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The Impact of Dreissena polymorpha _Pallas_ Invasion on Unionid

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The Impact of Dreissena polymorpha _Pallas_ Invasion on Unionid Powered By Docstoc
					                  Internat.Rev. Hydrobiol.      85      2000     5-6     529-541



    Lvusov E. Bunr-erovAl,     AI-BxnNosR Y. KARATAyEVI'* and DnNNe K. PRnu-le2

        rNew York StateMuseum, Cultural Education
                                                    Center,Albany, NY 12230 USA.
                                 huniver1@nycap.rr.com
2Department Ecology and Evolution,
          of                       StateUniversiiy of New York at Stony Brook, Stony
                       NY I I 794-5245. padrlla@life.bio.sunysb.edu


         The Impact of Dreissenapolymorpha (Pallas) Invasion
                         on Unionid Bivalves
           key words: Dreissenapolymorpha, zebramussel,Unionidae,unionids, impacts


                                             Abstract
   Dreissenapolymorpha, the zebramussel,is one of the most aggressive   and important invading@
tic speciesworld wide. Its spreadhas followed the path of human activity, initially following human
constructedcanals connectingthe Black Sea and Baltic Sea basins.One consequence this invasion
                                                                                      of
is the impact of zebra mussels on native bivalves. Overgrowth by Dreissena can cause a dramatic
decrease unionid density. The extent of this effect is determinedby severalfactors includins Dreis-
         in
sena density,time sinceinvasionby Dreissena,biomassof attachedD-rei\sena, type of bottJm sedi-
                                                                             and
ments (sand versus silt). We found a correlationbetweenoverall Dreissenadensity.   and the number of
zebra musselsper overgrown unionid, and between Dreissena density and the ratio of the mass of
attachedzebramusselsto the massof the host unionid. The extensiveovergrowthof unionids by Dreis-
sena, resulting in mass mortality, is characteristic periods of rapid population growth, when Dreis-
                                                    of
sena invade a new waterbody.



                                        1. Introduction

   Dreissenids, especiallyDreissenapolymorpha, can becomeenormouslyabundantin fresh-
water, where they are the only bivalves which.attachto hard substrates   and have a plankto-
nic larval stage.Within a short period of time they can obtain an order of magnitudehigher
biomass than that of all other native benthic invertebrates (SorolovA et al., 1980a;KHnn-
cHENKo,   1983;KRnerevsv et al., 1994;SrNrrsyNe     and Pnoresov, 1994).The zebramussel
is frequently competitively dominant over native freshwaterfauna, and has large impacts on
all parts of the ecosystem,especially benthic animals (WrrroR, 1969; SorolovR et al.,
1980b;Kennreyev and Bunr-erovn,1995a;KRnereynv et al.,1997\. Often the most direct
and severeimpact of zebramusselsis on unionid bivalves (SEnesrysN,      1937;Mecrre, l99l;
KRRereyBv et al., 1997).
   Prior to the zebramussel invasion, the major bivalves in freshwaterbenthic communities
were unionids. Unionids have a very different lifestyle and life history than zebramussels.
They live in soft sediment,crawl through the sedimentwith a large foot, and live solitary
or in groups, but not in as extreme densitiesas zebra mussels.Unionids have slow growth,
low fecundity, are long lived, and have parasitic glochidia larvae. Unionids can provide the
most abundantsourceof hard substratum colonizationby D. polymorpha rn many lakes,
                                           for
reservoirs, and rivers (LeweNoowsKr,  1916;KanRrRysv, 1983;Hssenr et a1..1989:Lyeru-
NovrcHet al., I99q.
   x This author has also publishedunder Ar-BxeNoeR Keneresv
                                                    Yu.
  ** Author to whom the correspondence  should be addressed
530                                    L. E. Bunltrove et al.

