Habitat influences snail community structure and trematode infection levels in a spring-fed river, Texas, USA

Transcription

1 Hydroiologi (28) 6:29 4 DOI 1.17/s PRIMARY RESEARCH PAPER Hitt influences snil community structure nd tremtode infection levels in spring-fed river, Texs, USA Lori R. Tolley-Jordn Æ J. Murry Owen Received: 26 Septemer 26 / Revised: 3 Septemer 27 / Accepted: 15 Octoer 27 / Pulished online: 3 Novemer 27 Ó Springer Science+Business Medi B.V. 27 Astrct The spring-fed Coml River in Texs, USA, hs een impounded nd chnnelized resulting minly in lentic environment with four hedwter spring runs. We smpled two spring runs (lotic sites) nd two lentic sites sesonlly from April 21 through April 22 to ssess (1) co-occurrence of ntive nd exotic snil species (s determined y interspecific ssocition), (2) the importnce of hitt conditions in structuring reltionships mong these species, nd (3) the distriution of snils infected with exotic tremtode prsites. Three exotic nd four endemic species of qutic snils were collected, ut only Elimi comlensis (Prosornchi: Pleuroceride, ntive), Melnoides tuercultus, nd Trei grnifer (Prosornchi: Thiride, exotic) were in sufficient densities for further nlyses. Hndling editor: K. Mrtens L. R. Tolley-Jordn J. M. Owen Ntionl Fish Htchery nd Technology Center, U.S. Fish nd Wildlife Service, Sn Mrcos, TX 78666, USA Present Address: L. R. Tolley-Jordn (&) Deprtment of Biologicl Sciences, Aqutic Biology Progrm, The University of Alm, P.O. Box 8726, Tuscloos, AL 35487, USA e-mil: tolle4@m.u.edu Present Address: J. M. Owen 324 West Mgnoli, Sn Antonio, TX 78212, USA Trei grnifer ws positively ssocited with oth M. tuercultus (v 2 = 18.5, P \.1) nd E. comlensis (v 2 = 7.3, P \.1), lthough the cooccurrence etween the two exotics ws much stronger. Melnoides tuercultus nd E. comlensis exhiited strong, negtive ssocition (v 2 = 1.9, P \.1). The weker co-occurrence etween E. comlensis with the thirids ppered to e driven y differences in hitt use y the thirids nd ntive E. comlensis. In lentic hitts, densities of M. tuercultus nd T. grnifer were similr ut differed significntly from E. comlensis whose densities were 2 times less thn the exotic snils. In lotic spring runs, densities of T. grnifer nd E. comlensis were similr, ut differed significntly from M. tuercultus whose densities were 1 times fewer. Lower densities of M. tuercultus nd T. grnifer in hitt conditions common to the spring runs my explin why exotic snil interctions were less with the ntive E. comlensis thn with ech other. The ntive snil, E. comlensis, ws not infected with ny tremtodes, while 6.1% of M. tuercultus nd 4.8% of T. grnifer were infected with exotic tremtodes. Distriutions of infected snils were ggregted; such tht most infected snils were found in lentic hitts with silt sustrtes nd moderte to high levels of detritus. Continued declines in spring-flows due to quifer withdrwls nd droughts will increse lentic hitts tht my led to incresed densities of T. grnifer nd M. tuercultus nd their concomitnt prsites.

2 3 Hydroiologi (28) 6:29 4 Keywords Snil Exotic Spring-fed river Tremtode Prsite Introduction The impcts of exotic snils on qutic systems in North Americ hve not een dequtely studied (Stryer, 1999; ut see Hll et l., 23; Kerns et l., 25). Successful estlishment of qutic snils into novel environments often results in exceptionl densities nd iomss compred to ntive invertertes, llowing exotic snils to hve competitive dvntge over endemic invertertes (Kerns et l., 25). In ddition, mny exotic snils re hosts to one or more tremtode prsites. The estlishment of exotic tremtodes long with their snil host hs een suggested to negtively ffect ntive verterte communities susceptile to infection y the exotic prsites (Font, 23). In novel hitts, these prsites re pthogenic to verterte hosts lcking coevolved defense mechnisms to keep densities of prsites t low level (Trschewski, 26). The rnge expnsion of two qutic prosornch Asin snils in the fmily Thiride (Melnoides tuercultus Müller nd Trei grnifer Lmrck) is ttriuted to life history chrcteristics tht fcilitte successful invsion into stle wrm wter systems in the western hemisphere (Pointier et l., 1993). Thirid snils re generlly prthenogenetic with long lifespns nd slow intrinsic growth rtes tht llow them to otin high popultion densities in stle tropicl strems (Pointier et l., 1993) nd wrm-wter springs of su-tropicl nd temperte regions (Duggn, 22; Rder et l., 23). These snils cn directly lter community structure through competition with endemic snils (Pointier, 1999). For exmple, in permnent wter odies, thirids hve een shown to successfully compete ginst snils chrcterized y short lifespns nd high intrinsic growth rtes (Pointier et l., 1993). However, quntittive studies of the interction of thirids with ntive snils tht lso thrive in permnent wter odies nd possess similr life history chrcteristics (e.g., slow intrinsic growth rtes, long life spns, nd ilities to chieve high densities), like North Americn snils in the fmily Pleuroceride (Dillon, 2), hve