On the ecology of the rotifer Cephalodella hoodi from an extremely acidic lake

1. The biovolume‐specific carbon content, relative egg volume (a measure of per‐offspring reproductive investment), growth and grazing rates, and the gross growth efficiency (GGE) of the rotifer Cephalodella hoodi, isolated from an extremely acidic habitat (pH 2.65), were determined and compared with literature values for rotifers living in circum‐neutral habitats in order to reveal potential special features or adaptations related to the extreme habitat of C. hoodi. 2. Of the two dominant phytoflagellates (Ochromonas sp. and Chlamydomonas acidophila) that occur in the natural habitat of C. hoodi, only C. acidophila promoted positive growth and reproduction and, thus, the following results were obtained with C. acidophila as a food alga. 3. The body volume‐specific carbon content of C. hoodi is in the range of that found in rotifers from circum‐neutral lakes, suggesting that no costly carbon investment, brought about by the thickening of the lorica, for example, was required to withstand low pH. 4. The egg volume of C. hoodi exhibited no phenotypic plasticity dependent on the food concentration and, thus, C. hoodi allocated a constant, absolute amount of energy to each individual offspring. No adaptation to low food densities was found. 5. A dome‐shaped type II functional response curve was found to describe the ingestion of Chlamydomonas as a source of food. 6. Compared with other rotifers, C. hoodi had a high threshold and half‐saturating food concentration (=low affinity) but also a high maximum growth rate and a relatively high GGE, suggesting no severe adverse effect of low pH.     

Dietary restriction in two rotifer species: the effect of the length of food deprivation on life span and reproduction

According to resource allocation theory, animals face a trade off between the allocation of resources into reproduction and into individual growth/maintenance. This trade off is reinforced when food conditions decline. It is well established in biological research that many animals increase their life span when food is in suboptimal supply for growth and/or reproduction. Such a situation of reduced food availability is called dietary restriction. An increase in life span under dietary restricted conditions is seen as a strategy to tolerate periods of food shortage so that the animals can start reproduction again when food is in greater supply. In this study, the effect of dietary restriction on life span and reproduction in two rotifer species, Cephalodella sp. and Elosa worallii, was investigated using life table experiments. The food concentration under dietary restricted conditions was below the threshold for population growth. It was (1) tested whether the rotifers start reproduction again after food replenishment, and (2) estimated whether the time scale of dietary restricted conditions is relevant for the persistence of a population in the field. Only E. worallii responded to dietary restriction with an increase in life span at the expense of reproduction. After replenishment of food, E. worallii started to reproduce again within 1 day. With an increase in the duration of dietary restricted conditions of up to 15 days, which is longer than the median life span of E. worallii under food saturation, the life span increased and the life time reproduction decreased. These results suggest that in a temporally (or spatially) variable environment, some rotifer populations can persist even during long periods of severe food deprivation.     

Temporal dynamics and growth of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic lake

1. The in situ abundance, biomass and mean cell volume of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001–03). 2. Actinophrys sol exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 × 10³ Heliozoa L^?1. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. Actinophrys sol cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems. 3. We determined the growth rate of A. sol using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single‐celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp. and the rotifers Elosa worallii and Cephalodella hoodi. Actinophrys sol fed over a wide‐size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. Actinophrys sol neither increased in cell number (k) nor biomass (k_b) when starved, with low concentrations of single‐celled bacteria or with the alga Ochromonas sp. Positive growth was achieved with single‐celled bacteria (k = 0.22 ± 0.02 d^?1; k_b = ?0.06 ± 0.02 d^?1) and filamentous bacteria (k = 0.52 ± <0.01 d^?1; k_b = 0.66 d^?1) at concentrations greater than observed in situ, and the alga C. acidophila (up to k = 0.43 ± 0.03 d^?1; k_b = 0.44 ± 0.04 d^?1), the ciliate Oxytricha sp. (k = 0.34 ± 0.01 d^?1) and in mixed cultures containing rotifers and C. acidophila (k = 0.23 ± 0.02–0.32 ± 0.02 d^?1; maximum k_b = 0.42 ± 0.05 d^?1). The individual‐ and biomass‐based growth of A. sol was highest when filamentous bacteria were provided. 4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. Actinophrys sol are omnivorous predators supported by a mixed diet of filamentous bacteria and C. acidophila in the epilimnion. Heliozoa are important components in the planktonic food webs of ‘extreme’ environments.     