   Overgrowth by zebramusselscan have negativeeffects on the host unionids (reviewedin
ScslonssnR et al., 1996; KeRerevev et al., 1997). By attaching to their valves, D. poly-
morpha can make it more difficult for unionids to burrow and move through sediment,and
the added mass of Dreissena can weigh down unionids, resulting in burial in very soft or
unconsolidated sediments  (SeepsrynN, 1937; KeRerRyev, 1983;MRCrre, l99l; Gurs and
Mrcrm, 1994: Scst-opssnnand NRLnen, 1994). Mussel attachmentto unionid shells can
increase drag and the likelihood of dislodgment by water motion for speciesliving near
shore (SnnnsrvnN,1937; KeRRrevBv, 1983; TucreR, 1994). Zebra mussel attachmentcan
occlude the openings in unionid valves, preventing opening for respiration, feeding and
reproduction,or preventing the closing the valves (Rtcclnnnr et al., 1996). D. polymorpha
may directly compete with unionids for food (Heec et al., 1993), occupy otherwise avail-
able space(Tucren, 1994), and induce shell deformities(LewnNnowsKl, 1976;HuNrEn and
BRLnY,1992).
   We measuredthe zebra mussel abundanceand unionid fouling rates in a wide range of
waterbodiesin Belarus with different characteristics.We tested whether fouling by zebra
musselshas negativeimpacts on unionids. We examinedwhether the mass of unionids was
negatively correlated with the mass of zebra mussels attachedto them, and whether the
extent of mortality seeminglycausedby zebramussel fouling was associated     with substrate
type and time since invasion. We contrastedtheseresults with similar data from two lakes
in North America and literature information for other lakes. Finally, we use these date to
test whether we can use zebramusseldensitiesin the environmentto predict the fouling rate
and impact of zebra musselson unionid bivalves.


                                           2. Methods

   To determinethe impact of zebramussels unionids, we studied nine lakes from the Naroch re-
                                                on
gion, Lake Lepelskoe,the Svisloch River, and two reservoirs(Drozdy and Chizhovskoe)on this river
in Belarus, and Lakes Clark and Vineyard in Michigan, USA (Table 1). The Svisloch River flows
through Minsk, the capital of Belarus. The Drozdy reservoir is upstreamof Minsk, is subject to low
levels of industrial and urban pollution, is well oxygenated,and has an abundance sand and rubble
                                                                                   of
substrates; silts are rare. The ChizhovskoeReservoir is downstreamof the main industrial regions of
the city and is heavily polluted with industrlal and metropolitan sewageand petroleum waste. Lake
Lepelskoeis 120 km northeastand Lake Naroch '110 km northwestof Minsk. Vineyard and Clark Lakes
are both in JacksonCounty, Michigan, U.S.A., approximately100 km west-southwest Detroit.
                                                                                      of
   The waterbodieswe studieddiffered in a number of limnological parameters,    time since zebramus-
sel colonization, and Dreissena density(Table l). D. polymorpha was first reportedfrom Lake Lepel-
skoe in 1929 (OvcHINNtKov,1933) but probably colonized this lake much earlier, shortly after con-
structionof the Dnieper-Zapadnaya    Dvina Canal in 1805.This canal connectsthe Dnieper River (Black
Sea basin) and the ZapadnayaDvina River (Baltic Sea basin) and was the route through which zebra
musselscolonizednorthern Belarus (Bunlarovn, 1998).The Svisloch River, Chizhovskoeand Drozdy
reservoirsand the rest of the Belarussian  lakes studiedwere colonizedwith zebramusselsin the 1980s.
In North America, zebra musselswere first detectedin Lakes Vineyard and Clark in 1994.
   In each waterbody,we hand collectedunionids at approximately 1.5-2.5 m depth by diving. Every
unionid within a 1 m wide transect(100 or 500 m, dependingon unionid density) was collected.In the
Svisloch River, unionids were collectedfrom 9 separate             All bivalves were identified to spe-
                                                          transects.
                         we
cies. Before dissection, cleanedDreissenaand other fouling organismsfrom the surfaceof the shell.
Unionid shell lengths (maximum linear dimension) were measured with calipers (+ 1 mm). Then
unionids and zebta musselswere cut open with a scalpel,to remove water from mantle cavities, and
weished.
                                                                                     lmpact of Dreissenaon Unionid                                                                                                            531
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532                                  L. E. Bunr-erovl' et al.

