DOI: 10.2478/s11686-008-0011-1 © 2008 W. Stefañski Institute of Parasitology, PAS Acta Parasitologica, 2008, 53(1), 85–92; ISSN 1230-2821 The effects of the trematode polymorphus on the reproductive cycle of the polymorpha in the Drava River

Jasna Lajtner1*, Andreja Luciæ1, Miljenko Marušiæ2 and Radovan Erben1 1Department of Zoology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, HR-10000 Zagreb; 2Department of Mathematics, University of Zagreb, Bijenièka 30, HR-10000 Zagreb; Croatia

Abstract The effects of the trematode on the reproductive cycle of the zebra mussel Dreissena polymorpha were examined in mussel populations from the Drava River. The reproductive cycle was studied by histological examination of the gonads and quantified by an image analysing system to determine changes in volume of the entire visceral mass, gonads, digestive glands and in particular the volume of trematodes. Results confirmed that (1) gonads of D. polymorpha were affect- ed by B. polymorphus infection more than any other organ and (2) development of cercariae in sporocysts of B. polymorphus coincides with host gonad maturation. This is the first study in which the image analysing system was used to determine the effect of trematodes on the reproductive cycle of D. polymorpha. Also, this is the first record of sporocysts of B. polymorphus in D. polymorpha in this part of Europe.

Keywords Bucephalus polymorphus, Dreissena polymorpha, sporocyst, cercariae, Drava River, Croatia

Introduction Many studies of biology and ecology of D. polymorpha have been conducted (Morton 1969, Walz 1973, Stañczykow- The freshwater mussel Dreissena polymorpha (Pallas, 1771) ska 1977, Lewandowski 1982). However, investigations of has become one of the most dominant species in many lakes diversity, distribution and significance of endoparasites of and rivers of Europe, since it began spreading from the Cas- D. polymorpha have become important only during the past pian area in the early 19th century (Stañczykowska 1977). In decade. These studies include Dreissena’s mantle-cavity cili- the mid-1980s, it could occasionally be found in the Great ates (Laruelle et al. 1999, Burlakova et al. 2000, Karatayev et Lakes of North America where it is suspected to have entered al. 2000), intracytoplasmic prokaryotes (Molloy et al. 2001) via ballast water discharge (Hebert et al. 1989), and today and endosymbiont assemblages detected in Dreissena popu- D. polymorpha spreads rapidly over the Nearctic region. This lations in Russia (Kuperman et al. 1994, Molloy et al. 1996) success of D. polymorpha could be attributed to the ability of and Belarus (Karatayev et al. 2000). adults to adhere to hard surfaces with their byssus, to devel- To date, seven genera of trematodes parasitic in D. poly- opment by free-swimming veliger larvae, and to their extraor- morpha have been described: Bucephalus (, Buce- dinarily high fecundity (Stañczykowska 1977, Borcherding phalidae), Phyllodistomum (Gorgoderidae), Echinoparyphium 1991). and Echinostoma (Echinostomatidae), Sanguinicola (Sangui- In Croatia, D. polymorpha is a new species which began to nicolidae), Leucochloridiomorpha (Brachylaemidae) and As- colonize the Drava River ecosystem during the 1980s (Erben pidogaster (Aspidogastrea, Aspidogastridae) (Conn and Conn et al. 2000, Lajtner et al. 2004). Since then, it has spread 1995, Molloy et al. 1997, Laruelle et al. 2002). upstream to the town of Varaždin and this process is still in In the life cycle of Bucephalus polymorphus (Baer, 1827), progress (Lajtner et al. 2004). D. polymorpha is the first intermediate host (Molloy et al.

*Corresponding author: [email protected] 86 Jasna Lajtner et al.