Review of rotifers and crustaceans in highly acidic environments of pH values ≤3

A review of the literature on rotifers and crustacean zooplankton in highly acidic environments revealed that data from eleven aquatic environments on three continents (America, Europe, Japan) with a pH 3 are available. Seven sites are influenced by volcanism or weathering processes in the catchment area, four others originated from human mining activities. Species richness was generally low. Only 16 species are found and 1–11 species are reported for each area. These studies clearly show that small littoral or benthic rotifers predominate over crustaceans under highly acidic conditions. In the Lusatian mining area (Germany), all lakes are colonized by zooplankton, even the most acidic one with a pH of 2.3. The core community consists of the rotifers Cephalodella hoodi, C. gibba, Elosa worallii and Rotaria rotatoria, with C. hoodi and E. worallii the most abundant. Larger species, such as the rotifer Brachionus sericus or the cladoceran Chydorus sphaericus, occur at a pH close to 3. A similar pattern is reported from acidic mining lakes in Illinois, U.S.A. Many of these species can also be found in less acidic softwater or even alkaline environments due to the tolerance of a broad range of pH values. Elosa worallii and Brachionus sericus are probably the most acidophilic rotifer species, though at least the latter can also grow at neutral pH in the laboratory. Clear understanding of the pH limits of B. sericus in nature may also have been complicated by the fact that it has probably in the past been wrongly named as B. urceolaris (phenotype `sericus'). The typical B. urceolaris cannot tolerate extremely low pH. Overall, generalist species with a worldwide distribution seem to play the major role in the colonization of anthropogenic highly acidic lakes.     

Multiple environmental stressors confine the ecological niche of the rotifer Cephalodella acidophila

Summary

• 1 The planktonic food web in extremely acidic mining lakes is restricted to a few species that are either acidophilic or acidotolerant. Common metazoans inhabiting acidic mining lakes with a pH below 3 include rotifers in the genera Cephalodella and Elosa.
• 2 The life history response of Cephalodella acidophila to three environmental key factors, pH (2, 3.5, 5.0 and 7.0), temperature (10, 17.5 and 25 °C) and food concentration (10 000, 35 000 and 50 000 algal cells per mL), was investigated in a full factorial design using life‐table experiments.
• 3 The effect of each of the three environmental variables investigated on the rotifer life cycle parameters (life span, fecundity and population growth rate) differed. C. acidophila is a stenoecious species with a pH optimum in the range 3–4 and a comparably high food threshold. Combining the laboratory results with field data, we conclude that C. acidophila is severely growth limited in its natural habitat. However, low pH alone is not harmful as long as temperatures are moderate to warm and food is abundant.
• 4 The population of C. acidophila in the field is maintained mainly due to release from competitors and predators.

     

Dynamics of planktonic food webs in three mining lakes across a pH gradient (pH 2–4)