                                          3. Results

   Lake Naroch - We studied Lake Naroch from 1990-1995. Since Lake Naroch was in-
vaded by zebra mussels in the mid-1980s, our study was during the initial stage of
colonization. The average density and biomass of zebra mussels across the whole lake
dramatically increasedfrom 1990 to 1995 (Table2).
   The percentof unionids infestedwith zebramusselsincreased     from 60 Voin 1990 (n = 93),
to l00%oin 1993 (n = 100) and 1995(n = 50). The average     numberand massof mussels     per
host unionid also increased, did the ratio of massof D. polymorpha to host unionid (Table
                             as
2). In 1990, all unionids found were alive. However, in 1993 the majority of the host-
unionids were dead; only 3Vo of unionids collected were alive. In 1995 all unionids were
still fouled by D. polymorpha and only 8 7o were alive.
   In 1990 zebra mussels were most abundanton unionids near a stream. Skema. which
flows into Lake Naroch from Lake Myastro. Lake Myastro was colonizedby zebramussels
before Lake Naroch, and the connectingstreamprovided the route for the zebramusselinva-
sion (BunLAKovA, 1998). The percentageof unionids fouled with D. polymorpha and the
abundanceof Dreissena attachedto unionids decreased       with increasingdistancefrom the
outflow of this stream(Figure 1).
   Lake Myadel-Lake Myadel was colonizedat the sametime as Lake Naroch.By 1998
zebramusselshad reachedvery high densities(Table 1) and we found no live unionids,only
empty shells.
   Lake Lepelskoe - In contrast,unionids are still abundantin Lake Lepelskoe,where zebra
mussels were first reported in 1929 (OvcurNNIKov,1933) but probably colonized shortly
after constructionof the Dnieper-Zapadnaya     Dvina Canal in 1805. Divers readily collected
hundredsof unionids within an hour. Although 92Vo were infested with zebra mussels,the
averageratio of massof D. polymorpha to host unionid was lower than that in Lake Naroch
(Table 2) and only l2%o of infested unionids were dead.
   Lake Volchin - Unionids in Lake Volchin on sandy sedimentswere completely over-
grown with Dreissena. The number of zebra mussels per unionid host was significantly
higher than on silty sediments(P = 0.049, ANOVA) and zebramusselswere found only on
the posterior third of unionid shells. The ratio of mass of D. polymorpha and host unionid
was not significantly higher on sand than on silt (P = 0.054, ANOVA), but due to unequal
sample sizes our statisticalanalysishas relatively low power.
   Svisloch River - The highestdensitiesof zebramusselswere found in the Svisloch River
(Table 1). In this river, sand and rubble sedimentsalternatewith silt. In sandy and rubble


Table 2.   The overgrowth of unionids by zebra musselsin Lake Naroch. Means + standard
                                         effors.

                                                                 1990       t993        1995

The number of sites sampled(n)                                   (37)        (4s)        (80)
Average density of D. polymorpha in the lake 1mr)               7.4+ 3.0   763+ 149   l52t + 451
Average biomassof D. polymorpha in the lake (gm-2)                 +
                                                                1.5 0.6     99+ 30     r07 + 44
Voof all unionids colonized by D. polymorpha                       60        100         100
7o of unionids found dead                                          097                    92
Average number of D. polymorpha per living infested             9.5+ 6.2      +
                                                                           135 35       36+11
  unionid bivalve
Biomass of D. polymorpha per living infestedunionid                +
                                                                1.8 0.9       +
                                                                           34.0 8.4      +
                                                                                      14.4 3.7
  bivalve (g)
Ratio of mass of D. polymorpha and living host unionid          0.3+ 0.2   2.8+ 1.4      +
                                                                                       1.1 0.4
  bivalve
                                  Impact of Dreissena Unionid
                                                     on                                              533

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                                     Distance from Stream Skema (km)
                                   '0-
                                          Number,r= -0.82,p=0.023
                                   \-     Percent,r= -0.87,P=O.O1O

Figure l.   The percent of unionids colonizedby Dreissena and density of D. polymorpha attachedto
                unionids at different distancesfrom the outflow of the StreamSkema.


areasthe unionids had high numbersof attachedzebramussels(up to 100 per unionid). In
contrast,unionids burrowed in silt were completely free of zebia mussels,even though at
severalof these sites the density of unionids was around 100 m-2.
   Reservoirs Drozdy and Chizhovskoe - High densitiesof zebramusselswere found in
the upstreamReservoir Drozdy, but not as high as in the Svisloch River (Table 1). Zebra
musselswere most abundantin the shallow areas,at 0.5 m depth, on sandy-rubble    substrate.
Half of the 54 unionids collected from this reservoir were dead and all had attached,zebra
mussels.Zebra mussels were found in low densities in the lower reservoir, Chizhovskoe
(Table 1), and primarily at the mouth of an inflowing stream.Unionids were abundantin
this reservoir, and none of approximately 100 unionids collected from silty sedimentshad
attachedzebramussels.
   Lakes Clark and Vineyard - We found dimilar resultsin North America. In Lake Clark
unionids were found on the surfaceof sandy seiiimentand were completely overgrown with
zebramussels.In Lake Vineyard, the unionids were partly burrowed in silty sedimentsand
zebra musselswere found only on the posterior half to one third of their shells. The aver-
age number of infested unionids and the ratio of mass of D. polymorpha and host unionid



Table 3. Percentof unionids fouled by zebramusselsin different waterbodies.Intact druses
do not have additional remains of damagedbyssal threads.Damageddruseshave additional
remainsof byssal threads,indicating that more zebramusselswere previously attached. The
remains of just byssal threads on a unionid indicates that zebra musselswere attachedat
                                  some time in the past.