Œl¹ski

1997). The life cycle starts with the development of the earli- digestive gland, byssus gland, foot, parts of the adductor mus- est larval stage, the miracidium, which enters the mantle cav- cle) were separated from the remaining tissues (gills, mantle, ity of the mussel via water and penetrates the visceral mass, heart, kidneys, parts of adductor muscle), fixed in Bouin’s fix- especially in the gonad area. Here, the miracidium develops ative, dehydrated in an ascending alcohol series and chloro- into the sporocyst which is of irregular shape with many form and embedded in Paraplast Plus (Sigma P-3683, melt- branches. The next developmental stage, the cercaria, devel- ing point 56°C). Each visceral mass was completely cut in ops within the sporocyst. After completing their development, transverse sections (10 µm); thereby enabling us to record the the cercariae exit the mussel mantle cavity and enter the sec- length of the visceral mass (distance from the first to the last ond intermediate host via water. Most frequently, these hosts section). During this process, 20 sections were taken along the are cyprinid , in which the metacercarial stage develops. whole visceral mass, stained with Mayer’s haemalaun (Merck) The final hosts are also fishes, infected with metacercariae and eosin and mounted in Canada balsam (Romeis 1968). The while eating infected . Metacercariae develop into adult stage of gametogenic development was described using a four- trematodes and become sexually mature. After fertilisation, a step qualitative evaluation, as reported by Gist et al. (1997): new life cycle starts (Molloy et al. 1997). stage 0 = gonad inactive; stage 1 = developing; stage 2 = pre- This paper focuses on infections of B. polymorphus in spawn; stage 3 = postspawn. D. polymorpha and is a further contribution towards under- For ten mussels, the areas of the whole visceral mass, go- standing the endoparasites of this mussel. It presents histo- nads, digestive gland and trematodes were measured using an logical photomicrographs illustrating the precise location of image analysing system (LUCIA G 4.81) according to Bor- trematodes in the visceral mass. Also, by measuring the vol- cherding (1991). To calculate volume, the mean tissue areas of ume of the gonads, digestive glands and other organs of the each mussel were multiplied by the corresponding length of visceral mass and comparing these data to data for uninfect- the visceral mass. In addition, indices of gonad, digestive ed mussels, an attempt was made to determine the impact of gland and other tissues of the visceral mass were calculated trematodes on the reproductive cycle of D. polymorpha. as the percentage of tissue to visceral mass volume (Borcherd- ing 1991).

Materials and methods Statistical analyses All statistical analyses were carried out using SAS System Field sampling for Windows. The MANOVA test was used to test for differ- Mussels were collected on eight field trips from January to ences between infected and uninfected mussels. The Wilcox- December 2000, each of approximately 35 to 40 days. The on rank sum test was used to establish differences for a par- collecting site was immediately offshore of the Drava River ticular variable. P-values less than 0.05 (p<0.05) were con- (46°19′N, 16°44′E), approximately 1 km downstream of the sidered statistically significant. dam of the Dubrava hydro-electric power plant, at depths ranging from 0.3 to 0.5 m. Current velocity was generally lower than 0.50 m/s. Mussels were scraped from small rocks, Results collected by wading. On each sampling date, 50 specimens of D. polymorpha were collected. Examination of the histological preparations indicated that of Physical and chemical parameters including water tem- 80 analysed mussels, 17 (21.3%) were infected with B. poly- perature, °C (range 8.20–22.00), pH (range 7.70–8.15), dis- morphus. Twelve (70.6%) of these were females, three were solved oxygen, mg/l (range 8.20–16.70), calcium, mg/l (range males (17.6%) and two (11.8%) were hermaphrodites. Infect- 38.40–46.40), total hardness, mg CaCO3/l (range 132.00– ed mussels were found at each sampling date. 168.00), alkalinity, ml 0.1 N HCl/1-m (2.30–2.70) and micro- biological parameters, chlorophyll a, mg/m3 (range 0.06–2.52) Reproductive cycle of Dreissena polymorpha were recorded at each sampling date. Gametogenesis (oogenesis and spermatogenesis) (gonad stage 1) started in autumn and continued through winter and Histological procedures and tissue measurements early spring. Gonad development was accelerated in spring Shell length, shell width and shell height of all mussels were with the rise of water temperature while first mature oocytes measured with vernier callipers. Afterwards, thirty randomly- and spermatozoa were recorded already in March (gonad stage selected mussels were submitted for length-dry mass analysis. 2). Spawning occurred in May and June, which was confirmed The results of these analyses are being prepared separately for by a large number of mussels at gonad stage 3. Temperature publication. Other mussels, with an approximate shell length and sufficient food resources are the main triggers that induce of 26.0 ± 2 mm, were chosen for histological analysis. The synchronized and massive spawning (Borcherding 1991, Gist visceral mass of mussels (containing gonads, stomach, gut, et al. 1997, Wacker and von Elert 2003). Already in April, the The effects of Bucephalus polymorphus on D. polymorpha 87

