
(Söderbäck 1991). If shelters are limited, competition for shelter may also
be an important factor in crayfish species replacements (Vorburger and Ribi
1999). Exclusion can lead to higher mortality for subordinate species due to
increased exposure to fish predation (Garvey et al. 1994).
Crayfish species within Orconectes naturally differ in their levels of aggressiveness
(Bovbjerg 1953, 1956, 1970; Capelli and Munjal 1982; Tierney
et al. 2000). Aggressive dominance and larger body size generally confer
a competitive advantage for shelter (Vorburger and Ribi 1999). Chela size
also can influence interspecific interactions (Garvey and Stein 1993). Large
chelae are energetically expensive to grow, but bring certain advantages.
2010 E.A. Bizwell and H.T. Mattingly 361
Crayfish with large chelae are less susceptible to fish predation and are able
to dominate similar-sized crayfish with smaller chelae. Further, males with
large chelae are better equipped to secure and hold females during copulation
and copulate longer (Garvey and Stein 1993).
Currently, there are no studies addressing competition or aggression
between O. shoupi and sympatric crayfish species. A better understanding
of aggressive interactions among these species could help explain species
distributions, provide insight on additional threats resulting from competition,
and guide future management decisions on translocations of O. shoupi.
Thus, the objectives of this study were to determine whether O. shoupi: (1) is
more or less aggressive than the other two species; (2) has larger or smaller
chelae than size-matched heterospecifics; and (3) demonstrates dominance
or subdominance as predicted by observed chela size differences.
Materials and Methods
Species collection and laboratory acclimation
Orconectes shoupi, O. placidus, and O. durelli were hand-collected, using
kick nets and small hand-held nets, from streams in which neither of the
other two species were present. This procedure ensured that a previously
established hierarchy between these species pairs would not affect laboratory
results. Orconectes shoupi individuals were collected from Mill Creek,
Davidson County, TN (36º01'12"N, 86º41'15"W) on 29 July and 10 September
2005; O. placidus from Richland Creek, Davidson County, TN (36º08'
42"N, 86º51'11"W) on 29 July and 10 September 2005; and O. durelli from
an unnamed tributary to Owl Creek, Williamson County, TN (35º58'54"N,
86º43'22"W) on 10 September 2005. Water temperatures were 25–26 °C
in July and 23–24 °C in September. Adult male and female crayfish with a
carapace length (CL) between 18–40 mm, having both chelae intact, were
transported to the laboratory. Most males were Form I, although reproductive
status was not considered in the experimental design. Females carrying
eggs were not collected nor used in experimental trials.
Upon arrival at the laboratory, crayfish were held in twelve 189-L
tanks separated by species and sex (e.g., female O. shoupi in a tank, male
O. shoupi in a tank, female O. placidus in a tank, etc.). Each tank was
equipped with gravel substrate, sponge filters already established with
bacteria for waste conversion, one Aqua-Tech 20-40 filter, and shelters that
consisted of cut portions of PVC pipe, bricks, and rocks. Crayfish were
given 7–10 d to acclimate to laboratory conditions (20 °C water temperature;
12 h light: 12 h dark photoperiod) before trials began. Diet consisted
of commercial catfish food offered every other day, with excess food removed
by siphoning between feedings. Dissolved oxygen, temperature,
pH, ammonia, and nitrite were measured three times per week to monitor
water quality. Following the completion of trials, crayfish were returned to
their original collection sites.
362 Southeastern Naturalist Vol. 9, No. 2
To test if one species of crayfish was aggressively dominant over another,
methods generally followed Vorburger and Ribi (1999). Observations were
made while reviewing videotaped aggression trials between hetero- and
conspecific pairs as follows: (1) male O. shoupi vs. male O. shoupi (n =
14 pairs); (2) female O. shoupi vs. female O. shoupi (n = 9 pairs); (3) male
O. shoupi vs. male O. placidus (n = 20 pairs); (4) female O. shoupi vs. female
O. placidus (n = 15 pairs); (5) male O. shoupi vs. male O. durelli (n =
8 pairs); and (6) female O. shoupi vs. female O. durelli (n = 11 pairs). Trials
between males and females were not conducted in this study.
All pairs were size-matched (±1 mm carapace length), and each animal
was only used once. Individuals of each pair were placed in separate 21-L
experimental chambers and allowed to acclimate for 24 h before trials began.
