Morphology and microhabitat use in stream fish

Morphology and microhabitat use in stream fish

Brian M. Wood and Mark B. Bain

Abstract: Microhabitat use and body morphology were compared among 15 warmwater stream fishes from the Alabama River (Alabama, U.S.A.) watershed. Morphological variation among separate populations of a species was detected in 14 of the 15 species, indicating that populations should be separated in analyses among species. Comparison of morphological variation between microhabitat generalist and specialist species suggested that all species may vary in morphology relative to their environment. Regression analysis showed that within two families, Cyprinidae and Percidae, morphology was related to specific microhabitat variables. In the Centrarchidae, morphology was not related to any microhabitat variables. Morphological differences among the species occurred along gradients that were similar to gradients of habitat utilization, indicating that within a family, species widely separated in microhabitat use were morphologically different and species using similar microhabitats were similar in morphology. Our results suggest that patterns of mgsrphological variation correspond to properties of the available habitat for warmwater stream fish species.

RCsnmC : Nous avons comparC 19utilisation des microhabitats et la morphologie corporelle chez 85 poissom d'eaux chaudes provenant du bassin de la rivikre Alabama (Alabama, hats-kinis). Nous avons repCrC une variation morphologique entre des populations sCparCes d'une meme espkce chez 14 des 15 espkces, ce qui indique qu'il faudrait tenir eompte des diverses populations dans les analyses portant sur Ies espkces. La comparaison de la variation morphologique entre les espkces gCnCralistes et spCcialistes sur le plan des microhabitats indique que, chez toutes Ies espkces, la morphologie semble varier en fonction de l'environnement. L'analyse de rCgression a montrC que dans deux familles, les CyprinidCs et les PercidCs, la morphologie Ctait like Zi des variables spicifiques aux microhabitats. Par contre, chez les CentrarchidCs, la morphologie n'ktait like B aucune telle variable. Les diffkrences morphologiques entre les espkces apparaissaient selon des gradients qui Ctaient similaires aux gradients Be 19utilisation de 19habitat, ce qui permet de penser que, dans une mCme famille, les esphces nettement skparCes quant B 19utilisation des microhabitats Ctaient morphologiquement diffdrentes, tandis que les espkces qui utilisaient des microhabitats simiIaires prksentaient des ressemblances morphologiques. Nos rCsultats indiquent que les profils de variation morphologique correspondent aux propriktks de I'habitat dont disposent les esp2ces de poissons des eaux chaudes. [Traduit par la RCdaction]

Introduction

The morphology of fish is regularly interpreted as a set of physical attributes linked to the use of habitat and food resources. Morphological investigations based upon this relationship include the function of characters (Keast and Webb 1946), community structure (Gatz 1979a), niche partitioning among species (Douglas 1987), distinctions among similar species (Baumgartnes e t al. 1988), and

discrimination among separate populations (Shepherd 199 1). Central to most of these studies is the assumption that morphology and ecology are directly related (Gatz 1979a; Felley 1984; Wimberger 1992). However, direct tests of this assumption are limited, and the generality of this assumption is not known.

When morphology and resource use have been inves- tigated, morphometric measurements were typically limited to select body structures such as fins (e.g., Gatz 1979b;

Received February 24, 1994. Accepted January 14, 1995. 5 12290

B.M. ~ o o d ' and M.B. ~ a i n . ' Alabama Cooperative Fish and Wildlife Research Unit,' Funchess Hall, Auburn University. Auburn, AL 34849, U.S.A.

' Present address: New York Cooperative Fish and Wildlife Research Unit, Department of Natural Resources, Fernow Hall, Cornell University, Ithaca, NY 14853, U.S.A. Send reprint requests to M.B.B. Cooperators are Auburn University (Department of Fisheries and Allied Aquacultures, Department of Zoology and Wildlife Sciences, Alabama Agricultural Experiment Station), Alabama Division of Game and Fish, the National Biological Survey, and the Wildlife Management Institute.

Can. J. Fish. Aquat. Sci. 52: 1487-1398 (1995). Printed in Canada I Imprime au Canada

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Felley 1984; Page and Swofford 1984) with poor or no ability to quantify body shape (Winans 1984). These measurements tended to concentrate on specific regions of the body, such as the head and caudal peduncle, causing analyses to be biased towards specific body regions (Winans 1984). More recently, comprehensive measurement of body shape has been refined (truss method of Strauss and Bookstein (1982)) and these data have been highly effec- tive for differentiating both species md populations (Winans 1984; Roby et al. 1991; Wimberger 1992). In addition, truss data provide even areal coverage of the entire f o m of the fish (Humphries et al. 1981; Strauss and Bookstein 1982; Bookstein et al. 1985). Because truss data may better elucidate differences among fishes, relations between morphology and habitat should be better assessed using truss data than conventional data sets.

