Biological Journal of the Linnean Society, 2015, 115, 842–858. With 6 figures.
Predictors of body shape among populations of a stream
fish (Cyprinella venusta, Cypriniformes: Cyprinidae)
TRAVIS C. HAAS*, DAVID C. HEINS and MICHAEL J. BLUM
Department of Ecology and Evolutionary Biology, Tulane University, 6823 St Charles Avenue, New
Orleans, LA, 70118, USA
Received 9 November 2014; revised 15 February 2015; accepted for publication 16 February 2015
Performance-related variation in fitness can manifest as morphological responses to ecological and evolutionary
pressures. Eco-morphological studies often utilize stark binary comparisons, such as lentic to lotic populations of
freshwater fishes, to characterize relationships between form and function despite possible complications from
confounding factors. In the present study, we compared body shape variation among lotic populations of a stream
fish (Cyprinella venusta Girard) to disentangle the influence of ecological and evolutionary drivers of phenotypic
change. We assessed the extent to which body shape corresponded to three key environmental factors (mean
channel velocity, mean discharge, and mean annual run-off), phylogeny (mitochondrial DNA divergence), and
body size (centroid size). We also examined relationships between these parameters and a fineness index, which
is a measure of streamlining and morphological optimization for steady swimming performance. All three
environmental variables had some explanatory power, although morphological characteristics were
predominantly associated with variation in mean annual run-off. Phylogeny was also a strong predictor of
morphological variation, whereas body size had little predictive power. Populations experiencing higher mean
annual run-off exhibited a shorter base of the dorsal fin, a more slender body and caudal peduncle, a smaller
head in both horizontal and vertical dimensions, and a more anterior placement of the eye. With some exceptions,
such as variation in jaw length, differences in body shape associated with phylogenetic history were similar to
those associated with run-off. Notably, all clades exhibited parallel responses to variation in run-off. Populations
experiencing high mean annual run-off approached a hydrodynamic optimum, suggesting a morphology optimized
for steady swimming performance. In contrast to previous studies that emphasize the importance of average
water velocity, the findings of the present study indicate that morphological variation among populations of
stream fishes is tightly linked to more complex aspects of hydrology and evolutionary history. © 2015 The
Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858.
ADDITIONAL KEYWORDS: blacktail shiner – geographical
steady swimming – streamlining – swimming performance.
INTRODUCTION
Morphological variation is one of the most apparent
forms of biological diversity. Consequently, the nature of relationships between form and function has
been well studied (Hora, 1935; Bock & von Wahlert,
1965; Bock, 1980; Losos, 1990; Amundson & Lauder,
1994), including whether morphology is attributable
to performance modulated by the environment
(Domenici, 2003). Among fishes, morphology affects
swimming performance (Ojanguren & Bra~
na, 2003),
*
Corresponding author. E-mail: [email protected]
842
variation – geometric
morphometrics –
and swimming performance varies with hydrological
conditions, where fitness differences emerge because
swimming performance is moderated by competition
and predation (Pough, 1989). Performance-related
variation in fitness therefore can manifest as morphological responses to extrinsic pressures (Hubbs,
1941; Gatz, 1979, 1981; Arnold, 1983; Wikramanayake, 1990; Hendry et al., 2006; Krabbenhoft, Collyer
& Quattro, 2009; Haas, Blum & Heins, 2010; Franssen, 2011). For example, fishes in high flow streams
may be more torpedo-shaped (Hubbs, 1941), allowing
more efficient steady swimming (fusiform fishes;
Langerhans & Reznick, 2010) or they may be more
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
PREDICTORS OF STREAM FISH BODY SHAPE
flattened from top to bottom (Hora, 1952) to reduce
sheer flow or avoid high flows through better positioning in the boundary layer (dorsoventrally compressed fishes; Carlson & Lauder, 2011; Statzner &
Holm, 2012). In structurally complex habitats, fishes
may exhibit deeper bodies (Videler, 1993), which can
enhance unsteady swimming (Domenici, 2003). Some
fishes inhabiting hypoxic habitats exhibit longer
heads and deeper bodies, reflecting configurations
that appear to have arisen from greater gill proliferation (Chapman, Albert & Galis, 2008). Yet, functional relationships between fish body shape and
prevailing environmental conditions are not always
evident, possibly including behavioural responses
(Toline & Baker, 1997; Binning & Chapman, 2010;
Welsh et al., 2013).
Several scenarios may result in spurious relationships (Prairie & Bird, 1989) or conclusions that environment and morphology are unrelated (Blob et al.,
2008). The absence of a clear relationship between
environment and morphology could be the result of
selection-driven adaptation lagging behind environmental change, where populations may exhibit maladaptive morphologies during transitional response
periods (Maynard Smith, 1978). For example, a population of stream fish inhabiting a recently
impounded river may not exhibit morphological characteristics typical of a species found in a lentic environment because it may require many generations
for advantageous attributes to emerge after
impoundment (Kinnison & Hendry, 2001; Smith &
Bernatchez, 2008; Cureton & Broughton, 2014).
Alternatively, an environmental condition might not
exhibit sufficient variation to serve as a significant
driver of phenotypic change or diversification (Binning & Chapman, 2010). Morphological characteristics also might not be subject to strong selective
pressures across the range of environmental conditions under consideration (Webb, 1975; Lande &
Arnold, 1983) or the traits may be under phylogenetic constraint (McKitrick, 1993).
Eco-morphological studies of fishes often utilize
binary comparisons between stream and lake environments to identify factors that drive phenotypic
change (Haas et al., 2010; Collin & Fumagalli, 2011;
Naspleda et al., 2012), even though this approach
may not be as informative as comparisons among
streams or among lakes (Francis et al., 1986; Hendry
et al., 2006; Kerfoot & Schaefer, 2006; Spoljaric & Re~ iga-Vega et al., 2011; Jacquemin,
imchen, 2007; Z
un
Martin & Pyron, 2013; Cureton & Broughton, 2014).
The lotic–lentic binary approach is statistically elegant because it provides a clear basis for low-flow vs.
high-flow contrasts and comparisons. Despite this
seeming elegance, lake-stream comparisons can be
complicated by confounding factors. Streams and
843
lakes often predictably differ in a number of potentially confounding characteristics that can result in
phenotypic change, such as predator regime, water
velocity, food availability, and habitat complexity
(Alvarez & Bell, 2007; Franssen et al., 2013a). Consequently, identifying the key determinants of body
shape variation can be difficult. The lotic–lentic
approach also does not effectively capture the nature
of relationships in intermediate flow environments,
which can afford opportunities to disentangle the
influence of multiple drivers of phenotypic change.
Furthermore, lotic–lentic comparisons typically do
not consider whether variation is attributable to evolution, even though observed morphological variation
in fishes is often heritable (Langerhans, 2008).