Experimental chambers were covered with black plastic to prevent
visual interactions between crayfish through the glass. Crayfish were not
fed during the 24-h acclimation period. Each individual was marked with a
small application of different-colored nail polish to distinguish individuals
After 24 h, pairs were placed in a new experimental chamber, without
the black plastic covering, and trials were filmed for 30 min using Panasonic
PV-GS9 and PV-GS19 Mini-DV camcorders equipped with 60-min mini-
DV tapes. Cameras were set on desktop tripods 15 cm from experimental
chambers allowing a direct, level view of the trial. Supplemental lights were
used to increase video quality. Trials took place during August and September
2005 between 0800 and 1200 h. Experimental chambers were separated
by screens so that paired crayfish in each experimental chamber could not
interact visually with other pairs. No one was present in the room after the
Videotaped trials were reviewed using a modified ethogram (Bergman
and Moore 2003, Bruski and Dunham 1987) to record fight intensities
reached during multiple encounters throughout a 30-min trial (Fig. 1). Intensity
points were used to assist in decisions regarding encounters won during
the 30-min trial and levels of aggressiveness (Bizwell 2006). Winners of
each encounter were determined according to what types of behaviors were
displayed during the encounter and which individual retreated from an encounter.
In most cases, the retreating crayfish was deemed the “loser” of that
encounter. However, in some instances, a crayfish would display dominating
behaviors (e.g., driving an opponent backward, cornering, or chasing), but
then retreat. This crayfish was deemed the “encounter winner” based on the
dominating behavior displayed during the encounter.
Within each trial, the two crayfish individually accrued aggression
points for each encounter won as illustrated (Fig. 1). For example, consider
a hypothetical trial with “crayfish 1 (C1)” and “crayfish 2 (C2)” as follows.
If C1 won the first encounter with 2.5 aggression points, C2 won the second
encounter with 1 point, C2 won the third encounter with 2 points, C1
2010 E.A. Bizwell and H.T. Mattingly 363
won the fourth encounter with 2.5 points, and C1 won the fifth encounter
with 3.5 points, then at the end of this trial, C1 would have accrued 8.5 aggression
points and C2 would have accrued 3.0 aggression points, and C1
would be declared the winner.
One-way (O. shoupi vs. O. shoupi) and two-way (O. shoupi vs.
O. placidus and O. shoupi vs. O. durelli) ANOVAs were used to analyze the
aggression points for each trial between species and gender treatments. For
heterospecific pairings, the number of trials in which O. shoupi won more
encounters and the number of trials in which O. shoupi lost more encounters
per trial were tallied. Chi-square tests were used to determine if there were
more trials in which O. shoupi won or lost more encounters than would be
expected by chance.
We made the following measurements to determine if chela sizes differed
between the three species (e.g., Stein et al. 1977). Carapace length
(rostral tip to the posteriomedian edge of the cephalothorax), chela
length (carpal joint to the distal tip of the propodus), and chela width
(greatest palm width) of all individuals were measured in millimeters
with vernier calipers. Only chelae with no evident signs of prior damage
or regeneration were measured. If chelae were of different sizes, the
larger one (assessed visually) was measured.
Differences in chela lengths and widths for individuals of each sizematched
trial were analyzed using paired t-tests. Data for O. durelli male
chela width did not meet the assumption of normality and were analyzed
with a Mann-Whitney U test. Bizwell (2006) reported linear models of
Figure 1. Flow chart developed to assign aggression points based on the behaviors
and outcomes of each encounter between size-matched pairs of the crayfishes Orconectes
shoupi, O. placidus, and O. durelli.
364 Southeastern Naturalist Vol. 9, No. 2
chela size as a function of body size (r2 > 0.71, P < 0.0022), but we provide
model slope coefficients here to illustrate sexual dimorphism evident in all
Effect of chelae size on dominance
To determine if chela sizes had an affect on dominance, therefore affecting
outcomes of the aggression trials, a table was constructed comparing
differences in chela sizes between each species and gender pairings and the
observed outcomes of the trials. Based on differences in chela lengths and
widths, hypotheses were made in regards to aggression points and number
of trials in which O. shoupi won more or less encounters per trial. Species
with a chela size advantage (larger chelae) would be predicted to win more
encounters. Hypotheses were then compared with aggressiveness results to
assess the role of chela sizes on dominance. If the majority of hypotheses
were consistent with observed aggression patterns, then it could be inferred
that chela sizes were potentially playing a role in structuring the outcomes
of aggressive interactions among the three species.