Before interspecific patterns in morphology can be interpreted, the extent of intraspecific morphological variation must be evaluated (Wiens and Rotenberry 1980). Sub- stantial intraspecific variation in morphology has been documented for many fishes. Intraspecific morphological variation has been used to separate stocks of fishes (e.g., Shepherd H99H), explain intraspecific patterns in microhabitat use (e.g., Ehlinger 1990), document the influence of rearing conditions on a single brood of fish (Swain et al. 1991), and explain mechanisms of defense against preda- tors (Br~nmark and Miner 1992), The substantial plasticity of fishes suggests that intraspecific differences in morphology may influence comparisons of morphology among species. Therefore, interspecific morphological differences may be secondary to intraspecific differences at the microhabitat level.

Fish communities have often been shown to be orga- nized primarily on the basis of habitat (Werner and Hall 1976; Werner et al. 1977; Schlosser and Toth 1984), and morphological investigations have indicated a relationship between habitat use and morphology (Keast and Webb 19666; Gatz 1979b; Felley 1984; Page and Swofford 1984). Habitat use by a species is generally consistent over the range of a species, indicating that many species found in small streams are habitat specialists (Gorman and Marr 1978). Although this generalization may hold for large habitat scales, microhabitat use by stream fishes often varies among individuals of a species (Moyle and Baltz 1985). Consequently, species may be classified as either microhabitat generalists (individuals differ in microhabitat use and use a wide range of available habitats) or specialists (individuals use specific and similar microhabitats). The link morphology may have with microhabitat use has received little attention, probably because few investigators of morphology have been able to obtain suitable data on microhabitat use.

In this pager, we use morphology to describe variation in microhabitat use of fish species found in warmwater streams. The specific objectives of this study were to (i) identify the extent of intraspecific morphological variation from collections in streams differing in habitat avail- ability, (ii) identify variation in morphology and microhabitat use among species of stream fish, and (iii) relate variation in morphology to microhabitat use.

Materials and methods

Fish were collected from nine study sites on six streams located within the Alabama River watershed in Alabama (Fig. 1). Streams chosen for study included a United States Geological Survey streamflow gage, were unregulated, and had similar mean annual flows (Table 1). Water chem- istry data from each stream indicated low conductivity (50-150 pmhos/cm; 1 mho = 1 S), low alkalinity (5-50 mgBL CaCO,), and neutral to slightly acidic pH (6-7.5). Mean summer water temperatures ranged from 24 to 27°C across the six streams.

There were large differences mong the six study streams in physical structure and flow regime (Table 1). Physical structure ranged from rocky, high-gradient streams with a stable, unchanging channel (e.g., Little River) to sandy, low-gradient streams with a shifting channel (e.g., Uphapee Creek). Flow regime among streams varied from highly unstable (e.g., Little River) with relatively great differences among peak, base, and mean Wows to highly stable (e.g., Terrapin Creek) with relatively minor variations in flow statistics.

To obtain fish samples in discrete microhabitats, we used prepositioned area electrofishers (Bain et al. 1985). Electrofishers were located along four or six fixed tran- sects selected to include common instream habitats at each study area. The electrofishers were placed along each transect to sample both midstream and margin areas. For midstream samples, electrofishers were placed at one third and two thirds of the transect width; stream width was measured from bank to bank perpendicular to the greatest amount of stream flow. Stream margins were sampled with one electrofisher with its outside edge directly at the water's edge and one electrofisher located on the opposite shore approximately 1 m from the water's edge. All sampling was done in May through September, during baseflow or near-baseflow conditions.

Fish were collected by electrifying the prepositioned area electrofishers approximately 30 min after placement. A 750-W AC generator and a variable voltage (maximum of approximately 908 V) transformer were used to power the electrofishers for approximately 20-30 s. The electrofishers measured 1.8 X 2.0 m and only sampled fish located between the electrodes. A 3.0 X 1.2 m seine with 3.2-mm mesh was placed immediately downstream of the electrofisher after power was activated to capture stunned fish, and then the seine was passed over the sample area. The substrate within the electrofisher was also disrupted by kicking to dislodge benthic fishes. Dip nets were used to capture fish in areas where the current would not sweep stunned fish into the seine. Fish were anesthetized with tri- cane methanosulfate (MS-222) and then placed into 10% Formalin. Approximately 1 week later the fish were transferred to a 50% isopropyl alcohol solution for preservation.

To quantify microhabitat conditions at each electrofisher location, a standardized sampling method was developed. Water depth, substrate type, and water velocity were recorded at each comer of the electrofisher and averaged for statistical analysis. A graduated wading rod was used to determine water depth. Current velocity was measured at

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0.6 times the water dep,th with a Marsh-McBirney electronic current meter. Substrate type was categorized as clay (<0.06 mm), silt (0.06-0.1 mm), sand (0.11-1 mm), gravel (1.1-60 mm), cobble (68.1-150 mm), boulder (>I50 mm), or bedrock (including hard clay bottoms). Cover was recorded as the number of objects with a diam- eter greater than 7 mm lying across the grid's perimeter.

