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1-1-1996

Zebra Mussel Infestation of Unionid Bivalves(Unionidae) in North AmericaDon W. SchloesserNational Biological Service

Thomas F. NalepaNational Oceanic and Atmospheric Administration, thomas.nalepa@noaa.gov

Gerald L. MackieUniversity of Guelph

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Schloesser, Don W.; Nalepa, Thomas F.; and Mackie, Gerald L., "Zebra Mussel Infestation of Unionid Bivalves (Unionidae) in NorthAmerica" (1996). Publications, Agencies and Staff of the U.S. Department of Commerce. Paper 368.http://digitalcommons.unl.edu/usdeptcommercepub/368

AMER. ZOOL., 36:300-310 (1996)

Zebra Mussel Infestation of Unionid Bivalves (Unionidae) inNorth America1

DON W. SCHLOESSER

National Biological Service, Great Lakes Science Center, Ann Arbor, Michigan 48105

THOMAS F. NALEPA

National Oceanic and Atmospheric Administration, Great Lakes EnvironmentalResearch Laboratory Ann, Arbor, Michigan 48105

AND

GERALD L. M A C K I E

University of Guelph, Department of Zoology, Guelph, Ontario N1G2W1, Canada

SYNOPSIS. In 1989, zebra mussels received national attention in NorthAmerica when they reached densities exceeding 750,000/m2 in a waterwithdrawal facility along the shore of western Lake Erie of the LaurentianGreat Lakes. Although water withdrawal problems caused by zebra mus-sels have been of immediate concern, ecological impacts attributed tomussels are likely to be the more important long-term issue for surfacewaters in North America. To date, the epizoic colonization (i.e., infesta-tion) of unionid bivalve mollusks by zebra mussels has caused the mostdirect and severe ecological impact. Infestation of and resulting impactscaused by zebra mussels on unionids in the Great Lakes began in 1988.By 1990, mortality of unionids was occurring at some locations; by 1991,extant populations of unionids in western Lake Erie were nearly extir-pated; by 1992, unionid populations in the southern half of Lake St. Clairwere extirpated; by 1993, unionids in widely separated geographic areasof the Great Lakes and the Mississippi River showed high mortality dueto mussel infestation. All infested unionid species in the Great Lakes (23)have become infested and exhibited mortality within two to four yearsafter heavy infestation began. Data indicate that mean zebra mussel den-sities >5,000-6,000/m2 and infestation intensities >100-200/unionid inthe presence of heavy zebra mussel recruitment results in near total mor-tality of unionids. At present, all unionid species in rivers, streams, andlakes that sympatrically occur with zebra mussels have been infested and,in many locations, negatively impacted by zebra mussels. We do not knowthe potential consequences of infestation on the 297 unionid species foundin North America, but believe zebra mussels pose an immediate threat tothe abundance and diversity of unionids.

INTRODUCTION zebra mussel (Dreissena polymorpha) inColonization of substrates, water with- Europe and now in North America (Clarke,

drawal facilities, and hard-bodied inverte- 1952; Nalepa and Schloesser, 1993). De-brates is a distinctive characteristic of the velopment of controls to keep water with-

drawal facilities free of zebra mussels was. ,' From'hl sy™P°s i u m Biol°sy- Oology andPhys- t h e i m m e d i a t e concern during the invasioniology of Zebra Mussels presented at the Annual Meet-ing of the American Society of Zoologists, 4-8 Janu- a n d e a r l y proliferation Ot mussel popula-ary 1995, at St. Louis, Missouri. tions (Clarke, 1952; Griffiths et al, 1989).

300

ZEBRA MUSSEL INFESTATION OF UNIONIDS 301

FIG. 1. Zebra mussel infested unionid (Megalonaias nervosa) removed from the Illinois River 1993 (photocourtesy of D. Blodgett, Illinois Natural History Survey, Havana, Illinois).

However, epizoic colonization (i.e., infes-tation) of snails, crayfish, and especiallyunionids is believed to be the most directand ecologically destructive characteristicof zebra mussels (Hebert et al., 1989, 1991;Schloesser and Kovalak, 1991; Hunter andBailey, 1992; Mackie, 1993). In addition,the discovery and spread of a second dreis-senid species (Dreissena (rostriformis) bug-ensis) has raised even more concern for thesurvival of unionids (Mills et al., 1993; Spi-dle et al, 1994).