To better understand drivers of morphological
diversity, we examined relationships between phenotypic variation of blacktail shiner (Cyprinella venusta
Girard) and multiple environmental variables, as
well as mitochondrial (mt)DNA-based phylogenetic
structure, across seventeen populations in the southeastern USA. Building from previous work demonstrating the relationship between morphology and
flow regimes in anthropogenically altered hydrological environments (Haas et al., 2010), we evaluated
the nature of relationships between morphology and
environmental variation in free-flowing lotic habitats. By examining both overall shape variation and
a fineness index, which is a proxy for steady swimming performance, we tested the hypothesis that
variation in morphology is attributable to prevailing
hydrological conditions and evolutionary history
(Langerhans, 2008; Langerhans & Reznick, 2010).
MATERIAL AND METHODS
S TUDY
SYSTEM AND SAMPLING
The blacktail shiner is a substrate and habitat generalist (Ross, 2002) most commonly found in the middle of the water column in sandy pools and runs of
small to medium size rivers in the south-eastern
USA (Page & Burr, 2011). In the uplands of Alabama, however, C. venusta commonly occurs over
gravel and rubble substrates (Gilbert & Burgess,
1979). The species attains a maximum total length of
19 cm and is abundant throughout much of its
range, which extends as far west as central Texas,
as far east as north central Florida, as far south as
southern Texas, and as far north as southern Illinois
(Page & Burr, 2011). Blacktail shiners occasionally
inhabit reservoirs and lakes (Boschung & Mayden,
2004), where they typically exhibit a divergent body
shape relative to populations inhabiting nearby
stream environments (Haas et al., 2010; Franssen
et al., 2013a; Franssen, Stewart & Schaefer, 2013b).
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
844
T. C. HAAS ET AL.
Figure 1. Distribution of 17 sampling localities for populations of Cyprinella venusta in the southern USA. Site numbers
correspond to those given in Table 1. Shapes adjacent to numbers indicate mitochondrial DNA-based clade membership,
sensu Kristmundsdóttir & Gold (1996). Diamonds, populations belonging to the Western clade; squares, populations
belonging to the Choctawhatchee clade; triangles, populations belonging to the Mobile clade. Lines overlaid on map illustrate the hypothesized ranges of subspecies, sensu Gibbs (1957a) and Gilbert & Burgess (1979). Solid line, Cyprinella
venusta venusta; dotted line, Cyprinella venusta stigmatura; dashed line, Cyprinella venusta cercostigma.
Morphological data were derived from seventeen
populations of C. venusta from free-flowing streams
in Texas, Mississippi, and Alabama (Fig. 1). All
specimens were collected for previous studies conducted by Heins & Baker (1987) and Haas et al.
(2010). Each population was sampled via seine from
an approximately 100 m stream reach. The total
number of individuals captured at each site ranged
from 12 to 326 and all specimens were fixed in 10%
formalin. Specimens collected by Heins & Baker
(1987), corresponding to populations 1–10 in Table ,
were transferred to 50% isopropanol, and then to
70% ethanol for storage. Specimens collected by
Haas et al. (2010) were stored in formalin. The seventeen populations examined in the present study
encompass much of the morphological and genetic
variation exhibited across the species’ distribution
(Fig. 1, Table 1). Samples were obtained from all
three subspecies identified by Gibbs (1957a, b). The
seventeen populations also correspond to the Western, Mobile, and Choctawhatchee mtDNA clades
identified by Kristmundsd
ottir & Gold (1996)
(Table 1).
A NALYSIS
OF MORPHOLOGICAL VARIATION
We examined 12–25 randomly selected adult specimens (i.e. ≥73 mm SL; Casten & Johnston, 2008)
from each study population. Although C. venusta are
sexually dimorphic (Hood & Heins, 2000), we did not
distinguish between males and females because preliminary analyses conducted on a subset of populations (results not shown) demonstrated that males
and females exhibit similar morphological shifts in a
given environment. This approach also helped ensure
consistency with similar ecomorphological studies of
C. venusta and other North American minnow species (Franssen et al., 2013a, b).
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
Mulberry Fork
of Black
Warrior River
Locust Fork of
Black
Warrior River
Conasauga River
15
Georgia
Alabama
Alabama
Alabama
Alabama
Alabama
Alabama
Alabama
Alabama
Mississippi
Mississippi
Mississippi
Texas
Texas
Texas
Texas
Texas
State
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Choctawhatchee
Choctawhatchee
Western
Western
Western
Western
Western
Western
Western
Western
Mitochondrial
DNA clade (sensu
Kristmundsdottir
& Gold (1996)
Cyprinella
venusta
venusta
Cyprinella
venusta
venusta
Cyprinella
venusta
venusta
Cyprinella
venusta
venusta
Cyprinella
venusta
venusta
Cyprinella
venusta
cercostigma
Cyprinella
venusta
cercostigma
Cyprinella
venusta
cercostigma
Cyprinella
venusta
cercostigma
Cyprinella
venusta
cercostigma
Cyprinella
venusta
stigmatura
Cyprinella
venusta
stigmatura
Cyprinella
venusta
stigmatura
Cyprinella
venusta
stigmatura
Cyprinella
venusta
stigmatura
Cyprinella
venusta
stigmatura
Cyprinella
venusta
stigmatura
Subspecies
(sensu Gibbs,
1957a)
25
25
25
2008
2008
2008
2008
2008
25
25
2008
2008
1984
1984
1984
25
25
25
24
25
1984
1984
25
25
1984
1984
1984
1984
1984
25
25
25
12
23
N
Year
collected
34.78322
34.07315
33.99774
33.51141
33.23166
32.49036
32.43337
31.27488
30.51199
30.81172
30.59477
30.40712
30.60541
30.28531
30.24965
32.22471
31.67164
Latitude
84.872643
86.505081
86.751137
86.652687
87.777397
85.742287
86.124657
85.678253
87.513573
88.458633
89.339622
89.538406
93.794228
94.19249
94.792183
94.226646
94.95295
Longitude
42859
42863
42856
42855
42860
42869
42871
42868,
42870
42866
42867
Museum
collection
number
0.415
0.432
0.383
0.436
0.322
0.480
0.530
0.620
0.424
0.338
0.625
0.166
0.377
0.327
0.547
0.386
0.351
Mean
channel
velocity
(m s 1)
29.08
15.35
23.80
16.62
27.49
44.29
230.03
90.92
24.33
46.16
44.80
68.78
6.47
37.37
284.76
82.89
34.02
Mean
discharge
(m3 s 1)
0.46
1.10
1.80
1.83
0.34
0.73
0.36
0.61
1.13
0.32
0.61
0.52
0.55
0.23
0.12
0.30
0.24
Gradient
(m km 1)
34.93
33.97
43.50
22.72
45.40
30.83
113.89
38.52
39.63
38.04
36.41
74.08
20.12
63.05
87.45
70.18
45.28
Mean
stream
width (m)
62.89
59.08
63.93
57.45
53.57
41.81
43.54
46.79
80.44
68.05
67.08
55.00
35.09
35.34
15.74
23.86
24.08
Mean
annual
run-off
(cm)
7.54
7.21
7.00
7.34
6.53
7.02
6.90
6.81
5.60
5.46
5.80
6.67
6.90
6.91
7.50
7.32
6.34
pH
92.01
88.96
59.17
112.39
50.50
46.05
34.71
35.26
18.00
26.66
34.82
404.51
120.00
61.50
270.77
219.28
166.83
Mean
dissolved
solids
(mg l 1)
0.0141
0.0228
0.0208
0.0167
0.0092
0.0141
0.0092
0.0095
0.0085
0.0008
0.0081
0.0047
0.0240
0.0301
0.0234
0.0186
0.0263
Mean
relative
warp 1
0.0077
0.0008
0.0030
0.0003
0.0079
0.0079
0.0053
0.0001
0.0042
0.0026
0.0092
0.0050
0.0051
0.0017
0.0014
0.0045
0.0035
Mean
relative
warp 2
0.939
0.862
0.896
0.924
0.972
0.943
0.952
0.942
0.983
0.984
0.964
0.954
0.879
0.852
0.890
0.893
0.852
Mean
fineness
index
0.0082
0.0074
0.0076
0.0091
0.0075
0.0074
0.0081
0.0081
0.0103
0.0087
0.0084
0.0091
0.0083
0.0087
0.0077
0.0078
0.0080
Mean
centroid
size
Museum collection numbers refer to lots housed at the University of Florida Museum of Natural History (lots without collection numbers are held privately).
Specimens from sites 11–17 were collected as part of the study detailed in Haas et al. (2010). Relative warp scores are a measure of overall shape variation; fineness index (FI) is a measure of streamlining, where FI = 1 corresponds to a body that most closely resembles a perfectly streamlined airfoil (Langerhans & Reznick, 2010); Centroid size, a measure of overall body size, is the square root of the sum of squared distances of a set of landmarks from their centroid.
17
16
Cahaba River
14
Blackwater
River
9
Sipsey River
Escatawpa
River
8
13
Wolf River
7
Uphapee
Creek
Catahoula
Creek
6
12
Big Cow Creek
5
Tallapoosa
River
Village Creek
4
11
Trinity River
3
Choctawhatchee
River
Sabine River
2
10
Angelina River
Stream
1
Site
number
Table 1. Environmental and morphological data for populations of Cyprinella venusta examined across the range of the species
PREDICTORS OF STREAM FISH BODY SHAPE
845
846
T. C. HAAS ET AL.
Figure 2. The 15 landmark (LM) locations identified in Cyprinella venusta photographs for geometric morphometric
analyses (Hood & Heins, 2000): (1) origin of dorsal fin at first dorsal fin ray; (2) nape of neck, posterior boundary of
supraoccipital bone; (3) tip of snout, upper margin of mouth; (4) posterior margin of opercular series; (5) origin of pelvic
fin at first pelvic fin ray; (6) origin of anal fin at first anal fin ray; (7) origin of anal fin at last anal fin ray; (8) origin of
caudal fin at ventral-most caudal fin ray; (9) origin of caudal fin at dorsal-most caudal fin ray; (10) origin of dorsal fin at
last dorsal fin ray; (11) center of eye; (12) on the midline, half way between the eye and the anterior edge of the caudal
spot; (13) on the midline, half way between landmark (LM) 12 and the anterior edge of the caudal spot; (14) on the midline at the anterior edge of the caudal spot; and (15) the fork of the tail. LMs 12–15 were used for specimen unbending
only and were not included in overall shape analysis.
Using geometric morphometric methods, we
derived measures of gross shape variation, referred
to as relative warps (RWs), for all seventeen populations and for a subset of fifteen populations that
excluded the two populations from the Choctawhatchee clade (Bookstein, 1991, 1996; Rohlf & Marcus,
1993; Adams, Rohlf & Slice, 2004; Zelditch, Swiderski & Sheets, 2012). Because geometric morphometrics examines overall shape variability instead of
particular traits or characteristics, it minimizes the
risk that a functionally important aspect of morphological variation will go unnoticed (Blob et al., 2008).
To capture landmark data, we first photographed the
left side of each specimen along with a size standard
on a light table using a DSC-H2 digital camera
(Sony). We then concatenated images of all specimens into a single file using the ‘Build tps file’ function in TPSUTIL (Rohlf, 2010a). Eleven landmarks
(Fig. 2) were assigned and a scale factor reference
length was defined for each specimen using TSPDIG
(Rohlf, 2004). The influence of unnatural bending of
specimens resulting from the effects of preservation
was removed using the ‘Unbend specimens’ procedure in TPSUTIL (Rohlf, 2010a). We placed four
additional landmarks on each photograph (LMs 12–
15) (Fig. 2) and used those landmarks in conjunction
with pre-existing LMs 3 and 15 to implement the
procedure. Because they were situated along the
midline, LMs 12–15 were not useful for the purposes
of overall shape analysis. LMs 12–15 were subsequently removed from the tps data file before further
shape analysis was conducted. The composite RWs
describing overall body shape were derived from the
resulting tps file with TPSRELW (Rohlf, 2010b). We
subsequently examined only the RWs that explained
at least 10% of overall morphological variation and
we used centroid size computed with TPSRELW (Rohlf, 2010b) as our measure of overall body size.
Trends in morphological variation were visualized by
producing D’Arcy Thompson (1942) style transformation grids based on thin-plate splines in TPSRELW
(Rohlf, 2010b). All subsequent analyses, unless otherwise noted, are based on the data set encompassing
seventeen populations.
In addition to overall shape variation, we characterized body shape variation relevant to swimming
performance by calculating a fineness index (FI)
(Langerhans & Reznick, 2010): FI = (1 – |1 – (FR/
4.5)|), where FR (fineness ratio) = (body length)/
(maximum diameter). A fineness index of FI = 1 corresponds to a body shape that exhibits the minimum
drag for the maximum volume. Fishes with a high
fineness index are expected to exhibit higher steady
swimming endurance, with an optimum at FI = 1
(Langerhans & Reznick, 2010). We used standard
length of each individual for body length and used
maximum body depth for maximum diameter (Lagler
et al., 1977). Standard length and body depth measurements were made in TPSDIG (Rohlf, 2004) using
the ‘Make linear measurements’ function, adding
each measurement to the listing as it was made, and
finally extracting the set of measurement data after
all measurements had been completed by viewing
the listing.
A NALYSIS
OF ENVIRONMENTAL VARIATION
For all sites, stream gradient (m km 1) was calculated in Google Earth (http://www.google.com/earth/)
using the ‘Show Ruler’ tool, tracing a distance of
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
PREDICTORS OF STREAM FISH BODY SHAPE
10 km upstream of the fish sampling site and measuring the altitude (m) at the site (s) and 10 km
upstream (u), which served as inputs for solving the
equation: gradient = [(u – s)/10 km]. Although the
stream reaches where fish were collected were only
100 m in length, we calculated stream gradient over
a much larger distance to account for the grain of
satellite imagery (Mohammed, Ghazi & Mustafa,
2013) and to provide a conservative estimate of
stream gradient experienced by the local population
at each site. The daily migratory distance and home
range of C. venusta is not well known, although it
has been suggested that most North American
stream fish probably have a home range of approximately 10–20 m (Hill & Grossman, 1987). A congener
of C. venusta, C. caerulea, has been known to travel
up to 332 m in a 2-month period; it was observed,
however, that the vast majority of individuals,
stayed within a single habitat patch (measuring 8–
160 m) over the study period (Johnston, 2000).
We also compiled records of hydrological conditions
(run-off, discharge, channel velocity, stream width,
pH, and dissolved solids) at each of the seventeen
sample locations from the nearest US Geological
Survey (USGS) gauging station (National Water
Information System Web Interface; http://waterdata.usgs.gov) (Heins & Baker, 1987; Peterson &
Kwak, 1999; Freeman et al., 2001; Angilletta et al.,
2008). Because of the variability in the time period
over which records of hydrological conditions were
available for all conditions at all sites, we calculated
an average annual value from all available daily
data (except for run-off, which was an annual mean)
for each variable at each site. Averages were based
on 1–90 years of data (mean = 44.01 years), depending on the variable and site. Gauges were typically
upstream of sampling sites (nine out of 17 cases) and
were located an average of 12.5 river km from the
sampling site (range = 0–33 km). A gauge was not
available for the Blackwater River, and so data were
obtained from a gauge installed on the nearest similar river (Styx River, located 11.0 straight-line miles
from the Blackwater River site; Heins & Baker,
1987). No dissolved solids data were available from
the gauge nearest the Conasauga River collection
site, and so we used dissolved solids data from the
next nearest gauge, located 46.2 river km downstream from the gauge from which all other environmental data were gathered for the Conasauga River
site. In other cases where environmental data were
not available from USGS gauge records (pH and dissolved solids at the Big Cow River and Blackwater
River; mean annual run-off at the Catahoula River),
we used data from Heins & Baker (1987).
From map-based and gauge-based records, we calculated site-specific mean values for seven environ-
847
mental variables that were selected on the basis of
the availability of data from all 17 streams: ‘Mean
annual run-off’, which is the average amount of
water (cm) that runs off a drainage basin into a
stream in a year; ‘Mean discharge’, which is the
average daily measurement of channel discharge
(m3 s 1); ‘Mean channel velocity’, which is the average daily measurement (m s 1); Gradient, which is
the slope of the stream described as the ratio of
change in stream altitude (m) per unit distance (km);
‘Mean stream width’, which is the average daily
measurement of channel width (m); ‘Mean pH’,
which is the average daily measurement of in-stream
pH; and ‘Mean dissolved solids’, which is the average
daily measurement (mg l 1) of dissolved organic and
inorganic substances in stream water. We examined
variables for normality using the Shapiro–Wilk test
in SPSS (IBM Corp, Chicago, IL, USA). Any variables that differed significantly (P < 0.05) from a
normal distribution were transformed as a natural
logarithm. To prevent multicollinearity and complications arising from covariance (Glantz & Slinker,
1990; Walters et al., 2008), we tested for correlations
among the environmental variables and discarded all
but one variable from groupings of co-linear variables (Pearson’s r > 0.8) or variables that were significantly correlated to one another (a < 0.05). Mean
discharge was significantly correlated with gradient
(Pearson’s r = 0.57, P = 0.02) and mean stream
width (Pearson’s r = 0.83, P < 0.001); therefore,
mean discharge was retained for further analyses.
Mean annual run-off was significantly correlated
with mean pH (Pearson’s r = 0.51, P = 0.04) and
mean
dissolved
solids
(Pearson’s
r = 0.60,
P = 0.01), therefore we retained mean annual run-off
for further analysis. Mean channel velocity was not
correlated with any other environmental variable.
Although mean discharge and mean run-off exhibited
a marginally significant correlation (Pearson’s
r = 0.491, P = 0.046), we nonetheless retained both
in subsequent analyses. Accordingly, three environmental variables (mean annual run-off, mean discharge, and mean channel velocity) were used in
general linear models of morphological variation.
A NALYSIS
OF PHYLOGENETIC VARIATION
Although the systematics of the blacktail shiner have
not been fully resolved, Boschung & Mayden (2004)
have suggested that three or more subspecies merit
recognition. Thus far, three subspecies have been
recognized on the basis of morphological variability
(Gibbs, 1957a, b) and four evolutionary lineages have
been identified on the basis of mtDNA variation
(Kristmundsd
ottir & Gold, 1996) (Fig. 1). The western blacktail shiner (Cyprinella venusta venusta)
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
848
T. C. HAAS ET AL.
possesses a deeper body, deeper caudal peduncle,
and longer head than the other subspecies. The slender blacktail shiner (Cyprinella venusta stigmatura)
is the most fusiform of the subspecies, with a more
slender overall body depth and more slender caudal
peduncle; the length of the head is also the shortest
of all the subspecies. The morphological characteristics of the eastern blacktail shiner (Cyprinella venusta cercostigma) are intermediate to those of the
other subspecies.
Subspecific delineations within C. venusta are not
congruent with phylogenetic hypotheses of evolutionary differentiation. The maximum-parsimony (MP)
and neighbour-joining analyses of Kristmundsd
ottir
& Gold (1996) recovered four well supported (> 50%
bootstrap support) mtDNA-based phylogeographical
clades. The Western clade includes not only populations that largely fall within the range of C. v. venusta, but also a portion of the range of C. v.
cercostigma. The Mobile clade, which is sister to the
Western clade, encompasses the range of C. v. stigmatura and a small portion of the range of C. v.
cercostigma. The Apalachicola clade is composed of
populations ranging from north central Florida to
central Georgia, which corresponds to the eastern
section of the range of C. v. cercostigma. The Choctawhatchee clade, which is basal to the other three
major clades, consists of a population from the Choctawhatchee River in southwestern Alabama, which
falls within the range of C. v. cercostigma. The geographical discordance between the clades identified
by Kristmundsdottir & Gold (1996) and the subspecies described by (Gibbs, 1957a, b) may be attributable to a variety of factors, including phenotypic
convergence, as well as intergradation as a result of
hybridization and incomplete sampling (Gibbs,
1957a; Boschung & Mayden, 2004).
To avoid potentially circular arguments, we chose
to use the mtDNA-based lineages described by
Kristmundsdottir & Gold (1996) to determine
whether morphological variation is attributable to
phylogenetic history because the subspecies
described by Gibbs (1957a, b) are largely delineated
on the basis of morphological variation, and because
levels of sequence divergence between the clades (i.e.
≥5.7%) are comparable to what has been observed
between closely-related fish species (Dowling &
Brown, 1989; Smouse et al., 1991; Dowling et al.,
1992). We characterized the phylogenetic history of
each population according to distributions of
mtDNA-based clades hypothesized by Kristmundsdottir & Gold (1996). Populations located within
the range of one of the clades were assigned to that
clade. The one population (Blackwater River) located
outside the areas sampled by Kristmundsd
ottir &
Gold (1996) was assigned to the Choctawhatchee
clade on the basis of river drainage patterns and geographical proximity to the Choctawhatchee population sampled by Kristmundsd
ottir & Gold (1996). To
determine whether the clades occupied streams with
distinctly different environmental conditions and to
address the possibility that phylogeny and environment are conflated, we conducted a multivariate
analysis of variance (MANOVA) with clade as the
fixed factor and the three down-selected environmental variables (mean annual run-off, mean channel
velocity, and natural log-transformed mean discharge) as the dependent variables.
P REDICTORS
OF MORPHOLOGICAL VARIATION
Using the geometric morphometric results based on
fifteen populations (the Choctawhatchee clade populations were excluded from this analysis as a result
of insufficient replication), we constructed general
linear models (GLMs) relating population averages
of morphological variation (RW1 and RW2) and fineness (FI) to the selected subset of annual averages of
environmental variables for each site, population
averages of body size, and the mtDNA clade membership of each population. Body size (measured as
centroid size) and mtDNA clade (Kristmundsd
ottir &
Gold, 1996) were included in GLMs to determine
whether allometry or phylogenetic history, respectively, are more dominant predictors of body shape
variation than are environmental variables. Models
were constructed using each of the variables, both
alone and in combination. Akaike’s information criterion corrected for small sample size (AICc) was used
to assess model goodness-of-fit, where smaller AICc
values indicate a more robust model (Burnham &
Anderson, 2002). For each model i, the difference (Di)
was calculated between the model’s AICc and the
minimum AICc value within the set. Candidate models were identified as those models with a value of
Di ≤ 2, which are considered to be indistinguishable
(Burnham & Anderson, 2002). A weight (wi) was also
calculated for each model (Burnham & Anderson,
2002), which can be interpreted as the probability
that model i is the best model within the set of models considered.
We considered Akaike weights (wi), relative variable importance, evidence ratios, and coefficients of
partial determination to distinguish between competing models. Relative variable importance was calculated by summing the Akaike weights for each model
that contained the parameter of interest and comparing the sums (Burnham & Anderson, 2002). Evidence
ratios were calculated by dividing the wi for the top
ranking model by the wi for each model in the set.
Coefficients of partial determination, which estimate
the magnitude of the contribution of a given variable
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
PREDICTORS OF STREAM FISH BODY SHAPE
to the explanatory power of a model, quantify the difference in variation explained by the model with all
predictors and the model with a specific predictor
omitted. The larger the coefficient of partial determination, the greater the net contribution of the variable to a given model’s explanation of shape
variation (Goodman, 1972).
RESULTS
M ORPHOLOGICAL
VARIATION
In the present study, we report outcomes of the geometric morphometric analysis based on the full data
set, unless otherwise noted. Exclusion of the two
Choctawhatchee clade populations did not result in
substantive differences in the shape variables produced by geometric morphometric analysis. For
example, RW1 and RW2 explained 43% and 15% of
overall shape variation, respectively, with all populations included in the analysis (Table ). When the two
Choctawhatchee populations were excluded, RW1
and RW2 explained 45% and 16% of the variation,
respectively.
Populations scoring higher on RW1 exhibited a
longer dorsal fin and more anterior placement of the
insertion of the dorsal fin at the basal junction of the
anterior-most dorsal fin ray (LM 1) (Fig. 2), greater
body depth (LMs 1, 5, 6, and 10), a longer and deeper head (LM 2), greater caudal peduncle depth
(LMs 7–9), and a more posterio-ventral position of
the eye (LM 11). Populations scoring higher on RW2
exhibited a more ventral position of the mouth (LM
3), a shallower head (LM 4), more anteriorly located
pelvic fins (LM 5), a longer anal fin and more anterior placement of the insertion of the anterior-most
ray of the anal fin (LM 6), a longer caudal peduncle
(LMs 8–9), and a more ventral placement of the eye
(LM 11). Assessment of inter-population allometry
revealed that populations with a higher mean centroid size had shorter heads in the horizontal dimension (LM 2), and more ventrally-positioned eyes (LM
11).
P REDICTORS
OF MORPHOLOGICAL VARIATION
The GLMs based on geometric morphometric outputs from fifteen populations (i.e. the data set
excluding the two Choctawhatchee clade populations) indicate that morphological variation among
C. venusta populations is a reflection of environment
and evolutionary history. Of the thirty-one RW1
models considered, two top-performing models were
identified according to the criterion Di < 2 (Table 2).
Run-off and clade were identified as important predictors of RW1 in both top models. One model also
849
included discharge, whereas the other model also
included velocity and body size. According to evidence ratios calculated from Akaike weights (wi),
the model containing discharge, run-off, and clade
was 2.06 times as likely to be the best explanation
for shape variation associated with RW1 compared
to velocity, run-off, clade, and body size. Run-off and
clade also were stronger predictors of body shape
variation in both top models than other variables in
the models; the coefficients of partial determination
were > 0.69 for run-off and clade in both models,
whereas the coefficients of partial determination
were < 0.22 for discharge, velocity, and body size
(Table. 3). Of the thirty-one RW2 models considered,
six top-performing models were identified (Table. 2).
Discharge was identified as an important predictor
of RW2 in four of the six plausible models and runoff was identified as an important predictor in two
of the models. Clade was also identified as an
important predictor in two of the models. According
to evidence ratios, the model containing only discharge and body size as predictors was 1.81–2.62
times as likely to be the best explanation for shape
variation associated with RW2 compared to the
other five top performing models. Relative variable
importance measures indicated that discharge had
greater explanatory power than any of the other
predictor variables. Relative importance of discharge
in the plausible models was 0.73, whereas relative
importance of body size, run-off, and clade each
were < 0.32. In all three of the plausible models
that included one or more other explanatory variable, discharge was a stronger predictor of body
shape variation than the other variable(s) in a given
model. Indeed, the coefficient of partial determination for discharge was greater than that of any of
the other explanatory variables in each model
(Table. 3).
Model selection results for FI were less complex
than model results for RW1 and RW2 (Table 2). Of
the thirty-one FI models considered, a single top performing model was identifiable according to AICc criteria. Consistent with the model results for RW1, the
most probable model for FI consisted of run-off and
discharge as predictors. Run-off made a greater net
contribution to explaining body shape variation, with
a coefficient of partial determination of 0.55, than
did discharge, which had a coefficient of partial
determination of 0.35 (Table. 3).
D’Arcy Thompson transformation grids based on
aligned population-averaged landmark coordinates
from the full data set revealed unique morphological
shifts associated with variation in mean annual runoff, mean discharge, mean channel velocity, and centroid size across the range of C. venusta (Figs 1, 2,
3). Individuals from streams with higher mean
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850
T. C. HAAS ET AL.
Table 2. Plausible predictive models for overall and fineness index measures of body shape variation in Cyprinella venusta selected using Akaike’s information criterion adjusted for small sample size (AICc)
Model
Predictor variables
K
RSS
Relative warp 1
Discharge + run-off + clade
Velocity + run-off + clade + size
5
6
0.0004
0.0003
Discharge
Discharge + run-off
Clade
Discharge + run-off + clade
Size
Discharge + size
3
4
3
5
3
4
0.0003
0.0003
0.0004
0.0002
0.0004
0.0003
Discharge + run-off
4
0.012
Relative variable
importance:
run-off = 0.72;
clade = 0.72;
discharge = 0.49;
body size = 0.24;
velocity = 0.24
Relative warp 2
Relative variable
importance:
discharge = 0.43;
run-off = 0.18;
clade = 0.17;
body size = 0.07
Fineness index
Di
wi
r2
Evidence ratio
Rank
147.2
145.75
0
1.44
0.49
0.24
0.93
0.94
1.00
2.06
1
2
153
151.81
151.48
151.43
151.2
151.07
0
1.19
1.52
1.57
1.8
1.93
0.18
0.10
0.08
0.08
0.07
0.07
0.19
0.32
0.1
0.49
0.08
0.28
1.00
1.81
2.14
2.19
2.46
2.62
1
2
3
4
5
6
0
0.56
0.57
1.00
1
AICc
94.93549
Run-off, mean annual run-off; discharge, mean discharge; clade, mitochondrial DNA-based phylogeographical clade as
defined by Kristmundsd
ottir & Gold (1996); K, the number of model parameters; RSS, residual sum of squares; Di, the
difference between the lowest AICc value and the AICc value of each model in the set (model i); wi, the Akaike weight,
or weight of evidence in favour of model i; evidence ratio, the wi for the top ranking model divided by the wi for each
model in the set. Model rankings are based on evidence ratios, and models with Di < 2 were considered plausible as a
result of model selection uncertainty.
Table 3. Net contributions of individual variables in candidate models as measured by coefficients of partial determination
Model
Predictor variable
Coefficient of partial determination
Relative warp 1 = discharge + run-off + clade
Discharge
Run-off
Clade
Velocity
Run-off
Clade
Body size
Discharge
Run-off
Discharge
Run-off
Clade
Discharge
Size
Discharge
Run-off
0.17
0.76
0.80
0.21
0.79
0.70
0.11
0.29
0.16
0.34
0.32
0.25
0.22
0.12
0.35
0.55
Relative warp 1 = velocity + run-off + clade + body size
Relative warp 2 = discharge + run-off
Relative warp 2 = discharge + run-off + clade
Relative warp 2 = discharge + size
Fineness index = discharge + run-off
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PREDICTORS OF STREAM FISH BODY SHAPE
851
Figure 4. Visualization of differences in average body
shape among three mitochodrial DNA-based clades of
Cyprinella venusta, as described by Kristmundsdóttir &
Gold (1996). Thin-plate spline visualizations illustrate the
direction and relative magnitude of differences in the
location of landmarks relative to the average body shape
among all three clades (magnified 4 to more clearly illustrate variation). A, Western clade. B, Choctawhatchee
clade. C, Mobile clade.
Figure 3. Thin-plate spline visualizations illustrating
predicted average body shape at 3 observed extremes of
environmental variables, based on fitted regressions
between overall body shape and environmental variables.
For each environmental variable, the top image illustrates predicted average body shape at minimum
observed environmental value, exaggerated 3 ; the lower
image illustrates predicted average body shape at maximum observed environmental value, exaggerated 3 . A,
mean annual run-off. B, mean discharge. C, mean channel velocity.
annual run-off were characterized by a shorter base
of the dorsal fin and more posterior placement of the
insertion of the dorsal fin at the basal junction of the
first dorsal fin ray (LM 1), a more slender body (LMs
1, 5, 6, and 10) and caudal peduncle (LMs 7–9), a
smaller head in both horizontal and vertical dimensions (LMs 2 and 4), and a more anterior placement
of the eye (LM 11). Individuals from higher discharge streams exhibited a longer base of the dorsal
fin and more anterior placement of the insertion of
the dorsal fin at the basal junction of the first dorsal
fin ray (LM 1), and a longer base of the anal fin and
more posterior placement of the insertion of the anal
fin at the basal junction of the first anal fin ray (LM
6). Individuals inhabiting streams with higher channel velocity were characterized by a shorter base of
the anal fin and more anterior placement of the origin of the anal fin at the basal junction of the last
anal fin ray (LM 7). Assessment of inter-population
allometry revealed that populations with a higher
mean centroid size had shorter heads in the horizontal dimension (LM 2), and more ventrally-positioned
eyes (LM 11).
Populations belonging to different clades differed
in body depth (LMs 1, 5, 6, and 10), vertical head
depth (LM 2), the length of the jaw and corresponding horizontal dimensions of the head (LM 4), horizontal placement of the pelvic fins (LM 5), length of
the base of the anal fin (LM 6), caudal peduncle
depth (LMs 7 and 9), and the placement of the eye in
both vertical and horizontal directions (LM 11)
(Figs 2, 4). Populations within the Western clade
generally scored higher on RW1, and were characterized by a deeper body (LMs 1, 5, 6, and 10) and caudal peduncle (LMs 7 and 9), a larger head in both
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
852
T. C. HAAS ET AL.
horizontal and vertical dimensions (LMs 2 and 4),
and a more ventral–posterior placement of the eye
(LM 11). By contrast, populations belonging to the
Mobile clade scored lower on RW1. Mobile clade populations were characterized by a more slender body
(LMs 1, 5, 6, and 10) and caudal peduncle (LMs 7
and 9), a smaller head in both horizontal and vertical
dimensions (LMs 2 and 4), and a more dorso-anterior
placement of the eye (LM 11). Populations belonging
to the Choctawhatchee clade were generally intermediate in body shape between the Western and Mobile
clades. Choctawhatchee clade populations were characterized by a body depth intermediate between that
of Western and Mobile populations at LM 1,
although deeper than that of Mobile clade populations at LMs 5, 6, and 10. Populations belonging to
the Choctawhatchee clade possessed a caudal peduncle depth similar to that of Western populations at
LM 7 and similar to that of Mobile populations at
LM 9. Head size of Choctawhatchee populations was
intermediate between Western and Mobile populations at LM 2 and similar to that of the Western
clade at LM 4. The horizontal placement of the eye
in Choctawhatchee populations was intermediate to
that of populations from the Western and Mobile
clades, and vertical placement was similar to that of
the Western clade (LM 11).
The attributes of several traits covaried with clade
and body size or run-off. With the exception of the
vertical orientation of the eye (LM11) varying with
both clade and body size (Figs 2, 4), most of the morphological differences associated with phylogeny (i.e.
between mtDNA-based clades) were similar to those
associated with run-off (Figs 3, 4). However, jaw
length and horizontal dimensions of the head (LM 4)
were attributable to phylogeny alone. Furthermore,
the clades differed in their relationship between body
shape and run-off. Although the MANOVA indicated
that populations from different mtDNA clades did
not inhabit streams with significantly different environmental conditions (F6, 24 = 1.12, P = 0.379)
(Fig. 5), populations from the Western clade typically
occurred in low run-off streams and scored high on
RW1, whereas populations from the Mobile clade
occurred in higher run-off streams and scored lower
on RW1 (Fig. 6). Choctawhatchee clade populations
were intermediate between Western and Mobile
clade populations in both RW1 scores and run-off
(Fig. 6).
DISCUSSION
Previous studies of the relationship between fish
body shape and physicochemical characteristics of
aquatic environments have predominantly focused
Figure 5. Boxplots illustrating variation in average environmental conditions among streams inhabited by populations of Cyprinella venusta belonging to different
mitochodrial DNA-based clades, sensu Kristmundsdóttir &
Gold (1996). Streams inhabited by populations belonging to
different clades do not differ significantly from one another
in mean annual run-off, discharge, or channel velocity (see
text). Outliers 3 and 11 shown in the middle (discharge)
correspond to Trinity River and Tallapoosa River populations, respectively.
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PREDICTORS OF STREAM FISH BODY SHAPE
853
Figure 6. Relationship between mean annual run-off and populations of C. venusta belonging to three mitochodrial DNAbased clades, sensu Kristmundsd
ottir & Gold (1996). Diamonds, populations belonging to the Western clade; squares, populations belonging to the Choctawhatchee clade; triangles, populations belonging to the Mobile clade. Populations scoring
higher on relative warp (RW) 1 exhibited a longer dorsal fin and more anterior placement of the insertion of the dorsal fin
at the basal junction of the anterior-most dorsal fin ray, greater body, a longer and deeper head, greater caudal peduncle
depth, and a more posterio-ventral position of the eye.
on water velocity as a key driver of phenotypic variability (Wikramanayake, 1990; Hendry et al., 2006;
but see also Wood & Bain, 1995; Blob et al., 2008).
Most studies have examined binary conditions on
water velocity, comparing body shape in low vs. high
velocity environments, often between lake and
stream populations (McGuigan et al., 2003; Langerhans, 2008; Foster, Bower, & Piller, 2015). Studies of
body shape variation among fish populations across
lotic habitat gradients are comparably uncommon.
One such study (Hendry et al., 2006) observed that
guppies (Poecilia reticulata) from high water velocity
sites were characterized by smaller heads and deeper
caudal peduncles. A second study (Jacquemin et al.,
2013) found that bluntnose minnow (Pimephales notatus) from streams with higher current velocity
exhibited a more fusiform body shape. Finally,
Schaefer, Duvernell & Kreiser (2011) observed that
the body shape of three topminnows (Fundulus spp.)
was related to stream size, where headwater populations exhibited a body shape approaching a hydrodynamically optimal fineness ratio. In the present
study, we aimed to identify predictors of morphological variation among stream populations of blacktail
shiner (C. venusta) across the south-eastern USA,
taking into account a range of hydrological and phylogenetic conditions. Unlike prior studies, we found
that mean channel velocity was not a strong predictor of morphological variation. Rather, mean annual
run-off was consistently identified as the strongest
hydrological predictor of C. venusta morphological
variation. We also found that phylogenetic history
was an important predictor of intraspecific body
shape variability.
E NVIRONMENTAL
PREDICTORS OF MORPHOLOGICAL
VARIATION
Water velocity can be a powerful driver of body
shape variation among stream fish populations (Hen~ igadry et al., 2006; Kerfoot & Schaefer, 2006; Z
un
Vega, Reznick & Johnson, 2007; Jacquemin et al.,
2013), but our findings support the deduction that
velocity is not always the most important pressure
acting on stream fish morphology (Machado, Heins &
Bart, 2002). Aspects of water velocity (e.g. stream
flow and flood severity) are arguably among the most
characteristic features of stream environments that
play a central role in the ecology and evolution of
freshwater fish (Hynes, 1970). Among the populations of C. venusta that we examined, however, only
variation in the length of the base of the anal fin
was found to correspond to average channel velocity
(Fig. ), which suggests that other factors, such as
variation in velocity or peak flow events (i.e. high
and/or low), may exert stronger pressure than velocity. Alternatively, factors acting independently or in
conjunction with velocity may have greater influence
on phenotypic variation among C. venusta populations, including sets of variables that are not as
readily interpretable (Machado, Heins & Bart, 2002).
Indeed, our findings suggest that multiple aspects of
morphology, including fineness, are associated with
combinations of hydrological conditions. For example,
© 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 842–858
854
T. C. HAAS ET AL.
the length of the anal fin base was associated with
average channel velocity, as well as discharge. Similarly, the length of the dorsal fin base was associated
with discharge, where populations of C. venusta from
streams with higher discharge generally exhibited a
longer dorsal fin base, yet the length of the dorsal fin
base was also associated with mean annual run-off.
Although multiple environmental factors may yield
a combined influence on phenotype, the observed
shifts in C. venusta morphology were most often
associated with variation in mean annual run-off
(Tables 2, 3). Individuals from streams with higher
mean annual run-off generally exhibited a shorter
dorsal fin base, a smaller head, a more slender caudal peduncle, a more slender body overall, and more
anteriorly positioned eyes. Run-off is defined as the
total quantity of water from precipitation that enters
a stream within a drainage basin in a given year,
where spatial variability in run-off reflects precipitation, evapotranspiration and soil-moisture storage
capacity (Wolock & McCabe, 1999). Thus, run-off is a
complex variable integrating across a number of
physical and chemical characteristics that could singularly or jointly act as drivers of body shape variation in stream fishes (Machado et al., 2002). In the
present study, for example, we found that run-off is
highly correlated with mean pH and mean dissolved
solids. Although we are not aware of any prior studies that have established a relationship between runoff and body shape, body shape is not the first characteristic to show such an association (Heins &
Baker, 1987; Heins, 1991; Machado et al., 2002).
Machado et al. (2002), who found that egg size and
other fitness-related traits in C. venusta correspond
to run-off conditions, noted that flows in higher runoff streams tend to be high in late summer and early
fall, when small juveniles are present. Thus, high
run-off is likely indicative of seasonally high flows
that could act as a selective agent for the evolution
of larger egg size (Machado et al., 2002). It is similarly plausible that seasonal variation in flow conditions might also result in morphological variation,
especially in traits that are important in locomotion
(e.g. dorsal fin length and depth of the body and caudal peduncle).
P ERFORMANCE
IMPLICATIONS OF ENVIRONMENTAL
AND MORPHOLOGICAL VARIATION
Fish species that commonly encounter high flows or
live in open water environments often possess morphological features that optimize steady swimming
performance (Blake, 1983, 2004; Vogel, 1996; Plaut,
2001; Domenici, 2003; Langerhans, 2008). Steady
swimming is characterized by movement in a
straight line at a constant speed. Drag is theoreti-
cally minimized by a streamlined body shape, characterized by a deep anterior body depth tapering to a
narrow caudle peduncle (Webb, 1975; Blake, 1983).
The body shape variation that we found among populations of C. venusta is largely consistent with theoretical expectations for optimization of steady
swimming performance. Populations of C. venusta
exhibited greater streamlining in streams where conditions might favour optimization of steady swimming performance. Populations of C. venusta from
higher run-off, higher discharge streams exhibited
shallower bodies and approached the optimal fineness index value, which is indicative of a morphology
that closely resembles a symmetric airfoil exhibiting
the minimum drag for the maximum volume (Langerhans & Reznick, 2010). Although this finding is
consistent with trends detected in other studies that
show more streamlined fish exhibit lower drag coefficients and higher steady swimming performance
(McHenry & Lauder, 2006; Langerhans & Reznick,
2010), it remains possible that the observed trend in
C. venusta is an outcome of factors acting on features
that are not necessarily linked to performance (Blob
et al., 2008). Further work, such as experimental trials, will be necessary to test whether the morphological variation observed in C. venusta corresponds to
variation in steady swimming performance (Ojanguren & Bra~
na, 2003; Tierney, 2011).
E VOLUTIONARY
HISTORY AND MORPHOLOGICAL VARIATION
The present study also indicates that morphological
variation among C. venusta is attributable to phylogenetic history. In some candidate GLMs, phylogenetic history had as much or greater explanatory
power as run-off. Morphological shifts associated
with phylogenetic history were very similar to those
associated with run-off. Shifts associated with mean
annual run-off occurred in the same morphological
features as those associated with clade (e.g. body
depth, caudal peduncle depth, eye position, and
length of the base of the anal fin), and in the same
direction (i.e. more slender body and caudal peduncle
associated with higher mean annual run-off). This
suggests that the observed relationship between phylogenetic history and morphology is partly attributable
to
the
geographical
distribution
and
corresponding habitat in which evolutionary lineages
within C. venusta reside. Habitat characteristics
among the sampled populations were not statistically
different, although populations belonging to the Western clade tend to inhabit lower run-off streams,
whereas populations belonging to the Mobile clade
tend to inhabit relatively higher run-off streams. We
also found evidence of clade-specific differences in
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PREDICTORS OF STREAM FISH BODY SHAPE
morphology, indicating that evolutionary history acts
independently of environmental pressures to shape
morphological diversity in C. venusta. For example,
populations from each of the three clades inhabiting
streams with mean annual run-off of approximately
20 cm generally had deeper caudal penduncles and
were deeper bodied than populations from the same
clade inhabiting streams with mean annual run-off
closer to 30 cm, although clade-specific differences
were also observed among the three populations (i.e.
the Mobile clade population was more slender than
either of the Choctawhatchee or Western clade populations). Thus, populations within different evolutionary lineages exhibit some morphological
convergence as a consequence of parallel responses
to mean annual run-off (Fig. 6), where within-clade
shifts in morphology are similar to trends observed
among all populations. Examination of a larger number of populations within each clade will be required
to determine the extent and drivers of morphological
convergence, which in turn would help resolve geographical incongruencies between subspecific delineations based largely on phenotype and DNA
sequence-based phylogenetic hypotheses of evolutionary differentiation (Gibbs, 1957a; Kristmundsd
ottir
& Gold, 1996; Boschung & Mayden, 2004).
The specimens examined from each evolutionary
lineage were collected in different decades and subjected to different types of preservative, thus presenting the possibility that the effect of evolutionary
lineage and artefacts as a result of preservation are
indistinguishable. This is unlikely to be the case, however, because ethanol is expected to cause greater
preservation-related shrinkage than formalin (Fowler
& Smith, 1983). Thus, the populations collected earlier and stored in ethanol (i.e. members of the Western
and Choctawhatchee clades) should have exhibited
more slender morphological characteristics than those
stored in formalin (i.e. members of the Mobile clade).
We observed just the opposite, suggesting that the
effects of preservation, if any, were overwhelmed by
those resulting from evolutionary lineage.
Evidence from previous studies suggests that, in
most cases, both genetic divergence and phenotypic
plasticity play a role in shaping morphological diversity (Langerhans, 2008). In Cyprinella lutrensis, a
closely-related congener of C. venusta, differences in
body shape between reservoir and stream populations were found to have a genetic basis (Franssen,
2011). In a meta-analysis of 19 studies, Langerhans
(2008) also found that body shape variation associated with flow regime was partially heritable in
almost all (84%) of the cases examined, although evidence of phenotypic plasticity was also widespread
(i.e. in 100% of the cases examined). Our results are
consistent with these findings, where body shape
855
variation among riverine populations of C. venusta
appears to be attributable to a combination of phylogenetic history and phenotypic plasticity, although
further work (i.e. common garden experiments) will
be necessary to assess the relative contribution of
genetic variation and plasticity to phenotypic diversity (i.e. variability, divergence, and convergence)
across C. venusta populations.
C ONCLUSIONS
Fishes have achieved a variety of morphologies to
meet challenges of life in fluid environments that
encompass a diverse range of physical and biological
characteristics (Videler, 1993; Vogel, 1996). Although
we largely considered physical environmental factors
in the present study, it is plausible that biological
factors that we did not examine, such as predation
(Burns, Di Nardo & Rodd, 2009) and aspects of trophic ecology (Jonsson, 2001; Franssen et al., 2013a),
are equally important or stronger predictors of morphological variation. Furthermore, the physical environmental factors that we identified as significant
predictors of body shape variation may merely be
correlated with biological factors that play a more
direct role in determining the morphological variation observed among stream populations of C. venusta. Thus, studies of the predictability of body shape
based on biological factors would represent important next steps for understanding the basis of morphological diversity in stream fishes.
ACKNOWLEDGEMENTS
We thank Rob Robins and the Florida Museum of
Natural History, Division of Ichthyology, for the loan
used in the present study. Justin Mann helped with
field sampling and Lee Attaway assisted in the field
and laboratory. We also thank Matt Sperber for providing laboratory assistance. Craig Hood provided
early inspiration and assistance in methodology.
Finally, we thank three anonymous reviewers for
providing helpful comments that greatly improved
the quality of the manuscript. Part of this research
was conducted when TCH was supported by a Louisiana Board of Regents graduate fellowship. Financial support for field work was provided by a
Louisiana Board of Regents grant to MJB.
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