Stanis³a water temperature in the Drava River was above 12°C, while Site selection of the parasites chlorophyll a values were at the highest level (2.52 mg/m3). Bucephalus polymorphus was identified in histological sec- Mussels in the rest stage (gonad stage 0) were already tions by the shape of its sporocysts, its position in the mussel found in August and some mussels remained in this stage until body and the morphological characteristics of the cercariae January. The greatest number of mussels in the rest stage was according to Laruelle et al. (2002) (Fig. 1). observed in October.

Fig. 1. Gonads of Dreissena polymorpha filled with sporocyst branches containing several developmental stages of cercariae of Bucephalus polymorphus: A. Accumulation of haemocytes at the site of first infection. B. Early stage of cercarial development. C. Late stage of cercar- ial development. D-E. Hermaphrodites of D. polymorpha. Abbreviations: bt – bifurcate tail, c – well-developed cercaria, ct – connective tis- sue, e – early stage of cercarial development, ff – female follicle, h – haemocytes, hf – hermaphroditic follicle, mf – male follicle, o – ovum, s – sporocyst, se – sporocyst epithelium, uic – unbifurcated intestinal caecum. Scale bars = 100 µm. Collection dates of sectioned mussels: 1A – on 25 October, 1B and 1D – on 25 March, 1C – on 5 July, 1E and 1F – on April 27 88 Jasna Lajtner et al.

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Infection started in the connective tissue of the visceral metogenesis (gonad stage 1) while all other females from this mass before spreading towards the gonads. In some speci- site were already in the prespawn stage (gonad stage 2). Male mens, an accumulation of haemocytes was observed around follicles occupied most of the gonad, especially in the poste- the infection site, indicating activation of the mussel defensive rior part of the body where no trematodes were detected. mechanisms (Fig. 1A). The sporocyst had numerous branches which were visible as tubules of irregular shape in which cer- Morphometric analysis cariae were developing (Fig. 1B, C). Also, young cercariae Mean values of all morphometrical measurements of unin- were dominant in mussels sampled during autumn and winter fected mussels are presented in Table I. During the winter (Fig. 1B) while mature cercariae were dominant in mussels months, when the gonads of most mussels were in the early sampled in spring and summer (Fig. 1C). In mature cercariae, gametogenesis stage (gonad stage 1), gonad volumes were unbifurcated intestinal caecum and forked tail were visible small, while in spring gonad volumes increased with the (Fig. 1C). In all analysed mussels, the gonads were the organs increase in water temperature. The largest gonad volume was most affected, which was also confirmed by morphometrical measured in April, just before the beginning of the spawning measurements. (gonad stage 2), followed by a decline of gonad volume Both hermaphrodites found in the sample were also infect- (gonad stage 3). The smallest gonad volumes were measured ed by trematodes (Fig. 1D-F). It was interesting that some in October and December when gonads of most mussels were gonad follicles included only oocytes and some included only in the inactive stage (gonad stage 0) (Table I). spermatozoa; however, mixed “hermaphrodite” follicles were Lengths of the visceral mass and other measured volumes also observed and included both oocytes and spermatozoa were highest in April. After spawning, all measured volumes (Fig. 1F). Of the hermaphrodite mussels sampled in March, decreased while the lowest values were measured in mussels females were dominant while in mussels sampled in April, in the resting stage (Table I). males were dominant. This was also confirmed by analysis of In mussels infected by trematodes, the greatest volumes gonad volume in these specimens. Mussels sampled in March were also measured in April (Table II), as was the highest vol- were undergoing gametogenesis (gonad stage 1) as were the ume of trematodes. The minimum length and volume of vis- other mussels from the same sampling site (Fig. 1D). In spec- ceral mass and gonad volume were established in December imens sampled in April (Fig. 1E), follicles were filled with when all other measured volumes were also at lower, though mature spermatozoa (gonad stage 2) which also characterized not minimum, values. all other males from the same site. However, it was interesting Analysis of the hermaphrodite found in March indicated that oocytes from the same sample were just undergoing ga- that the gonad volume was 15.2 mm3 (13.1%) while trema-

