The zooplankton of the lower river Meuse, Belgium: seasonal changes and impact of industrial and municipal discharges

Between 1991 and 1993, samples were collected upstream and downstream of the industrial basin and urban centre of Liège. Rotifers and crustaceans (cladocerans and copepods) were identified and counted. Their population dynamics were related to physical and chemical factors (temperature, oxygen, ammonium, nitrates, nitrites, phosphates) and to phytoplankton biomass. The zooplankton was dominated by rotifers; crustaceans (cladocerans and copepods) were less abundant. There was a succession of groups and species, some thriving in the spring and others in summer or autumn. The dominant rotifer species were Brachionus calyciflorus Pallas, Brachionus angularis Gosse, Keratella cochlearis (Gosse) and Synchaeta spp.; B. calyciflorus and B. angularis are spring species. K. cochlearis was present between May and November. Crustacean biomass was important in summer and autumn, but the faunal spectrum and biomass also varied with sampling location. Low spring and summer discharges allowed the phytoplankton to develop significantly. The zooplankton development followed a similar pattern. During low flow, when plankton populations become established, some declines in phytoplankton could only be explained by sedimentation and grazing pressure by zooplankton. Although these factors provided a good explanation of the longitudinal variation, some local conditions (e.g. oxygen deficit, high level of phosphate) also induced changes (e.g. industrial and municipal waste water discharge).     

Comparative population dynamics of the rotifers Brachionus angularis and Keratella cochlearis

The results indicate that the two rotifer species Brachionus angularis and Keratella cochlearis have different life history strategies. Brachionus angularis appeared to be an r-strategist, showing higher reproductive, mortality and population growth rates than K. cochlearis. Additional evidence is offered suggesting that K. cochlearis is a K-strategist. Although K. cochlearis had a lower growth rate than B. angularis, this was compensated by a lower mortality rate, especially in pre-reproductive individuals. In addition, K. cochlearis allocated energy to reproduction more efficiently than B. angularis.     

Effectiveness of the caudal spine as a defense mechanism in Keratella cochlearis

The temporal distribution of spined and unspined forms of Keratella cochlearis in Cubillas reservoir during an annual cycle, and the stomach contents of Asplanchna girodi, were studied. The results suggest that the length of the caudal spine plays a critical role in the defense against predation by Asplanchna girodi. Short-spined forms showed a higher susceptibility to predation than unspined forms, which may have important consequencies for spine length variation in Keratella cochlearis populations.     

Keratella cochlearis (Gosse) in Africa

Keratella cochlearis occurs over the complete latitudinal range of Africa, and over a considerable altitudinal range, but not in the highest mountain tarns. Physical, chemical, and biological factors may affect the distribution of this species. Lack of suitable food, and competition or predation by larger zooplankters are probably important limiting factors. Morphometric variation in samples from Zimbabwe is much less than in samples from the Auvergne. In particular, posterior spine length shows a much lower relative growth in relation to lorica size.     

Influence of temperature on morphological variation in populations of Keratella cochlearis (Gosse) in Rybnik Reservoir

Morphological changes in a population of Keratella cochlearis were investigated in a reservoir of changed temperature and high trophy. Four morphological forms were distinguished: Keratella cochlearis f. cochlearis, K. cochlearis var. tecta f. micracantha, K. cochlearis var. tecta f. micracantha, K. cochlearis var. tecta f. typica and K. cochlearis f. macracantha. A correlation between the length of the lorica and the posterior spine, and the temperature of water was observed. These four forms of Keratella cochlearis occurred during the entire period of investigations.     

Eine neue tropische Unterart von Keratella cochlearis (Rotatoria)

A new subspecies of Keratella cochlearis, viz. Keratella cochlearis pachyacantka is described. It has been recorded from Africa and South America.     