Morphological measurements Specimens for morphometric measurements were obtained from the preserved fish collections. Species with large numbers of individuals in more than one stream were selected for study. As a result, 15 species from three different families were selected (see Appendix). An attempt was made to measure 30 individuals at each site; however, the lack of suitable specimens reduced sample sizes at many sites. Individuals measured were taken randomly from the pool of suitable specimens at each site. At sites where collections were made in both 1989 and 1990, specimens from both years were combined. Differences in morphology between years were not tested because when specimens were combined, specimens from 1990 made up at least 80% of the sample. Overall, 1554 fish were measured.

Fish were measured and archived using the truss method of Strauss and Bookstein (1982) and the digitizing tech- nique of Winans (1984). Ten identifiable and distinct landmarks were chosen to structure the truss system, result- ing in a total of 21 morphometric measurements (Fig. 2). Each specimen was placed on water-resistant paper and landmark locations were recorded with a pin. Fish total length (measured with dial calipers), weight (measured with an electronic scale), and identification number were recorded beside the pinned form of each fish. The pin holes were then digitized as X-Y coordinates into a spread- sheet program where morphometric distances were calculated by computer using the Pythagorean theorem.

The precision of this approach was calculated using one species from each of the three families studied: orangefin shiner, bluegill, and greenbreast darter. Twenty specimens of each species were measured and digitized independently three times. Using a one-way analysis of variance, variation was separated into among and within groups. Repeatability for each measurement was calculated as the ratio of the among-group variation to the total variation (Falconer 1981; Baumgartner et al. 1988). Measurement repeatability ranged from 0.968 to 0.999, showing that the error of each measurement was very low.

Statistical analysis Microhabitat data were standardized and morphological data were transformed prior to analysis. Habitat variables for each stream were standardized to account for flow regime and channel form differences among the different study streams. Using the mean (of the four measurements for each habitat variable) habitat measurements taken from the electrofishers, a grand mean and standard deviation for each habitat variable were calculated for all individuals of a species collected at each site. The mean habitat measurements for the electrofisher where each specimen was collected were then subtracted from the grand mean and divided by the standard deviation. The result was a new

Fig. 1. Location sf study sites within the Alabama River basin. Study sites denoted by circles indicate sites sampled in both 1989 and 1998, triangles indicate sites sampled only in 1989, and squares indicate sites sampled only in 1998. Study site numbers are denoted under the column study site code in Table 1.

score for each habitat variable for each individual. The standardization scaled each stream's habitat conditions similarly: extremes in habitat conditions were given the highest values (both positive and negative) and the midrange conditions were given the lowest values (near zero). Morphological data were transformed using logarithms (base 10) before analysis because the transformation pre- serves allometries and produces scaling-independent covari- ances (Bookstein et al. 1985; Baumgartner et al. 1988). Analyses were performed using two statistics packages (Data ~ e s k " 3.0 1989 and Systat 5.1 1991).

Multivariate analysis of covariance was used to test for differences in morphology within and among species. Multi- variate analysis of covariance, prior to testing for significant differences in population centroids, uses a covariate to adjust the set of dependent variables (Pimentel 1979). Total length was used as a covariate in the multivariate analysis of covariance to eliminate the confounding effect of size from the tests. A significant Wilks' lambda statistic indicated that significant shape differences existed. Because of the large morphological differences among the three family groups, separate analyses were conducted for each group.

Principal components analysis was used to illustrate microhabitat use and morphological differences among the species. Principal components from the standardized microhabitat features were computed using the correlation matrix,

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Table 1. Stream flow data and physical characteristics of each study site.

Stream

USGS gage data Field data

Mean Drainage Peak Base annual Site

Study area flow flow discharge CV gradient site code (krn2) (m3/s) (m3/s) (m3/2;) (96) (cmlm) Common substrates"

Little River LWN (1) 515 150.7 0.1 Terrapin Creek TER (2) 635 25.9 1.7 Hillabee Creek HILl (3) 492 54.2 0.3

HIE2 (4) Hatchett Creek HAT1 (5) 681 66.3 0.6

HAT2 (6 ) Oakmullgee Creek OAK (7) 570 15.5 0.5 Uphagee Creek UPHl (8) 862 212.4 0.4

UPH2 (9)

Cob, Grvl, Snd Cob, Grvl Grvl, Snd, Cob Grvl, Cob, Snd, BId Grvl, Snd, Cob, BId Cob, Bld, Grvl, Bed GwB, Snd Grvl, Snd, Cly, SIt Snd, Grvl, Slt

Note: United States Geological Survey (USGS) data were taken from a representative and comparable set of records (water year 2984) for pages located on each study stream. Field data were based on habitat surveys conducted during base-flow conditions. Peak flow indicates the highest daily discharge measured during the water year while base flow was the typical mean daily discharge during the dry period (May-November). The coefficient of variation (CV) for mean annual discharge was calculated from the daily discharge data. Site gradient was calculated as the elevation change from the upstream to the downstream end s f study site divided by site length.