Table I. Mean values (± SD) of length of visceral mass (LVS), visceral mass volume (VSV), gonad volume (GV), digestive gland volume (DGV) and volume of other tissues of visceral mass (VOT) of mussels Dreissena polymorpha

Month n LVS (mm) VSV (mm3) GV (mm3) DGV (mm3) VOT (mm3) 1 9 10.3 ± 0.5 91.0 ± 15.5 49.0 ± 11.6 21.1 ± 4.1 20.9 ± 4.1 3 5 11.2 ± 1.6 129.6 ± 45.8 80.3 ± 39.2 29.3 ± 6.2 20.0 ± 4.5 4 8 14.9 ± 0.9 288.8 ± 31.4 211.3 ± 24.9 48.7 ± 9.3 28.8 ± 4.3 6 9 13.4 ± 1.4 234.2 ± 88.5 170.8 ± 84.1 35.7 ± 9.3 27.7 ± 3.8 7 8 11.0 ± 0.6 108.5 ± 30.7 64.1 ± 24.1 24.6 ± 6.5 19.8 ± 2.7 8 7 9.8 ± 0.9 80.8 ± 18.3 41.8 ± 12.2 18.9 ± 4.0 20.1 ± 2.8 10 9 10.2 ± 1.2 86.6 ± 20.4 46.6 ± 17.5 19.6 ± 4.5 20.6 ± 4.2 12 8 9.7 ± 1.3 77.6 ± 24.9 39.7 ± 17.4 18.1 ± 5.6 19.8 ± 4.0

Table II. Mean values (± SD) of length of visceral mass (LVS), visceral mass volume (VSV), gonad volume (GV), digestive gland volume (DGV), volume of other tissues of visceral mass (VOT) and volume of trematodes (VT) of Dreissena polymorpha infected with Bucephalus polymorphus

Month n LVS (mm) VSV (mm3) GV (mm3) DGV (mm3) VOT (mm3) VT (mm3) 1 1 11.4 113.6 11.6 24.2 29.6 48.2 3 5 11.8 ± 1.3 145.4 ± 32.8 19.4 ± 18.7 24.9 ± 6.4 22.2 ± 2.9 78.9 ± 38.3 4 2 12.8 ± 2.0 217.0 ± 111.6 22.6 ± 31.8 45.6 ± 27.5 29.4 ± 18.3 119.4 ± 97.6 6 1 11.8 160.7 20.6 27.5 16.3 96.3 7 2 10.9 ± 0.1 101.3 ± 1.2 7.5 ± 5.4 16.9 ± 2.5 15.7 ± 3.7 61.2 ± 5.4 8 3 11.6 ± 0.6 128.9 ± 12.8 10.4 ± 11.9 16.2 ± 4.2 20.5 ± 3.1 81.8 ± 7.0 10 1 10.9 108.5 34.4 20.2 21.4 32.5 12 2 10.3 ± 0.6 97.5 ± 12.3 0.8 ± 0.2 17.6 ± 3.4 16.8 ± 2.3 62.3 ± 11.0 The effects of Bucephalus polymorphus on D. polymorpha 89

Fig. 2. Relative frequencies of gonad index (GI), digestive gland Fig. 3. Relative frequencies of index of trematodes Bucephalus poly- index (DGI) and index of other tissues of visceral mass (IOT) of non- morphus (IT), gonad index (GI), digestive gland index (DGI) and infected Dreissena polymorpha index of other tissues of visceral mass (IOT) of infected Dreissena polymorpha

