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A genetic and morphological comparison of
shallow- and deep-water populations of the
introduced dreissenid bivalve Dreissena
bugensis
W. Trevor Claxton, Anthony B. Wilson, Gerry L. Mackie, and Elizabeth G.
Boulding
Abstract: The discovery of a morphologically distinct dreissenid mussel in the profundal zone of Lake Erie suggests
the presence of either a third dreissenid mussel species in the Great Lakes or a previously unknown morphological
phenotype of an existing dreissenid species. We examined the morphometrics and molecular systematics of the zebra
mussel (Dreissena polymorpha) and the profundal and epilimnetic forms of the quagga mussel (Dreissena bugensis)
from Lakes Erie and Ontario. In an attempt to resolve the taxonomic status of the profundal form of the quagga
mussel, we sequenced a 710 base pair fragment of the cytochrome oxidase subunit I mitochondrial gene of the two
forms of the quagga mussel. No nucleotide differences were found, supporting the hypothesis that the profundal form
of the quagga mussel is a phenotype of D. bugensis, not a separate species. In contrast, the second and third principal
component scores from an analysis of the morphological variables shell length, shell width, shell height, and shell
mass separated the epilimnetic and profundal forms of the quagga mussel into two groups, but grouped zebra mussels
from all depths together. The most parsimonious explanation for our results is that D. bugensis shows plasticity in shell
morphology with respect to depth, whereas D. polymorpha does not.
Résumé : La découverte d’une moule dreissénide morphologiquement distincte dans la zone profonde du lac Érié
indique la présence d’une troisième espèce de moule dreissénide dans les Grands Lacs ou la présence d’un phénotype
morphologique particulier encore inconnu d’une espèce de dreissénide déjà présente. Nous avons étudié la
morphométrie et la systématique moléculaire de la Moule zébrée (Dreissena polymorpha) et des formes épilimnétique
et profonde de la Moule quagga (Dreissena bugensis) des lacs Érié et Ontario. Dans l’espoir de tirer au clair le statut
taxonomique de la forme d’eau profonde de la moule quagga, nous avons procédé au séquençage d’un fragment de 710
paires de base du gène mitochondrial COI chez les deux formes de la moule. Nous n’avons pas trouvé de différences
entre les nucléotides des deux formes, ce qui supporte l’hypothèse selon laquelle la forme d’eau profonde est un
phénotype de D. bugensis et pas une espèce distincte. En revanche, la deuxième et troisième composantes principales
d’une analyse des variables morphologiques, longueur, largeur, hauteur et masse de la coquille, ont séparé les formes
épilimnétique et profonde de la Moule quagga en deux groupes, mais regroupé les Moules zébrées de toutes les
profondeurs. L’explication la plus parcimonieuse des résultats est que D. bugensis fait preuve de plasticité quant à la
morphologie de sa coquille en fonction de la profondeur, alors que cette plasticité n’existe pas chez D. polymorpha.
[Traduit par la Rédaction]
Claxton et al.
1276
Since the discovery of the zebra mussel (Dreissena
polymorpha (Pallas, 1771)) in Lake St. Clair in 1988
(Hebert et al. 1989), two new taxa of dreissenid bivalves
Received May 25, 1997. Accepted February 11, 1998.
W.T. Claxton,1 A.B. Wilson,2 G.L. Mackie, and E.G.
Boulding.3 Department of Zoology, University of Guelph,
Guelph, ON N1G 2W1, Canada.
1
Present address: Centre for Forensic Sciences, Ministry of
the Solicitor General and Correctional Services, 25
Grosvenor Street, Toronto, ON M7A 2G8, Canada.
2
Present address: Department of Biology, University of
Konstanz, Universität Straße 10, D-78464, Germany.
3
Author to whom all correspondence should be addressed
(e-mail: [email protected]).
Can. J. Zool. 76: 1269–1276 (1998)
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have been reported in the Laurentian Great Lakes. The first
of these new taxa was found in Lake Ontario by May and
Marsden (1992) and was given the working name “quagga”
mussel. Rosenberg and Ludyanskiy (1994) later used museum samples to identify it as Dreissena bugensis Andrusov,
1897. Currently, the North American distribution of the
quagga mussel includes Lake Erie, Lake Ontario, and the St.