Acidic mining lakes (ML) in Lusatia (Germany) are characterised by their geogenically determined chemistry. The present study describes the structure, main components and relationships within the food webs of three acidic mining lakes with different pH values (ML 111: pH 2.6; ML 117: pH 2.8; ML Felix: pH 3.6) in order to show their typical characteristics. The investigation covered the period 1995–1997. The number of species and the biomass are both low, but increase with increasing pH. Planktonic components in the most acidic ML 111 (pH 2.6–2.9) comprise bacteria, Ochromonas spp. and Chlamydomonas spp. and a few rotifers (E. worallii, C. hoodi). Heliozoans are the top-predators. In ML 117 (pH 2.8–3) Gymnodinium sp., ciliates, the rotifer B. sericus and the pioneer crustacean Chydorus sphaericus join the pelagial community. Heliozoans were not found in ML 117 or ML Felix (pH 3.4–3.8). ML Felix had the most taxa. The benthic food chain of all three lakes includes phytobenthic algae as producers, chironomids as primary consumers and corixids as top predators in the profundal. Corixids predate on small cladocerans inhabiting the pelagial in lakes with a pH above 2.8 such as ML Felix. They invade the pelagial and act as a connecting link between the benthic and the pelagic food chains, which are isolated in lakes with a lower pH. Occasionally primary producers and consumers were abundant in all three lakes. These organisms do not depend on the degree of acidity, but on the availability of essential ressources. Mass variations covered up any seasonal variation in the extremely acidic ML 111 (0.9 mm³ l^–1), while in the other two lakes seasonal patterns of biomass were found.     

The outcome of competition between the two chrysomonads Ochromonas sp. and Poterioochromonas malhamensis depends on pH

We investigated the effect of pH on the competition of two closely related chrysomonad species, Poterioochromonas malhamensis originating from circumneutral Lake Constance, and Ochromonas sp. isolated from a highly acidic mining lake in Austria (pH ∼2.6). We performed pairwise growth experiments between these two species at four different pH ranging from 2.5 to 7.0. Heterotrophic bacteria served as food for both flagellates. Results were compared to growth rates measured earlier in single species experiments over the same pH range. We tested the hypothesis that the acidotolerant species benefits from competitive release under conditions of acid stress. The neutrophilic strain numerically dominated over the acidotolerant strain at pH 7.0, but was the inferior competitor at pH 2.5. At pH 3.5 and 5.0 both strains coexisted. Surprisingly, P. malhamensis prevailed over Ochromonas sp. under moderately acidic conditions, i.e. at the pH where growth rates of the latter peaked when grown alone. Since bacterial food was not limiting, resource competition is improbable. It appears more likely that P. malhamensis ingested cells of its slightly smaller competitor. Adverse effects mediated via allelopathy, either directly on the competing flagellate or indirectly by affecting its bacterial food, might also have affected the outcome of competition.     

Mixotrophy and the toxicity of Ochromonas in pelagic food webs

1. Toxic compounds produced by many phytoplankton taxa are known to have negative effects on competitors (allelopathy), anti‐predatory effects on grazers (mortality or impaired reproduction) or both. Although mixotrophs of the genus Ochromonas are known to be toxic to zooplankton, it has often been assumed in studies of plankton community processes that all flagellates in the size range of this taxon are edible to typical zooplankton grazers (i.e. cells ≤30 μm for Daphnia, ≤6 μm for rotifers). 2. We explored the toxicity of a species of Ochromonas to other planktonic taxa, including its competitors (two species of phytoplankton and protists) and consumers (two species of zooplankton). To test if mode of nutrition by this mixotroph influences its toxicity to other taxa, we exposed each test species to Ochromonas cultured in chemostats under four different nutritional regimes: osmotrophy (labile dissolved organic carbon) and phagotrophy (bacterial prey) in both light and dark conditions (i.e. with or without photosynthesis). 3. Filtrate from osmotrophically fed Ochromonas had a significant negative effect on the population growth rate of two obligate phototrophic phytoplankton, Cryptomonasozolini and Chlamydomonas reinhardtii. The protists Tetrahymena tetrahymena and Paramecium aurelia were also negatively affected by Ochromonas filtrate. Ochromonas cells were toxic to both the rotifer Brachionus calicyflorus and the cladoceran Daphnia pulicaria, with the toxic effects significantly more severe when fed at high cell densities (75 000 cells mL⁻¹) than at low densities (7500 cells mL⁻¹). Ochromonas cultured osmotrophically in the light was more toxic to the Daphnia than cells cultured under other conditions. In contrast, Ochromonas from all nutritional conditions was equally highly toxic to Brachionus. 4. Our findings support the view that Ochromonas can be toxic to other components of the food web with which it interacts. It is especially toxic to zooplankton that directly consume it, although the effect depends upon Ochromonas cell density and whether or not a good food source is simultaneously present. Our results call into question the common practice of pooling flagellates into a single ‘functional group’ included in an ‘edible phytoplankton’ category of cells <30 μm in diameter.     

