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22001166 SOUTHEASTERN NATURALIST 1V5o(3l.) :1552,3 N–5o3. 33
Fundulus catenatus (Northern Studfish) Response to the
Potential Alarm Cue Chondroitin Sulfate
Sarah Farnsley1, Bernard Kuhajda2, Anna George2, and Hope Klug1,*
Abstract - The evolution of organismal cue-response systems can allow for an effective
behavioral reaction to various environmental signals. In aquatic habitats, the reception of
certain chemical cues can increase individual fitness of organisms by serving as an indicator
of predation threat. In some fish species, damage to an individual’s epidermal cells causes
release of a substance that functions as an alarm cue and consequently initiates defense
responses in neighboring prey. Recent research on the chemical makeup of the substance
that elicits an anti-predator response in Danio rerio (Hamilton) (Zebrafish) revealed that
chondroitin fragments were a key component in this substance. The goal of our study was
to investigate the ability of chondroitin to elicit an alarm response in Fundulus catenatus
(Storer) (Northern Studfish). This species is a small-bodied killifish native to southeastern
to south-central USA and is associated with topwater habitats near aquatic and/or overhanging
vegetation. We hypothesized that reduced movement and/or a change in position in the
water column would be a likely response of the Northern Studfish to chondroitin. We experimentally
observed Northern Studfish behavior before and after the addition of chondroitin
and a control substance, and compared the fishes’ behavioral responses. Our results show
that the Northern Studfish that were exposed to chondroitin tended to reduce their movement
by sevenfold and were more likely to move to the bottom of the aquarium relative to
the control group, suggesting that chondroitin potentially serves as an alarm-cue component
in this species. Our study represents the first demonstration of Northern Studfish response to
a chemical cue and the first time that chondroitin sulfate has been shown to elicit a component
of alarm behavior in a stream fish. We discuss our findings in relation to potential uses
of chondroitin as an alarm cue in the conservation of imperiled stream fishes.
Introduction
Organisms assess and respond to their environment based on detection of visual,
auditory, and/or olfactory cues; specific ecological conditions often dictate the most
effective mode of signal transmission (Meuthen et al. 2012). In aquatic environments,
chemical cues mediate essential behavioral interactions between individuals
when visual cues are limited, e.g., where the degree of turbidity is high or vegetation
is dense (Mirza and Chivers 2000). The olfactory reception of chemical cues in
aquatic habitats can increase individual fitness by allowing individuals to identify a
conspecific mate (Rafferty and Boughman 2006), locate spawning sites (Sorensen
et al. 2005), and establish social structure (Moore and Bergman 2005). Chemical
1Department of Biology, Geology and Environmental Science, University of Tennessee-–
Chattanooga, Department 2653, 615 McCallie Avenue, Chattanooga, TN 37403. 2Tennessee
Aquarium Conservation Institute, 201 Chestnut Street, Chattanooga, TN 37402. *Corresponding
author - Hope-Klug@utc.edu.
Manuscript Editor: Andrew Rypel
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cues can also act to decrease the likelihood of predation because various groups of
aquatic organisms interpret certain chemical cues as a threat of predation and consequently
respond with anti-predator behavior (Ferrari et al. 2010). The odor of a
nearby predator (Tollrian and Harvell 1999), the urinary ammonia discharged from
startled prey (Kiesecker et al. 1999), and/or substances released following injury
inflicted by a predator (Wisenden 2003) have all been shown to serve as alarm signals
that initiate defense responses in prey.
In many prey species of fishes, such as small-bodied minnows (Cyprinidae) and
other members of the superorder Ostariophysi, predatory damage to an individual’s
epidermal cells causes release of a substance that functions as a chemical alarm-cue
to neighboring conspecifics (Ferrari et al. 2010, Pfeiffer 1977). Originally known as
schreckstoff, this alarm substance was first discovered to elicit a fright response in
minnows (von Frisch 1941) and has been the focus of several evolutionary, physiological,
and chemical studies (Chivers et al. 2007, Mathuru et al. 2012, Pfeiffer
et al. 1985). Evidence of response to an alarm cue has now been found in fish species
outside the superorder Ostariophysi, including sculpins (Cottidae; Bryer et al.