"Bed, bedrock; Bld, boulder; Cly, Clay; Cob, cobble; Grvl, gravel; Slt, silt; Snd, sand.

because the habitat variables were measured on different scales and the standardization of variation that occurs with the correlation matrix was required (Bimentel 1979). The important habitat components (eigenvalues > 1.0) were then used to plot species locations in multivariate habitat space. The variance-covariance matrix was used to obtain principal components from the morphological data. The variance-covariance matrix measures absolute variation and was used because each variable was measured on an equal scde (Pimentel 1979). Components with eigenvalues > 0.7 were considered as important components because they provided ecologically interpretable information (Matthews 2985). The shear procedure described by Haarnphries et al. (1981) and Bookstein et d. (1985) was utilized to ensure that the morphology components were not confounded with size infomation. The shear did not produce a difference in coefficients, and therefore the morphology components were assumed not to be confounded with size variation.

Multivariate analysis of variance was used to evaluate association between habitat attributes and the presence of a species. For each species, the hypothesis that habitat composition was not different among samples with and without a species was tested. Initially a complete model with all habitat variables was tested and variables were eliminated on the basis of canonical loadings and changes in the F statistic for the model. Different models were tested until the simplest model (less variables) was found that best identified species absence or presence. Models including only one variable were reported as an analysis of variance model, while models including more than one variable were reported as a multivariate analysis of variance model.

Regression analysis was utilized to develop relationships between morphology and microhabitat use. Mor- phological variables created in principal components analysis

for each family group were regressed against the variables from the principal components analysis for the microhabitat data. The result was three separate regression analyses that determined if relationships were present between morphology and microhabitat use for each family.

Multivariate statistical analyses include two assumptions that must be met for the analyses to be valid. These assumptions are that the dependent variables are multi- normally distributed and that the within-group dispersion matrices are equal across the treatment groups (Hair et al. 1984). Although these assumptions were not directly tested in this study, they were accepted as valid because similar studies have confirmed that morphological data meet the assumptions. For example. Pimentel (1979) and Misra and Ni (1983) state that multivariate nomality is closely approx- imated by the logarithms of morphometric measurements taken on continuous variables. Baumgartner et al. (1988) tested the nomality of ino~phometric variables and principal component scores and found that 84% of the variables had no departure from normality and the other 16% were only slightly non-normal. The homogeneity sf dispersion matrices was tested by Taylor and McPhail (1985). Although they found differences among matrices, they felt that their analyses were not compromised because of their large sample sizes of 25. Because our sample sizes were equally large and logarithmically transformed, we feel that the assumptions of our multivariate analyses were adequately met.

Results

Significant intraspecific morphological variation was found among stream populations in 14 of 15 species using multivariate analysis of covariance (P < 0.01, Table 2). In the Cyprinldae and Percidae, each species varied in morphology among populations. In the Centrarchidae, one species,

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bluegill, did not vary in morphology among stream populations. The observed variation in morphology among populations indicated that each study streah has morphologically unique fish within a species, and therefore populations should not be pooled for the study of morphology-habitat relations.

Multivariate analysis of covariance indicated that within each family, species differed in morphology (P < 0.81). Principal components analysis illustrated morphological differences within each family. In each analysis, the first component was considered a size component because it was comprised of coefficients with relatively equal and similar values (Humphries et al. 1981; Winans 1984; Bookstein et al. 1985). Subsequent bipolar components were characterized as shape components. These bipolar shape components discriminated species within each family. Shape components from each of the family-group principal component analyses were not correlated with either fish total length or body weight (P > 0.05). Pearson correlation coefficients between shape principal components and fish total length were very small ( r 5 0.09). The lack of correlation between the shape components and total length and weight indicates that these analyses were measuring actual shape differences among the species and not simply differences in length or weight.

The pattern of morphometric variation among cyprinids was best illustrated by the second principal component of cyprinid morphology. The first component (92% of the total variation) largely explained size variation. The second component (2% of the total variation) separated the species along a morphological gradient (Fig. 3). Five truss measurements largely explained the variation among the species: mouth shape and the relative positions of the dorsal and anal fins. Positive and negative principal component loadings defined a gradient in shape with extremes marked by species with smaller, subterminal mouths with a relatively large distance between the origins of the dorsal and anal fins (e.g., largescale stoneroller), to species with larger, superior mouths and a small distance between the origins of the dorsal and anal fins (e.g., silverstripe shiner).