Calculated aggression points did not differ between genders in Orconectes
shoupi vs. O. shoupi trials (F(1,44) = 1.04, P = 0.31; Fig. 2a). Orconectes
shoupi was significantly more aggressive than O. placidus (F(1,66) = 7.81,
P = 0.007), with O. shoupi having more aggression points than O. placidus
(Fig. 2b), but there was no gender (F(1,66) = 0.19, P = 0.67) nor interaction
Figure 2. Mean aggression points (± SD) for trials involving size-matched pairs
of the crayfishes Orconectes shoupi (circles), O. placidus (squares), and O. durelli
(triangles). Open symbols indicate trials with males against males and solid symbols
represent trials with females against females. (2a) O. shoupi vs. O. shoupi,
(2b) O. shoupi vs. O. placidus, (2c) O. shoupi vs. O. durelli.
2010 E.A. Bizwell and H.T. Mattingly 365
effect (F(1,66) = 0.11, P = 0.74). There were no significant species (F(1,34) =
1.93, P = 0.17), gender (F(1,34) = 0.02, P = 0.89), nor interaction effects (F(1,34)
= 2.75, P = 0.11) for O. shoupi vs. O. durelli trials (Fig. 2c).
The numbers of encounters won per trial were not significantly different
for three of the four species and gender pairings (χ2 < 2.58, P > 0.05). For
both male and female pairings in O. shoupi vs. O. placidus trials, and male
pairings in O. shoupi vs. O. durelli trials, the number of encounters won per
trial for each species did not differ. Female O. durelli, however, won signifi-
cantly more encounters than female O. shoupi (9 vs. 2 trials, respectively;
χ2 = 4.46, P < 0.05).
Chela lengths and widths differed between some species and gender combinations.
There were no significant differences in chela lengths or widths
between male O. shoupi and O. placidus (t < 1.40, P > 0.18). Orconectes
shoupi and male O. durelli did not differ in chela lengths (t = 0.94, P =
0.38), but O. durelli had significantly wider chelae (U = 50, P = 0.05). Both
chela lengths and widths were significantly different between O. shoupi and
O. placidus females, with O. shoupi having both longer and wider chelae (t >
3.96, P < 0.0008). Female O. shoupi and female O. durelli were significantly
different in both chela lengths and widths, with O. shoupi having longer
chelae and O. durelli having wider chelae (t > 2.77, P < 0.02). Chela sizes
for O. shoupi vs. O. shoupi pairings were not significantly different (t < 1.20,
P > 0.25), as would be expected.
Sexual dimorphism was evident between males and females of each
species grouping, and chela length and width slope coefficients illustrated
differences in chela growth with body size (Fig. 3). Female chela lengths
grew at about a 1:1 ratio with body size for all three species. Chela
lengths grew disproportionately more rapidly with body size in males
than in females, although it was less pronounced in male O. durelli. Chela
widths obviously grew less rapidly than chela lengths with body size, but
in male O. durelli, chela widths grew more rapidly with body size than
did those of O. shoupi and O. placidus.
Effect of chelae size on dominance
Chela lengths and widths were not different between O. shoupi and
O. placidus males. It was hypothesized that neither aggression points nor
the number of trials in which one species won more encounters would be
significantly different because neither species had a chela size advantage
(Table 1). However, one of those predictions was not supported because
O. shoupi accrued significantly more aggression points than O. placidus.
Chela lengths and widths were significantly different between female
O. shoupi and O. placidus. Because O. shoupi had longer and wider chelae, it
was hypothesized that female O. shoupi would have more aggression points
and would also have more trials in which they won more encounters. This
366 Southeastern Naturalist Vol. 9, No. 2
prediction was not supported because neither species won more encounters
nor was there any significant difference in aggression points.
No significant difference was observed between chela lengths in either
male O. shoupi and O. durelli, but O. durelli had wider chelae. Our a priori
hypothesis was that differences in width would give O. durelli males an
advantage; thus, O. durelli would earn more aggression points and there
would be more trials in which they won more encounters. However, neither
hypothesis was supported. Both chela lengths and widths were significantly
different between O. shoupi and O. durelli females. Orconectes shoupi
Figure 3. Chela length (top panel) and width (bottom panel) slope coefficients from
linear models of chela size regressed on carapace length (Bizwell 2006) for male and
female Orconectes shoupi, O. placidus, and O. durelli.
2010 E.A. Bizwell and H.T. Mattingly 367
females had longer chelae, whereas O. durelli females had wider chelae. No
hypotheses were made because neither species had a clear chelae size advantage.