Lake                         Sample   Volntact     VoDamaged           %o With byssal 7o No zebra mussels
                              size     druses        druses             threadsonly    or byssal threads

Bolshye Shvakshty              r47    11                   29                3t                23
Dolzha                          31    7r                   16                t3                 0
Spory                           26238                                        31                38
Bolduk                          25    53                   27                l2                 8
534                                                      L. E. Bunlnrove. et al.

            160
                               =o....
            140                       Regression              95%confid,
                                                    *
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                                                       Dreissena         per
                                                                    (number m";
                                                              density
Fisure 2.       The number of Dreissenaper unionid bivalve for Belarussianlakes with different densities
                                             of Dreissena.          \


bivalve in Lake Clark were significantly higher than in Lake Vineyard (P = 0.026, ANOVA)
(Table 4). In addition, the unionids in Lake Vineyard were significantly larger ( P < 0.001,
ANO V A ) and h e a v i e r(7 3 .1 + 4 .4 g v s . 1 8 .8 +2.2 g, P < 0.001, A N OV A ) than i n Lake
Clark. The average length of unionids from Belarus lakes was significantly smaller
(P < 0.001, /-test) than in North American lakes Clark and Vineyard (56.6 + 0.56 mm in
Belarus,74.5 + 3.2 mm in North America).



                                   'o'..
                                       Regression     95% confid.
            p                     y =0,2490,0023'(r= 0.84,p = 0.002)
                                         +      x
            .E 2,2

            3 1,8
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                    -0,2
                      -100                    100
                                                         Dreissena         per
                                                                      (number m';
                                                                density
Figure 3. The mass ratio of attachedDreissena and host unionid bivalve (mass of zebra mussels
 attachedper mass of unionid bivalve), for different densitiesof zebramusselsin Belarussianlakes.
                                Impact of Dreissena Unionid
                                                   on                                     s35
   Byssal Thread Remains - We frequently found unionids with the remainsof Dreissena
byssal threadson their shells (Table 3). In all lakes,the majority of unionids had at least the
remains of byssal threadsattachedto their shells (Table 3). The number of D. polymorpha
per unionid with intact druses(without additionalremainsof byssalthreads)was higher, than
on those with drusesand byssal remains (damageddruses).In the Lake Bolshye Shvakshty
the average  number of zebramussels    per host unionid with intact druses(7.211.3, n -li)
was significantlyhigher (P < 0.001, r-test)than for those with damageddruses(1.8 +0.2,
n = 42).In Lake Dolzha we found an averageof 8.9 + 1.5 (n = 6) zebramusselsper unionid
with intact druses and 2.6 + 0.8 (n = 20) on unionids with damageddruses.
   Zebra Mussel Density and Unionid Overgrowth - To determine if there was a rela-
tionship between zebramusseldensity and degreeof overgrowthof unionids in a waterbody,
we used data from Lake Naroch (1990 and 1993) and eight lakes in the Naroch region
studied in 1998 (Table 1). Both the number of zebramusselsand the ratio of mass of p.
polymorpha and host unionid were correlatedwith zebra mussel density (Figure 2 and,3).