The pattern of variation among centrarchids was best illustrated by using one shape component (2% of total variation). This component poorly discriminated the three species (Fig. 4), which was expected given the mixed significance results in Table 2. Ninety-five percent confidence intervals for the mean of the shape component for each species had a high amount of overlap, indicating little variation in morphological scores among the three species. Principal component loadings defined variation in two different truss measurements: mouth length and distance between the pectoral fin origin and pelvic fin origin. However, these measurements were not sufficient to separate the three species.

Three components were identified in the principal components analysis for the four darter species. The two shape components (each 3% of the total variation) illustrated the differences among the species (Fig. 5). The second component primarily separated one species from the other three, while the third component almost completely separated the four species. Principal component loadings on the second component (first shape component) contrasted two

Fig. 2. Locations of the 10 truss landmarks on specimens from each family studied. Landmarks are denoted by circles along the outline of specimen drawings. For landmarks not on the outside of body form, points were made at the closest point to the body on a line perpendicular to the horizontal axis of the specimen (Winans 1984). Landmarks refer to (1) most posterior point of maxillary, (2) most anterior point of jaw, (3) origin of pectoral fin, (4) most posterior part of neurocranium, (5) origin of pelvic fin, (6) origin of dorsal fin (or spinous dorsal fin), (7) origin of anal fin, (8) insertion of dorsal fin (or beginning of soft dorsal or origin of soft dorsal fin), (9) anterior attachment of ventral membrane from caudal fin, and (18) anterior attachment of dorsal membrane from caudal fin. Specimen sketches taken from Wobison and Buchanan (1988).

length truss measurements and four body-depth truss measurements (Fig. S i ) , indicating body slenderness. Five truss measurements largely explained variation in shape for the third component: compression of the body about the pectoral and pelvic fins and a concurrent elongation of the body (Fig. 5).

Multivariate analysis of variance identified microhabitat variables that predicted the presence of a species (Table 3). In general, most species showed a significant association with some microhabitat attribute at each study site, although

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Table 2. Probability values from tests of intraspecific morphological variation among study sites using univariate and multivariate analysis of covariance of rnorphornetric variables.

G yprinidae Centrarchidae Percidae

Measurement LSS ALS BTS ST'S PT% OFS %L% MIS RED BLU LON GBD SPD BBD BZD

1-2 2-4 1-3 2-3 1-4 3-4 4-6 3-5 4-5 3-6 5-6 6-8 5-7 6-7 5-8 7-8 8- 10 7-9 8-9 7- 10 9- 10

Multivariate

Note: Body length was used as the covariate. Measurements are shown in Fig. 2. Species cores are in the Appendix. Probability values s f 0.01 indicate a value less than OH equal to 0.51.

T h e variable would be significant at the species-wide sequential Bonferroni (Wise 1989) probability value sf 0.00227.

no significant associations occurred in 10 analyses out of 55 species-site combinations. The significant associations with habitat suggested that species used specific microhabitats at each study site. However, specific microhabitat use did vary among study streams, reflecting intraspecific variation in this behavior.

Principal components analysis characterized general microhabitat use by each species. Variation in the microhabitat data was represented by two components com- prising approximately 60% of the total variation in microhabitat conditions across all sites. The first habitat component was dominated by two microhabitat variables: current velocity and substrate type. The first component defined a gradient from fast water with coarse substrate to slow water with fine substrate, whereas the second component contrasted water depth and amount of cover (Fig. 6). The mean position (centroid) of each species was plotted within this microhabitat space. Species centroids illustrated patterns of microhabitat use among the species and families (Fig. 6). The cyprinid species were distributed in a gradient from shallow, high-cover microhabitats with medium current speeds and substrate sizes to deep microhabitats with less cover. However, they also showed some differences in use of available current speeds and substrate types. In contrast, the three sunfish species based similar rnicrohabitats. Centroid locations for all three were

concentrated in areas of slow current velocity and fine substrate with moderate depth and cover. The four darter species were distributed in a gradient from fast current and coarse substrates to slower current speeds with finer substrates. These patterns reflected the general use of available microhabitat conditions by each family, although they obscure intraspecific morphological variation.

For Cyprinidae and Percidae, regression analysis indicated significant ( P < 0.05) relationships between variables defined in principal components analyses of morphology and the standardized microhabitat variables from the analysis of habitat components. Results from regression analyses were similar if species were combined over all sites or if sites were separated. However, sites were separated because most species exhibited intraspecific variation among the populations. The second habitat component, contrasting water depth and the amount of cover, was inversely related to cyprinid morphology (Fig. '7). In contrast, both components from the percid analysis were sig- nificantly related to the first habitat component, a gradient from fast cun-ent speeds and coarse substrates to slow current speeds and fine substrates (Fig. 8). However, the third component was more strongly correlated to current velocity thaw was the second component. Regression analysis did not show a significant relationship between sunfish morphology and microhabitat use.