Lawrence River (Mills et al. 1993). Like the zebra mussel,
the quagga mussel is native to large rivers in the Black Sea
region (Rosenberg and Ludyanskiy 1994) and is thought to
have been introduced into North America by the discharge
of ballast water from transoceanic shipping (Hebert et al.
1989; May and Marsden 1992).
Recently, Dermott and Munawar (1993) reported another
new mussel in the genus Dreissena, which they gave the
working name “profunda.” Dermott and Munawar (1993)
noted that like the taxa from shallow water given the work© 1998 NRC Canada
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Can. J. Zool. Vol. 76, 1998
Fig. 1. Quagga mussels used in this study. (A) Epilimnetic form. (B) Profundal form. Scale bar = 0.60 mm.
ing name quagga by May and Marsden (1992), this
dreissenid mussel has a rounded ventral margin and lacks
the carina of the zebra mussel. However, Dermott and
Munawar (1993) noted that the profunda mussel differs from
the quagga mussel in that it has an obviously anteroventral
swelling (Fig. 1), is predominantly white, and is restricted to
the deep waters of Lake Erie and Lake Ontario (>40 m)
(Dermott and Munawar 1993). The species of this white,
deep-water mussel has not been formally determined. For
the purposes of this paper, the working name of the mussel
referred to as profunda by Dermott and Munawar (1993)
will be quagga mussel profundal form (quagga mussel p.f.).
The designation of D. bugensis will be quagga mussel
epilimnetic form (quagga mussel e.f.).
In their paper, Dermott and Munawar (1993) compared
the morphometry of the epilimnetic and profundal forms of
the quagga mussel. They compared the ratios of several
morphometric variables and showed significant phenotypic
differences between these forms. In general, the disadvantage of this type of analysis is that ratios are not constant
within a group that shows allometry or substantial plasticity
(Reyment et al. 1984). Therefore, Dermott and Munawar
(1993) were only able to analyze mature adult mussels of a
single size rather than of a range of sizes.
Establishing the taxonomic status and the potential impact
of this new deep-water dreissenid mussel is important to understanding how the ecology of the Great Lakes is changing
because of these invaders. In the past decade, dreissenid
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Claxton et al.
mussels have had a considerable impact in North America.
The zebra mussel has caused declines in native unionid bivalves in Lake St. Clair (Mackie 1991; Gillis and Mackie
1994; Schloesser et al. 1996). In addition, dreissenid mussels
have reduced turbidity and chlorophyll a concentration over
their entire range (Hebert et al. 1990). The profundal form
of the quagga mussel has an even greater potential impact
than the other two dreissenid taxa because it inhabits the biologically and limnologically sensitive hypolimnion. This
mussel appears to be able to thrive in this cold, low-nutrient
environment. Dermott and Munawar (1993) have reported
that 20–90% of the Lake Erie profundal benthic community
is now composed of quagga mussel p.f.
Molecular markers have been helpful in distinguishing
morphologically similar dreissenid mussel species in the
Laurentian Great Lakes (e.g., Claxton et al. 1997). Spidle et
al. (1994) investigated genetic differences between dreissenid mussels and, based on allozyme analysis, concluded
that quagga mussel p.f. was a phenotype of D. bugensis.
However, Baldwin et al. (1996) reported that there were genetic differences (4 out of 619 base pairs (bp), or a sequence
divergence of 0.65%) in the cytochrome oxidase subunit I
(COI) mitochondrial gene of the two forms of quagga mussels, based on two individuals of each taxa. Owing to the
small sample size used by Baldwin et al. (1996), it is currently unclear whether these two mussels are separate
species or whether the observed variation represents intraspecific variation within D. bugensis.
Two hypotheses have been developed relating to the taxonomic status of quagga mussel p.f. in the Great Lakes. The
first, proposed by Baldwin et al. (1996), suggests that this
mussel is a third species of dreissenid that was previously
unreported in the Great Lakes. The second, proposed by
Spidle et al. (1994), suggests that this mussel is a deep-water
phenotype of D. bugensis. The objective of this study was to
test these hypotheses using morphometrics and molecular
systematics. To do this, we used principal components analysis to quantitatively analyze four morphometric variables
measures on all three dreissenid taxa. This technique allowed us to analyze differences in mussel shape independently of mussel size and allometry (Reyment et al. 1984). As
a result, we were able to analyze juvenile and adult mussels
of a wide range of sizes.
We also sequenced and carried out restriction analysis of a
710-bp fragment of the COI mitochondrial gene of both
forms of quagga mussels from Lake Erie and Lake Ontario.
Fixed nucleotide differences between these two mussel
forms would support the hypothesis that they represent different species. No nucleotide differences would support the
hypothesis that quagga mussel p.f. was a phenotype of
D. bugensis.
Field sampling of mussels
Three taxa of dreissenid mussels were collected from Long
Point Bay, Lake Erie, for morphological analysis. Samples were
collected on July 15, 1992, at three sites. Site 1 was immediately
offshore of the Nanticoke Thermal Generating Station at a depth of
1 m (42°44.80′N, 80°12.30′W). Sites 2 and 3 were located approximately 15 and 25 km south of site 1 at depths of 13 and 40 m
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(42°41.30′N, 80°05.30′W and 42°39.10′N, 80°02.40′W), respectively. We collected the zebra mussel from all three sites, quagga
mussel e.f. from site 1, and quagga mussel p.f. from sites 2 and 3.
The profundal form of the quagga mussel was not present at site 1,
and the epilimnetic form of the quagga mussel was not present at
sites 2 and 3. Mussels from site 1 were scraped from small rocks,
collected by wading. Mussels collected from sites 2 and 3 were
collected using an Ekman grab because the bottom was composed
of silt. Ekman samples were cleaned using a sieve box with 1-mm
mesh.
All mussels were placed on ice and immediately transported to
the University of Guelph. Mussels to be used for morphometric
analysis were stored in a 70% ethanol solution for 1 week and then
placed in a drying oven for 48 h prior to analysis. Mussels to be
used for density analysis were kept alive on ice.
Samples collected for genetic analysis were taken from sites 1
and 3 on August 20, 1995. We collected 20 quagga mussels e.f.
from site 1 and 20 quagga mussels p.f. from site 3. The sampling
method was the same as used for samples collected in 1992. Mussels were immediately placed on ice and transported to the University of Guelph where they were stored at –70°C. The portion of the
specimens remaining after DNA extraction was preserved in alcohol and deposited at the Canadian Museum of Nature (catalogue
numbers for quagga mussels e.f. from site 1: CMNML 93059–
93068; catalogue numbers for quagga mussels p.f. from site 3:
CMNML 93069–93078). In addition, samples of both forms of the
quagga mussel from the Lake Ontario collections of Baldwin et al.
(1996) were provided by O. Sanjur.
Morphometric characters
Four morphometric variables were measured in all mussels used
in this study: shell length (SL), the maximum anteroposterior dimension of the shell; shell width (SW), the maximum dorsal–
ventral dimension of the shell; shell height (SH), the maximum lateral dimension of the shell; and shell mass (SM), the mass of the
dry shell after the body was removed. Measurements were made
using a Hi-pad digitizing tablet interfaced to an IBM computer.
Mussels were measured at 10× magnification using a stereoscope
equipped with a drawing tube, which superimposed the LED cursor of the digitizing pad onto the mussel image. Mussels that were
used for genetic analysis were measured using calipers.
To estimate their relative buoyancy, we also measured total
organism density (TOD), which we defined as the wet mass per
unit mussel volume of an intact live mussel. Wet mass was calculated the same way for all mussels; the mussels were weighed on
an analytical balance after blotting them dry. Mussel volume was
measured by observing the displacement of water in a 10-mL graduated cylinder. Density measurements were made on 30 quagga
mussels e.f. and 30 quagga mussels p.f.
Statistical analysis of morphometric characters
Principal components analysis was carried out using the correlation matrix of the log-transformed morphometric variables (SL,
SW, SH, and SM) using the program SYSTAT (Wilkinson et al.
1992). A total of 300 D. polymorpha collected from sites 1, 2, and
3, 100 quagga mussels e.f. collected from site 1, and 200 quagga
mussels p.f. collected from sites 2 and 3 were analyzed. In addition, the morphological data from the shells of all mussels for
which we obtained DNA sequences were also analyzed using principal components analysis. We inspected the loadings on the principal components to determine which represented “size” and had
coefficients of the same sign and which represented “shape” and
had coefficients of mixed signs (Reyment et al. 1984). The scores
from PC1 were plotted against those from PC2 and those points for
each form of mussel from each depth were labelled with a unique
letter. We used the ellipse option in SYSTAT (Wilkinson et al.
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Table 1. Loadings of variables on the first three principal
components.
Shell
Shell
Shell
Shell
% of
mass (SM)
length (SL)
height (SH)
width (SW)
total variance
PC1
PC2
PC3
0.965
0.950
0.888
0.887
85.210
0.218
–0.235
0.442
–0.428
12.046
0.042
0.197
–0.083
–0.173
1.943
1992) to estimate 95% confidence ellipses for the principal component scores of each form and then visually inspected the graphs to
observe the amount of overlap of the ellipses. When the 95% confidence ellipses of two shape principal components show little
overlap, the two forms are considered to be morphologically distinct (Reyment et al. 1984). To help understand the relative contribution of the morphometric variables to the morphologically
distinct groups shown in the plot of PC2 and PC3, we also did all
possible bivariate plots of the variables SL, SW, SH, and SM and
labelled the points with a different letter for each taxon.
Analysis of variance (ANOVA) using the program SYSTAT
(Wilkinson et al. 1992) was used to compare the TOD of the
dreissenid taxa from sites that were at different depths. The least
significant difference (LSD) test was then used to determine which
of the three sites was significantly different from the others.
Isolation of dreissenid DNA
Isolation of DNA was carried out using the method described by
Claxton et al. (1997).
Development of dreissenid mtDNA primers and
polymerase chain reaction (PCR)
A set of two PCR primers for the mitochondrial gene COI were
used to amplify the mtDNA fragment for all mussels. These primers produced a 710-bp fragment of mtDNA. The PCR primer pair
used consisted of Folmer A (Folmer et al. 1994) and a new primer,
dreissenid B, which is specific for dreissinid mussels (Claxton and
Boulding 1998). IUB codes (5′ to 3′): GGATCTCCTAACCCTGTWGGATCAA.
The PCR was performed as described by Innis et al. (1988). One
microlitre of the isolated DNA was used for the template for the
PCR. The final concentration of MgCl2 in the PCR reaction was
2.0 mM. PCR was carried out for 33 cycles at an annealing temperature of 50°C, and the extension was carried out for 45 s.
Sequencing of the COI gene
Samples were prepared for sequencing using a Sephaglas
BandPrep Kit (Pharmacia). The COI mitochondrial gene was sequenced with an ABI Prism 377 DNA sequencer using dye terminator cycle sequencing with Amplitaq DNA Polymerase FS.
Manufacturer’s instructions were followed for all samples. Cycle
sequencing was carried out at an annealing temperature of 50°C.
Two sequencing primers were used to produce the composite sequences presented in this study. A total of 10 Lake Erie and 4 Lake
Ontario quagga mussels e.f. and 10 Lake Erie and 8 Lake Ontario
quagga mussels p.f. were sequenced using the dreissenid B primer.
In addition, Folmer’s A primer was also used to sequence the opposite strand of two quagga mussels of each form from Lake Erie.
Restriction analysis
As it would be prohibitively expensive to sequence large numbers of mussels, we designed an assay that would allow us to pick
up the DNA haplotype for quagga mussel p.f. reported by Baldwin
et al. (1996). The first of four base pairs that are reported to differ
between the epilimnetic haplotype and profundal haplotype
(Baldwin et al. 1996) is part of a polymorphic restriction site for
the enzyme BstNI (see Fig. 4). BstNI recognizes the site
CC(A,T)GG and would therefore cut at the CCTGG they reported
for quagga mussel e.f. but not at the CCTAG they reported for the
profundal form. Our restriction assay of the PCR-amplified COI
gene fragment was carried out on an additional 20 quagga mussels
e.f and 20 quagga mussels p.f. Restriction enzyme digests consisted of 4.0 µL of PCR product, 3.5 µL of sterile distilled water,
1.0 µL of BSA, 1.0 µL of 10X buffer, and 0.5 µL of enzyme.