not een conducted. In the Coml River, Coml County, Texs ( spring-fed, wrm-wter river), M. tuercultus nd T. grnifer hve successfully invded nd serve s hosts to two introduced Asin tremtodes, Centrocestus formosnus nd Philophthlmus grlli (Nollen & Murry, 1978; Mitchell et l., 2). Centrocestus formosnus is digenetic tremtode tht ws first recorded in Mexico in the 198s (Scholz & Slgdo- Mldondo, 2) nd susequently spred to wrmwter spring systems in Texs in the 199s (Mitchell et l., 2). The lifecycle of C. formosnus requires M. tuercultus s its first intermedite host, ny of wide vriety of fishes s second intermedite hosts, nd piscivorous wding irds s definitive hosts (Scholz & Slgdo-Mldondo, 2). The spred of C. formosnus into wrm-wter spring systems of the United Sttes is of mjor concern due to its ility to infect gills nd compromise the helth of endemic fishes; severl of which re listed s federlly endngered (e.g., Etheostom fonticol; McDermott et l., 2; Mitchell et l., 2). Despite C. formosnus ility to spred to wide rnge of verterte hosts (Scholz & Slgdo-Mldondo, 2; Font 23), not ll wrm-wter systems in North Americ contining thirids hve C. formosnus (McDermott, 2). Philophthlmus grlli is nother digenetic fluke tht uses oth M. tuercultus nd T. grnifer s intermedite hosts nd wterfowl s definitive hosts (Murry & Hines, 1969; Nollen & Murry, 1978). Philophthlmus grlli does not use fishes s n intermedite host nd is not considered to e s gret thret to the qutic community. In their ntive hitts, tremtode-infected snils represent smll proportion of the totl snil popultion (generlly less thn 5% in dequtely smpled hitts), nd infected snils tend to e sptilly ggregted. Thus, infection hot-spots (i.e., where ggregtes of infected snils occur) gretly contriutes to tremtode trnsmission. Fctors tht contriute to hotspot development include: susceptiility of snils to infection, likelihood of contct with vile tremtode eggs, nd hitt suitility for snils (Anderson & My, 1979). The infection sttus of non-thirid snils y tremtode prsites is relted to wter temperture (Anderson & My, 1979), proximity to definitive hosts (Smith, 21), wter depth (Jokel & Lively, 1995; Kziwe et l., 26), nd distnce from shore (Spp & Esch, 1994; Jokel & Lively, 1995).

3 Hydroiologi (28) 6: Thirid snils nd their prsites hve een shown to dversely ffect communities in the tropics nd re expected to disrupt stle, wrm-wter ecosystems in North Americ. Thus, it is necessry to ddress (1) the lck of quntittive nlyses descriing the interctions of thirid snils with endemic snils nd (2) hitt conditions tht influence distriutions of infected thirids in North Americ. The spring-fed Coml River in centrl Texs serves s unique loction to study the interction of thirids with North Americn endemic snils. We predict tht the river s stle, spring-fed flow regimes will llow thirids to successfully estlish stle popultions nd interct with ntive snils. In ddition, the heterogeneous hitts present in the Coml River llow for investigtion of the importnce of hitt conditions in structuring infected snil hotspots. The ims of this work were to ssess the co-occurrence of ntive snils nd exotic thirids nd to ssess the importnce of hitt conditions in structuring these reltionships. Further, we ddressed if the distriutions of infected snils re ggregted due to differences mong hitts. Study site The Coml River (29 71 N, W) is the shortest river in Texs (1.6 km length); yet its verge nnul dischrge of 9, L s -1 (Brune, 1981) forms the stte s lrgest spring complex. The river is chrcterized y stle temperture regime (22 24 C), low turidity, nd reltively constnt flows. Hydrologic ltertions to the river from oth chnneliztion nd dm construction hve converted much of the upper river from free-flowing lotic hitts to lentic ones. The impoundment of the hedwters of the river resulted in n 8.4 h reservoir (Lnd Lke) with four spring-runs tht flow into the lke (Ogden et l., 1986; Crowe & Shrp, 1997). Two spring runs of comprle size nd dischrge were chosen to serve s lotic sites (Fig. 1). Spring Run 1 (SR1) is the lrgest spring run with n verge dischrge of 6 L s -1, length of 18 m nd n verge width of 1 m. Spring Run 3 (SR3) hs n verge dischrge etween 3 nd 6 L s -1, length of 138 m, nd n verge width of 8 m (Crowe nd Shrp, 1997). The confluence (Confluence) of SR1 nd Spring Run 2 (Spring Run 2 ws not used in this study due to use s pulic swimming re) served s one lentic site nd is chnnelized, deep rech with length of 11 m nd n verge width of 2 m. The second lentic site ws chosen within Lnd Lke ner n islnd (Islnd) nd smples from this loction were tken long 1 m trnsect (Trnsect 1) nd perpendiculr 25 m trnsect etween the islnds (Trnsect 2). Methods Field smpling nd snil collections Smpling occurred in April, July, nd Octoer 21, nd Jnury nd April 22 in the two lotic (Spring Runs) nd two lentic (Confluence nd Islnd) sites. Smples were collected using strtified rndom smpling pproch to ensure tht vrious types of hitts were represented (Fig. 1). Spring Run 1 ws divided into four reches of equl