Thus, only two of six predictions were supported (Table 1), which
suggests that chela size may play a role in determining dominance, but it is
not the sole factor influencing outcomes of aggressive interactions.
Aggression and competitive exclusion are mechanisms usually proposed
to explain crayfish distribution and replacements (Bovbjerg 1970, Capelli
and Magnuson 1983, Capelli and Munjal 1982, Söderbäck 1995, Vorburger
and Ribi 1999). Söderbäck (1991), for example, reported that the introduced
North American crayfish Pacifastacus leniusculus leniusculus Dana (Signal
Crayfish) clearly dominated the Swedish native species Astacus astacus L.
(Noble Crayfish), which contributed to the reduction of the distribution of
the native species. Also, Capelli and Munjal (1982) found that the invasive
Rusty Crayfish Orconectes rusticus was more aggressive than another invasive
species, Orconectes propinquus Girard (Northern Clearwater Crayfish),
and a Wisconsin endemic species, Orconectes virilis Hagen (Virile Crayfish), and was able to displace both species in areas of Wisconsin.
Animals in conflict situations minimize costs related to energy loss and
risk of injury by assessing their opponents’ fighting ability and comparing
it with their own (Parker 1974, Schroeder and Huber 2001, Vorburger and
Ribi 1999). Dominance between two species of crayfish is often governed by
size (Söderbäck 1991, Vorburger and Ribi 1999), and body size differences
usually correspond to chelae size differences (Garvey and Stein 1993). Chelae
serve as weapons during agonistic interactions and play a large role in
fighting strategies, especially when they differ in size (Schroeder and Huber
2001). Garvey and Stein (1993) found that differences in body and chela
size allowed O. rusticus to outcompete both O. propinquus and O. virilis by
Table 1. Summary of predictions and observed outcomes of calculated aggression points and
number of trials in which Orconectes shoupi won more encounters based on differences in chela
lengths and widths. Abbreviations are S = O. shoupi, P = O. placidus, D = O. durelli, and NP =
Chela length Chela width Prediction
Trial difference? difference? Prediction Observed supported?
O. shoupi vs. O. placidus
Males No No Encounters: S = P S = P Yes
Aggression pts: S = P S > P No
Females S > P S > P Encounters: S > P S = P No
Aggression pts: S > P S > P Yes
O. shoupi vs. O. durelli
Males No S < D Encounters: S < D S = D No
Aggression pts: S < D S = D No
Females S > D S < D Encounters: NP S < D NP
Aggression pts: NP S = D NP
368 Southeastern Naturalist Vol. 9, No. 2
forcing exposure to fish predation. In addition, O. rusticus hybridized with
the native species, which caused the displacement of both.
The goal of this study was to identify potential competitive and displacement
threats to O. shoupi by determining the level of aggressiveness
of O. shoupi compared to co-existing O. placidus and O. durelli. Male and
female O. shoupi were more aggressive than O. placidus. Female O. durelli
were more aggressive than female O. shoupi, but no differences were detected
between male O. shoupi and O. durelli. We also documented differences
in chela size between female O. shoupi and O. placidus and both male and
female O. shoupi and O. durelli. However, chelae size was apparently not
the only factor influencing the outcomes of the aggression trials. If dominance
were based solely on chelae size differences, differences in aggression
would have been detected in male O. shoupi vs. O. durelli trials and would
not have been detected during male O. shoupi vs. O. placidus trials.
Based on these findings, no initial competition or displacement threats
to Orconectes shoupi were detected from O. placidus or male O. durelli.
However, Capelli and Munjal (1982) stated that caution should be taken
when applying laboratory results to field situations. It is unknown to what
extent aggressive interactions naturally occur in the wild and to what extent
resources are limited. Furthermore, studies have shown that size-matched
trials in a laboratory can result in a greater number of fights that occur longer
and reach greater maximum intensity levels (Bergman and Moore 2003,
Vorburger and Ribi 1999) than field observations that yield shorter, less
intense interactions (Bergman and Moore 2003). Also, Rorer and Capelli
(1978) found Orconectes spinosus Bundy (Coosa River Spiny Crayfish) to
be subdominant to Cambarus bartonii bartonii Fabricius (Common Crayfish) in laboratory tests, even though O. spinosus has completely displaced
C. bartonii in Mountain Lake, VA.
Forty-six percent (168 of 363) of named crayfish taxa in the US and
Canada are designated as having a “restricted range” (Taylor et al. 2007).