tode volume was 61.5 mm3 (52.9%). “Female” follicles occu- lowest gonad index was detected in December (51.2%). The pied 11.4 mm3 (75.3%), male follicles occupied 3.3 mm3 digestive gland index did not change significantly during the (22%) while hermaphroditic follicles occupied only 0.5 mm3 year, except in April and May, while the index of other tissues (2.7%) of visceral mass. gradually increased toward their highest values in December In the hermaphrodite sampled in April, gonad volume was (25.5%). Relative frequencies of the gonad index, digestive 45.1 mm3 (32.7%) while trematode volume was 50.4 mm3 gland index and index of other tissues of visceral mass are (36.5%). Within the overall gonad volume, male follicles shown in Figure 2. occupy as much as 43 mm3 (95.4%); female follicles occupy Comparison of indices between months indicated that in 1.4 mm3 (3.2%) while hermaphroditic follicles occupy only infected mussels, the trematode index was highest among all 0.6 mm3 (1.4%). indices in all observed months, except in October when the Comparison of the mean values in Tables I and II show gonad index value was somewhat higher than the trematode that the mean values of length and volume of visceral mass index (30% and 31.7%, respectively). Values of the digestive were larger in mussels infected with trematodes in all months, gland indices and indices of other tissues of the visceral mass except in April, June and July when spawning occurs. were lower in infected mussels than in healthy mussels in all In addition to volume, the gonad index, index of the diges- observed months. However, the greatest impact was observed tive gland and other tissues of the visceral mass were also in the gonad index. Relative frequencies of the gonad index, compared among mussels. Gonad index values were highest digestive gland index, index of other tissues of visceral mass in April and June (73.2% and 72.9%, respectively), at the time and index of trematodes are presented in Figure 3. when the lowest values of the digestive gland index (16.9% The applied MANOVA test has shown that infected and and 15.2%, respectively) and index of other tissues of viscer- uninfected mussels differ significantly (p<0.001). The Wil- al mass were measured (10% and 11.8%, respectively). The coxon rank sum test established differences for particular vari-

Table III. Comparison between mean values (± SD) of mussels Dreissena polymorpha infected with Bucephalus polymorphus and mussels without trematodes (groups differ significantly, p<0.001 for MANOVAtest)

Mussels without Mussels infected with p1 trematodes (n = 63) B. polymorphus (n = 17) Length of visceral mass (mm) 11.3 ± 2.1 11.5 ± 1.1 ns2 Visceral mass volume (mm3) 138.4 ± 86.5 136.9 ± 48.8 ns Gonad volume (mm3) 89.1 ± 72.8 15.0 ± 15.7 <0.0001 Digestive gland volume (mm3) 26.9 ± 11.9 23.9 ± 12.0 ns Volume of other tissues of visceral mass (mm3) 22.4 ± 5.1 21.4 ± 6.8 ns Gonad index (%) 58.4 ± 11.4 11.9 ± 12.7 <0.0001 Digestive gland index (%) 21.7 ± 5.2 17.1 ± 2.9 0.0002 Index of other tissues (%) 19.9 ± 7.1 16.1 ± 3.8 0.0243

1p-value for Wilcoxon rank sum test; 2ns – not significant. 90 Jasna Lajtner et al.