lengths while SR3 nd the Confluence were divided into three reches of equl lengths (designted s upstrem, middle nd downstrem). At ech smpled loction we mesured depth (m), velocity (m s -1 ), dominnt sustrte type (modified from Sttzner et l., 1988), percent vegettion cover, nd percent detritus cover. Live snils were collected y hnd from ech of four rndomly chosen qudrts (.25 m 2 ) in ech rech of oth spring runs (SR1 n = 16, SR3 n = 12). Within ech qudrt, ll stones were overturned, surfces scrped, nd the sustrte ws gitted y hnd nd snils removed. An Eckmn gr (.2 m 2 ) ws used to smple t the Confluence nd Islnd (lentic) sites since oth re comprised minly of silt sustrtes. Two rndom loctions per rech were chosen to plce two trnsect lines. On ech trnsect, four-rndomly selected loctions were used s smpling sites (eight smples per rech). At the islnd, two trnsects were used. One trnsect ws set up cross the front of the two islnds, nd the second trnsect ws plced perpendiculr to the first trnsect etween the islnds. Five rndom Eckmn smples were collected from ech trnsect, for totl of 1 smples. Live snils from ech smple were plced in seprte continers with river wter nd trnsported on ice to the Sn Mrcos Ntionl Fish Htchery nd Technology Center lortory. Spirl shell lengths

4 32 Hydroiologi (28) 6:29 4 Fig. 1 Mp of the Coml River, Coml County, Texs, USA. Spring Run 1, Spring Run 3, nd the Confluence re shown divided into upper, middle, nd lower reches. The Islnd ws divided into two trnsects (T1 cross the islnds nd T2 etween the islnds) (mm) were mesured from the pex of the spire to the sl lip. Lengths (mm) of live plnospirl snils were mesured cross the gretest dimeter of the snil. Mortlity in trnsported snils ws ssessed y touching the foot of ech individul with proe. All snils were identified ccording to Burch (1989). Snil infection sttus Tremtodes were surveyed y removing entire digestive trcts nd gonds, smering the tissues onto microscope slide, nd viewing the smer t 19 mgnifiction with compound light microscope. Percent infection [(numer infected snils/ totl numer of snils) 9 1] ws determined for ech species of snil nd prsite. Exotic tremtode prsites were identified ccording to Chen (1948) nd Alict (1962). Tremtodes in Elimi comlensis (Prosornchi: Pleuroceride) were compred to those found y Lindholm nd Huffmn (1979) in snils of the nery spring-fed Sn Mrcos River. Initilly, evlution of prsite presence ws conducted only in M. tuercultus [ 17 mm, since this is the minimum length recorded for infection in the Coml River (Mitchell et l., 2). We oserved two dditionl species of tremtodes, P. grlli nd tenttively identified tremtode, Hplorchis pumilio* (*denotes tenttive identifiction, Fust nd Nishigori, 1926), which led to further investigtion of the

5 Hydroiologi (28) 6: presence of tremtodes in T. grnifer nd the ntive snil, E. comlensis, during smpling months of Octoer 21, nd Jnury nd April 22. Only E. comlensis [ 1 mm were exmined, since this is the minimum length recorded for infection in nery spring-fed river (Lindholm nd Huffmn, 1979), nd only T. grnifer [ 16 mm were exmined for the presence of infection. Percent infection vlues pply only to snils greter thn the minimum length used for dissection. Significnce levels were djusted with Bonferroni correction fctor (/numer of comprisons per fmily of tests) All nlyses were conducted using SYSTAT 1 (SPSS Inc. Chicgo, IL. 2) nd MINITAB (Minit Inc. Stte College, PA. 22) with significnce levels set to =.5. Distriutions of infected snils were ssessed y recording the percentges of smples with infected snils mong the four smpling sites nd for ech hitt vrile. Sttisticl nlyses Tests of interspecific ssocition (the ffinity for two species to occur in the sme loction) were used to determine if two species co-occur. Presence/sence vlues of ech species from ech smple (n = 31) were rrnged in chi-squre contingency tle following Ludwig nd Reynolds (1988). Cooccurrences were significnt with v 2 [ 3.84, =.5, such tht two species positively co-occurred if the species were found together more often thn if they were rndomly ssocited nd negtively co-occurred if the species were found together less often thn if they were rndomly ssocited (Ludwig & Reynolds, 1988). Overll differences in densities of snils etween lentic nd lotic hitts were descried using multivrite nlysis of vrince (MANOVA), tht included the fctors of smpling dte, loction (two lentic nd two lotic sites), rech (upper, middle, nd lower of ech loction), nd densities of ech snil species (numer of individuls m -2 ). To ddress sptil vritions of snil popultions t finer scle (within ech loction), single fctor nlysis of vrince (ANOVA) were conducted for ech rech of ech loction for ech species. Hitts used y snils (s descried y depth, velocity, sustrte, percent vegettion cover, nd percent detritus cover) were ssessed with one-fctor ANOVA for ech species y ech hitt vrile. Velocity nd depth were converted from continuous to ctegoricl dt sets y inning depths into.1 m increments nd velocities into.3 m s -1 