Infestation of unionids is beginning tooccur throughout the range of the zebramussel in North America (Tucker et al.,1993; personal communications). Zebramussels have invaded and are becomingabundant in major river systems of theUnited States such as the Illinois, Missis-sippi, Tennessee, Ohio, Arkansas, and Hud-son (Schloesser, 1995). In the Illinois River,where zebra mussels first invaded and be-came abundant outside the Great Lakes wa-ter basin, infestation at some locations is as

heavy as that found in the Great Lakes (Fig.1). Similar heavy infestations have been ob-served in the Mississippi River and someinland lakes (personal communication, D.Garton, Indiana University, Kokomo, Indi-ana). To date, low to medium infestationshave been observed in all large rivers wheremussels and unionids are sympatrically oc-curring.

The present study reviews the coloniza-tion of zebra mussels on unionids in NorthAmerica. A major portion of this review isdevoted to the impacts of mussels on union-ids as documented in the Great Lakes be-cause infestation first occurred and has beenmost documented in this area.

ZEBRA MUSSEL ATTACHMENT

Zebra mussels have four life-historycharacteristics that allow them to attach toand successfully colonize unionids: 1) Thereproductive and planktonic distribution pe-riod of zebra mussels occurs at a time whenunionids extend their shells out of the sed-

GANSER LIBRARY' MILLERSVILLE UNIVERSITY

302 D. W. SCHLOESSER ET AL.

iment to feed and reproduce, thus promot-ing direct colonization of unionid shells(Schloesser and Kovalak, 1991; Nalepa andSchloesser, 1993). 2) Zebra mussels havehigh reproductive and, when habitat con-ditions are favorable, survival potential(Walz, 1978; Sprung, 1993). 3) Early set-tled pediveligers and juveniles activelysearch for and select hard substrates, suchas unionid shells, for attachment (Lewan-dowski, 1976; Martel, 1993; Yankovich andHaffner, 1993). 4) Juvenile and adult zebramussels produce byssal threads that allowthem to attach to firm substrates and main-tain physical contact with unionids (Eckroatet al., 1993).

INFESTATION

Unionid speciesTo date, infestation of unionids has been

documented for 31 species in North Amer-ica (Table 1). In the Great Lakes, 23 specieshave been infested. In major rivers, 24 spe-cies have been infested (Tucker et al., 1993;Strayer et al., 1994; Tucker, 1994). It islikely that there will be many more speciesinfested as zebra mussels continue to in-vade and colonize rivers, streams, and in-land lakes (Stolzenburg, 1992; Schloesser,1995). Potentially, zebra mussels could in-fest all 297 unionid species in North Amer-ica (Williams et al., 1993).

MortalityInfestation and resulting mortality of

unionids has been most intensively studiedin Lake St. Clair and western Lake Erie ofthe Great Lakes (Hebert et al., 1989, 1991;Schloesser and Kovalak, 1991; Hunter andBailey, 1992; Mackie, 1993; Gillis andMackie, 1994; Nalepa, 1994; Schloesserand Nalepa, 1994). This is attributed to thefirst discovery of zebra mussels in Lake St.Clair in 1988, but first introducted into bothLake St. Clair and western Lake Erie in1986 (as determined by life-history char-acteristics), and an exponential increase inabundance in the summer of 1989 (Hebertet al., 1989; Griffiths et al., 1989; Nalepaand Schloesser, 1993). To date, all 18 spe-cies that have been intensively studied inLake St. Clair and western Lake Erie haveexhibited high mortality attributed to infes-

tation by zebra mussels (Table 1; Gillis andMackie, 1994; Nalepa, 1994; Schloesserand Nalepa, 1994). In addition, near totalmortality of five additional species inPresque Isle Bay in east-central Lake Eriehas also occurred (DWS, unpublished data).To date, all species that have experiencedheavy infestation for several years have ex-hibited near total mortality.