ables (Table III). The gonad volume of infected mussels is sig- er than the gonad volume of uninfected mussels. On the con- nificantly lower than that of uninfected (p<0.0001, trary, other measured volumes were not significantly different Wilcoxon rank sum test). Variables such as length of the vis- between infected and uninfected mussels. Measured values of ceral mass, total volume of the visceral mass, volume of diges- the gonad index also confirm that gonads are the target organ tive glands and volume of other tissues of the visceral mass of trematode infection. were not statistically significant different between infected Molloy et al. (1997) assumed that cercariae enter the sur- and uninfected mussels. Comparison of the indices indicated rounding water over a longer period of time, which also has that all values were significantly lower in infected mussels been confirmed by our study of zebra mussels from the Drava than in uninfected mussels and that the greatest difference was River. It is interesting that, in our sample, young cercariae among the gonad indices of the two groups of mussels predominate in mussels sampled in autumn while mature cer- (p<0.0001, Wilcoxon rank sum test). This indicates that go- cariae predominate in spring and summer. Baturo (1978) nads were attacked by trematodes more than any other organ showed that the release of cercariae is positively correlated of the visceral mass. with water temperature, i.e. as water temperature increases, the greater the number of cercariae released into the water. The same author considered that this process is also connect- Discussion ed with the development of fry, which are the subsequent hosts and are intensively developing at this time of year. Although geographically widely distributed, infection of B. Moreover, in spring and in early summer, when the cercariae polymorphus in zebra mussel populations is not common and are released into the water, the spawning of zebra mussels in the prevalence of infection can vary widely. During this study, the Drava River occurs. Therefore, it is possible that cercar- a prevalence of 21.3% infected zebra mussels was recorded in iae development is synchronized with the mussel reproductive the Drava River. In the most extensive field study conducted cycle. to date, very high prevalence rates (up to 73%) were record- There are several possible explanations as to why trema- ed in D. polymorpha in South-Eastern France (Wallet and todes are primarily located in the mussel gonads. First, fat and Lambert 1986). However, low to moderate rates of prevalence glycogen content of uninfected bivalves is markedly higher in of infection have been typically reported: 1% (Kuperman et the gonad than in the mantle, gills, foot or the rest of the soft al. 1994), 1–4% (Baturo 1977), 2–5% (Smirnova and Ibra- tissues (Jokela et al. 1993). Second, the gonad is assumed to sheva 1967), 9% (Molloy et al. 1996) and 13–28% (de Kin- be less critical for survival of the host in comparison with the kelin et al. 1968). gills, digestive gland and kidney region, including the heart, Gonads were the primary organs infected by trematodes of all of which have vital functions (Jokela et al. 1993). There- the family , as described by Laruelle et al. (2002). fore, by locating in the gonad, the parasites may combine Strong infection by these trematodes led to complete castra- low risk of host mortality with the availability of abundant tion and to host sterility, as well as to occurrence of protandry energy (Taskinen et al. 1997). The authors found that Ano- and hermaphroditism (Tripp 1973, Stadnychenko 1974, Bow- donta piscinalis infected with Rhipidocotyle species do not er et al. 1994, Laruelle et al. 2002). Image analysis of the reproduce due to parasitic castration, but that the uninfected histological sections of the zebra mussel in our study con- individuals develop their offspring, presumably, during the firmed that the gonads were the organs predominantly attack- period of most abundant resources in the seasonally-fluctuat- ed by the trematodes. Moreover, as both D. polymorpha her- ing environment. The authors confirmed that, if the parasites maphrodites found in the Drava River were infected by trema- adjust their reproduction to the same period as the uninfected todes, it might be thought that this condition is induced by clams, they minimize mortality caused by an energetic stress trematode infection. However, the number of hermaphrodites of the host, which has also been confirmed in this study. As is insufficient to prove such an assumption with statistical gonads of the zebra mussel from the Drava River were most methods and further investigation is required. infected compared to other vital organs, we believe that infec- In the majority of infected mussels, the digestive gland, tion is not lethal for the mussel and could be prolonged into very close to the gonads, was not affected by trematode infec- the next year. tion and histologically did not differ from digestive glands of Survival of the host is important for the trematodes as uninfected mussels. Identical circumstances were found in bivalves cannot get rid of bucephalid trematodes once infec- other organs of the visceral mass. Similar results were found tion has occurred (Lauckner 1983). Infection is maintained by Molloy et al. (1996) and Laruelle et al. (2002). Our find- from one year to the next with infected gonads producing cer- ings once again confirm the presumption that the gonads are cariae instead of gametes (Taskinen et al. 1994). In research the primary site of trematode infection. dealing with this problem, long-lived bivalves were mostly In this study, computer image analysis was applied for the studied (e.g., A. piscinalis which lives 17 to 19 years). D. poly- first time to obtain quantitative data on the negative effect of morpha can live up to 7 years; however, most European pop- B. polymorphus on the reproductive cycle of the zebra mussel. ulations live around four to five years (Stañczykowska 1977). Analysis of the volumes of visceral mass organs indicates that North American populations live only one and half to two the gonad volume of infected animals was significantly small- years (Mackie and Schloesser 1996). Based on our findings, The effects of Bucephalus polymorphus on D. polymorpha 91

we conclude that although D. polymorpha does not belong to sena polymorpha in the Drava river (Croatia). Biologia (Bra- the long-lived bivalves, the trematode B. polymorphus settles tislava), 59, 595–600. and develops only in the mussel gonads, which are less harm- Laruelle F., Molloy D.P., Fokin S.I., Ovcharenko M.A. 1999. His- tological analysis of mantle-cavity ciliates in Dreissena poly- ful for the host than other vitally important organs. morpha. Their location, symbiotic relationship, and distin- guishing morphological characteristics. Journal of Shellfish Acknowledgements. This study was supported by the Ministry of Research, 18, 251–257. Science, Education and Sports of the Republic Croatia (project No. Laruelle F., Molloy D.P., Roitman V.A. 2002. Histological analysis of 0119 128). We thank anonymous reviewers whose suggestions great- trematodes in Dreissena polymorpha: their location, patho- ly improved the manuscript. genicity, and distinguishing morphological characteristics. Journal of Parasitology, 88, 856-863. DOI: 10.2307/3285521. Lauckner G. 1983. Diseases of : . In: (Ed. O. Kin- ne) Diseases of marine animals. Biologische Anstalt Hel- References goland, Hamburg, 477–961. Lewandowski K. 1982. The role of early developmental stages in the Baturo B. 1977. Bucephalus polymorphus Baer, 1827 and Rhipi- dynamics of Dreissena polymorpha (Pall.) (Bivalvia) popu- docotyle illense (Ziegler, 1883) (, Bucephalidae): lations in lakes. II. Settling of larvae and the dynamics of morphology and biology of developmental stages. Acta Para- numbers of settled individuals. Ekologia Polska, 30, 223– sitologica Polonica, 24, 203–220. 286. Baturo B. 1978. Larval bucephalosis in artificially heated lakes of the Mackie G.L., Schloesser D.W. 1996. Comparative biology of zebra Konin region, Poland. Acta Parasitologica Polonica, 25, 307– mussels in Europe and North America: An overview. Inte- 321. grative & Comparative Biology, 36, 244–258. DOI: 10.1093/ Borcherding J. 1991. The annual reproductive cycle of the freshwa- icb/36.3.244. ter mussel Dreissena polymorpha Pallas in lakes. Oecologia, Molloy D.P., Giamberini I., Morado J.F., Fokin S.I., Laruelle F. 2001. 87, 208–218. DOI: 10.1007/BF00325258. Characterization of intracytoplasmic prokaryote infections in Bower S.M., McGladdery S.E., Price I.M. 1994. Synopsis of infec- Dreissena (Bivalvia: Dreissenidae). Diseases of Aquatic Orga- tious diseases and parasites of commercially exploited shell- nisms, 44, 203–216. DOI: 10.3354/DAO044203. fish. Annual Review of Fish Diseases, 4, 1–199. Molloy D.P., Karatayev A.Y., Burlakova L.E., Kurandina D.P., Burlakova L.E., Karatayev A.Y., Padilla D.K. 2000. The impact of Laruelle F. 1997. Natural enemies of zebra mussels: Preda- Dreissena polymorpha (Pallas) invasion on unionid bivalves. tors, parasites, and ecological competitors. Reviews in Fish- International Review of Hydrobiology, 85, 529–541. DOI: eries Science, 5, 27–97. 10.1002/1522-2632(200011)85:5/6<529::AID-IROH529 Molloy D.P., Roitman V.A., Shields J.D. 1996. Survey of the para- >3.0.CO;2-0. sites of zebra mussels (Bivalvia: Dreissenidae) in north west- Conn D.B., Conn D.A. 1995. Experimental infection of zebra mus- ern Russia, with comments on records of in Europe sels Dreissena polymorpha (Mollusca: Bivalvia) by metacer- and North America. Journal of the Helminthological Society cariae of Echinoparyphium sp. (Platyhelminthes: Trematoda). of Washington, 63, 251–256. Journal of Parasitology, 81, 304–305. DOI: 10.2307/32839 Morton B.S. 1969. Studies on the biology of Dreissena polymorpha 39. Pall. I. General anatomy and morphology. Proceedings of the Erben R., Lajtner J., Luciæ A., Maguire I., Klobuèar G.I.V. 2000. Malacological Society of London, 38, 301–321. Attachment of the zebra mussel on the artificial substrates in Romeis B. 1968. Mikroskopische technik. 16 Aufl Oldenbourg, the reservoir Dubrava (River Drava, Croatia). Limnological München, 695 pp. Reports, 33, 225–231. Smirnova V.A., Ibrasheva S.I. 1967. Larval trematodes from fresh- Gist D.H., Miller M.C., Brence W.A. 1997. Annual reproductive water molluscs in the western Kazakhstan. Trudy Instituta cycle of the zebra mussel in the Ohio River: a comparison Zoologii Akademii Nauk Kazakhskoy SSR, 27, 53–87 (In with Lake Erie. Archiv für Hydrobiologie, 138, 365–379. Russian). Hebert P.D.N., Muncaster B.W., Mackie G.L. 1989. Ecological and Stadnychenko A.P. 1974. The infection of Unio pictorum and Ano- genetic studies on Dreissena polymorpha (Pallas): a new mol- donta piscinalis (Mollusca, Lamelibranchia) with parthenites lusc in the Great Lakes. Canadian Journal of Fisheries and of Bucephalus polymorphus Baer (Trematodes) and the effect Aquatic Sciences, 46, 1587–1591. of the parasites on the host’s organism. Parazitologiya, 8, Jokela J., Uotila L., Taskinen J. 1993. Effects of the castrating trema- 420–425 (In Russian). tode parasite Rhipidocotyle fennica on energy allocation of Stañczykowska A. 1977. Ecology of Dreissena polymorpha (Pall.) fresh-water clam Anodonta piscinalis. Functional Ecology,7, (Bivalvia) in lakes. Polskie Archiwum Hydrobiologii, 24, 332–338. DOI: 10.2307/2390213. 461–530. Karatayev A.Y., Burlakova L.E., Molloy D.P., Volkova L.K. 2000. Taskinen J., Mäkelä T., Valtonen E.T. 1997. Exploitation of Ano- Endosymbionts of Dreissena polymorpha (Pallas) in Belarus. donta piscinalis (Bivalvia) by trematodes: parasite tactics and International Review of Hydrobiology, 85, 539–555. DOI: host longevity. Annales Zoologici Fennici, 34, 37–46. 10.1002/1522-2632(200011)85:5/6<543::AID-IROH543 Taskinen J., Valtonen E.T., Mäkelä T. 1994. Quantity of sporocysts >3.0.CO;2-3. and seasonality of two Rhipidocotyle species (Digenea: Buce- Kinkelin P. de, Tuffery G., Leynaud G., Arrignon J. 1968. Étude épi- phalidae) in Anodonta piscinalis (Mollusca: Bivalvia). Inter- zootiologique de la bucéphalose larvaire a Bucephalus poly- national Journal for Parasitology, 24, 877–886. DOI: morphus (Baer 1827) dans le peuplement piscicole du bassin 10.1016/0020-7519(94)90014-0. de la Seine. Recherches Vétérinaires, 1, 77–98. Tripp M.R. 1973. Hermaphroditism in Bucephalus-infected oysters. Kuperman B.I., Zhochov A.E., Popova L.B. 1994. Parasites of Journal of Invertebrate Pathology, 21, 321–322. DOI: Dreissena polymorpha (Pallas) molluscs of the Volga basin. 10.1016/0022-2011(73)90221-8. Parazitologiya, 28, 396–402 (In Russian). Wacker A., von Elert E. 2003. Food quality controls reproduction of Lajtner J., Marušiæ Z., Klobuèar G.I.V., Maguire I., Erben R. 2004. the zebra mussel (Dreissena polymorpha). Oecologia, 135, Comparative shell morphology of the zebra mussel, Dreis- 332–338. DOI: 10.1007/s00442-003-1208-5. 92 Jasna Lajtner et al.

Wallet M., LambertA. 1986. EnquLte sur la répartition et l’evolution Walz N. 1973. Untersuchungen zur Biologie von Dreissena poly- du parasitisme a Bucephalus polymorphus Baer, 1827 chez morpha Pallas in Bodensee. Archiv für Hydrobiologie, 42, le mollusque Dreissena polymorpha dans le sud-est de la 452–482. France. Bulletin Français de la Pe$ che et de la Pisciculture, 300, 19–24.

(Accepted November 12, 2007)