increments. Snil densities of ech species nd velocities (m/s) were fourth root trnsformed to meet the ssumptions of prmetric ANOVA (Quinn & Keough, 22). Post-hoc nlyses were conducted with Tukey s HSD tests ( =.5). Results Interctions mong ntive nd exotic snils Three exotic snil species: Mris cornurities (Pulmont: Pilide), M. tuercultus nd T. grnifer, nd four ntive species: Heetncylus excentricus (Pulmont: Ancylide), Phys virgt (Pulmont: Physide), Cincinnti comlensis (Prosornchi: Hydroiide) nd E. comlensis, were collected in the upper Coml River. On verge, less thn one individul m -2 of Mris cornurities, C. comlensis, P. virgt, nd H. excentricus were collected nd no further nlyses were conducted with these species. The three species sufficient for further nlyses, M. tuercultus, T. grnifer (exotic), nd E. comlensis (ntive), hd overll men densities of 24, 298, nd 3 individuls m -2, respectively. All species comintions were significntly ssocited. Melnoides tuercultus positively co-occurred with oth T. grnifer (v 2 = 18.5, P \.1, expected vlue present = 9, oserved vlue present = 18) nd E. comlensis (v 2 = 7.3, P \.1, expected vlue present = 73, oserved vlue present = 84), while M. tuercultus negtively occurred with E. comlensis (v 2 = 1.9, P \.1, expected vlue present = 61, ctul vlue present = 47). Distriutions of snils mong lotic nd lentic sites Significnt differences mong lotic nd lentic sites (MANOVA k =.4, P \.1) nd reches within these sites (MANOVA k =.95, P =.4) were oserved mong species. Smpling dte showed no significnt influence (MANOVA k =.94, P =.29) on densities of M. tuercultus (men monthly rnge

6 34 Hydroiologi (28) 6: individuls m -2 ), T. grnifer ( individuls m -2 ), or E. comlensis (18 45 individuls m -2 ). Densities of ech species in lotic nd lentic hitts showed tht E. comlensis were found primrily in lotic spring runs (59 individuls m -2 in spring runs versus 3 individuls m -2 in the confluence nd islnd). Melnoides tuercultus ws limited to lentic sites s shown y densities tht were lmost 1 times greter in the confluence nd islnd sites (458 individuls m -2 ) thn in the spring runs (5 individuls m -2 ). Trei grnifer densities were significntly lower in lotic sites (35 individuls m -2 ) thn in lentic sites (66 individuls m -2 ). In lotic sites T. grnifer densities did not significntly differ from E. comlensis. Susequent nlyses of snil popultions within lotic nd lentic sites (i.e., mong reches of ech loction) showed tht species were not uniformly distriuted within the sites (Fig. 2) nd tht sites did not hve uniform hitt fetures (Tle 1). The reltionship of ech species to ech hitt vrile showed tht densities of E. comlensis were significntly greter in velocities [.1 m/s, depths \ 1.2 m, sustrtes of grvel size or lrger, nd detritus coverge less thn 25%. Melnoides tuercultus densities were significntly greter in velocities \.1 m/s, depths etween.3 nd 1.2 m, silt sustrtes, detritus coverge greter thn 25%, nd vegettion coverge less thn 5%. Trei grnifer densities were greter in velocities \.3 m/s, depths, etween.6 nd.9 m, silt sustrtes, detritus cover greter thn 25%, nd vegettion coverge greter thn 5% (Fig. 3). Prsite occurrence nd hitt use y infected snils Of 99 E. comlensis [ 11 mm exmined, none were infected with tremtode prsites. Thirteen of 271 (4.8%) dissected T. grnifer were found infected with Philophthlmus grlli. All infected T. grnifer were found in the Confluence. Thirty-nine of 61 (6.4%) dissected M. tuercultus were found infected with C. formosnus, ten (4.1%) were infected with P. grlli, nd four (1.6%) were infected with third heterophyid tremtode of Asin origin tenttively identified s Hplorchis pumilio* (* indictes tenttive identifiction). Infected individuls were found with only one species of tremtode. Infected snils hd ggregted distriutions. Of 31 smples collected in the study, 29 smples hd infected M. tuercultus nd 5 smples hd infected T. grnifer. No infected snils were found in SR3, nd only the lower rech of SR1 hd infected snils. All of these smples, regrdless of their loction (SR1, Confluence, or Islnd), hd similr lentic hitt chrcteristics with the mjority of smples found in velocities less thn.1 m/s, silt sustrtes, greter thn 5% vegettion nd 25% detritus cover (Tle 2). Discussion Interctions of ntive nd exotic snils nd their distriutions in lentic nd lotic hitts The results from this study nd previous survey of the upper Coml River (Cule, 1998) showed tht pulmonte snils comprised less thn 1% of the snil fun. Although there re no records of quntittive snil surveys in the Coml River efore Cule (1988), gstropod survey ws conducted y Lindholm & Huffmn (1979) in the nery upper Sn Mrcos River. The Sn Mrcos River is spring-fed river tht origintes from the sme quifer nd geologicl formtion s the Coml River, so tht the two rivers shre similr physicl nd chemicl chrcteristics (Brune, 1981; Ogden et l., 1986). The survey of the upper Sn Mrcos River ws not quntittive, ut pulmonte snils, including plnorids nd physids, were collected in sufficient densities ( individuls) for prsitology nlyses. Plnorid snils were collected only from two lentic hitts within n impoundment devoid of thirid snils (Lindholm & Huffmn, 1979). No plnorid snils were collected during our study of the Coml River. The stle, lentic hitts creted from impoundment nd chnneliztion of the upper Coml River should support higher densities of pulmonte snils. However, these stle chrcteristics likely fcilitted high popultion densities chieved y M. tuercultus nd T. grnifer tht my hve resulted in competition. Experimentl studies re needed to test if competition etween Coml River pulmontes nd exotic thirids is