Infestation was first observed in Lake St.Clair in 1988 during a preliminary surveyto determine the abundance and distributionof zebra mussels shortly after their discov-ery (Hebert et al., 1989). In 1988, the max-imum infestation intensity was 38 zebramussels/unionid and the maximum sub-strate density was 200 mussels/m2. By1989, the maximum infestation intensitywas 10,500/unionid and the maximum sub-strate density was 4,500/m2 (Hebert et al.,1991). Unionids were described as beingcovered with a 4- to 6-cm-deep layer overthe entire unionid shell that extended out ofthe sediments. In western Lake Erie, themaximum infestation intensity was48/unionid and the maximum substrate den-sity was about 100/m2 in winter 1989 andby August 1989, the maximums had in-creased to 10,700/unionid and 750,000/m2

(Schloesser and Kovalak, 1991; Nalepa andSchloesser, 1993). In both Lake St. Clairand western Lake Erie, increased infesta-tion intensities in the summer of 1989 wereprimarily attributed to young-of-the-yearrecruitment. This period included onlyabout four weeks of post-planktonic veligersettling of zebra mussels (Garton and Haag,1993).

High to near total mortality of unionidsoccurred in Lake St. Clair within three tofive years after heavy infestation intensitiesbegan in summer 1989 (Gillis and Mackie,1994; Nalepa, 1994). Gillis and Mackie(1994) intensively studied infestation ofunionids at two sites in southern Lake St.Clair from 1990 to 1992. Densities ofunionids decreased from 1 to 0/m2 at onesite between 1990 and 1992 and from 2 to<l/m2 at another site between 1991 and1992. At a 4-m depth contour, unionids de-creased from 8/m2 in 1986, to 2/m2 in 1990,to 0/m2 in 1992. In 1986, 10 species werefound, in 1990, 12 were found, in 1991, 4

ZEBRA MUSSEL INFESTATION OF UNIONIDS 303

TABLE 1. Species list of unionid mollusks infested by zebra mussels in the Great Lakes and major connectingrivers in North America 1989-1994.*

*Amblema plicata plicataAnodonia implicala

*Anodontoides ferussacianusArcidens confragosusEllipsaria lineolata

*Elliptio complanta*Elliplio dilatata*Fusconia ftava*Lampsilis cardium*Lampsilis siliquoidea (& radiata)Lampsilis teres

*Lasmigona complanata complanata*Lasmigona costata*Leptodea fragilisLeptodea ochracea

*Ligumia nasuta*Ligumia rectaMegalonaias nervosa

*Obliquaria reflexa*Pleurobema coccineum*Potamilus alatus*Potamilus ohiensis* Ptychobranchus fasciolaris*Pyganodon grandisQuadrula nodulata

*Quadrula pustulosa pustulosa* Quadrula quadrula*Strophitus undulatus*Truncilla donaciformis*Truncilla truncata

Utterbackia imbecillus

LSC

LSCLSCLSCLSCLSC

LSC

LSC

LSCLSC

LSCLSCLSC

LSCLSC

LSC

Great Lakes

WLE

WLEWLE

WLE

WLEWLE

WLE

WLE

PI

PI

PIPIPIPI

PIPI

PIPI

PI

PIPI

PIPIPIPIPI

Major nvers

IMR

IMRIMR

IMR

IMRIMRIMRIMR

IMR

IMRIMR

IMRIMR

IMRIMRIMRIMR

IMRIMRIMR

HR

HR

HR

HRHR

* An asterisk indicates species where mortality has been attributed to infestations. LSC = Lake St. Clair; Gillisand Mackie (1994); Hunter and Bailey (1992). WLE = western Lake Erie; Haag el al. (1993); Schloesser andNalepa (1994). PI = eastern Lake Erie, Presque Isle; unpublished, DWS. IMR = Illinois and Mississippi rivers;Tucker et al. (1993); Tucker (1994). HR = Hudson River; Strayer et al. (1994); Strayer, personal communication.