Species with small ranges, such as O. shoupi, are extremely vulnerable to
extinction. One criterion necessary for downlisting O. shoupi to threatened
status is that two viable populations must exist: one through protection of
the existing Mill Creek basin populations, and another either by (re)introduction
of the species into the Richland Creek system, or by discovery of
an additional distinct population (USFWS 1987). Thus, our understanding
of competition between O. shoupi and O. placidus is key to the recovery of
Two specimens of O. shoupi were collected in 1895 from Richland Creek
(Bouchard 1984a). Since that collection, no other O. shoupi specimens have
been collected outside the Mill Creek system. Because O. placidus dominates
Richland Creek (O’Bara et al. 1985), it has been suggested that O. placidus
may have caused the extirpation of O. shoupi from Richland Creek (Bouchard
1984a, O’Bara et al.1985, USFWS 1987). It is not known whether O. shoupi
specimens collected from Richland Creek in 1895 were part of an established
2010 E.A. Bizwell and H.T. Mattingly 369
population and O. shoupi was displaced, or if it was an error in locality
(Bouchard 1984b) and Richland Creek was never part of the historic range of
O. shoupi. On the other hand, if the 1895 Richland Creek collection was valid,
then it remains a possibility that O. placidus was better able to exploit a key
resource and extirpate O. shoupi from Richland Creek through some form of
competition. In our study, no evidence of chela size advantage was detected
for O. placidus, nor was there any evidence of O. placidus being more aggressive;
in fact, O. shoupi was somewhat more aggressive. If Richland Creek was
indeed part of O. shoupi’s former range, then the mechanism for its displacement
remains unknown, given the uncertainty of applying laboratory results to
the greater complexity of natural conditions.
This study investigated basic levels of aggression between species in
size-matched trials; it did not assess aggression levels during competition for
key resources such as food or shelter. When key resources are the source of
aggressive interactions between species, outcomes may be different because
of the higher reward. Although this study has led to a better understanding
of basic aggression between species, a logical next step would be to conduct
studies of resource competition between O. shoupi and both O. placidus and
O. durelli to provide a more comprehensive understanding of competition
among these species. Studies investigating competition for key resources
could determine if aggression levels change with the presence of vital resources.
Also, growth and reproduction should be studied in detail for all
three species. Söderbäck (1995), for example, found that P. l. leniusculus
was able to replace the native A. astacus through competition advantages in
combination with P. l. leniusculus' greater capacity for population increase.
Pacifastacus l. leniusculus has a higher individual growth rate, reaches
sexual maturity at a smaller size and younger age, and has a higher per capita
Orconectes durelli was included in this study because it is found in many
of the same Mill Creek tributaries as O. shoupi. Orconectes durelli appears
to be more abundant in smaller streams, but can be found with O. shoupi in
Sevenmile and Owl creeks, two major tributaries in which O. shoupi populations
are not limited to areas near the mouth. No chela size advantage or
higher levels of aggression that could threaten O. shoupi were detected in
O. durelli males. However, O. durelli females had wider chelae and were
more aggressive than O. shoupi females, although O. shoupi females had
longer chelae. This could possibly pose a problem during the breeding season
or during the time that females are carrying eggs. If O. durelli females
can better compete for resources than O. shoupi, this could potentially affect
fecundity and recruitment for O. shoupi.
Many studies have found that larger crayfish dominate smaller crayfish
(Bovbjerg 1953, 1956; Söderbäck 1991; Vorburger and Ribi 1999).
If US Fish and Wildlife Service personnel decide to follow recovery plan
objectives as written, and introduce O. shoupi into Richland Creek (USFWS
1987), stocking of O. shoupi into Richland Creek should be based
370 Southeastern Naturalist Vol. 9, No. 2
on the length frequency of the present O. placidus population so that small
O. shoupi are not introduced into a population of larger O. placidus, leaving
O. shoupi vulnerable to competition.
A Faculty Research Grant from Tennessee Technological University (TTU) supplied
primary funding for this research. The Department of Biology at TTU and the
US Fish and Wildlife Service Cookeville Field Office provided additional funding
and technical support. We thank numerous students who volunteered time in the field
and laboratory. The manuscript was improved by comments from L.A. Barclay, C.A.
Brown, D.L. Combs, two anonymous reviewers, and the editor. The findings and
conclusions in this article are those of the authors and do not necessarily represent
the views of the US Fish and Wildlife Service.
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