7 Hydroiologi (28) 6: Fig. 2 Men (+SE) densities (numer individuls m -2 )of Melnoides tuercultus (lck rs), Trei grnifer (white rs), nd Elimi comlensis (grey rs) in the upper, middle, nd lower reches of Spring Run 1, Spring Run 3, Confluence, nd Islnd sites of the upper Coml River. Different letters ove error rs indicte significnt differences t P \.9 (Bonferroni correction). Note difference in scles etween pnels Men Density Spring Run 1 5 Confluence 4 c c Upper Middle Lower Upper Middle Lower 15 Spring Run 3 2 Islnd 15 1 Men Density c Upper Middle Lower Trnsect 1 Trnsect 2 plusile explntion for the lck of pulmonte snils in lentic hitts. In the Coml River, there hve historiclly een two endemic species of prosornchs, C. comlensis nd E. comlensis. However, very few C. comlensis were found during this study, or in Cule s (1998) survey, nd the cuses for their low numers re unknown. Elimi comlensis densities were much greter thn ll other ntive snils, leit their highest densities were 1 times less thn the highest densities of the exotic thirids, M. tuercult nd T. grnifer. Elimi comlensis nd M. tuercultus negtively co-occurred. Results from this study suggest tht the negtive ssocition etween these species is not due to competition ut rther differences in hitt use. The sence of E. comlensis from lentic hitts creted from dmming nd chnneliztion of the springs is likely due to the preference of Elimi for flowing wter hitts (Dillon, 2; Lindholm & Huffmn, 1979). Our results showed tht M. tuercultus ws generlly restricted to lentic hitts, which hs een oserved in other tropicl systems (Liu & Resh, 1997; Smdi et l., 1997; Duggn, 22). The sence of M. tuercultus from lotic hitts in the Coml River my e due to the inility of M. tuercultus to form stle popultions in lotic hitts ecuse dislodgement from sptes cuses significnt reductions in densities (Giovnelli et l., 25). Trei grnifer nd E. comlensis distriutions positively co-occurred, which is likely due to the ility of Trei grnifer is le to tolerte greter flows thn M. tuercultus (Dussrt & Pointier,

8 36 Hydroiologi (28) 6:29 4 Tle 1 Men of ll smpling dtes for velocities (±SD), depths (±SD), nd percent of smples collected from ech sustrte, vegettion coverge nd detritus coverge ctegory in reches of the lotic nd lentic sites Loction Men velocities nd depths Velocity (m/s) Depth (m) Spring Run 1.37 ± ±.14 Spring Run 3.33 ± ±.28 Confluence.7 ± ±.35 Islnd.7 ±.6.58 ±.12 Loction Percent of smples in ech sustrte ctegory Silt/snd Grvel Cole Boulder Spring Run Spring Run Confluence Islnd Loction Percent of smples in ech vegettion ctegory 25% 26 5% 51 75% 76 1% Spring Run Spring Run Confluence Islnd Loction Percent of smples in ech detritus ctegory 25% 26 5% 51 75% 76 1% Spring Run Spring Run Confluence Islnd ), which llowed these snils to persist in spring runs in the Coml River. However, the preference of T. grnifer for lentic hitts ws evident s shown y densities tht were 2 times greter in the lentic hitts (Confluence nd Islnd) thn in the spring runs. Therefore, high dischrges from the springs tht crete fster flows could restrict T. grnifer from chieving higher densities in lotic hitts where ntive E. comlensis chieve their highest densities. Conversely, when spring flow declines due to pumping of the underground quifer nd/or droughts tht cuse reduced spring flows (Bowles & Arsuffi, 1993; Crowe & Shrp, 1997), the conditions could ecome more lentic. Under incresingly lentic conditions, T. grnifer densities could increse in the reduced flows of the spring runs, inducing stronger interctions nd possile competitive exclusion of E. comlensis. Further experimentl studies re needed to test if competition cn occur etween E. comlensis nd T. grnifer. The exotic thirids positively co-occurred. While some studies hve found tht lentic hitts colonized y T. grnifer nd M. tuercultus, T. grnifer is dominnt (Pointier et l., 1998) others hve found similr numers of T. grnifer nd M. tuercultus within the sme hitts (Pointier et l., 1994). In our study, strong, positive co-occurrence nd similr densities of M. tuercultus nd T. grnifer within lentic hitts suggests tht these snils re not directly competing with ech other. Perhps differences in food preferences of M. tuercultus, tht consumes mostly detritus with some lge nd ditoms (Mdsen, 1992), nd T. grnifer, which is thought to grze lgl complexes (Chniotis et l., 198), llow similr numers of these snils to exist in the sme lentic hitts.