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Table 3. Attributes of habitat ass~ciated with presence ~f a species as determined by multivariate and univariate analysis of variance.

Study stream

LRN TER HIL l HIL2 HAT1 HAT2 OAK UPHl UPH2

Largescale stoneroller Alabama shiner Blacktail shiner Striped shiner Pretty shiner Orangefin shiner Silverstripe shiner Mimic shiner

Redbreast sunfish Bluegill Longear sunfish

Greenbreast darter Speckled darter Blackbanded darter Bronze darter

Cyprinidae

SH,CO n s CO,SH SH SL,FI -

ns -

DP -

-

ns -

- -

Centrarehidae

%L,FI - - - - -

Pereidae

FA,CO,SH FA,SH SM,SL ns -

SH,CO ns - -

Note: Abbreviations for habitat attributes are as follows: SH, shallaw; DP, deep; FI, fine substrate; 60, coarse substrate; SL, slow velocity; FA, fast velocity; CP, cover present; N6, no cover; ns, no attributes significant. Dashes indicate that a species was not present. Habitat attributes are listed in order of importance. Study stream abbreviations are denoted in Table 1.

Intraspecific morphological differences among separate populations were observed in our study and in previous studies on many different fish species (Berg 19'79; Gatz B979a; Ihssen et al. 198 1; Widdell and Eeggett 198 1; Winans 1984; Beacham 1985; Taylor and McPhail 198%; Layzer and Clady 198'7; Matthews 1987; Ehlinger 1990; Karakousis et al. 1991; Shepherd 1991; Swain et al. 1991). Many authors have noted that, within a species, morphological features are responsive to local habitat conditions. Swain et al. (199%) suggested that coho salmon (Oncorhynchus kisutch) have a phenotypic plasticity that allows them to adapt to their local environment. Riddell and Leggett (1981) hypothesized that morphological differences in juvenile Atlantic salmon (Salrno salar) were adaptive responses to long-term flow conditions. Similarly, Beacham (198%) con- cluded that morphometric variation in pink salmon (Oncs- rhynchus g ~ r b u s c h a ) reflected adaptation to local water velocity in spawning streams. Our data indicated morphometric differences among separate populations, including populations located within the same stream. Conse- quently, the populations should be separated when testing for relationships between morphology and habitat at the interspecific level.

Variation in morphology among populations did not appear to confound the analysis of morphology among species. Within a species, populations were relatively

similar in morphology compared with other species, and populations grouped together when compared with populations of other species. Recently, Douglas and Matthews (1992) suggested that the phenotype of fishes acts as a constraint on the ecology of an individual, therefore limiting the ability of an individual to adapt to local conditions. Our data support and elucidate this conclusion. While individuals of a species may vary in morphology depending on attributes of their local environment, their morphology is restricted by the phylogenetic processes that made the species unique.

Multivariate analysis of variance indicated that 11 of the 15 species could be classified as microhabitat specialists because they used a statistically distinct subset of microhabitat conditions and they often used similar types of microhabitats in different streams. Four other species (blacktail shiner, striped shiner, silverstripe shiner, and mimic shiner) sometimes used a statistically distinct microhabitat, and the type of microhabitat differed among streams. However, these generalist species did not vary at a greater degree than did the specialists in the morphology-habitat relations analysis (Fig. 7). Therefore, although a microhabitat specialist may use very distinct microhabitats, they still have the ability to vary in morphology relative to their environment, an ability that generally would be given to a microhabitat generalist.

Cyprinid species have, in previous studies, been shown to partition habitat by stratifying the water column into

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Can. J. Fish. Aquat. Sci. Vol. 52, I995

Fig. 3. Histogram of the distribution of the principal components scores from the Cyprinidae principal components analysis. Shaded areas indicate bars that include the 95% confidence intervals for the mean principal components score for each species. Sample sizes are denoted for each species. Highlighted landmark lines identify the measurements loading most heavily in the principal components analysis and indicate measurements separating the species. Measurements from one sketch are coitrasted with ieasurements from the other sketch to define morphological differences among the species.

lo A Mimic shiner

-1.5 4.5 0.5 1.5

PC 2 (Shape component)

vertical zones (Goman and Karr 1978; Douglas 1987). In addition, Gorman and Karr (1978) pointed out that habitat segregation might be based upon horphological adapta- tions, and Felley (11984) found significant correlations between specific-morphological variables and water depth. In this study, cyprinids segregated the available range of habitat by the depth of water and presence of cover possibly owing to vertical separation of habitat. Five morphological measurements were important in associating cyprinid morphology and use of habitat. The divergence of these measurements, showing differentiation in mouth type and in the relative locations of the dorsal and anal fins among the species, suggests some adaptive advantages of morphology relative to the depth of water inhabited. Mouth types ranged from subterminal in the largescale stoneroller to-superior in the pretty shiner and silverstripe shiner.