species were found, and by 1992 no live low-infested regions, the average number ofspecies were found (Nalepa and Gauvin, zebra mussels per unionid was <3 in 1990,1988; Gillis and Mackie, 1994). Nalepa and unionids were still present in 1992.(1994) conducted a lake wide survey of However by 1994, unionids in Lake St.unionids in Lake St. Clair in 1986, 1990, Clair were nearly extirpated (TFN, unpub-and 1992. Mean lake-wide densities of lished data).unionids remained relatively stable at 2/m2 Unionid mortality also occurred in west-between 1986 and 1990 and then declined ern Lake Erie, but it occurred within one toto 1/m2 in 1992. The total number of union- two years after heavy infestations instead ofids collected declined from 281 to 248, and three to five years as documented in Laketo 99 in each of the three years, respective- St. Clair. Schloesser and Nalepa (1994)ly. Because of physical characteristics and documented the swift decline of unionids atwater current patterns, infestation and sub- one site in offshore waters of western Lakesequent unionid mortality varied substan- Erie (Fig. 2). In fall 1989 when all livetially within Lake St. Clair. In high-infested unionids were infested, the unionid collec-regions, unionids were still present in 1990, tion consisted of 53% live and 47% deadbut the average number of zebra mussels individuals. In May-June 1990, the collec-per unionid was about 400. By 1992, no tion was 17% live and 83% dead, and byunionids were collected in this region. In September 1990, no live unionids could be

304 D . W . SCHLOESSER ET AL.

100

O

o

Sept 1989 May-June 1990 Sept 1990

Sampling PeriodJuly 1991

FIG. 2. Percent live and dead unionids collected at one site in offshore waters of western Lake Erie 1989-1991 (Schloesser and Nalepa, 1994).

found. Maximum infestation intensitieswere 11,550/live unionid and 14,393/deadunionid. In addition, Schloesser and Nalepa(1994) sampled 17 historical stations locat-ed throughout western Lake Erie. In 1991,they found only 4 live and 187 dead union-ids. The near total absence of unionids inmost of western Lake Erie, like that in LakeSt. Clair in the early 1990s, is unprece-dented because unionids had existed inthese waters for centuries.

Zebra mussels have been found outsidethe Great Lakes water basin in several riv-ers including, the Illinois, Mississippi, Ten-nessee, Ohio, Arkansas, and Hudson(Schloesser, 1995). Many of these sightingswere of isolated, single individuals thatprobably did not represent established pop-ulations that threatened unionids at the timeof their discovery. However, establishedpopulations of zebra mussels were presentin the lower Illinois River and adjoiningarea of the Mississippi River in 1991. Thesepopulations probably became established in1990 from veligers contained in water flow-

ing in canals that connect the Illinois Riverto Lake Michigan (Schloesser, 1995). If in-festations and resulting impacts similar tothose observed in the Great Lakes are tooccur outside the Great Lakes, one wouldexpect them to be found in the area of con-fluence near the Illinois and Mississippi riv-ers before they occur in other inland waters.

Indeed, zebra mussels have infested 20species of unionids near the confluence ofthe Illinois and Mississippi rivers, and ob-servations and descriptions of infestationsare similar to those observed in the GreatLakes (Table 1; Schloesser and Kovalak,1991; Nalepa and Schloesser, 1993; Tuckeretal, 1993; Tucker, 1994). Length-frequen-cy distributions of zebra mussels collectedin the area of confluence of the two riversindicate that mussels probably became es-tablished in the study area in 1990 (Tuckeretal, 1993). Between 1991 and 1993, mus-sel densities increased dramatically; in theIllinois River, maximum densities of mus-sels on substrates were about 10/m2 in 1991and about 100,000/m2 in 1993 (unpublished

ZEBRA MUSSEL INFESTATION OF UNIONIDS 305

data, S. Whitney, Illinois Natural HistorySurvey, Havana, Illinois). In 1992, zebramussels in Alton pool of the MississippiRiver infested 9 of 25 unionid species, oc-curred on 27% of the unionids, and aver-aged about 2 mussels/unionid (Tucker etal., 1993). In 1993, infestation in Altonpool occurred on all 18 species collectedand on 100% of the unionids and averaged37/unionid (Tucker, 1994). Several otherpools located around the confluence of theIllinois and Mississippi rivers showed sim-ilar, but lower average infestation intensitiesthan that observed in Alton pool of the Mis-sissippi River (Tucker et al., 1993). At onesite (pool 26 of the Mississippi River),Tucker (1994) described substrate densitiesas " . . . zebra mussels not only colonizedunionids but also essentially covered theentire surface of the gravel bar. . . . Theyformed a pavement made up of zebra mus-sel shells interconnected by byssalthreads." Data presented by Tucker (1994)indicate that during the early infestation pe-riod in the Mississippi River, there was ahigh degree of variability of infestation in-tensities based on unionid species. Thishigh variability of infestation has beencharacteristic of early infestation periods inthe Great Lakes (Hebert et al., 1989;Schloesser and Kovalak, 1991; Nalepa,1994).