9 Hydroiologi (28) 6: Fig. 3 Men (+SE) densities (numer individuls m -2 )of Melnoides tuercultus (MT), Trei grnifer (TG), nd Elimi comlensis (EC) in ech velocity (m/s), depth (m), sustrte, vegettion nd detritus cover ctegory smpled from the upper Coml River. Different letters ove error rs indicte significnt differences t P \.2 (Bonferroni correction) Men Density Men Density A) Velocity D) Vegettion to.1 m/s.11 to.2 m/s 5.21 to.3 m/s.31 to.4 m/s 4.41 to.6 m/s >.6 m/s 3,c c c 6 B) Depth to.3 m 5.31 to.6 m c.61 to.9 m 4 3 c c.91 to 1.2 m 1.21 to 1.6 m 2,, E) Detrtitus -25% 26-5% 51-75% 76-1% MT TG EC c -25% 26-5% 51-75% 76-1% Men Density C) Sustrte,d c,d c,d,d c,d c,d silt/snd grvel cole oulder MT TG EC Tle 2 Percentges of the 29 smples contining Melnoides tuercultus (%MT) infected with Centrocestus formosnus, Philopthlmus grlli nd Hplorchis pumilio (tenttive identifiction) nd percentges of five smples contining Trei grnifer (%TG) infected with P. grlli in ech velocity (m/s), depth (m), sustrte, percent vegettion cover nd percent detritus cover ctegory Velocity %MT %TG Depth (m) %MT %TG Sustrte %MT %TG Vegettion(%) %MT %TG Detritus(%) %MT %TG Silt Grvel Cole Boulder [.6

10 38 Hydroiologi (28) 6:29 4 Densities nd distriutions of infected snils The presence of P. grlli nd C. formosnus within the upper Coml River ws expected since P. grlli ws reported y Murry & Hines (1969) in oth T. grnifer nd M. tuercultus, nd C. formosnus infecting M. tuercultus ws recently documented in the Coml River y Mitchell et l. (2). However, the presence of the third tenttively identified Asin tremtode, H. pumilio*, tht infects muscle tissue of fishes, hs not een previously documented within the Coml River. Hplorchis pumilio* ws recently reported in M. tuercultus nd fishes in the fmily Cyprinodontide (Scholz et l., 21) in Mexico nd could e moving into regions of the southern United Sttes vi migrtory wding irds tht serve s the definitive hosts. The estlishment of tremtodes in novel molluscn hosts is rre due to the host specificity of tremtodes for specific snils (Cri et l., 21). Therefore we did not expect E. comlensis to e infected with ny exotic tremtodes. Although lotic hitts were ville in this study, the distriutions of M. tuercultus infected with C. formosnus were ggregted into lentic hitts with the confluence yielding the highest levels of infection snils. The mjority of infected snils were found in flows less thn.1 m/s, silt sustrtes, greter thn 5% vegettion nd 25% detritus coverge. Studies tht focused on tremtode infection within snils in lentic hitts showed tht infected snils were ggregted due to hitt conditions. Jokel nd Lively (1995) found tht shllower depths nd nerness to nks significntly influenced the presence of tremtode infection within Potmopyrgus ntipodrum. Willims & Esch (1991) found most infected Helisom nceps in shllow depths nd in sustrtes comprised primrily of leves. These microhitt vriles my e necessry for successful trnsmission of eggs into the snil hosts, or the success of other stges of prsite development, tht cuse the ggregted ptterns of infected snils oserved in this study. Conclusions Owing to the lck of historicl gstropod surveys in the Coml River, it is unknown if the sence, or very low densities (less thn 1 individul m -2 ), of five endemic snil species historiclly found in the river (Cule, 1998) were due to the introduction nd successful competition y the exotic M. tuercultus nd T. grnifer, or other fctors. The positive cooccurrence of M. tuercultus nd T. grnifer densities within lentic hitts suggests tht these species re not competing with ech other. Melnoides tuercultus nd E. comlensis generlly did not co-occur, s M. tuercultus ws mostly found in lentic hitts nd E. comlensis in lotic hitts. The positive co-occurrence of T. grnifer nd E. comlensis suggests tht these species re not competing. However, this study ws conducted during period of norml spring flows so tht the high flow velocities in lotic hitts, where E. comlensis mintins the highest densities, my hve prohiited T. grnifer from reching densities gret enough to strongly interct with E. comlensis. Declining flows from pumping of the quifer in concert with droughts could result in n opportunity for T. grnifer to rech high densities in the lotic hitts incresing interction with E. comlensis popultions. We recommend conducting lortory experiments to determine if competitive exclusion is possile etween E. comlensis nd T. grnifer. Our dt suggests tht lentic hitts re