Decline of Clear-Water Rotifer Populations in a Reservoir: The Role of Resource Limitation

The relative importance of and changes in resource limitation of herbivorous rotifers were assessed during the clear-water phase in the Rímov Reservoir, Czech Republic, using in situ manipulative experiments. Resource limitation was tested experimentally as the difference in population growth rate (Δr) among various experimental treatments on four occasions. The reservoir community of rotifers was exposed to three treatments: (i) control, (ii) diluted and (iii) diluted and fertilized. Significant responses to these experimental manipulations were shown by Synchaeta spp., Polyarthra spp. and Keratella cochlearis. Growth rate was usually highest during the spring rotifer maximum and decreased during the clear water phase. The highest intensity of food limitation (expressed as ‚Chlorophyll-a’ limitation) was found in Synchaeta spp. K. cochlearis had low food limitation during the spring peak, high food limitation during the second experiment and low food limitation, again, during the later experiment. In contrast, Polyarthra spp. had the same Chlorophyll-a limitation throughout the whole experimental period. Linear regression was used to estimate the relative proportion of Δr variability explained by Chlorophyll-a concentration and rotifer density in all of the experiments. Chlorophyll-a concentration explained 89, 97 and 92% of the resource limitation in Synchaeta spp., Polyarthra spp. and K. cochlearis, respectively. The proportion of variability explained by rotifer density-dependent factors was lower: 60% for Synchaeta spp. and 68 % for Polyarthra spp.     

Comparison of predator–prey interactions with and without intraguild predation by manipulation of the nitrogen source

Theory predicts that intraguild predation leads to different community dynamics than the trophic cascades of a linear food chain. However, experimental comparisons of these two food‐web modules are rare. Mixotrophic plankton species combine photoautotrophic and heterotrophic nutrition by grazing upon other phytoplankton species. We found that the mixotrophic chrysophyte Ochromonas can grow autotrophically on ammonium, but not on nitrate. This offered a unique opportunity to compare predator–prey interactions in the presence and absence of intraguild predation, without changing the species composition of the community. With ammonium as nitrogen source, Ochromonas can compete with its autotrophic prey for nitrogen and therefore acts as intraguild predator. With nitrate, Ochromonas acts solely as predator, and is not in competition with its prey for nitrogen. We parameterized a simple intraguild predation model based on chemostat experiments with monocultures of Ochromonas and the toxic cyanobacterium Microcystis. Subsequently, we tested the model predictions by inoculating Ochromonas into the Microcystis monocultures, and vice versa. The results showed that Microcystis was a better competitor for ammonium than Ochromonas. In agreement with theoretical predictions, Microcystis was much more strongly suppressed by intraguild predation on ammonium than by top–down predation on nitrate. Yet, Microcystis persisted at very low population densities, because the type III functional response of Ochromonas implied that the grazing pressure upon Microcystis became low when Microcystis was rare. Our results provide experimental support for intraguild predation theory, and indicate that intraguild predation may enable biological control of microbial pest species.     

Long-term Annual and Seasonal Changes of Meta- and Protozooplankton in Lake Müggelsee (Berlin): Effects of Eutrophication,Grazing Activities,and the Impact of Predation

Annual changes of rotifers, copepods, cladocerans, the ciliate Epistylis rotans, and larvae of Dreissena polymorpha were analysed for the period 1908–1990. Though food resources increased 6–10 fold in the course of eutrophication, only rotifers and Epistylis increased accordingly. Probably as a result of increased predation pressure crustaceans increased only twice. The seasonal pattern of metazoans and protozoans (flagellates, sarcodines, ciliates) were analysed for 12 and 3 years, resp. During winter and spring, large heterotrophic flagellates and ciliates dominated the zooplankton and were responsible for a pronounced - formerly underestimated - grazing pressure on phytoplankton. In early summer, metazoan filter-feeders were often able to cause a significant reduction of phyto- and protozooplankton. However, during some years, phytoplankton declined in the absence of a pronounced grazing pressure. Field data and experiments revealed that predators were able to regulate the density of cladocerans in early summer (mainly cyclopoids) and summer (mainly Leptodora, smelt and fish juveniles).     