2000), trout (Salmonidae; Mirza and Chivers 2000), and sunfish (Centrarchidae;
Marcus and Brown 2003) (see also Ferrari et al. 2010 for a review). Studies comparing
alarm-cue responses between closely related fishes suggest that the alarm cues
are likely evolutionarily conserved (Mirza et al. 2003).
Chemical alarm-cue recognition is hypothesized to lead to increased survival of
neighboring individuals by eliciting an ecologically appropriate defense behavior
(Lönnstedt et al. 2012, Mirza and Chivers 2000, Smith 1992). Fishes may respond
to alarm-cue detection with area avoidance (Golub et al. 2005), increased shoaling
(i.e., grouping together for social reasons, Pitcher 1983), shelter use (Mathis and
Smith 1993), altered feeding activity (Mirza and Chivers 2001), and/or complete
lack of motion (Reed 1969). A single species may be capable of exhibiting various
anti-predator behaviors; the specific response depends on the perceived level of
threat (Ferrari et al. 2010).
Previous research has focused on the chemical makeup of the cell contents
responsible for producing the alarm signal, and some studies have suggested
that hypoxanthine-3-N-oxide accounts for an alarm response in Ostariophysan
fishes (Brown et al. 2000). Recently, Mathuru et al. (2012) utilized biochemical
fractionation to show that chondroitin fragments are a key component of the
alarm-substance compound that elicits an anti-predator response in Danio rerio (F.
Hamilton) (Zebrafish).
Despite a large amount of research on alarm behavior in fishes (Chivers and
Smith 1998, Ferrari et al. 2010, Smith 1992), it remains unclear in most species
whether chondroitin acts as a component of alarm cues, and if so, what response
it elicits. The goal of our study was to investigate the ability of chondroitin to
elicit behavior consistent with an alarm response in Fundulus catenatus (Storer)
(Northern Studfish), a stream fish widely distributed in the Ohio and Mississippi
River drainages (Page and Burr 2011). Although the species that prey upon
Northern Studfish in the wild are not well documented, potential predators include
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Micropterus dolomieu Lacepède (Smallmouth Bass) and Micropterus salmoides
Lacepède (Largemouth Bass) (Tennessee Wildlife Resource Agency 2012).
Alarm responses to various chemical stimuli have been well documented in
stream fishes. For example, Salvelinus fontinalis (Mitchill) (Brook Trout) respond
to damage-released alarm signals of conspecifics as well as alarm cues released from
closely related heterospecifics (Mirza and Chivers 2000, 2001), Cottus cognatus
Richardson (Slimy Sculpin) respond to predator odor and damage-released alarm
cues of conspecifics (Bryer et al. 2001), Etheostoma exile (Girard) (Iowa Dater)
respond to epidermal club-cell damage of conspeifics (Smith 1979), and Fundulus
olivaceus (Storer) (Blackspotted Topminnow) respond to skin extract from conspecifics
(Reed 1969). As such, if chondroitin functions generally as an alarm cue in
fishes, we would expect chondroitin to elicit a behavioral response in the Northern
Studfish. Other topwater prey-fishes that live near aquatic or overhanging terrestrial
vegetation have been shown to cease movement in the presence of predators (Reed
1969), and we therefore hypothesized that reduced movement would be a likely
response of the Northern Studfish to this substance. We also hypothesized that Northern
Studfish might additionally alter their behavior in response to chondroitin by
changing their position in the water column (e.g., by moving near the bottom or top of
the water column). To assess these hypotheses, we observed and quantified behavior
before and after addition of chondroitin, and compared changes in behavior to those
following the addition of a control substance. We discuss our findings in relation to
potential uses of the alarm-cue chondroitin in the conservation of stream fishes.