                                       4. Discussion

  _We found a positive correlation betweenDreissena density in a waterbody and number
of zebramusselsattachedto unionids, and betweenD. polymorpha densityand the ratio of
the mass of attachedmusselsto the mass of the host unionid bivalve. LsweNoowsKr(1976)
found similar correlations for lakes in Poland, as did RrccrRnnret al. (1995) for the St.
Lawrence River. RrccrRRnr al. (1995) also found that unionid mortality (reflectedby the
                             et
proportion of dead unionids) correlated with Dreissena field density, and a significant re-
lationship betweenthe ratio of mass of D. polymorpha and, host living unionid and the pro-
portion of freshly killed unionids.
   In Lake Naroch, shortly after invasion,zebramusselscauseda dramaticdecline in unionid
bivalve density. In contrast, in Lake Lepelskoe, where zebra mussels have been for far
longer, unionids coexist with D. polymorpha. Although overgrowth by zebramusselshas
caused some host mortality, unionids not only persist, but also maintain high densities.
Unionids and zebra musselsalso coexist in {-ake Lukomskoe (KenerAyEV, tl8:) and the
TsimlyanskoeReservoir (Russia) (MnosHNrcHENKo al., 1984; MrRosHNrcHENKo,
                                                    et                              1987).
There have been detectablenegative impacts of zebra musselson unionid populations in
other lakes, including Lake Balaton (Hungary) (Por.rvr, 1992). However, ihe decline in
abundanceof unionids in Lake Hallwil (Switzerland) from the 1910s to the 1980s was
causedby a decrease the number of host fish for unionid larvae and an increasein eutro-
                       in
phication in addition to the influence of D. polymorpha which colonized in the 1970s
(ARrnR, 1989). Similarly, the drop in the number of unionid speciesin Mikolajskie Lake
from 1972 to 1987 was not a direct result of zebramusselimpact, but was due to increased
eutrophicationand pollution (LnweNoowsKr,1991).
 _ Extensive overgrowth of unionids by D. polymorpha, rcsulting in mass mortality can be
characteristicof periods of rapid population growth of zebramussels,when they invade a
new waterbody.The dramatic decline of unionids after zebramusselinvasion is well docu-
mented both in Europe (SennsrveN,1937; KeRereyev and Bunr-erove, 1995b; Bunr-ero-
ve, 1998) and North America (Heac et al., 1993; NlLerR, 1994; RrccrRRor al., 1996).
                                                                             et
However, D. polymorpha coexistwith native bivalves in Europeanwaters with established
zebta musselspopulations (LnweNDowsKr,1976; KRnnreyev, 1983; PoNyr, 1992: BuRr-e-
KovA,1998).
  In sandy and rubble areas of the Svisloch River, we found that unionids were heavily
overgrown with D. polymorpha.In silty areasunionids were very abundant,were buried in
sediments,and were completely free of zebramussels.Similar results were found in Lake
536                                                                              L. E. Bunltxovn et al.



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538                               L. E. BunI-AK:IYA et al.