structuring infection hot-spots s indicted y the ggregted distriutions of infected T. grnifer nd M. tuercultus. Thus, monitoring efforts of exotic tremtode invsions to wrm-wter spring systems should e focused on lentic hitts where highest levels of infection re most likely to occur. These results re importnt for mngement considertions of wrm-wter spring systems, such s the Coml River, where reductions in dischrge due to drought or over-pumping of groundwter crete lentic conditions more suitle for incresed densities of exotic snils nd exotic prsites. These fctors my led to negtive consequences for ntive snils nd fishes, not only in the Coml River, ut in other spring-fed systems experiencing hitt modifictions, declines in spring-flows nd invsions of exotic snils nd their concomitnt prsites. Acknowledgments Funding for this project ws provided y the U.S. Fish nd Wildlife Service. We thnk NFHTC stff nd volunteers including: Vl Cntu, Lynley Doyen, Rndy Gison, Lene Griego, Ben Grod, Connie Johnson, nd Jenn Lueg nd for their ssistnce with dt collection nd dt entry. We lso thnk Michel Longnecker nd John Swek for

11 Hydroiologi (28) 6: ssistnce with experimentl design nd nlysis. Tom Brndt, Joe Fries, Mike Levy, Richrd Mnning, Jen-Pierre Pointier, Roin Overstreet, nd Tomáš Scholz grciously reviewed proposls nd provided helpful suggestions throughout the course of the study. Tom Brndt, Michel Chdwick, Joe Fries, Alexnder Huryn, nd Heidi Rntl provided mny useful comments nd suggestions to previous versions of the mnuscript. The comments of two nonymous reviewers gretly improved the qulity of this mnuscript. The views in this rticle re the uthors nd do not necessrily reflect the views of the US Fish nd Wildlife Service. References Alict, J. E., Life cycle nd developmentl stges of Philophthlmus grlli in the intermedite nd finl hosts. Journl of Prsitology 48: Anderson, R. M. & R. M. My, Prevlence of schistosome infections within molluscn popultions: Oserved ptterns nd theoreticl predictions. Prsitology 79: Bowles, D. E. & T. L. Arsuffi, Krst qutic ecosystems of the Edwrds Plteu region of centrl Texs, USA. A considertion of their importnce, threts to their existence, nd efforts for their conservtion. Aqutic Conservtion: Mrine nd Freshwter Ecosystems 3: Brune, G., Springs of Texs, Vol. I. Brnch-Smith, Inc. Fort Worth, Texs. Burch, J. B., Freshwter Snils of North Americ. USEPA 6/ Cule, M. K., Aundnce nd distriution of the freshwter snil fun in Coml Springs-Lnd Lke, New Brunfels, Coml Co., TX. Msters Thesis. Southwest Texs Stte University, Sn Mrcos. Chniotis, B., J. M. Butler, Jr., F. Ferguson & W. Join, 198. Bionomics of Trei grnifer (Gstropod: Thiride) in Puerto Rico, n Asitic vector of Prgonimisis westermni. Crien Journl of Science 16: Chen, H. T., Some erly lrvl stges of Centrocestus formosnus (Nishigori, 1924). Lingnn Science Journl 22: Cri, T. H., R. A. Bry & D. T. J. Littlewood, 21. The nture nd evolution of the ssocition mong digenens, molluscs nd fishes. Interntionl Journl for Prsitology 31: Crowe, J. C. & J. M. Shrp, Hydrogeologic delinetion of hitts of endngered species: The Coml Springs/ River System. Environmentl Geology 3: Dillon, R. T., 2. The ecology of freshwter molluscs. Cmridge University Press, Cmridge: Duggn, I. C., 22. Short communiction: First record of wild popultion of the tropicl snil Melnoides tuercult in New Zelnd nturl wters. New Zelnd Journl of Mrine nd Freshwter Reserch 36: Dussrt, G. B. J. & J.-P. Pointier, A comprtive study of the hydrodynmic performnce of shells of Trei grnifer (Lmrck, 1822) nd Melnoides tuercult (Müller, 1774). Mlcologicl Review, Supplement 8: Fust, E. C. & M. Nishigori, The life cycles of two new species of Heterophyide, prsitic in mmmls nd irds. Journl of Prsitology 13: Font, W. F., 23. The glol spred of prsites: Wht do Hwiin strems tell us? Bioscience 53: Giovnelli, A., C. d Silv, G. Lel & D. Bptist, 25. Hitt preference of freshwter snils in reltion to environmentl fctors nd the presence of the competitor snil Melnoides tuercultus (Müller, 1774). Memóris do Instituto Oswldo Cruz 1: Hll, R. O., J. L. Tnk & M. F. Dydhl, 23. Exotic snils dominte nitrogen nd cron cycling in highly productive strem. Frontiers in Ecology nd the Environment 1: Jokel, J. & C. M. Lively, Sptil vrition in infection y digenetic tremtodes in popultion of freshwter snils (Potmopyrgus ntipodrum). Oecologic 13: Kziwe, F., B. Mkng, C. Ruire-Akiiki, J. Oum & C. Kriuki, 26. Ecology of iomphlri (Gstropod: Plnoride) in Lke