Available information indicates that atlow infestation intensities unionids can sur-vive in the presence of zebra mussels (Le-wandowski and Stanczykowska, 1975; Le-wandowski, 1976). However, critical infes-tation intensities and corresponding sub-strate densities of zebra mussels at whichimpacts on unionids occur are unknown.Lewandowski (1976) indicates a direct re-lationship between non-lethal infestation in-tensities (<200/unionid) and substrate den-sities below about 2,000/m2. Our review ofavailable data in North America indicatesthat substrate densities >5,000-6,000/m2

and infestation intensities > 100-200 juve-nile and adult mussels/unionid in the pres-ence of heavy zebra mussel recruitment,which may substantially increase numbersof mussels in summer and fall, causes mor-tality of unionids (Hebert et al, 1989, 1991;Schloesser and Kovalak, 1991; Nalepa,

1994; Schloesser and Nalepa, 1994; Ric-ciardi et al., in press).

To date there have been several tech-niques to evaluate real and potential im-pacts of infestation including: 1) numbersof infesting zebra mussels, 2) visual esti-mate of infestation intensity, 3) weight ofinfesting zebra mussels, 4) energy reservesof unionids, 5) deformities of unionidshells, 6) growth of unionids, and 7) mor-tality of unionids. Unfortunately, the mostcommon evaluation technique used to date{i.e., mortality) occurs too late for mitiga-tion of impacts on unionids. More evalua-tion techniques are needed during the earlyinfestation period. Such techniques wouldindicate the potential for negative impactsand the need to take active measures to pre-serve the affected unionid population. Atpresent, the earliest warning of negative im-pacts of infestation on unionids appears tobe the presence of heavy encrustations onexposed unionid shells {e.g., Fig. 1). Evi-dence indicates that unionid populations ex-posed to such heavy infestations will ex-perience near total mortality.

CAUSES OF MORTALITY

At present, the causal mechanism(s) ofunionid mortality as a result of infestationby zebra mussels is(are) unknown. Severalpossible mechanisms have been proposed(Schloesser and Kovalak, 1991). Existingevidence for proposed mechanisms of mor-tality is derived primarily from field andlaboratory observations and some labora-tory analysis of indicators of unionidhealth, such as energy reserves {e.g., gly-cogen content). More research is needed tocharacterize the causal mechanism(s) ofunionid mortality; such efforts will be use-ful to determine if impacts can be mitigatedthrough active conservation efforts to saveunionids.

Valve movement and deformitiesVisual observations of infested unionids

indicate that zebra mussels restrict and/orprevent normal valve movement of union-ids (Fig. 1). In addition, observations ofunionid shells cleaned of infesting musselsindicate that unionid shell deformities areoften observed (Fig. 3). Hunter and Bailey

306 D. W. SCHLOESSER ET AL.

FIG. 3. Posterior views of Lampsilis siliquoidea exibiting varying degrees of shell deformities attributed toinfestation by zebra mussels in Lake St. Clair of the Great Lakes, (photo courtesy of R. D. Hunter, OaklandUniversity, Rochester, Michigan).

(1992) reported the most extensive obser-vations of the mechanical disruption ofvalve closing and opening, smothering ofsiphons, and shell deformities. Descriptionsindicate that mussels colonize unionids atsuch high intensities that unionid valvescannot open and close. Unionid valves werepulled by byssal threads with such tensionthat new growth and old shell material wasdeformed for 78% of the unionid popula-tion.

Available foodsThe reduction of available foods for in-

fested unionids as a result of zebra musselfeeding has been partially studied in theGreat Lakes (Hebert et al, 1989; Haag etal., 1993). Both elimination of foods in thewater column by zebra mussels and directinterference of food gathering siphons ofunionids by zebra mussels are possiblemechanisms that may contribute to lowerfood supplies reaching infested unionids.When found at substantial densities, zebra

mussels have been found to reduce avail-able phytoplankton in entire lake systems(Maclsaac et al., 1992; Madenjian, 1996).Gillis (1993) reported interference compe-tition between zebra mussels and unionidsand showed that the position of attached ze-bra mussels, rather than the number at-tached, was the largest contributing factorto altering the filtration activities of hostunionids. Infestations of up to 2,000 zebramussels/unionid resulted in total occlusionof the siphon openings of host unionids.Hebert et al. (1991) and Haag et al. (1993)showed physiological food reserves of in-fested unionids to be lower than non-in-fested unionids. Infested unionids had lipidreserves 50% lower than non-infestedunionids. Similarly, Haag et al. (1993) re-ported lower glycogen and cellulase activityof infested unionids than non-infestedunionids (Table 2).

Impair movementImpairment of movement of unionids

caused by infestation by zebra mussels has

ZEBRA MUSSEL INFESTATION OF UNIONIDS 307

TABLE 2. Percent survival, glycogen content (i.e., index of fitness), and enzyme activity (i.e., index of stress)of two species of infested and non-infested unionids held in cages (ca. 90 days) in near shore waters of westernLake Erie July-October 1990. (Haag et al., 1993).

Species

Lampsilis siliquoideamale

female

Amblema plicata plicata

Treatment

InfestedNon-infestedInfestedNon-infested

InfestedNon-infested

Percentsurvival

64681768

100100

Glycogencontent(mg/g)

0.080.130.040.08

0.671.90

Enzymeactivity

(units/ jig)

111.5200.5151.6399.6

87.8257.4

been observed and is believed to be a com-mon cause of unionid mortality. Gillis andMackie (1994) attributed the presence ofmany dead, thin shelled, alate unionids{e.g., Potamolus alatus and Leptodea fra-gilis) found on their sides with large colo-nies of zebra mussels attached, as evidencethat they were dislodged from the substrateby infestations. In contrast, rounded species{e.g., Pyganodon grandis) were found lyingon their sides less frequently than thin-shelled, alate species. Hunter and Bailey(1992) observed infestations where musselsformed layers on unionids in excess of 2cm and extended from the sides of the in-festation layers laterally on to adjacent sed-iments probably immobilizing unionids.Tucker (1994) observed infested unionidslying on their sides along the shores inwindrows in Pool 26 of the MississippiRiver. He believed "a pavement" like bot-tom covered by zebra mussels preventedunionids from burrowing into sedimentsonce they were dislodged. Unionids werethus stranded and exposed to fluctuatingwater levels that Tucker (1994) believedcaused unionid mortality. Schloesser andNalepa (1994) attribute mortality of infest-ed unionids to smothering by sedimentscaused by the inability of unionids to main-tain themselves at the mud-water interfacedue to the additional weight of infestingmussels. Weight of infesting zebra musselsmay exceed host unionid weight by a factorof four, but has been noted to be as high as8.5 (Hebert et al, 1991; Schloesser and Ko-valak, 1991; Nalepa, 1994).

Accumulation of toxic metabolic fecesZebra mussels bio-deposit much of what

they filter as feces and pseudofeces. Accu-mulation of deposited materials by infestingmussels around unionid beds may cause de-graded water quality unsuitable for unionidsurvival. Direct observations support rap-idly changing benthic habitat conditionsnear heavily infested unionids (Schloesserand Kovalak, 1991; Gillis and Mackie,1994). In Lake St. Clair, there was a no-ticeable increase in the overlying deposi-tional layer between 1990 and 1991 whenhigh mortality of infested unionids occurred(Gillis and Mackie, 1994). In near-shorewaters of western Lake Erie, benthic habitatshowed evidence of anoxia {e.g., bubbles,black surface sediment, and decaying ben-thic fauna) near heavily infested unionidsand high densities of accumulating zebramussels (SCUBA observations, Schloesserand Kovalak, 1991).

