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Hydrobiologia 442: 89–99, 2001. M. Boersma & K.H. Wiltshire (eds), Cladocera. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 89 Biotic and abiotic preferences of the cladoceran invader Limnosida frontosa Thomas C. Jensen1 , Dag O. Hessen1 & Bjørn A. Faafeng2 1 Department 2 Norwegian of Biology, University of Oslo, P.O. Box 1027, Blindern, N-0316 Oslo, Norway Institute for Water Research. P.O. Box 173, Kjelsås, N-0411 Oslo, Norway Key words: Limnosida frontosa, abiotic parameters, food spectrum, particle selection Abstract During the late 1980s and early 1990s, the cladoceran Limnosida frontosa invaded several lakes within its natural range in southeastern Norway. In this project, we wanted to study the types of lakes preferred by Limnosida. We also wanted to evaluate the potential competitive effects on other zooplankton species. In a survey of 65 Norwegian lakes, Limnosida showed preference for lakes of low Ca concentrations and low productivity. This is probably due to decreased competition from species with higher Ca requirements and a lower fish predation on zooplankton in these lakes compared to more productive lakes. Particle size preferences of Limnosida were studied and compared with those of the microfiltrator Daphnia magna, as there was no published information on the food preference of Limnosida. When fed monodisperse fluorescent latex beads (0.5, 1.0, 6.0 µm), Limnosida strongly selected the largest beads, while D. magna had a more nonselective feeding behaviour. Mesh sizes of Limnosida’s filtering appendages were 0.4–1.2 µm depending on the animal size, and the particle selection was correlated with the filter mesh sizes. Limnosida should thus be considered a low efficiency grazer on bacteria and µ-algae. Hence, this species probably does not interfere significantly with microfiltrators like Diaphanosoma brachyurum and most daphnids. This was supported by community analysis of lakes with and without Limnosida. In general, Limnosida commonly co-occurred with a number of filter-feeding cladocerans, and we found no sign of competitive exclusion in lakes where the species has become established. Introduction During the late 1980s and early 1990s, the cladoceran Limnosida frontosa Sars invaded several lakes in southeastern Norway. (Walseng & Karlsen, 1997; Faafeng, unpubl. data). Invasion of species has attracted much attention, because ecosystem properties, such as productivity and community structure may be influenced by invading species (Mooney & Drake, 1989). Zooplankton invaders may also affect the native zooplankton communities. For example, after invading Lake Naivasha, Kenya, Daphnia pulex outcompeted other zooplankton species (Harper, 1984), and the invasion of Bythotrephes cederstroemii in lakes in the northeastern U.S.A., strongly affected the native invertebrate predator Leptodora kindti as well as the herbivorous zooplankton (Lehman & Cáceres, 1993; Yan & Pawson, 1997). A species’ distribution depends on interactions between a number of biotic and abiotic factors, which together form the multidimensional niche of the species. Though invasion may lead to extinction of native species through competitive exclusion (Hardin, 1960), competing zooplankton species may also coexist (Jacobs, 1977; Chow-Fraser & Maly, 1992). Differing abiotic demands among zooplankton species promote coexistence (Havens et al., 1993; Hessen et al., 1995a). Temporal or spatial niche differentiation will also allow competitors to coexist (Leibold, 1991; Angeli et al., 1995). Furthermore, biotic factors such as size selective predation (Brooks & Dodson, 1965; Zaret, 1980), age structure (Lynch, 1978) and intraspecific competition (Hu & Tessier, 1995) have been used to explain coexistence between zooplankton species. Food is also an important factor as far as the competitive outcome between species is 90 concerned, as different species utilize different kinds of food (Neill, 1975; Chow-Fraser & Maly, 1992) or food size spectra (Burns, 1968; Geller & Müller, 1981; Hessen, 1985). This kind of niche differentiation will promote coexistence. Limnosida was originally described by Sars (1993 [1861]). The species is planktonic (Lilljeborg, 1982 [1901]; Sars, 1993 [1861]) found from May to October, though it is mainly present in mid summer (Lilljeborg, 1982 [1901]; Vekhov, 1987). Pejler (1965) found the species in lakes with relatively low productivity. Limnosida is a palearctic species (Sars, 1865, 1993 [1861]; Behning, 1912; Lilljeborg , 1982 [1901]; Vekhov, 1987). The objectives of this study was to examine whether the distribution of Limnosida could be related to abiotic and biotic factors such as calcium concentrations, lake productivity parameters and fish predation on zooplankton. Co-occurrence of Limnosida with other cladocerans was also investigated. Since there is no information on the food preferences of Limnosida in the literature, an assay of particle size preferences was undertaken in order to obtain some basic information on food size selection and filtering rates to evaluate the potential competitive effects on other zooplankton species. Materials and methods A survey of nutrient levels and plankton communities in a large number of Norwegian lakes has been conducted since 1988 (Faafeng & Hessen, 1993). For the present study, only lakes within the distribution area of Limnosida were chosen. This included a set of 65 lakes in southeastern Norway, geographically delimited to the north and east within the area where Limnosida occurred. Only lakes with pH>5 were included in order to avoid lakes with severe acidification. Samples for total phosphorus (total P), total nitrogen (total N) and chlorophyll a from July and August from 1988 to 1993 were included in this study. Samples for the analysis of calcium (Ca) were taken from August or September from 1988 to 1993. Further description of the sampling and analysis is given in Hessen et al. (1995a, b). The relationship between distribution of Limnosida and Ca concentration and tentative lake productivity (total P) was assessed by logistic regression. This relates presence/absence of a species to continuous or discrete independent variables (Jongman et al., 1995). Univariate logistic regression describes the probability (p) of an event (presence/absence) by a probability ratio as a linear function of the independent variable (x): p = b 0 + b1 · x log10 1−p This expression describes a situation where an organism has an optimum for either high or low values of the independent variable. Logistic regression can also be used to assess species preferences along the gradient axis of the independent variable (Jongman et al., 1995). Furthermore, the univariate model can be extended to describe the probability as a function of two independent variables (Jongman et al., 1995). This has been done in previous zooplankton studies (Hessen et al., 1995a). Here, the above expression is extended to a quadratic polynomial. The bivariate model then becomes an expression of the form: p b 0 + b1 · x1 + b2 · x1 2 log10 1−p +b3 · x2 + b4 · x2 2 + b5 · x1 · x2 where x1 and x2 are independent variables. In the bivariate regression in this study, the interaction between Ca and total P is considered. The criterion for accepting an effect in the logistic models was p < 0.05. JMP 3.2.2 (SAS Institute Inc. Cary, NC) was used for statistical calculations. The presence/absence data for Limnosida was mainly based on this lake survey. In a few lakes, the species was not registered in our lake survey, but it presence has nevertheless been verified in later studies. For the statistical treatment, the species was considered as present in these lakes as well. The co-occurrence between Limnosida and other cladocerans was assessed from observations on presence/absence of the species in the survey for the same lakes during the years 1988–1993. In order to examine whether Limnosida was associated with some of the other species the coefficient of association, Yule’s Q, was calculated (Freeman, 1987). Q varies from −1 to 1. A value of 0 indicates no association, values >0 indicate positive association, and values <0 indicate negative association. The fish species composition of the lakes was categorised on basis of questionnaires and reports. Six fish community categories were identified, ranging from category 1 dominated by brown trout (Salmo trutta) to category 6 dominated by various cyprinids, also representing a supposed increase in fish predation 91 on zooplankton (Hessen et al., 1995b). The occurrence of Limnosida as related to fish community was then assessed, based on these categories. The particle size selection of Limnosida was assessed both by feeding experiments and examination of the filtering appendages. In order to study particle size preferences of the species, feeding trials with fluorescent latex beads were performed (Hessen, 1985). The particle size preferences of Limnosida were compared with those of the well known microfiltrator Daphnia magna (Geller & Müller, 1981; Brendelberger & Geller, 1985). Limnosida for the experiments was collected from Lake Gjersjøen and D. magna originated from a culture at the Department of Biology, University of Oslo. The animals from the lake were collected by vertical net hauls (95 µm) from the epilimnion at different dates during the summer of 1997. They were transferred to the laboratory in lake water, where they were allowed to acclimatize for 24 h (19 ◦ C) in 12 l aquaria and fed on a suspension of the green algae Selenastrum capricornutum. Limnosida and D. magna were exposed to fluorescent latex beads (Fluoresbrites, Polysciences) of 0.5 µm, 1.0 µm and 6.0 µm at concentrations of approx. 2.24·106– 3.07·106, 7.56·105 – 1.43·106, 2.88·104 – 6.50·104 beads ml−1 in separate 100 ml beakers. The food concentration (S. capricornutum) in these experiments was approx. 0.08 mg C l−1 . The number of animals in the beakers varied from 10 to 21, and all grazing experiments lasted for 10 min. The animals were then killed with hot water (60 ◦ C), filtered on 90 µm gauze, washed and sorted according to size under a dissecting binocular. Depending on size, from 3 to 13 animals of each size group were placed in vials with distilled water and sonicated for five minutes (MSE Soniprep 150), which caused a breakdown of the animal tissue. The suspension of particles was filtered on black 0.2 µm polycarbonate filters and counted with a fluorescence microscope (Leitz Laborlux S). A second series of experiments was performed to compare the feeding rate and selection of latex beads of Limnosida to that of more natural food. 14 C-labelled S. capricornutum (cell size of approx. 3 × 7 µm) was added to the beakers (approx. 0.8 mg C l−1 ), together with the latex beads (0.5 µm, 1.0 µm and 6.0 µm). In this experiment, 200 ml beakers were used and the number of animals was doubled. About 10% of the animals in both experiments did not ingest the beads. These were probably dead or unhealthy specimens. As for many cladocerans, the gut passage time (GPT) for different types of natural food Table 1. Major characteristics for the 65 lakes. Data on total P, total N and chlorophyll a are from July and August samples from 1988 to 1993. Data on calcium are from August and September samples. Area as km2 ; total P, total N and chlorophyll a as µg l−1 ; Ca as mg l−1 Area Total P Total N Chlorophyll a Ca Average Median Max. Min. 12.6 29 669 16.2 7.8 32.1 15 510 7.5 4.3 362.8 301 3061 121.7 37.3 0.2 3 237 1.2 1.7 is more than 15 min (Peters, 1984), we assumed that the incubation time of 10 min was shorter than GPT. The selectivity index, Is (−1 to 1), of each bead size was calculated according to Ivlev (1961). Is varies from −1 to 1. Values >0 denotes a positive selection, and values <0 a negative selection. Formalin-preserved Limnosida from Lake Gjersjøen were sorted according to size, critical-point dried, mounted on aluminium stubs, covered with gold-palladium (Polaron SEM coating unit) and examined by scanning electron microscopy (SEM) (Jeol JSM-6400 scanning microscope). The intersetulae distances (ISD) were measured on the 5 first thoracic appendages on several individuals from each size group. Results The occurrence of Limnosida in Norway is restricted to the southeastern part of the country (Fig. 1). It has been recorded in 45 lakes, and 40 of these were included in our survey. The 65 lakes in our survey represented a range from oligotrophy to eutrophy, with a total P concentration from 3 to 301 µg l−1 , but with the majority of lakes being oligotrophic (Table 1). The Ca concentration ranged from 1.7 to 37.3 mg l−1 (Table 1). Both Ca and total P were skewed towards lower values, and became more symmetric after log transformation. Total P was correlated with chlorophyll a (log (TP) vs log(Chl. a): r = 0.93, p < 0.0001), justifying the use of total P as the single productivity variable. Total P was also correlated with Ca (log(TP) vs log(Ca): r = 0.54, p < 0.0001). Univariate logistic regression testing the occurrence of Limnosida along gradients of Ca and total P (Fig. 2) showed a decreased probability of Limnosida 92 Figure 1. Distribution of L. frontosa in Southern Norway (filled dots) related to all lakes surveyed (open dots). Data is mainly based on the national eutrophication survey (cf. Faafeng & Hessen, 1993; Hessen et al., 1995b), with supplementary information from various sources (Borgstrøm et al., 1974; Walseng & Karlsen, 1997; Wærvågen, 1998; Kjellberg unpubl. data; Walseng unpubl. data). occurrence with increasing concentrations of Ca and total P (Ca: p = 0.0003, total P: p = 0.039). Bivariate logistic regression for the occurrence of Limnosida in relation to Ca and total P gave significant effect of Ca only (p = 0.0003). This indicates that Ca was more important than total P for the occurrence of Limnosida. The biotic preferences of Limnosida were related to fish and zooplankton communities as well as to the preferred food size spectra. The fish communities in most of the 65 lakes from the survey were dominated by cyprinids and had thus an assumed high fish predation on zooplankton. Limnosida almost exclusively occurred in lakes belonging to category 6, suggesting a relatively high resistance towards fish predation. Limnosida commonly co-occurred with a number of abundant cladocerans (Fig. 3). Most frequent cooccurrence was recorded with Daphnia cristata, Diaphanosoma brachyurum, Bosmina longispina, Bosmina coregoni and Bosmina longirostris. Yule’s Q was positive for Limnosida and each of the species D. cristata, Holopedium gibberum and B. coregoni (Table 2). The occurrence of Limnosida was thus positively associated with these three species, of which at least the two latter are species preferring rather large particles. 93 Figure 2. (a) Logistic regression of L. frontosa’s occurrence in relation to Ca. Based on data from 65 lakes. Line: probability of occurrence as predicted by the model. Dots: ranked concentrations of Ca in lakes with L. frontosa (below line) and without L. frontosa (above line). (b) Logistic regression of L. frontosa’s occurrence in relation to total P. Based on data from 65 lakes. Line: probability of occurrence as predicted by the model. Dots: ranked concentrations of total P in lakes with L. frontosa (below line) and without L. frontosa (above line). Figure 3. Distribution of the 65 lakes from the lake survey divided into 4 groups according to presence or absence of L. frontosa and the other cladocerans. The 4 groups are: L. frontosa present and the species Y present (L. fro. +/Spec. Y +), L. frontosa absent and the species Y present (L. fro. -/Spec. Y +), L. frontosa present and the species Y absent (L. fro. +/Spec. Y -), L. frontosa absent and the species Y absent (L. fro. -/Spec. Y -). Abbreviations are: L. fro., Limnosida frontosa; D. cri., Daphnia cristata; B. log., Bosmina longispina; D. bra., Diaphanosoma brachyurum; B. cor., Bosmina coregoni; B. lon., Bosmina longirostris; D. cuc., Daphnia cucullata; C. spp., Ceriodaphnia spp.; H. gib., Holopedium gibberum; Chy., Chydorids; D. hya., Daphnia hyalina; D. gal., Daphnia galeata; D. lon., Daphnia longispina; D. log., Daphnia longiremis. 94 Table 2. The coefficient of association, Yule’s Q (with 95% confidence interval), for L. frontosa and each of the other species in the lake survey. Significant values (p<0.05) in bold. Q varies from −1 to 1. A value of 0 indicates no association, values > 0 positive association, and values < 0 negative association. Abbreviations as in Figure 3. D. longiremis and D. longispina occurred in less than 5 of the 65 lakes, so Yule’s Q was not calculated for these and L. frontosa Species combination L. fro. and D. cri. L. fro. and H. gib. L. fro. and B. cor. L. fro. and B. log. L. fro. and Chy. L. fro. and D. bra. L. fro. and C. spp. L. fro. and B. lon. L. fro. and D. cuc. L. fro. and D. gal. L. fro. and D. hya. Yule’s Q 0.57±0.39 0.48±0.33 0.43±0.29 0.19±0.43 0.14±0.57 0.05±0.51 −0.14±0.60 −0.19±0.61 −0.19±0.61 −0.26±0.94 −0.48±1.02 effective consumer of bacteria and the smallest algal species. Clearance rates calculated from the ingested latex beads also varied with bead size and animal size (Fig. 5). Both Limnosida and Daphnia showed increased clearance rates with increasing particle size. There was also a tendency for increased clearance rates with increasing animal size for both species. Limnosida had considerably lower clearance rates than D. magna (Fig. 5a,c). The combined experiments with beads and 14 C-labelled algae gave similar clearance rates for the two largest sizes of beads and algae (Fig. 5b), indicating that the feeding response of Limnosida to beads and to natural food were comparable. An increase in Limnosida clearance rates was observed during the summer. The animals collected at the different dates have different histories with respect to parameters, such as food conditions and temperature. There might be an inherent seasonal effect on the clearance rates even though the animals in all the experiments were temperature adapted during a 24 h period prior to the experiments. All size categories of Limnosida showed a strong preference for the larger beads. In all the experiments with Limnosida, we observed a shift from negative to positive selectivity indices between 0.5 and 1.0 µm particle size (Fig. 6a, b). Furthermore, the indices for 6.0 µm beads were larger than for 1.0 µm beads. There were no marked differences in particle size selection between the different size groups of animals. D. magna also showed a shift from negative to positive selectivity indices between 0.5 and 1.0 µm particle size (Fig. 6c), and a preference for the largest beads. Yet compared with Limnosida, D. magna experienced a rather nonselective feeding behaviour. Discussion Figure 4. Cumulative frequencies of intersetulae distances (ISD) of the 4 size groups of L. frontosa. Pooled data in each size group for the 5 first appendages. The food size preference in Limnosida was inferred from filter mesh sizes and the size spectrum of ingested particles. The intersetulae distances (ISD) of Limnosida ranged from 0.4 to 1.2 µm depending on animal size and site on the filters. ISD increased with animal size, and variations in ISD were less in the smallest animals (Fig. 4). Based on these morphological studies, Limnosida would not be considered an The 65 lakes from the survey covered a limited range of Ca and lake productivity. However, Limnosida showed a statistically strong preference for lakes with low Ca and total P. The interpretation of the logistic regression is complicated by the correlation of Ca and total P. Bi- and multi-variate logistic regression takes this into account (Jongman et al., 1995), and Ca appeared to be a more important parameter than total P for the occurrence of Limnosida. Limnosida’s preference for lakes of low productivity with low Ca concentrations corresponds well with studies on the Ca content of different cladocerans, 95 Figure 5. (a) Clearance rates for different size groups of L. frontosa given 3 sizes of beads. Concentration of Selenastrum capricornutum during the experiments was approx. 0.08 mg C l−1 . Means and standard deviations refer to the number of experiments in which the different size groups are represented. (b) Clearance rates for different size groups of L. frontosa given 3 sizes of beads and 14 C-labeled algae. Concentration of S. capricornutum during the experiments was approx. 0.8 mg C l−1 . Means and standard deviations as in (a). (c) Clearance rates for different size groups of D. magna given 3 sizes of beads . Concentration of S. capricornutum during the experiments was approx. 0.08 mg C l−1 . Means and standard deviations as in (a). 96 Figure 6. Particle size selection of L. frontosa and D. magna, expressed as the selectivity index Is . (a) Experiments where L. frontosa was given beads (food concentrations: 0.08 mg C l−1 ). Means and standard deviations refers to all experiments at this food concentration. (b) Experiments where L. frontosa was given beads and 14 C-labeled algae (food concentrations: 0.8 mg C l−1 ). Means and standard deviations refers to all experiments at this food concentration. (c) Experiments where D. magna was given beads (food concentrations: 0.08 mg C l−1 ). Means and standard deviations refer to all experiments at this food concentration. which indicates that Limnosida has low Ca requirements compared to other species (Wærvågen et al., unpubl. data). The observed preference of Limnosida for lakes with low Ca concentrations could be an indirect effect of decreased competitive abilities from species with higher Ca demands, such as certain daphnids, at low Ca levels (Hessen et al., 1995a; Wærvågen et al., unpubl. data). Considering the biotic properties of the lakes, Limnosida apparently preferred lakes dominated by different cyprinids. Migration barriers restrict the occurrence of these fish to southeastern Norway (HuitfeldtKaas, 1924). Consequently, the distribution of lakes in category 6 coincide with that of Limnosida. These lakes had a relatively high supposed fish predation pressure on zooplankton. However, Limnosida also preferred lakes of low productivity. Lakes of increasing productivity will have an increasing biomass of planktivorous fish (Johansson & Persson, 1986; Jeppesen, 1998). As a consequence, these lakes could also have an increasing predation pressure on zooplankton. Limnosida thus seems to be fairly resistant to predation from fish and are surely able to coexist with cyprinids. Nevertheless, the predation pressure from fish in the most productive lakes might be too high to allow for persistence of Limnosida in these lakes. The spreading of Limnosida do apparently not cause strong effects on the zooplankton communities. The differences in co-occurrence and association between Limnosida and other cladocerans in the lake survey (Fig. 3) is probably partly caused by the 97 geographic distribution of species and partly by the differences between species in demands to abiotic parameters, resistance to predation and general competitive abilities. The occurrence of Limnosida was positively associated with H. gibberum, which has a more restricted occurrence. Like Limnosida, H. gibberum is mainly found in lakes of low productivity and Ca levels (Pejler, 1965; Hessen et al., 1995a). H. gibberum is probably also a superior competitor at low Ca, because of its low Ca requirements (Wærvågen et al., unpubl. data). This could explain this species’ positive association with Limnosida. The associated occurrence of the two common species D. cristata and B. coregoni with that of Limnosida could be partly explained by the vulnerability of these species to fish. Like Limnosida, D. cristata and B. coregoni are fairly resistant to predation (Hessen et al., 1995b), and they would therefore be less preyed upon in the lakes dominated by cyprinids. In the feeding experiments, Limnosida had by far lower clearance rates for all sizes of beads than D. magna, which is known to have rather high sizespecific clearance rates (McMahon & Rigler, 1965; Kersting & Van Der Leeuw, 1976). Though clearance rates determined with artificial particles in laboratory experiments does not mimic natural conditions, it still indicates that Limnosida has lower clearance rates than D. magna. Larger individuals of Limnosida had high clearance rates compared with other cladocerans (Burns & Rigler, 1967; Ganf & Shiel, 1985). Filter area (Kořínek & Macháček, 1980; Ganf & Shiel, 1985) and mesh size (Kořínek and Macháček, 1980; Geller & Müller, 1981; Brendelberger & Geller, 1985) both increase with body size. According to Crittenden (1981), the discharge rate increases disproportionately with mesh size. Furthermore, the thoracic beat rate (Watts & Young, 1980) depends on animal size. All this may contribute to increased clearance rates with animal size. The differences in mesh size between size groups of Limnosida were too small to cause a difference in particle size selection between size groups. However, larger individuals of Limnosida will probably have a broader food size range than smaller ones, since larger individuals will be able to ingest larger particles (Burns, 1968). Particle size selection of Limnosida seemed to correlate well with filter mesh size, and this study thus gives support to the sieving model, where mechanical filtering is supposed to be the most important mechan- ism in the particle capture of many cladocerans (Geller & Müller, 1981). Limnosida has previously been found in lakes of relatively low productivity (Pejler, 1965). The present study supports this observation. Decreased competition from cladocerans with higher Ca demands as well as moderate fish predation may be important factors. The food preferences of the species could add to this. It has been suggested that high efficiency bacteria feeders could have a competitive advantage in more productive lakes (Geller & Müller, 1981). Species capable of utilising small particles will be favoured, when the distribution of food particles changes from being dominated by medium sized algae in relative oligotrophic lakes to microalgae and bacteria combined with large inedible algae in eutrophic lakes (Hessen et al., 1986). Low efficiency bacteria feeders, like Limnosida, could thus be better adapted to the food regime in less productive lakes. We found no sign of competitive exclusion by Limnosida. In general, zooplankton communities did not differ between lakes with or without Limnosida. In Lake Gjersjøen Limnosida have coexisted with the native zooplankton since it invaded the lake in 1985, although it may occur in fairly high biomasses in some samples (Faafeng, unpubl. data). In 1997, Limnosida also showed a pronounced seasonal and/or spatial niche overlap with D. brachyurum, D. cristata, B. longispina and B. longirostris in this lake (Jensen, 1999). One mechanism that could reduce resource competition and thus promote coexistence between Limnosida and other species is niche differentiation with respect to food size spectra. Since Limnosida was found to be a low efficiency bacteria feeder, the invasion of the species may not interfere significantly with high efficiency bacteria feeders like D. brachyurum and many daphnids (Geller & Müller, 1981; Hessen, 1985). The effect on other zooplankton species with high Ca requirements might be most adverse in lakes with low Ca where these species have reduced competitive abilities due to the suboptimum calcium concentrations. It seems that the invasion of Limnosida does not pose a serious threat to native zooplankton species. Invasions of so-called ‘exotic species’, not naturally occurring in that geographical area, have often had major consequences for the system they invaded (Baskin, 1992; Yan & Pawson, 1997). In the cases cited above, species have come together recently, and so the evolutionary forces have had a short time to bring balance to species interactions. When Limnosida 98 appears in new lakes, it is often found together with species naturally occurring within the same geographical range. In this case, species interactions are old, and the evolutionary forces might have brought balance to the interactions between the species. The prehistory of the species interactions may explain why Limnosida does not seem to affect native cladocerans upon invasion, though this remains speculative. In conclusion, Limnosida preferred lakes with low Ca and low total P. Invading new lakes it will most likely be found in soft water lakes of low productivity. This is probably due to reduced competition from species with higher Ca requirements and a lower fish predation on zooplankton in these lakes compared to more productive lakes. Furthermore, Limnosida was found to be a low efficiency bacteria feeder, so the invasion of the species may not interfere significantly with microfiltrators like D. brachyurum and many daphnids. This was supported by community analysis. Limnosida commonly co-occurred with other filter-feeding cladocerans. Acknowledgements We would like to thank Gösta Kjellberg and Bjørn Walseng for providing unpublished data on zooplankton from previous investigations. Tom Andersen and Ørnulf Borgan provided valuable suggestions on the statistical methods. Torulv Tjomsland is acknowledged for help on graphic representation. The english has been improved by Ingunn Myhre. References Angeli, N., B. Pinel-Alloul, G. Balvay & I. Ménard, 1995. Diel patterns of feeding and vertical migration in daphnids and diaptomids during the clear water phase in Lake Geneva (France). Hydrobiologia 300/301: 163–184. Baskin, Y., 1992. Africa’s troubled waters. Bioscience 42: 476–481. Behning, A., 1912. 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