Rotifers as predators on components of the microbial web (bacteria,heterotrophic flagellates,ciliates) — a review

Recent investigations have shown that processes within the planktonic microbial web are of great significance for the functioning of limnetic ecosystems. However, the general importance of protozoans and bacteria as food sources for rotifers, a major component of planktonic habitats, has seldom been evaluated. Results of feeding experiments and the analysis of the food size spectrum of rotifers suggest that larger bacteria, heterotrophic flagellates and small ciliates should be a common part of the food of most rotifer species. About 10–40 per cent of rotifers' food can consist of heterotrophic organisms of the microbial web. Field experiments have indicated that rotifer grazing should generally play a minor role in bacteria consumption compared to feeding by coexisting protozoans. However, according to recent experiments regarding food selection, rotifers should be efficient predators on protozoans. Laboratory experiments have revealed that even nanophagous rotifers can feed on ciliates. Preliminary microcosm and chemostat experiments have indicated that rotifers, due to their relatively low community grazing rates compared to the growth rates of bacteria and protozoans, should generally not be able (in contrast to some cladocerans) to suppress the microbial web via grazing, though they may structure it. Filter-feeding nanophagous rotifers (e.g. brachionids) seem to be significant feeders on the smaller organisms of the microbial web (bacteria, flagellates, small ciliates), whereas grasping species (e.g. synchaetids and asplanchnids) seem to be efficient predators on larger organisms (esp. ciliates). Another important role of rotifers is their feedback effect on the microbial web. Rotifers provide degraded algae, bacteria and protozoans to the microbial web and may promote microbial activity. Additional experimental work is necessary for a better understanding of the function of rotifers in aquatic ecosystems.     

Environment-dependent metabolic investments in the mixotrophic chrysophyte Ochromonas

Mixotrophic protists combine photosynthesis and phagotrophy to obtain energy and nutrients. Because mixotrophs can act as either primary producers or consumers, they have a complex role in marine food webs and biogeochemical cycles. Many mixotrophs are also phenotypically plastic and can adjust their metabolic investments in response to resource availability. Thus, a single species's ecological role may vary with environmental conditions. Here, we quantified how light and food availability impacted the growth rates, energy acquisition rates, and metabolic investment strategies of eight strains of the mixotrophic chrysophyte, Ochromonas. All eight Ochromonas strains photoacclimated by decreasing chlorophyll content as light intensity increased. Some strains were obligate phototrophs that required light for growth, while other strains showed stronger metabolic responses to prey availability. When prey availability was high, all eight strains exhibited accelerated growth rates and decreased their investments in both photosynthesis and phagotrophy. Photosynthesis and phagotrophy generally produced additive benefits: In low-prey environments, Ochromonas growth rates increased to maximum, light-saturated rates with increasing light but increased further with the addition of abundant bacterial prey. The additive benefits observed between photosynthesis and phagotrophy in Ochromonas suggest that the two metabolic modes provide nonsubstitutable resources, which may explain why a tradeoff between phagotrophic and phototrophic investments emerged in some but not all strains.     