FACTORS AFFECTING INFESTATION

Available data indicate that life historycharacteristics of species, sex, and/or shellmorphology of unionids may play an im-portant part in determining when impacts ofzebra mussels occur on unionids (eventhough the result of heavy infestations is thesame over a two to four year period, i.e.,near total mortality, discussed above). Databy Haag et al. (1993) suggest that theremay be differences in impacts of infestationbased on life history characteristics ofunionid species. The subfamilies Lampsili-nae and Anodontinae {e.g., Lampsilis sili-

308 D. W. SCHLOESSER ET AL.

quoidea) are long-term brooders {i.e., usu-ally 9-12 months), while Ambleminae{e.g., Amblema plicata plicata) are shortterm brooders {i.e., usually <2 months) ofyoung (Clarke, 1981). Over the short term,infested individuals of the Lampsilinae hadlower energy reserves {i.e., glycogen con-tent) than individuals of the Ambleminae(Table 2). Field surveys confirmed highermortality for lampsilines than amblemines.Mortality was also greater for female thanmale L. siliquoidea, indicating that mortal-ity was linked to stress of brooding young.Haag et al. (1993) also mentioned shellmorphology as a possible contributing fac-tor to differences in mortality of subfamiliesof unionids. Lampsilines and anodontineshave thin shells, and amblemines have thickshells. Some data from Gillis and Mackie(1994) and Nalepa (1994) support the hy-pothesis that thin alate species are impactedsooner than species with inflated and roundshells. However, for another species, A. p.plicata, no mortality was observed, but fit-ness and stress were substantially affected.Surveys of infested unionid populationssupport the generalization that species ofthe Lampsilinae subfamily {i.e., L. siliquo-idea, Leptodea fragilis, and Potamilus ala-tus) were more likely to have lower energyreserves and experience mortality soonerthan species of the subfamily Ambleminae{A. p. plicata and Quadrula pustulosa)(Haag et al., 1993). Of the heavily infestedunionids found in a survey of Lake St. Clairby Nalepa (1994), species in the subfami-lies Anodontinae and Lampsilinae declinedsooner than species in the subfamily Am-bleminae. However, results of Hunter andBailey (1992) at several sites in Lake St.Clair suggest that lampsilines were less sus-ceptible to impacts of infestation than sev-eral other species including species of Am-bleminae.

Personal observations and communica-tions indicate that some unknown factor(s)reduces the intensity of unionid infestationby zebra mussels, thus decreasing unionidmortality in some areas where unionids andzebra mussels co-exist. For example, at onesite in near-shore western Lake Erie all in-dividuals of Amblema plicata plicata be-came heavily infested and were extirpated

within two years, whereas at another nearbysite the same species were heavily infestedand have not shown high mortality in fouryears (DWS, unpublished data). A similarobservation has been made in a large bayof Lake Erie (Presque Isle Bay), a large in-terconnecting river of the Great Lakes (St.Clair River), and a small river in Michigan(Clinton River) (personal communications,E. Masteller, Penn State University, Erie,Pennsylvania, R. Smithee, Detroit Edison,Detroit, Michigan, and J. Lazar, TennesseeTechnological University, Cookeville, Ten-nessee). At present, it is believed that thereis(are) a unique characteristic(s) of thesehabitats that allow unionids to escape heavyinfestation and resulting mortality.

SUMMARY

To date, infestation and the causualmechanisms leading to impacts on unionidsis not a well documented observation inNorth America. The Great Lakes experi-ence shows us that zebra mussel densities>5,000/m2 cause infestation intensities>150 one-year-old and older zebra mus-sels/unionid. This infestation level in thepresence of heavy zebra mussel recruitmentseems lethal to all unionid species withinseveral years after infestation begins. Thismay be why a review of the European lit-erature by Lewandowski (1976) only foundmean infestation intensities below180/unionid.

If zebra mussel infestation occurs at rel-atively low intensities {i.e., <200/unionid)in some areas of North America, then thereis reasonable expectation, based on the co-existence of zebra mussels and unionids inEurope, that impacts of infestation onunionids in North America may vary dra-matically from that documented in theGreat Lakes.

ACKNOWLEDGMENTS

Contribution number 952 of the NationalBiological Survey, Great Lakes ScienceCenter and Contribution number 938 of theGreat Lakes Environmental Research Lab-oratory, Ann Arbor, Michigan.

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ZEBRA MUSSEL INFESTATION OF UNIONIDS 309

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