Methods
Fish acquisition and maintenance
This study was approved by the University of Tennessee Chattanooga IACUC
(IACUC #:1017HMK-01), and the Tennessee Wildlife Resources Agency (Nashville,
TN) issued a scientific collection permit (1691). We used seines to collect
adult and juvenile Northern Studfish from the Collins River in McMinnville, TN
(35°48'0''N, 85°37'12''W) on 23 May 2013. When not in use in experimental trials,
fish were randomly housed together in groups of 8 (regardless of sex, age, or size)
in 75.7-L aquaria at the University of Tennessee Chattanooga, Chatanooga, TN. We
fed frozen bloodworms to the fish daily, and maintained a 14:10-hour light:dark
cycle, and a water temperature of ~18 °C in the aquaria.
Experimental trials
We conducted our experiments during June and July 2013. To evaluate the effect
of the potential alarm-substance chondroitin on Northern Studfish behavior, we
performed an experiment that consisted of 2 treatments: (1) exposure to chondroitin
and (2) exposure to a control substance (distilled water). Just prior to use in an
experimental trial, we measured the total length of each fish and categorized it as
either a male, female, or juvenile (Table 1). We began each replicate by randomly
selecting 1 fish and placing it in a 75.7-L experimental observation tank, which contained
only water and tubing for aeration and substance addition. The observation
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tank was isolated from all other tanks (i.e., fish in the observation tank did not have
any contact with other fish) and from the behavioral observer. Specifically, we made
observations from behind an opaque-plastic barrier through eye slits and with the
aid of mirrors positioned above the experimental tanks. This set-up allowed us to
minimize exposure of the fish to any cues other than the substance added to the
tank. Fish were allowed to acclimate for ~1 hour. We observed fish behavior for 10
minutes following acclimation and prior to substance addition. During this time,
we recorded movement (i.e., whether the fish was motionless or in motion) and
vertical distribution (top, middle, or bottom third) in the tank every 30 sec via scan
sampling. After 10 min, we added the experimental substance (either chondroitin
or distilled water) through ~5-mm–diameter tubing that had previously been run
into the tank. Preliminary studies in which we added dye to the substance confirmed
that this method led to the substance being dispersed throughout the tank within
seconds. The chondroitin treatment consisted of 0.07 mg of commercially available
chondroitin (Sigma C4384, St. Louis, MO) dissolved in 5 mL of distilled water
and the control treatment substance was 5 mL of distilled water. This concentration
of chondroitin is consistent with levels used in the Mathuru et al. study (2012)
that elicited an alarm response in Zebrafish. We conducted a 10-min observation
period followed substance addition, during which we assessed the same behaviors
as before substance addition. We used each fish only once; a total of 18 trials per
treatment were completed (Table 1).
Statistical analysis
We calculated the proportion of time the fish spent in motion prior to and after
substance addition so that we could determine the change in motion for each
Table 1. Descriptive data of individuals used in the experiment. We provide an overview of the status
of each fish (juvenile, male, or female), the number of fish, the mean ± standard error, and range of
the total length of individuals used in each treatment and overall in the study (i.e., in both treatments
combined).
Treatment Status Number of fish Mean total length ± SE (mm) Range in total length (mm)
Control
Juvenile 1 44.0 -
Male 6 100.7 ± 4.15 85–110
Female 11 81.5 ± 4.85 52–106
Total 18 85.8 ± 4.57 44–110
Chondroitin
Juvenile 4 47.5 ± 3.57 42–58
Male 7 103.9 ± 6.30 86–128
Female 7 86.6 ± 7.34 61–117
Total 18 84.6 ± 6.32 42–128
Both combined
Juvenile 5 46.8 ± 2.85 42–58
Male 13 102.4 ± 3.77 85–128
Female 18 83.4 ± 4.03 52–117
Total 36 85.2 ± 3.84 42–128
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individual fish following the addition of the substance (chondroitin or distilled
water). We also calculated the proportion of time spent on the bottom third of the
tank before and after substance addition for each trial to assess the change in use of
lower third of the tank for all individuals.