Hallwil, where Unio tumidus is usually buried in the sedimentsand is rarely overgrown by
zebramussels,however,Anodonta cygneais often only partly buried and is colonized more
often by zebramussels(Anren, 1989).Basedon thesefindings and data from reservoirswith
different levels of pollution (Dr. V. VmoruRov, pers. comm.), we hypothesizethat under
certain circumstances   unionids, being more resistantto low oxygen and pollution than zebra
mussels,may survive in watersunsuitablefor Dreissena.ln addition, some unionids may be
capable of removing zebra musselsfrom their valves. In lakes where we found unionids
heavily fouled by D. polymorpha, we found some individuals completely free of zebra
mussels,but with byssal threadsstill attached.
   Overgrowth by D. polymorpha adversely affects host unionids. The extent of this
effect dependson a number of factors including zebramussel density in a waterbody,time
since invasion, and type of bottom sediment.Although the appearance zebra mussels
                                                                          of
in the North-American waterbodies has been correlated with negative impacts on the
native unionids (Haec et al., 1993; NeLnrR, 1994; ScHI-oessnR NRLeen,1994),a large
                                                                 and
decline in biodiversity and abundance unionids was detectedlong before the appearance
                                         of
of zebra mussels in the North America (Nnr-nee et al., 19911'     ScHI-oBsseR NALelt,
                                                                               and
 1994). Since 1989, all unionids in Western Lake Erie have been colonized by zebra
mussels (ScHr-oessnnand NRLeeA, 1994). North American scientists have reported
extremely high densities of D. polymorpha, greater than several thousand per unionid
(Henenr et al., l99l; ScHr-osssen     and KovaLer, l99l; Scrnopsssn and NRLtrl, 1994).
These densities are much higher than those reported by European scientists (Table 4).
Do these differences constitute a significant difference iq the impact of zebra mussels
on native unionids? From the perspectiveof the unionid, the mass of attachedD. poly-
morpha, or the ratio of the mass of attached zebra mussels to the mass of the host
unionid bivalve is probably more important than density (Hnnnm et al., 1991; KenRrevsv
et al., 1997). Of the studies which we could compare, the mean ratio of the biomass of
attachedD. polymorpha and the host unionid bivalve was very similar to that found in
Europe(Table 4).
    Moreover, high densitiesof zebra musselson unionids are typical mainly for the Great
Lakes. In other waterbodies,averageDreissenadensitiesper host unionid bivalve are very
similar to those in Europe (Table 4). Differences between the density of attachedmussels
reported by North American and European scientists could result if the size-frequency
composition of zebra mussel populations,is different in North America, North American
studies include smaller mussels in density estimates than European studies, or North
American unionids are larger than Europeanbivalves. In general,Europeanscientistsdo not
include mussels smaller than 1-2 mm in density estimates (Lvovn, 1980; KRR.r.tRYev,
 1983; Lvov.q. et al., 1994), and sometimesthey do not include mussels <5 mm (Btl nr
VRrre, 1991) or 8 mm (HevnuncER et al., 1990). The overwinter mortality of young-of-
the-yearand one-year-oldmusselsis very high, and by the spring the number of live mussels
 is greatly reduced. For example, in western Lake Erie in February 1989 the density of
D. polymorpha was 24 + 4 per unionid bivalve, and in August, after larval settlement,
 the densityof mussels   averaged  6,777 + 811 per unionid (ScHI-orssBR KovRLRt<,
                                                                      and             1991).
 In addition, we found that the average length of unionids from Belarussian lakes was
 significantly smaller than in North American Lakes Clark and Vineyard. The mean
 unionid bivalve shell length in North America used by RIcceRoI et al. (1995) was even
 greater (9.5cm), and the unionids from western Lake Erie (114+5.6-119+5.6mm)
 (ScHr-oesspn Kover-Rr, 1991) were more than twice as large as those from Belarussian
               and
 waterbodies.
    Is it possible that zebramusselswill have greater effects on unionids in North America
 than in Europe? In pre-glaciation Europe, zebra mussels and unionids coexisted. The
 distribution of zebra musselswas restrictedto areassurroundingthe Caspianand Black Sea
 only after the Pleistocene glaciation.BecauseNorth American native species   have no evolu-
                                   Impact of Dreissenaon Unionid                                539
tionary history of coexistence with zebramussels,we might expect zebramusselsto have a
larger impact on North American unionids.The species    composilionof unionids in the North
America is much different than in Europe, and we might expect species-specific  differences
in responseto fouling. The bivalve fauna of North American freshwatersis the most diver-
se in the world, with >250 native and 6 introduced species.Historically, there are records
of 297 species,>21 of which are now extinct, and,    >42 are endangered threatened,all
                                                                         or
which belong to the superfamily Unionacea (families Margaritiferidae and Unionidae)
(BoceN, 1993). In contrast, the bivalve fauna in Europe coniists of 62 species,and only
 14 speciesin the superfamily Unionacea(Jnrcrnr, 1967).
   Currently, North America is in the early phaseof zebramusselinvasion, and populations
are growing rapidly. At this stageof invasion, D. polymorpha causeda dramatic decline in
the abundanceof unionids in Europe (SnnrsrvrN, 1937; DussRRr, 1966; KRRereyev and
BuRr-Rrovn,1995b;Bunlerove, 1998).However,to our knowledge,the zebramusselin-
vasion did not result in the complete elimination of unionids in any Europeanlakes. After
initial peaks in zebta mussel abundance, polymorpha coexist wiih unionids in all lakes,
                                         D.
reservoirs and rivers studied. Will this pattern hold true for North America? With time,
perhapsthe impact of D. polymorpha on unionids will decrease.   Our hypothesismay be sup-
ported by ScUIoESSER al. (1996) who indicate that some unknown factor(s) reducesthe
                        et
intensity of unionid infestation by zebramussels,thus decreasingmortality ir, ,o-" North
American waterbodieswhere unionids and zebramusselscoexist.

                                                                       \
                                     5. Acknowledgments
   We would like to thank Devm W. GenroN and Leoo E. JouNsoNfor facilitating the study of
unionids in North American lakes, and RoNnln D. OBscHfor the identification of Nbrth American
unionid bivalve species.
   This researchwas supportedby grants from the University of Wisconsin Sea Grant Institute under
grants from the National Sea Grant College Program,National Oceanic and Atmospheric Administra-
!iqn, U.S. Department of Commerce, and from the State of Wisconsin. Federal grant NA9OAA-D-
SG469, and grant number R/LR65 to DKP. In the Republic of Belarus the research-*ur supportedby
grant numbet 444/50 from Belarussian                 (to
                                    StateUniverrsity AYK and LEB). We gratefully actnowledge
Icon RunaKovsKIY(Belarussian   StateUniversity) for technical assistance



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Manuscript receivedJanuary 24th,2000; revised May\25th, 2000; acceptedJuly 3rd, 2000

				
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