Fig. 4. Histogram of the distribution of the principal components scores from the Centrarchidse principal components analysis. Shaded areas indicate bars that include the 95% confidence intervals for the mean principal components score for each species. Sample sizes are denoted for each species. Highlighted landmark lines identify the measurements loading most heavily in the principal components analysis and indicate measurements separating the species. Measurements from one sketch are contrasted with measurements from the other sketch to define morphological differences among the species.

-8 '5 6.5 0.5 t.5

PC 2 (Shape mcwponent)

Because these species were on opposite ends of the morphological and habitat spectrums, mouth type was assumed to indicate where a species obtained its food (Keast and Webb 1966; Aleev 1949; Gatz 1979b; Helley 1984). The positions s f the dorsal and anal fins showed similar relationships. In shallow water forms, such as largescale stoneroller and Alabama shiner, the dorsal fin is corn- pletely anterior of the anal fin while in deeper water forms, such as pretty shiner and silverstripe shiner, the insertion of the dorsal fin is posterior to the origin of the anal fin. The positions of these fins indicate a change from maneuverability in shallow water forms, where the more anterior dorsal fin acts as a rudder, to stability in deeper water forms, where the more caudal dorsal fin acts as a stabi- lizer (Aleev 1969; Felley 1984). The species with inter- mediate forms, such as blacktail shiner, that used midrange water depths, generally had terminal mouths for foraging at midwater depths and moderately separated dorsal and anal fins that could be used both for stability and as a rudder.

Strong correlations between darter morphology and specific habitat variables indicated a strong relationship betweem habitat use and morphology (Page and Swofford 1984). Winn (1958) and G o m m and K m (6978) noted that daters

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Fig. 5. Histograms of the distributions of the principal components scores from the second and third components of the Percidae principal components analysis. Shaded areas indicate bars that include the 95% confidence intervals for the mean principal components score'for each species. Sample sizes are denoted for each species. Highlighted landmark lines identify the measurements loading most heavily in the principal components analysis and indicate measurements separating the species. Measurements from one sketch are contrasted with measurements from the other sketch to define morphological differences among the species.

Pdndpal &rnponent 2 (Shape mmpnen?)

-1 .8 -0.5 0 5 1.8

Principal Component 3 (Shape component)

subdivide riffles horizontally on the basis of current and substrate. Fisher and Pearson (1987) noted that four Ethes- stoma species found in riffles were segregated by current velocity, but not substrate. In our study, regression analysis related darter morphology to current velocity and substrate. Therefore, it appears that darter species horizontally segregate habitat by current velocity and substrate type, and, like the cyprinids, have morphologies related to their microhabitat use.

Two sets of morphological measurements separated the four species of darters and were related to current velocity.

Fig. 6. Species centroid locations from principal components analysis on standardized microhabitat data from samples where fish were captured. Microhabitat attributes that characterize each component are shown along the axes. Species codes are defined in the Appendix.

Shallow MQE COVt2P GBDA

- N STS - c 0.5 LseA ALS •

ABBD MIS. Q BZD

St% SPD BtU

BTS RED 8 0.0 LON

w .- & -0.5

Deep Less cover

-0.5 0.0 0.5 I .Q

Fast Principal Component 1 Slew Coarse substrate Fine substrate

Because substrate type is highly dependent on current velocity, current velocity was assumed to be most important in influencing the selection of habitat by darters. The first set, a body slenderness component, contrasted body depth and body length. It was comprised of four depth and two length measurements that revealed a shift from short, compact species (greenbreast darter) to longer bodied, nmower species (blackbanded darter). This set of measurements was related to current velocities as shown by Page and Swofford (1984), who noted that darters found in swifter waters (riffles) have a deep body and deep caudal peduncles (greenbreast darter) while darters found in moderate-speed waters (runs) have more fusifom bodies and nmower caudal peduncles (speckled darter). The second set of measurements, denoting compression about the pectoral and pelvic fins and a lengthening of the body, was also related to current velocity. It not only denoted a gradient from short and compact to long and narrow, but also indicated a small change in distance between the pectoral and pelvic fins. In the greenbreast darter, a short-bodied darter, there was a greater distance between the two fins than in the speckled darter, a longer bodied darter. The more anterior position of the pectoral fin in the greenbreast darter could aid in maintaining position on the substrate by acting to push the darter down when the water flow hits the fin. Page and Swofford (1984) suggested that the current flow- ing against the pectoral fins of riffle-dwelling darters pushes down and helps the darter maintain its position. In contrast, the closer proximity of the pectoral and pelvic fins of the speckled darter, a slower water darter, would permit greater maneuverability and hence more movement.