We used 2 separate ANCOVA analyses to examine the effect of chondroitin
on the change in (1) activity (i.e., the change in the proportion of the time spent
in motion) and (2) the change in the proportion of time spent in the bottom
third of the aquarium before and after substance addition. Data for both variables
were normally distributed (i.e., the data did not differ significantly from
a normal distribution; Shapiro-Wilk Test: P = 0.32 and P = 0.24, respectively);
thus, the data did not require transformation. We used treatment (chondroitin
or distilled water) as a fixed factor in both analyses. Because we hypothesized
that size might influence predation risk in the wild and therefore potentially influence
the response to chondroitin in our study, we included total length as a
covariate in our statistical analyses, and examined the interaction between total
length and treatment to assess the relationship between size and behavior for
each treatment group.
We also examined the relationship between total length and age class and sex using
an ANOVA in which status (juvenile, male, female) was a fixed factor and body
length was the response variable. Body length was normally distributed (Shapiro-
Wilk Test: P = 0.11) and did not require transformation prior to use in this analysis.
We employed a Tukey’s HSD test to detect any significant differences among treatment
groups.
Results
Chondroitin significantly decreased motion in the Northern Studfish (treatment
effect: F1, 32 = 10.1, P = 0.003; Fig. 1). Following substance addition, fish that were
exposed to chondroitin tended to reduce their movement by sevenfold relative to
the control group, with change in proportion of time spent in motion as -0.354
versus -0.050 (see also Table 2 for additional details of time spent in motion across
treatments before and after substance addition). Overall, there was no significant
relationship between total length and the change in movement (F1, 32 = 3.90, P =
0.057). However, there was a significant interaction between treatment and total
length (F1, 32 = 5.92, P = 0.021). Specifically, in our control group, only relatively
long fish tended to decrease movement after the addition of our control substance
(water) (Fig. 2A), whereas the majority of fish, regardless of size, reduced movement
after the addition of chondroitin (Fig. 2B).
Fish exposed to chondroitin were significantly more likely than fish in the control
group to increase the proportion of time spent on the bottom third of the tank
following substance addition (treatment effect: F1, 32 = 4.37, P = 0.045; Table 2,
Fig. 3). However, there was no relationship between length and the change in the
use of the bottom third of the tank (F1, 32 = 0.10, P = 0.76) and no significant interaction
between treatment and total length (F1, 32 = 3.27, P = 0.08).
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There was a significant relationship between the total length of a fish and whether
that fish was a juvenile, male, or female (F1, 33 = 25.3, P < 0.001). Specifically,
juveniles were significantly shorter than both males (Tukey’s HSD: mean difference
= -55.6, P < 0.001) and females (Tukey’s HSD: mean difference = -36.6, P less than
0.001), and females were significantly shorter than males (Tukey’s HSD: mean
difference = -18.9, P = 0.004; Table 2). Given this result, the relationship between
body length, treatment, and activity described above was reflective of maturation
(i.e., juvenile versus adult) or sex dif ferences among individuals.
Table 2. Mean ± standard error and the range of the proportion of time spent in motion and in the
bottom third of the aquarium before and after the addition of distilled water (control treatment) and
chondroitin mixed with distilled water. n = 18 for both treatment groups.
Proportion of time in motion Proportion of time in bottom third of tank
Treatment Before addition After addition Before addition After addition
Control
Mean ± SE 0.380 ± 0.079 0.320 ± 0.091 0.630 ± 0.079 0.660± 0.092
Range 0.000–1.000 0.000–1.000 0.095–1.000 0.048–1.000
Chondroitin
Mean ± SE 0.670 ± 0.084 0.310 ± 0.80 0.440 ± 0.079 0.620 ± 0.078
Range 0.000–1.000 0.000–1.000 0.000–1.000 0.000–1.000
Figure 1. Mean change in the proportion
of time spent in motion
before substance addition and after
substance addition for control
(distilled water) and chondroitin
treatment. Whiskers represent ±
standard error.
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2016 Vol. 15, No. 3
Figure 2. The relationship between total length and the change in the proportion of time
spent in motion before substance addition and after substance addition for (A) control (distilled
water) and (B) chondroitin treatments. There was a significant relationship between
total length and the change in the proportion of time spent active for (A) control fish (linear
regression: r2 = 0.36, P = 0.009) but not for (B) fish exposed to chondroitin (linear regression:
r2 = 0.008, P = 0.72).