Alert, Western Ugnd: Snil distriutions, infections nd temporl ssocitions with environmentl dynmics. Hydroiologi 568: Kerns, B. L., M. F. Dydhl, M. M. Gngloff & J. E. Jnnot, 25. Potmopyrgus ntipodrum: Distriution, density, nd effects on ntive mcroinverterte ssemlges in the Greter Yellowstone ecosystem. Journl of the North Americn Benthologicl Society 24: 138. Lindholm, J. T. & D. G. Huffmn, The gstropods of the upper Sn Mrcos River nd their tremtode prsites. Reserch Report No from the Aqutic Sttion. Southwest Texs Stte University, Sn Mrcos. Liu, H. T. & V. H. Resh, Aundnce nd micro distriution of freshwter gstropods in three strems of Moore, French Polynesi. Annls de Limnologie 33: Ludwig, J. A. & J. F. Reynolds, Sttisticl Ecology: A Primer on Methods nd Computing. John Wiley & Sons, New York: Mdsen, H., Food selection y freshwter snils in the Gezir irrigtion cnls, Sudn. Hydroiologi 228: McDermott, K., 2. Distriution nd infection reltionships of n undescried digenetic tremtode, its exotic intermedite host, nd endngered fishes in springs of west Texs. Msters Thesis. Southwest Texs Stte University, Sn Mrcos. Mitchell, A. J., M. J. Slmon, D. G. Huffmn, A. E. Goodwin & T. M. Brndt, 2. Prevlence nd pthogenicity of heterophyid tremtode infecting the gills of n endngered fish Etheostom fonticol in two centrl Texs spring-fed rivers. Journl of Aqutic Animl Helth 12: Murry, H. D. & D. Hines, Philophthlmus sp. (Tremtod) in Trei grnifer nd Melnoides tuercultus in South Texs. Annul Report to the Americn Mlcologicl Union: Nollen, P. M. & H. D. Murry, Philophthlmus grlli: Identifiction, growth chrcteristics, nd tretment of n

12 4 Hydroiologi (28) 6:29 4 Orientl eyefluke of irds introduced into the continentl United Sttes. Journl of Prsitology 64: Ogden, A. E., R. A. Quick & S. R. Rothermel, Hydrochemistry of the Coml, Hueco, nd Sn Mrcos Springs, Edwrds Aquifer, Texs (pp ). In Aott, P. L. & C. M. Woodruff, Jr. (eds), The Blcones Escrpment, Geology, Hydrology, Ecology, nd Socil Development in Centrl Texs. Geologicl Society of Americ Annul Meeting, Sn Antonio, Texs, 1 2. Pointier, J. P., Invding freshwter gstropods: Some conflicting spects for pulic helth. Mlcologi 41: Pointier, J. P., A. Theron & G. Borel, Ecology of the introduced snil Melnoides tuercult (Gstropod: Thiride) in reltion to Biomphlri glrt in the mrshy forest zone of Gudeloupe, French West Indies. Journl of Molluscn Studies 59: Pointier, J. P., R. N. Incni, C. Blzn, P. Chrosciechowski & S. Prypchn, Invsion of the rivers of the littorl centrl region of Venezuel y Thir grnifer nd Melnoides tuercult (Mollusc: Prosornchi: Thiride) nd the sence of Biomphlri glrt, snil host of Schistosom mnsoni. Nutilus 17: Pointier, J. P., S. Smdi, P. Jrne & B. Dely, Introduction nd spred of Thir grnifer (Lmrk, 1822) in Mrtinique, French West Indies. Biodiversity nd Conservtion 7: Quinn, G. P. & M. J. Keough, 22. Experimentl Design nd Dt Anlysis for Biologists. University Press, Cmridge: Rder R. B, M. C. Belk & M. J. Keleher, 23. The introduction of n invsive snil (Melnoides tuercult) to spring ecosystems of the Bonneville Bsin, Uth. Journl of Freshwter Ecology 18: Smdi, S., C. Blzn, B. Dely & J. P. Pointier, Locl distriution nd undnce of thirid snils in recently colonized rivers from the Crrien re. Mlcologicl Review 3: Spp, K. K. & G. W. Esch, The effects of sptil nd temporl heterogeneity s structuring forces for prsite communities in Helisom nceps nd Phys gyrin. Americn Midlnd Nturlist 132: Scholz, T. & G. Slgdo-Mldondo, 2. The introduction nd dispersl of Centrocestus formosnus (Nishigori, 1924) (Digene: Heterophyide) in Mexico: A review. Americn Midlnd Nturlist 143: Scholz, T., M. L. Aguirre-Mcedo & G. Slgdo-Mldondo, 21. Tremtodes in the fmily Heterophyide (Digene) in Mexico: A review of species nd new host nd geogrphicl records. Journl of Nturl History 35: Smith, N. F., 21. Sptil heterogeneity in recruitment of lrvl tremtodes to snil intermedite hosts. Oecologi 127: Sttzner, B., J. A. Gore & V. H. Resh, Hydrulic strem ecology: Oserved ptterns nd potentil pplictions. Journl of the North Americn Benthologicl Society 7: Stryer, D. L., Effects of lien species on freshwter mollusks in North Americ. Journl of the North Americn Benthologicl Society 18: Trschewski, H., 26. Hosts nd prsites s liens. Journl of Helminthology 8: Willims, J. A. & G. W. Esch, Infr- nd component dynmics in the pulmonte snil Helisom nceps, with specil emphsis on the hemiurid tremtode Hlipegus occidulis. Journl of Prsitology 77: