2008 NORTHEASTERN NATURALIST 15(1):111–122
Patterns of Spatial Variability in the Morphology of
Melinda A. Coleman1,2,* and Jessica F. Muhlin1
Abstract - Morphology is one of the most common methods used to identify among
congeneric species, but its use can be problematic when spatial patterns of phenotypic
variability are unknown. We examine spatial variability in the morphology of two
sympatric species of algae (Fucus vesiculosus and F. spiralis) on large (>100 km) and
small (<10 km) scales and determine the extent to which they can be identified based
on morphology. These species could generally be distinguished based on gross morphology
on large spatial scales, but on small scales some individuals exhibited much
overlap in the gross morphology. This finding is not surprising given that hybridization
and introgression in these species is common. Although there were some consistent
patterns in morphology between species, many were species-specific. Similarly, there
were few common spatial patterns when individual measures of morphology were
analyzed, suggesting that selective pressures may act on each species independently.
These results have implications for the use of morphology in identifying congeneric
species. Given the existence of individuals of intermediate morphology, it is likely that
species are often misidentified when spatial variability in morphological distinctness
is not considered. In particular, stipe width may prove to be a valuable predictor of
species identification for non-reproductive and young individuals, as it was the only
morphological variable to vary consistently between the species.
Morphology is one of the most common methods used to identify and
classify species. Congeneric species, however, are often difficult to distinguish
based on morphology because it can vary greatly in space. Moreover,
in closely related or congeneric species, hybridization can result in individuals
of “intermediate” morphology. Using morphology to identify and
distinguish among species is often problematic when spatial patterns of
morphological variability are unknown (Mathieson et al. 1981).
Morphological variability within species of marine macroalgae can occur
on small and large spatial scales. Field experiments have demonstrated that
the processes that cause this variability are sometimes genetic (e.g., Chapman
1974, Roberson and Coyer 2004) but often environmental (e.g., Fowler-
Walker et al. 2006). Before we can understand the processes that structure
morphology and whether such processes are general or specific to particular
places, morphological variability must be quantified on a number of spatial
scales. Although there exists a long history of such sampling for many species
(see Mathieson et al. 1981 and references therein), rarely are patterns of
1321 Hitchner Hall, School of Marine Sciences, University of Maine, Orono, ME
04469. 2Current address - 501B Biological Sciences Building, Center for Marine Biofouling
and Bioinnovation, University of New South Wales, NSW 2052, Australia.
*Corresponding author - firstname.lastname@example.org.
112 Northeastern Naturalist Vol. 15, No. 1
morphological variability simultaneously assessed across more than a single
species over multiple spatial scales. Understanding the extent to which the
morphology of closely related or morphologically similar species co-varies in
space is particularly important where morphological characteristics are used
to determine species identity.
Fucus vesiculosus L. and F. spiralis L. are sympatric macroalgae that are
common on the intertidal rocky shores of the northeastern USA and Canada.
Fucus vesiculosus occurs in mid-intertidal areas, with F. spiralis occupying
the adjacent substratum higher on the shore. Common morphological traits
used to distinguish between these species in the field include the presence of
a receptacle ridge (F. spiralis) and vesicles (F. vesiculosus); however, these
traits are not always present (e.g., in non-reproductive and young individuals,
respectively) making species identification problematic (Burrows and
Lodge 1951). Moreover, there is much morphological variability in these
and other traits within each species (Jordan and Vadas 1972, Kalvas and
Kautsky 1998, Knight and Parke 1950, Scott et al. 2001), which has often
been hypothesized to be a result of hybridization (Burrows and Lodge 1951,
Kniep 1925, Scott and Hardy 1994), different selective regimes (Chapman
1995), or genotypic differences (Anderson and Scott 1998). Despite great
spatial variability in morphology of each individual species and subsequent
difficulties in species identification, we have little knowledge of the extent to
which the morphology of these congeneric species co-vary in space. Indeed,
this is hampered by the fact that few studies have quantified morphological
variability in each species across more than one spatial scale.
Here, we examine spatial variability in the morphology of these two
congeneric species of algae and determine the extent to which these species
can be identified and distinguished based on morphology. We tested the hypothesis
that on large spatial scales (10s to 100s of km) F. vesiculosus and F.
spiralis can generally be separated based on gross morphology and individual
measures of morphology, but at small spatial scales (i.e., within sites), these
differences break down, potentially due to site-specific levels of hybridization
and introgression. Further, we determined whether spatial differences in the
morphology of each species are common to both species or are species-specific. Due to the occurrence of apparent widespread dispersal (Coleman and
Brawley 2005; Muhlin et al., in press) and hybridization and introgression
(Engel et al. 2005, Wallace et al. 2004) between these species, we thought it
most likely that selection or differing levels of hybridization were the driving
forces behind any differences in gross morphology among sites therefore
we also tested the hypothesis, that for each species variability in morphology
among sites is not related to spatial distance. That is, variability in morphology
between sites that are close together will be similar to, or greater than,
morphological variability between sites that are far apart.
Materials and Methods
Fucus vesiculosus and F. spiralis were collected from each of 4 sites
(separated by km) at each of two coastal points, Pemaquid Point (43o83.702'N,
2008 M.A. Coleman and J.F. Muhlin 113
69o50.64'W) and Schoodic Point (44o21.928'N, 68o04.613'W), >100 km of
coastal distance apart on the coast of Maine, from August to November 2002
(see Coleman and Brawley 2005 for map). F. spiralis were also collected
from 3 sites (separated by 10s of meters) at Avery Point in Connecticut (CT,
41o18.95'N, 72o3.99'W), approximately 565 km from sites in Maine (linear
distance). These Maine sites were Ledges (L, 44o20.585'N, 68o02.632'W),
Marks (M, 44o21.928'N, 68o04.613'W), Great Pool (GP, 44o20.032'N,
68o03.527'W), and Navy (N, 44o20.311'N, 68o04.021'W) at Schoodic Point
and Krezgy (K, 43o50.108'N, 69 o30.833'W), Pemaquid Lighthouse (PL,
43o50.220'N, 69o30.428'W), Yellow Head (YH, 43o51.481'N, 69o30.40'W) and
West Side (WS, 43o51.481'N, 69o31.130'W) at Pemaquid Point. Distances between
all sites within a point were approximately equidistant and were chosen
so that two were located on either side of each coastal point. Different sites on
these coastal points may represent different hydrodynamic regimes (i.e., more
exposed sites versus more sheltered sites) that have the potential to influence
the morphology of fucoids (Bäck 1993, Kalvas and Kautsky 1993). Individuals
of each species (n = 20 to 30 adults) were randomly collected along a 60-m
transect line, which was laid down in the middle of their respective “zones” in
the intertidal. This avoided any potential “hybrid zone” near where the species
distributions overlap. Only individuals that could be reliably identified by the
presence of vesicles (F. vesiculosus) or receptacle margin (F. spiralis) were
collected (see Fig. 114 in Fritsch 1945 for illustrations). Specimens were returned
to the laboratory on ice for measurement. We measured and compared
5 morphological variables that have previously been shown to be highly variable
in fucoids (Bäck 1993). We measured thallus length (cm) and width (cm),
stipe length (cm) and width (mm), and midrib width (mm) (see Bäck 1993 for
descriptions of how each variable was measured). For F. vesiculosus, we also
counted the number of vesicles and determined whether each individual was
male or female. Samples were pressed and stored in the University of Maine
Herbarium (MAINE). Plaster of Paris clod cards (as described by Thompson
and Glenn 1994) were used to measure relative time-integrated water motion
at 4 sites at Schoodic Point over 7 days of low to moderate water motion from
September to November 2003 (n = 3 replicate clod cards per site per day) to
test whether morphology was correlated with water motion. Clod cards were
cast from plaster of paris (calcium sulfate, Mallinckrodt Baker, Inc., Paris,
KY) in ice cube trays, air dried for 72 hours, pre-weighed (ca. 22 g), then securely
glued onto perspex backing plates. Cards were fastened by cable ties
to polypropylene lines on bolts in the mid-Fucus vesiculosus zone. Time of
immersion and emersion of each clod card was noted. After the fucoid bed was
exposed after high tide, clod cards were collected, allowed to dry for 72 h, and
reweighed to establish weight lost by dissolution. Water motion was expressed
as weight loss (grams) per hour that each clod card was submerged over a single
Spatial variability in gross morphology was visualized using non-metric
multidimensional scaling (nMDS) plots generated from Bray-Curtis dissimilarity
matrices. The significance of apparent groupings was tested using
114 Northeastern Naturalist Vol. 15, No. 1
a 3-factor PERMANOVA (Anderson 2001, McArdle and Anderson 2001)
on Bray-Curtis dissimilarity values calculated using raw data. Permutations
(n = 4999) were of residuals under the reduced model (Anderson and Legendre
1999, Anderson and ter Braak 2003). The factors were species (fixed),
point (fixed), and site (nested in point, fixed). Point and site were treated as
fixed factors because they were specific distances apart, on specific sides of
each point, and constituted a more comprehensive genetic sampling program
(e.g., see Coleman and Brawley 2005). Pairwise post hoc tests were done for
significant factors, and the Bonferroni correction was used (P < 0.008). Spatial
relationships for each individual measure of morphology were analyzed
using 3-factor analysis of variance (ANOVA). The factors were as above,
and n = 20 replicates were used. Data were not transformed in the few cases
where variances were heterogeneous because ANOVA is robust to departures
from this assumption when sample sizes are large (i.e., n = 20) or where
residual degrees of freedom are greater than 30 (see Underwood 1997).
Student-Newmann-Keuls (SNK) tests were done for terms where ANOVA
showed significant differences to reveal the exact nature of these differences.
For all SNK tests, P < 0.01 was used. For sites with 30 replicates (K, M, and
GP for F. spiralis), we randomly selected 20 replicates to balance our design
for PERMANOVA and ANOVA. For PERMANOVA analyses that included
“sex” as a factor, we randomly selected n = 6 replicates of each sex from
each site because this was the lowest number of replicates of any one sex at
a site. All replicates were used, however, to generate nMDS plots. For each
species, correlations between each pair of variables were assessed using all
data (n = 244 for F. spiralis and n = 160 for F. vesiculosus). We used the
Bon-Ferroni correction, and P < 0.005 was used for correlations. Relative
levels of water motion among sites were analyzed using a one-way ANOVA
(n = 21 replicate measures per site over all days), and differences determined
using SNK tests. Data were square root (x + 1) transformed to conform to
homogeneity of variances.
Separating F. vesiculosus and F. spiralis based on morphology
As predicted, nMDS plots revealed that F. vesiculosus and F. spiralis
clearly separated based on gross morphology (Fig. 1). SIMPER showed that
thallus length contributed most to this difference. The significance of these
groupings was confirmed using PERMANOVA (Table 1). nMDS plots also
revealed varying levels of separation between species within each site, with
some overlap in gross morphology (Fig. 2). At all sites, there was significant
separation between species (P < 0.01); however, this pattern was weaker at
YH (P < 0.05). Moreover, from nMDS plots it is clear that some individuals
have morphologies that are more similar to individuals belonging to the
other species than they are to other individuals of their own species (e.g.,
many individuals at M, L, YH, and K). At all sites, thallus length contributed
between 63% and 83% of dissimilarity between species.
2008 M.A. Coleman and J.F. Muhlin 115
We tested for differences in each morphological variable between species
at each site (Table 2). F. vesiculosus had a greater stipe length than
F. spiralis, and this pattern was consistent at all points and sites (Table 2).
Similarly, stipe width was greater in F. vesiculosus than in F. spiralis, but
only at approximately half the sites (M, GP, L, and PL; Table 2). Thallus
length was greater in F. vesiculosus compared to F. spiralis at all sites,
except YH, where there were no differences between species (Table 2). Interestingly,
midrib width was greater in F. spiralis than in F. vesiculosus at
all but 3 sites (M, GP, and WS; Table 2), and thallus width was greater in F.
spiralis than F. vesiculosus at all sites except GP, PL, and WS (Table 2).
Figure 1. nMDS
plot showing relationships
sites and between F.
vesiculosus and F.
between points on
the plot represent
how similar (points
close together) or
different (points far
apart) sites are from
one another. Each
a site centroid or
average. Site abbreviations
specified in Materials
sites were PL, K,
YH, and WS; Schoodic sites were L, M, GP, and N; and Connecticut sites were CT1,
CT2, and CT3. The letters “V” and “S” before site abbreviations represent F. vesiculosus
and F. spiralis, respectively. All data were used to generate plots, and CT sites
are included for F. spiralis.
Table 1. Results of PERMANOVA for all 5 morphological variables for both species. Factors
are as specified in Material and Methods section. n = 20 replicate individuals per site. ** = P <
0.01, *** = P < 0.001. Results of SNK tests are given in Results section.
Source d.f. SS MS F P
Species (Sp) 1 61,656.97 61,653.97 259.66 ***
Point (Po) 1 2282.49 2282.49 9.61 ***
Site (Po) 6 8152.24 1358.71 5.72 ***
Sp x Po 1 1365.04 1365.04 5.75 **
Sp x site (Po) 6 16,335.53 2722.59 11.47 ***
Residual 304 72,186.55 11.47
116 Northeastern Naturalist Vol. 15, No. 1
Spatial patterns in morphology of each species
For F. vesiculosus, there was no difference in gross morphology between
points (Fig. 1, Table 1). This pattern can be explained by large differences
among sites at Pemaquid but not at Schoodic Point. At Schoodic Point, gross
morphology did not differ among sites after the Bonferroni correction (Fig. 1,
Table 1). At Pemaquid, patterns were difficult to resolve, but in general, gross
morphology at K and YH differed from PL and WS. Thallus length contributed
Figure 2. nMDS
and F. spiralis
(s) at each site
points on the
(points close to
each other) or
far from each
are from one
are as specified
in Materials and
sites were PL,
K, YH, and WS;
sites were L, M,
GP, and N.
2008 M.A. Coleman and J.F. Muhlin 117
to at least 68% of these differences. When each morphological variable was
analyzed separately, there were few clear patterns between points, among
sites within points, or even among different variables. First, stipe length and
width did not vary at any point or site (Fig. 3, Tables 2 and 3). Some variables
(e.g., thallus length) differed among sites at one point (Pemaquid) but not the
other (Schoodic; Fig. 3, Tables 2 and 3). Other variables varied among sites
at both points (e.g., thallus width and midrib width) (Fig. 3, Tables 2 and 3).
Where there were significant differences among sites at both points, rank
orders of differences among sites were similar. Midrib width was positively
correlated with thallus width, and thallus length was positively correlated with
Table 2. Analyses of variance (ANOVA) for each morphological variable. Factors are as specified in Materials and Methods section. n = 20 replicate individuals per site. * = P < 0.05, ** =
P < 0.01, *** = P < 0.001. See Table 3 for Sp x Si (Po) SNK results. Pemaquid sites were PL,
K, YH, and WS. Schoodic sites were L, M, GP, and N.
Source d.f. SS MS F P SNK
(a) Thallus width
Species (Sp) 1 2.11 2.11 10.82 * F. spiralis > F. vesiculosus
Point (Po) 1 0.13 0.13 1.76
Site (Po) 6 0.45 0.07 2.40 * PL > K = WS = YH, PL = YH
Sp x Po 1 0.11 0.11 0.58
Sp x site (Po) 6 1.17 0.20 6.27 *** See Table 3
Residual 304 9.48 0.03
(b) Thallus length
Species (Sp) 1 18,107.10 18,107.11 22.46 ** F. vesiculosus > F. spiralis
Point (Po) 1 145.44 145.44 2.17
Site (Po) 6 402.78 67.13 1.06
Sp x Po 1 121.61 121.61 0.15
Sp x site (Po) 6 4836.56 806.09 12.72 *** See Table 3
Residual 304 19,267.76 63.38
(c) Stipe width
Species (Sp) 1 9.77 9.76 12.16 * F. vesiculosus > F. spiralis
Point (Po) 1 0.00 0.00 0.00
Site (Po) 6 2.93 0.49 3.90 *** WS > K, other pairs ns
Sp x Po 1 0.80 0.80 1.00
Sp x site (Po) 6 4.82 0.80 6.42 *** See Table 3
Residual 304 38.01 0.13
(d) Stipe length
Species (Sp) 1 472.64 472.64 64.23 *** F. vesiculosus > F. spiralis
Point (Po) 1 26.85 26.85 2.93
Site (Po) 6 54.92 9.15 2.04
Sp x Po 1 0.27 0.27 0.04
Sp x site (Po) 6 44.15 7.36 1.64
Residual 304 1360.95 4.48
(e) Midrib width
Species (Sp) 1 13.61 13.61 15.56 ** F. spiralis > F. vesiculosus
Point (Po) 1 1.13 1.13 0.49
Site (Po) 6 13.80 2.30 7.31 *** GP > M = L = N, PL > K = WS = YH
Sp x Po 1 2.42 2.42 2.76
Sp x site (Po) 6 5.25 0.87 2.78 * See Table 3
Residual 304 95.63 0.32
118 Northeastern Naturalist Vol. 15, No. 1
stipe width (Table 4). For F. vesiculosus, we did further multivariate analyses
including an extra variable (number of vesicles). We also stratified our data
according to sex (male versus female, n = 6 of each per site) to determine
whether gross morphology varied between male and females at some or all
sites. PERMANOVA found no differences in gross morphology between sexes
at any point or site (Table 5).
Table 3. SNK results for significant Sp x Si (Po) terms. All terms are significant at P < 0.01. ns
= not significant. Site abbreviations are as specified in Materials and Methods section.
Fucus vesiculosus Fucus spiralis
Schoodic Pemaquid Schoodic Pemaquid
Thallus width GP > L, PL > K, ns ns
other pairs ns other pairs ns
Thallus length ns K = YH < PL = WS, ns K = YH > WS = PL
but K = PL
Stipe width ns ns M = GP < N = L, YH > WS,
but N = GP other pairs ns
Midrib width GP > M = N = L PL > K = YH = WS ns K = YH = PL > WS,
but WS = K
Table 4. Matrix of correlations among all pairs of variables. The upper right of the matrix are
Pearson correlation estimates for F. vesiculosus, and the lower left are values for F. spiralis.
Significant terms are identified with an “*.” P < 0.005 was used. L = length, W = width.
Thallus L Thallus W Stipe L Stipe W Midrib W
Thallus L 0.009 0.001 0.278* 0.055
Thallus W 0.169 0.129 0.112 0.505*
Stipe L 0.359* -0.021 0.049 0.174
Stipe W 0.497* 0.151 0.183 0.089
Midrib W 0.250 0.458* 0.178 0.124
for (a) Fucus
and (b) F.
a b b r e v i a -
tions are as
M a t e r i a l s
CT sites are
sites were PL, K, YH, and WS; Schoodic sites were L, M, GP, and N; and Connecticut
sites were CT1, CT2, and CT3.
2008 M.A. Coleman and J.F. Muhlin 119
Gross morphology of F. spiralis varied between Schoodic and Pemaquid
Points (Fig. 1, Table 1) and thallus length contributed most to this difference
(66% of dissimilarity between points). Within each point, there were differences
in morphology among sites. The morphology of F. spiralis at Schoodic
Point showed little variability among most sites (Fig. 1), but GP was different
from all other sites. There was greater variability in F. spiralis morphology
among sites at Pemaquid; morphology at K and YH differed from PL and WS.
Again, thallus length contributed most to these differences (at least 63%). As
with F. vesiculosus, when each morphological variable was analyzed separately
for F. spiralis, there were no clear patterns between points, among sites
within points, or among variables. There was no difference in stipe length or
thallus width between points or among sites (Fig. 3, Tables 2 and 3). Stipe
width varied among sites at both points, and midrib width and thallus length
varied among sites at Pemaquid but not at Schoodic (Fig. 3, Tables 2 and 3).
For F. spiralis, midrib width was positively correlated with thallus width, and
thallus length was positively correlated with stipe width (Table 4). In addition,
stipe and thallus length were positively correlated. At Schoodic Point, water
motion was generally greater at GP than at any other site (ANOVA: 3, 83 d.f.;
F = 9.26; P < 0.00001).
Fucus vesiculosus and F. spiralis can be distinguished based on gross
morphology because all morphological variables that were measured varied
between species at all spatial scales. Despite this difference, at smaller spatial
scales (km) individuals exhibited a gradient of morphological variability
with overlap in the gross morphology of some individuals. That is, there
were always a few individuals that were morphologically more similar to
the opposite species than they were to members of their own species. This
overlap in morphology is not surprising. F. vesiculosus and F. spiralis occur
sympatrically on intertidal shores in Maine and are known to hybridize in
the laboratory (Burrows and Lodge 1953, McLachlan et al. 1971) and in the
field (Engel et al. 2005, Wallace et al. 2004). Even if individuals of intermediate
morphology are not recent (F1) hybrids, it is likely that some amount
of introgression has occurred between F. vesiculosus and F. spiralis (Engel
et al. 2005, Wallace et al. 2004). Indeed, data from microsatellite markers
Table 5. PERMANOVA results for F. vesiculosus data that were separated by sex. n = 6 individuals
per sex per site. Although the term Site (point) was significant, it is not relevant to this
particular analysis (see Table 1).
Source d.f. SS MS F
Point 1 1246.28 1246.28 2.61
Site (point) 6 13,043.51 2173.92 4.55
Sex 1 1082.87 1082.87 2.27
Point x sex 1 347.66 347.66 0.73
Site (point) x sex 6 1037.62 172.94 0.36
Residual 80 38,193.24 477.42
120 Northeastern Naturalist Vol. 15, No. 1
has revealed that there is much overlap in allele sizes and no unique alleles
to distinguish between F. vesiculosus and F. spiralis at the sites studied here
(M.A. Coleman and J.F. Muhlin, pers. Observ.), indicating that hybridization
and introgression are likely to occur.
The amount of variability in gross morphology at the scale of less than 10 kilometers
was not consistent on larger scales. That is, at Schoodic Point,
variability in gross morphology among sites was small (F. spiralis) or nonexistent
(F. vesiculosus), but at Pemaquid, the morphology of both species
varied significantly among sites. This suggests that either dispersal among
sites at Schoodic is great preventing the formation of genetic differences
(due to drift, mutation, etc) in morphology, or that regardless of the scales
of dispersal, selective pressures on morphology are similar across all sites
at Schoodic and result in similar morphologies. We have some evidence to
support the first of these models. Microsatellite data on gene flow in F. spiralis
demonstrated that dispersal is likely to occur across large spatial scales
with few genetic differences among individuals from sites at Schoodic, or
between pairs of sites at Schoodic versus Pemaquid Points (Coleman and
Brawley 2005, cf. Table 3). Large-scale dispersal cannot explain, however,
the greater variability in morphology at Pemaquid Point, because there was
also little genetic differentiation among sites at this point (Coleman and
Brawley 2005, cf. Table 3). Although it appears that dispersal occurs across
large spatial scales, it is possible that selective pressures differ among sites
at Pemaquid and not at Schoodic Point.
Consistency in patterns between species
Although most patterns in morphology were often species-specific, some
were consistent between species. For example, the few correlations that did
exist between variables were mostly common to both species, for both species’
gross morphology was generally more variable among sites at Pemaquid
than among sites at Schoodic, and spatial patterns in gross morphology at
Pemaquid Point were similar for both species. Regardless of why these specific patterns occurred, the factors that influence some morphological traits
of fucoids appear to be common. Interestingly, however, measures of thallus
length for F. vesiculosus and F. spiralis at some places (K and YH) showed
opposite patterns. That is, thallus length was shorter at K and YH than at other
Pemaquid sites for F. vesiculosus, but longer at the same sites for F. spiralis
(see Table 3). In contrast, this suggests that selective pressures may act on each
species independently because patterns in morphology among sites were species-
specific. The differing tidal heights that these species occupy may present
different selective pressures on morphology and, combined with site-to-site
differences in biotic and abiotic conditions, may account for varying patterns
in morphology between species. The lack of correlation between morphology
and distances among sites is further evidence to suggest that selection
drives morphological variability in these species. For F. vesiculosus, differing
levels of water motion among sites may be the driving force behind any
such selection since the site with significantly greater water motion (GP) also
2008 M.A. Coleman and J.F. Muhlin 121
exhibited morphological differences where they occurred (midrib width and
thallus width). Carefully designed reciprocal transplant experiments would be
required to test hypotheses about selection.
The results of this study have implications for the use of morphology in
identifying these congeneric species. Given the existence of many individuals
of intermediate morphology at some places, it is likely that at least some individuals
are regularly misidentified, which may result in an increase in levels of
variability in experiments, decreasing the power to detect real differences at
some places. The presence of a ridge around the receptacle of F. spiralis when
an individual is fertile is the most commonly used predictor of field species
identification to date, but cannot be used when individuals are non-reproductive.
Similarly, the presence of vesicles is used to identify F. vesiculosus in
the field, but vesicles are often absent, particularly on young individuals.
Stipe width was the only morphological variable to vary consistently between
species at all sites and may prove to be a valuable predictor of species identification
if spatial variability in this variable is taken into account. Consideration of
spatial variability in patterns of morphological distinctness among congeneric
species will provide a greater understanding of the genetic and environmental
processes that determine species morphology and identity.
We are grateful to the National Science Foundation for support of this work
(OCE- 99043). We also thank Susan Brawley for discussion, Dave Abelson for help
with collecting and measuring F. spiralis, and 3 anonymous reviewers for thoughtful
comments on the manuscript.
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