Digests were incubated at 60°C for 4 h, mixed with xylene cyanol
at 14%, and then loaded onto 2% agarose gels containing 0.04 mg
ethidium bromide/100 mL. Electrophoresis was carried out for 2 h
at 60 mV in 1× TBE buffer. Gels were visualized under UV light
and photographed.
Morphometric characters
The first three principal components explained 99% of the
total variance (Table 1). The first component loadings were
strongly positive and approximately equal with respect to
the four morphometric variables. This indicates that PC1
was a measure of the overall size of the shell. This component explained 85% of the total variance, indicating that
variance in mussel size accounted for most of the variance
of the data (Table 1).
The loadings on PC2 were strongly positive with respect
to SH, moderately positive with respect to SM, strongly negative with respect to SW, and moderately negative with respect to SL (Table 1). The unequal signs of the loadings
indicate that PC2 was a measure of shell shape. For example, mussels with a large SH and a large SM but a small SW
and a small SL would have a large, positive score on PC2. In
contrast, mussels with a small SH and a small SM but a
large SW and a large SL had a large, negative score on PC2.
This component explained 12% of the total variance, which
explained most of the variance associated with shell shape
(Table 1).
The loadings on PC3 were strongly positive with respect
to SL, weakly positive with respect to SM, strongly negative
with respect to SW, and weakly negative with respect to SH
(Table 1), indicating that this component was also a measure
of shell shape. For example, mussels with a large SL and a
small SW would have a high, positive score on PC3. This
component explained 2% of the total variance (Table 1).
The plot of PC2 versus PC3 separated the mussels used in
this study into three morphological groupings based on their
shape. The first such grouping was occupied by all zebra
mussels, regardless of collection site or depth (Fig. 2). The
second morphological grouping was occupied by quagga
mussel p.f. collected at site 2 at a depth of 13 m and at site 3
at a depth of 40 m (Fig. 2). The third morphological grouping was occupied by quagga mussel e.f. collected at site 1 at
a depth of 1 m (Fig. 2). The quagga mussels used for the
genetic analysis grouped with others of the same taxa that
were collected at the same depth.
The scores on PC2 (Fig. 3) and PC3 (Claxton 1997) did
not show any trends with respect to PC1 within a taxon, indicating that these measures of shape were independent of
shell size. This shows that the three morphological groupings designated by the plot of PC2 versus PC3 are also inde© 1998 NRC Canada
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Fig. 2. Relationship between scores on PC2 and PC3. A and B
represent quagga mussel p.f. collected from sites 2 and 3,
respectively, C represents quagga mussel e.f., and D, E, and F
represent Dreissena polymorpha collected from sites 1, 2, and 3,
respectively. Data points are enclosed by a 95% confidence
ellipse that was estimated separately for each taxon at each
depth.
Fig. 3. Relationship between scores on PC1 and PC2. A and B
represent quagga mussel p.f. collected from sites 2 and 3,
respectively, C represents quagga mussel e.f., and D, E, and F
represent Dreissena polymorpha collected from sites 1, 2, and 3,
respectively. Data points are enclosed by a 95% confidence
ellipse that was estimated separately for each taxon at each
depth.
pendent of size and are therefore useful for distinguishing
these taxa.
The bivariate plots of the morphometic variables SL, SW,
SH, and SM suggested that (i) SH increased fastest with SW
for zebra mussels, next fastest for quagga mussel e.f., and
most slowly for quagga mussel p.f., (ii) SM increased more
slowly with SL for quagga mussel p.f. than for the other
taxa, and (iii) SW increased faster with SL for quagga mussel e.f. than for the other taxa.
The mean TOD differed significantly among the different
sites (ANOVA, p < 0.05). The mean TOD of quagga mussels
e.f. collected at site 1 was significantly greater than the
mean TOD of quagga mussels p.f. collected at sites 2 and 3
(LSD, p < 0.05). The mean TOD of quagga mussels e.f. collected at site 1 and quagga mussels p.f. collected at sites 2
and 3 was 1.40 g/cm3 (SD = 0.157 g/cm3), 1.07 g/cm3 (SD =
0.246 g/cm3), and 1.01 g/cm3 (SD = 0.222 g/cm3), respectively. There was no significant difference in TOD in quagga
mussels p.f. collected at sites 2 and 3 (LSD, p > 0.25).
Our assay with the restriction enzyme BstNI showed no
restriction site polymorphism between the two forms of
quagga mussels. Both forms showed the same restriction
pattern, which was composed of three fragments of approximately 100, 250, and 350 bp. This means that the enzyme
always cut at the first site, as would be expected for the
epilimnetic form haplotype reported by Baldwin et al.
(1996). Thus, both our DNA sequencing and our assay with
BstNI show that none of the quagga mussel specimens that
we analyzed had the haplotype reported by Baldwin et al.
(1996) for the profundal form.
Genetic analysis of the COI gene fragment
We obtained 525 bp of COI nucleotide sequence from
both forms of the quagga mussel (Fig. 4, GenBank accession
numbers nuc 1 AF096765). We did not observe the haplotype with the differences at four sites reported by Baldwin et
al. (1996) for the profundal form in any of the individuals
we sequenced (Fig. 4). Indeed, we found no nucleotide differences in the COI gene fragment of these two mussel
forms (0 out of 525 bp, or 0% sequence divergence). All of
our sequences matched the sequence for this fragment reported by Baldwin et al. (1996) for the epilimnetic form.
Shell morphology
Our analysis showed that the three dreissenid taxa used in
this study could be separated into three morphological
groupings. We believe that the three morphological groups
represent phenotypes that are best adapted to different habitats. The first such grouping was occupied by zebra mussels
from all three sites (1, 13, and 40 m), indicating that
D. polymorpha showed no shell plasticity with respect to
depth or site location within Lake Erie. This was somewhat
surprising, as previous workers have reported that zebra
mussels can demonstrate considerable phenotypic plasticity,
with respect to shell morphology and colour (Pathy and
Mackie 1993; Rosenberg and Ludyanskiy 1994), although
the specimens they examined were from different lakes and
rivers. The shell morphology of our zebra mussels had a
greater rate of increase of SH with SW relative to the other
dreissenid taxa. The zebra mussel has an epifaunal mode of
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Fig. 4. COI nucleotide sequences for all the specimens that we sequenced of quagga mussel e.f. and quagga mussel p.f. Position 1 of
our sequence corresponds to position 1597 of the Drosophila yakuba sequence (Clary and Wolstenholme 1985). The four differences in
the nucleotide sequence of quagga mussel p.f. reported by Baldwin et al. (1996) are indicated in bold face type below our sequences.
life in which the shell morphology, particularly the flattened
ventral surface, is specialized for byssal attachment to hard
substrate (Mackie 1991; Dermott and Munawar 1993). Although zebra mussels have been found in the Great Lakes at
depths exceeding 50 m, they are primarily a shallow-water
species, most prevalent in the wave zone where wave energy
is at a maximum (Mills et al. 1993).
The second morphological grouping was occupied by
quagga mussel p.f., which was found at the two deepest sites
(13 and 40 m). It represents the opposite morphological extreme to the zebra mussel. Quagga mussel p.f. shells showed
a relatively greater rate of increase of SW with SH than the
other two groupings of dreissinids. This agrees with the
morphological results found by Dermott and Munawar
(1993), who determined that quagga mussel p.f. had a large
SW/SL ratio and a small SH/SL ratio relative to quagga
mussel e.f. In addition, SM increased more slowly with SL
than for the other two groupings, indicating that quagga
mussel p.f. has a relatively lighter shell. Quagga mussel p.f.
is infaunal, living partially buried in soft substrate, with little
or no byssal attachment (W.T. Claxton, unpublished data;
Dermott and Munawar 1993). The shell morphology of these
mussels is adapted to colonization of soft mud substrate. Indeed the TOD was significantly less for quagga mussel p.f.
than for quagga mussel e.f., suggesting that the profundal
form would be less likely to sink into soft mud. Parallel
shell morphological adaptations are present in Modiolus
(Stanley 1972; Dermott and Munawar 1993). These infaunal
mytilid bivalves also have elongate shells, similar to quagga
mussel p.f., for living in a soft substrate (Stanley 1972). It
should be noted that we found quagga mussel p.f. at a depth
of 13 m, much shallower than the >40 m depth that Dermott
and Munawar (1993) indicated as characteristic of this
taxon.
The third morphological grouping was occupied by
quagga mussel e.f., which was found only at the shallowest
site (1 m). The epilimnetic form of the quagga mussel shows
a SH/SL ratio which is more similar to that of D. polymorpha than to that of the profundal form. However, quagga
mussel e.f. shells generally show a larger rate of increase of
SW with SL than those of D. polymorpha. The high rate of
increase of SM with SL in these two dreissenid taxa may reflect their similar modes of life. Like the zebra mussel,
quagga mussel e.f. is an epifaunal mussel that attaches to
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Claxton et al.
hard substrate using byssal threads (May and Marsden 1992)
and may require a heavy shell to occupy the wave zone,
where it is exposed to maximum wave energy (Griffiths et
al. 1991).
Analysis of shell morphology supports the hypothesis that
there are three morphologically distinct species of
dreissenids in the Great Lakes. However, considerable
phenotypic plasticity of all morphological characters is
known to exist in dreissenids, even within single populations
(Pathy and Mackie 1993; Rosenberg and Ludyanskiy 1994).
Phenotypic plasticity in shell form can be influenced by environmental factors such as growth rate and sediment type,
as has been demonstrated for the marine bivalve Mya
arenaria (Newell and Hidu 1982). Indeed, phenotypic plasticity can even be adaptive and result in the same genotype
expressing different phenotypes to suit the environmental
conditions (Via et al. 1995). Therefore, our molecular systematics analysis of the three taxa must also be considered.
Genetic analysis of the COI gene
Neither sequencing nor restriction analysis showed any
genetic differences between the two forms of the quagga
mussel in this region of the COI mitochondrial gene. The
mussels of these two forms, collected from Lake Erie and
Lake Ontario, showed a single haplotype which is the same
as that reported for quagga mussel e.f. by Baldwin et al.
(1996). We never observed the haplotype reported as characteristic of quagga mussel p.f. by Baldwin et al. (1996), who
analyzed the same region of COI for mussels collected from
the same site at the same time. Their second haplotype may
represent either a new dreissenid sibling species or a rare
haplotype that we did not encounter in our investigation. It
is difficult to determine which hypothesis is more likely because of the small sample size of each taxon (n = 2) used by
Baldwin et al. (1996).
Baldwin et al. (1996) compared several species of the
family Dreissenidae in this region of the COI gene. For example, D. polymorpha and Mytilopsis leucophaeta showed
16 and 18% sequence divergence, respectively, from D. bugensis. This suggests that this region of the COI gene is suitable for differentiating species of this family.
We did not find sequence differences in the COI gene
fragment between these taxa, which supports the hypothesis
that quagga mussel p.f. is a deep-water phenotype of
D. bugensis, not a separate species. However, COI is the
most conservative protein-coding gene in the mitochondrial
genome. Sequence and restriction analysis of this gene is unlikely to differentiate species that are the product of a
speciation event that occurred <1 million years ago (Avise
1994). Therefore, further study, using faster evolving molecular markers, such as microsatellites, may finally clarify the
taxonomic status of quagga mussel p.f.
This research was supported by research and collaborative
grants from the Natural Sciences and Engineering Council
of Canada (NSERC) to E.G. Boulding, an NSERC research
grant to G.L. Mackie, and an Ontario Graduate Scholarship
to A.B. Wilson. We thank O. Sanjur, B.S. Baldwin, and R.C.
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