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Discussion
Chondroitin significantly reduced the time Northern Studfish spent in motion and
caused fish to spend a greater proportion of time near the bottom of the aquarium
than those fish without exposure to chondroitin. Further, in contrast to the case in
which only water was added to the tank, nearly all fish, regardless of size, reduced
activity when chondroitin was added to the aquarium. Together, these findings suggest
that chondroitin potentially serves as a component of alarm cues in this species.
Northern Studfish live in or near aquatic or overhanging terrestrial vegetation;
thus, decreasing motion and moving towards the bottom of the water column are
likely adaptive responses to a predation threat (Reed 1969). Ours is the first study
to demonstrate Northern Studfish response to a chemical cue, and it is the first time
that chondroitin sulfate has been shown to elicit behavior consistent with an alarm
response in a stream fish. In the future, additional work that compares the behavioral
response elicited by chondroitin with the behavioral response elicited by the epidermal
cells of con- and/or hetero-specific fish would be useful and would provide a
more-complete picture of how chondroitin potentially functions as an alarm cue.
The finding that chondroitin potentially functions as an alarm cue is consistent
with work by Mathuru et al. (2012), who found that chondroitin fragments trigger
a fear response in Zebrafish. Likewise, the finding that the Northern Studfish exhibits
a form of anti-predator behavior in response to a chemical cue is consistent with
Figure 3. Mean change in the
proportion of time spent in
the bottom one-third of the
aquarium before substance
addition and after substance
addition for control (distilled
water) and chondroitin treatment.
Whiskers represent ±
standard error.
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2016 Vol. 15, No. 3
studies of other stream fishes. For example, Bryer et al. (2001) found that Slimy
Sculpins increased shelter use in response to damage-released cues from conspecifics
and that the presence of these cues intensified alarm response to predator odor. Brook
Trout have been shown to respond to alarm signals released upon injury to conspecifics
by decreasing activity and reducing foraging (Mirza and Chivers 2001).
Interestingly, we also found that a change in the amount of movement of fish
in the control group following the treatment was correlated with body size but the
same was not true for fish in the group that was exposed to chondroitin. Specifically,
fish of all sizes exposed to chondroitin tended to reduce activity and to the
same degree. It is possible that in the absence of predation threat, larger fish naturally
exhibit a greater response to disturbance, and in nature, this might be due to
differences in feeding or territorial behavior. However, our finding that individuals
of all sizes reduced activity when exposed to chondroitin further suggests that
chondroitin elicits anti-predator behavior.
Our results have implications for the conservation of imperiled fishes, many of
which are stream-dwelling species. We hypothesize that by pairing a predatory cue
with a potential alarm substance, captive or hatchery-raised fish can be conditioned
to associate a predator with an anti-predator response. Once released back into
their natural habitat, survival of these fish may increase as a result of this associative
learning. Mirza and Chivers (2000) showed increased survival among juvenile
Brook Trout conditioned to associate a damage-released alarm cue with predator
odor. These conditioned fish retained predator recognition in response to predator
odor for 10 days following conditioning (Mirza and Chivers 2000). Other species
for which staged predator-encounters led to increased survival based on past
experience with predator cues include juvenile Oncorhynchus mykiss (Walbaum)
(Rainbow Trout; Mirza and Chivers 2003) and Pimephales promelas Rafinesque
(Fathead Minnow; Gazdewich and Chivers 2002). Predator-recognition conditioning
is particularly useful for conservation when predators are an unfamiliar,
introduced species and represent a considerable threat to prey-species survival.
Following conditioning, prey are no longer naïve to a predator and are better
equipped to assess risk of predation and exhibit ef fective defensive behaviors.
Future studies should aim to determine whether threatened and endangered
fishes respond to chondroitin as an alarm cue, and, if so, whether the use of this
chemical cue can be used in conditioning those fishes to associate it with a predation
threat.
Acknowledgments
We are grateful to Kathlina Alford and David Neely for their advice on fish acquisition
and care. We also thank the editor and 2 anonymous reviewers for their comments, which
greatly improved the manuscript.
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