Food limitation in a wild cyclopoid copepod population: direct and indirect life history responses

We studied the effects of food limitation on the population dynamics of the freshwater cyclopoid copepod Diacyclops thomasi in Oneida Lake, New York. In the field population, maximum juvenile abundance coincided seasonally with high phytoflagellate concentration. During the clear-water phase (a seasonal period of low algal density), D. thomasi disappeared from the water column, but fourth-instar copepodids (CIV) were found encysted in developmental arrest in the sediment. Laboratory assays of the effect of the density of two types of food on copepod life history parameters showed that temporal variation in the concentration of relatively small phytoflagellates significantly affected stage-specific development times. This food limitation was most pronounced during the clear-water phase, and supplementation of the diet with a laboratory-cultured phytoflagellate, Chlamydomonas, prevented food limitation. Although developmental arrest appears to be controlled primarily by photoperiod, availability of the larger, more mobile food, Euglena, also influenced the percentage of individuals entering developmental arrest in the laboratory. An investigation of the spatial and temporal emergence pattern in the field revealed that CIV copepodids started to emerge in late autumn and that emergence rates were significantly greater at deep-water locations (9–12 m water depth) compared with shallow-water locations (5–7 m). The clear-water phase in Oneida Lake is an annual event, probably produced by intense grazing by Daphnia pulicaria and Daphnia galeata. Food limitation is thus very likely a recurrent phenomenon for D. thomasi. This apparent seasonal competitive impact of Daphnia on Diacyclops affects both nauplii and immature copepodids. Diacyclops shows two types of responses to the food limitation: (1) the physiological response of slowed active development, and (2) the adaptive response of developmental arrest. Received: 3 November 1997 / Accepted: 1 March 1998     

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: changes in nutrient concentrations,phytoplankton and zoobenthos

Large bag-type (75 m³) and tube-type (105 m³) enclosures were set up in the shallow eutrophic Lake Suwa and were each stocked with exotic planktivorous whitefish (Coregonus lavaretus maraena). The release of whitefish caused the increase in nutrient concentration in the tube-type enclosure whereas no such increase was observed in the bag-type enclosure. Bottom sediment seemed to be an important source of chironomid food for whitefish. The proportion of phytoplankton measuring<10μm and 20–40μm, which respectively corresponded toOchromonas spp. andCryptomonas sp., were lower in the fish enclosures than in the control, which might have been caused by high grazing pressure by rotifers. The predation by whitefish might have affected the species composition of phytoplankton through reducing copepod predation on rotifers, not through reducing the densities of cladocerans which directly feed on phytoplankton as many investigators have reported. The phytoplankton biomass was not affected much by the release of fish. Possible reasons are that the increase in density of rotifers reduced the biomass of available phytoplankton and also that inedible Cyanophyceae were in the decreasing phase of their seasonal succession and could not increase successfully in spite of elevated nutrient levels.     

Use of Genetically Marked Minicells as a Probe in Measurement of Predation on Bacteria in Aquatic Environments

Minicells produced by Escherichia coli M2141 were used as probes to measure predation on pelagic bacteria in situ. The minicells, labeled with [³⁵S]methionine in one specific protein, were shown to disappear in the presence of a microflagellate (Ochromonas sp.), as seen by a decrease in the amount of labeled marker protein with time. Incubation in filtered (pore size, 0.2 μm) and autoclaved seawater did not affect the amount of labeled marker protein in the minicell. The generation time of flagellates feeding on minicells was determined to be similar to that found for flagellates grown on seawater bacteria or living E. coli NC3. Data indicate that minicells are seen as true food particles by the flagellates. The minicell probe was used in recapture experiments, in which predation in situ on pelagic bacteria was demonstrated. The rate of bacterial production showed a clear covariation with the rate of predation, both in different sea areas and in depth profiles. The obtained results (11 field experiments) showed that the rate of predation, on average, accounts for the consumption of 62% of the bacteria produced.     

Comparative and experimental approaches to top-down and bottom-up regulation of bacteria

The regulation of bacterial community biomass and productivity by resources and predators is a central concern in the study of microbial food webs. Resource or bottom-up regulation refers to the limitation of bacteria by carbon and nutrients derived from allocthonous inputs, primary production, and heterotrophic production. Predatory or top-down regulation refers to the limitation of bacteria below levels supportable by resources alone. Large scale comparative studies demonstrate strong correlations between bacterial productivity and biomass, suggesting significant resource regulation. Comparisons of the abundances of heterotrophic flagellates and bacteria, however, imply that in some cases there may be top-down regulation of bacteria in eutrophic environments. Experimental studies in lakes support the importance of resource regulation and reveal little top-down control from protozoans. Increases in bacterial abundance and production with nutrient enrichment were limited in enclosure experiments with high abundances of the cladoceran, Daphnia. Regulation of bacteria by Daphnia may occur in many lakes seasonally and prevail in some lakes throughout the year where these animals sustain dense populations. In most situations, however, bacteria appear to be limited primarily by resources.     

Effect of Filtered Cultures of Flagellate Ochromonas sp. on Colony Formation in Microcystis aeruginosa

The possible effect of filtered cultures of flagellate Ochromonas sp. on colony formation in M. aeruginosa was investigated in this paper. The results show that filtered cultures of flagellates fed with M. aeruginosa could induce colony formation in M. aeruginosa. Furthermore, induction strength is clearly dependent on the concentration of flagellates and filtered cultures. However, no colonial M. aeruginosa was found in the treatments of filtered cultures of flagellates fed with Microcystis wesenbergii, filtered cultures of flagellate fed with Chlorella pyrenoidosa, and algae homogenates. This suggests that infochemicals released from flagellates fed with M. aeruginosa may be a trigger for colony formation in M. aeruginosa. The clearance rates of flagellates on algae were markedly decreased when they were cultivated with induced colonial M. aeruginosa. These indicate that colony formation in M. aeruginosa is a predator‐induced defense which could reduce predation risk from flagellates (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Suppression of microzooplankton by zebra mussels: importance of mussel size

1. The zebra mussel (Dreissena polymorpha) is amongst the most recent species to invade the Great Lakes. We explored the suppressive capabilities of mussels 6–22-mm in size on Lake St Clair microzooplankton (< 240)μm) in laboratory experiments. 2. Absolute suppression of rotifers and Dreissena veliger larvae was proportional to mussel shell length for individuals larger than 10 mm; larger zooplankton, mainly copepod nauplii and Cladocera, were not affected. Mussel clearance rates on rotifers generally exceeded those on veligers, although rates for both increased with increasing mussel size. Rotifer-based clearance rates of large (22 mm) mussels approached published values for phytoplankton food. 3. Most zooplankton taxa, particularly rotifers, declined significantly in western Lake Erie during the late 1980s concomitant with the establishment and population growth of zebra mussels in the basin. Densities of some taxa subsequently increased, although rotifers and copepod nauplii densities remained suppressed through 1993. Available evidence indicates that direct suppression by Dreissena coupled with food limitation provides the most parsimonious explanation for these patterns.     

PHYLOGENY OF CHLAMYDOMONAS (CHLOROPHYTA) BASED ON CLADISTIC ANALYSIS OF NUCLEAR 18S rRNA SEQUENCE DATA1

The genus Chlamydomonas Ehrenberg may contain as many as 450 described species. Morphological, physiological and molecular data show that variation among some Chlamydomonas species can he great, leading to speculation that multiple, generic-level lineages exist within this genus. The most recent systematic studies of Chlamydomonas have led to proposals of nine distinct morphological and 15 distinct sporangial autolysin groups. Partial sequences from the nuclear small subunit rRNAs from 14 Chlamydomonas species representing 12 autolysin and four morphological groups, and from three flagellates thought to he related to Chlamydomonas were determined in a phylogenetic study of relationships among these algae. Sequence comparisons among some Chlamydomonas species revealed differences comparable to the sequence divergence between soybeans and cycads. Cladistic analysis of the sequence data suggests that multiple lineages exist among species of Chlamydomonas. Some of these lineages represent alliances of both Chlamydomonas and non-Chlamydomonas taxa; thus, the current taxonomy does not reflect natural, or monophyletic, groups. Collectively, these lineages may represent distinct families or even orders.