Although the truss data set did not produce differences among the three sunfish species, many studies have documented variation in morphological characters among kpoanis species (Keast and Webb 1966; Gatz 1979s, 1979b; Douglas and Avise 1982; Mittelbach 1984; Kieffer and Colgan 1992). Two reasons may account for the lack of morphological differences among the sunfish species. First, one truss measurement, distance between the origins of

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Fig. 7. Plot of regression analysis between cyprinid principal compsnent 2 centroids and the standardized habitat principal component 2 centroids. Microhabitat attributes that characterize the component are shown along the horizontal axis.

I I I -0.6 -0.2 0.2 0.6

D V Habfat Principal Shalhw < ~ e s a cower Component 2 Sewe More cower

the pectoral and pelvic fins, had a higher amount of measurement variation than the other 20 measurements. Because the body morphology of these three species appears to be very similar, a high degree of variation in one measurement could possibly conceal slight differences in morphology. Secondly, the truss measurements used in this study may not be sufficient to separate the three species. Douglas and Avise (1982) stated that, morphologically, the members of the Cyprinidae family were no more dif- ferentiated from one another than were the kepsmis species. However, Felley (1984) noted that morphological relationships pertinent for one species group may not be relevant to other species groups. Previous morphological studies separating sunfish have often used up to 50 morphological characters, including several fin, body, and internal measurements not incorporated within this study. We conclude that the truss system was not sufficient to separate the three species because characters used to separate the three species in other studies were not included in ours.

Although our approach to measuring morphology differed from that of previous studies using stream fish species, the conclusions we have drawn from the truss morphometric data are similar. Like Satz (%979b), Felley (1984), and Wikramanayake (19901, we found that morphology can be predicted from microhabitat use and therefore support the assumption that morphology and ecology are related. Our results also indicate that although a species

Fig. 8. Plots of regression analysis between percid principal component 2 and principal component 3 centroids and the standardized habitat principal component 1 centroids. Microhabitat attributes that characterize the component ate shown along the horizontal axis.

Morphology Principal Csmp@nent 2

I i I -0.5 -0.25 0.0 0.25

Fsss t Habitat Paiwdpal SBo w

Coarse Component I Smre - may vary in morphology among sites, the variation is not sufficient to confound interspecific rnorphometric analyses.

Morphological differences were greatest among species that differed most in microhabitat use. Species ranging greatly in use of microhabitat, such as the eight cyprinid species, were simultaneously highly separated in morphological space. In contrast, species in the same microhabitats, like the sunfish, showed little separation in morphology. This finding relates well to previous studies using different morphological measurements. Felley (1984) noted that cyprinid fin sizes and placements were indicative of pre- ferred position in the water column. Page and Swofford (1984) reported that taxonomically distant darters occu- pying similar habitats resembled one another morpholog- icdly more than they resembled closely related (e.g., same genus) species. Overall, habitat is considered to be a primary factor in the organization of fish communities (Schoener 1974; Werner and Hall 1976; Werner et al. 1977; Bain et al. 1988), and our results indicate that patterns of morphological variation also correspond to the properties of available habitat for many species and populations.

This study could not have been completed without the dili- gent work of Jeremy Knight and his technicians, whose

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carefully catalogued fish collection and data sheets made this contribution possible. Sincere appreciation is also expressed to Dennis DeVries and Michael Maceina for use of their digitizing pad and software. Helpful comments were given by John Grizzle, Michael Maeeina, Michael Wosten, Johnie Crance, Mary Freeman, Cliff Webber, Jeff Walker, and one anonymous reviewer during the study and on earlier drafts of the manuscript. This study was funded from a research grant by the United States Fish and Wildlife Service, National Ecology Research Center, under eon- tract No. 14-16-0089-1550, No. 9, with Auburn Univer- sity, through the Alabama Cooperative Fish and Wildlife Research Unit.

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Appendix

Names of species used in this study.

Code (used in

Common name Fig. 6) Scientific name

Cypriwidae

Largescale stoneroller LSS Campostorna oldgobepis Alabama shiner ALS Cjprinella calldstia Blacktail shiner BTS Cyprirzebba venusbn Striped shiner STS Luxikus ehrysocephalus Pretty shiner PTS Lyrhrurus beklus Orangefin shiner OFS Nsbropis ainrnophilus Silverstripe shiner SLS Notropis srilbius Mimic shiner MIS Notropis vsllucellus

Centrarehidae

Redbreast sunfish RED Lepornis auritus Bluegill BLU Eepomis snacwchirus Longear sunfish LON Lepornis megalobis

Percidae

Greenbreast darter GBD Ethecpstsma jol-diani Speckled darter SPB Ebhe~stoma! sbigmaeum Blackbanded darter B B D Bercina nigrofasciaba Bronze darter BZD Percicina galmaris

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Documents

Morphology and microhabitat use in stream fish