The Spatial Segregation of Zooplankton Communities with Reference to Land Use and Macrophytes in Shallow Lake Wielkowiejskie (Poland)

The spatial distribution of zooplankton in relation to two types of land‐use (forested and pastoral‐arable) of a lake's surroundings and to various habitats (helophytes, elodeids, nymphaeids and open water) was examined along 16 parallel transects on a macrophyte‐dominated lake (area – 13.3 ha; mean depth – 1.4 m). The type of habitat was the main determinant of zooplankton community structure. Dissected‐leaved elodeids harboured the richest and most abundant community with typically littoral (e.g., Colurella uncinata) and pelagic species (e.g., Keratella cochlearis). Two species (Polyarthra major and P. vulgaris) selectively chose the open water and one (Lecane quadridentata) the Typha stand. No spatial differentiation in zooplankton abundance was recorded between the two types of the catchment area. One possible explanation may be the shallowness and small area of this lake which may support full mixing and no difference in physical‐chemical gradients. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Regulation of Rotifer Community by Predation of Cyclops vicinus (Copepoda) in the Římov Reservoir in Spring

The structuring role and predation impact of Cyclops vicinus on the rotifer community was studied using in situ enclosure experiments in Římov Reservoir during the spring of 2000. Seasonal changes in abundance and birth rates of the rotifer species Synchaeta lakowitziana , Polyarthra spp., Keratella cochlearis and Kellicottia longispina were related to predation pressure, i.e. selectivity and cropping rates of C. vicinus . All four rotifer species were found to be consumed by C. vicinus whose predation rates showed a marked succession through the spring period and corresponded to changes in the relative abundance of the pelagic species. The highest predation rates exhibited by C. vicinus were on Synchaeta lakowitziana and Polyarthra spp., being 11.3 and 6.4 ind Cyclops ^–1 day^–1 respectively. The activity of the predator switched among three types of preferred food during the spring period. The most preferred, Synchaeta, was replaced by Polyarthra after Synchaeta became extinct and these species were replaced by Keratella when both soft‐bodied forms were not available in sufficient quantities. Loricated species (K. cochlearis and K. longispina) were consumed at a slower rate. We calculated the proportion of rotifer production cropped day^–1 in order to estimate the predation impact of C. vicinus , which ranged from 11.6 to more than 100% over time and rotifer species. This shows that relative predation impact can be high not only on species most strongly selected, but also on species which are not selected. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Food limitation and seasonal population declines of rotifers

Summary Keratella cochlearis exhibited consistent seasonal abundance patterns during a four-year study in Little Rock Lake, Wisconsin, U.S.A. In each year, spring population peaks were followed by strong summer reductions. Concomitant with population declines, there were reductions in rotifer egg ratios. Keratella taurocephala abundance patterns were similar to those of K. cochlearis during 1984 and 1985, but not in 1986 and 1987, when spring peaks and summer declines were not apparent. However, summer declines in the egg ratio of K. taurocephala were observed during each year. The reduction in rotifer populations simultaneously with decreased egg ratios suggested that population declines were caused by food limitation. Food-addition experiments conducted in situ in small enclosures indicated that food was limiting for K. cochlearis when its populations were declining, but not during other periods of the year. Keratella taurocephala did not show a consistent response to food addition.     

The longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea)

The longitudinal distribution and seasonal dynamics of zooplankton were examined along a 200-km section of the middle to lower Nakdong River, Korea. Zooplankton was sampled twice a month from January 1995 to December 1997 at five sites in the main river channel. There was considerable longitudinal variation in total zooplankton abundance (ANOVA, p < 0.001). All major zooplankton groups (rotifers, cladocerans, copepodids and nauplii) increased significantly with distance downstream along the river. There also were statistically significant seasonal differences in zooplankton abundance at the sampling sites (ANOVA, p < 0.01). Zooplankton abundance was high in spring and fall and low in summer and winter. The seasonal pattern of rotifers was similar to that of total zooplankton. This reflected the fact that rotifers (Brachionus calyciflorus, B. rubens, Keratella cochlearis and Polyarthra spp.) strongly dominated the zooplankton community at all locations. Among the macrozooplankton, small-bodied cladocerans (e.g. Bosmina spp.) dominated; the abundance of large-bodied cladocerans (e.g. Daphnia) was negligible (0–5 ind. l^–1). Among the environmental variables considered, partial residence time seemed to play the most important role in determining characteristics of the river zooplankton community.     

Use of fluorescently labelled algae to measure the clearance rate of the rotifer Keratella cochlearis

1. Fluorescently labelled algae (FLA) were used to measure clearance rates of the rotifer Keratella cochlearis. The freshwater algae Chlorella vulgaris and Stichococcus bacillaris were labelled with a fluorescent dye, 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF), following a modified staining procedure. 2. Keratella cochlearis ingested both algal species. Clearance rates on tracer foods varied between 2.4 and 6.9 μl ind^?1 h^?1, which are comparable with those determined using other techniques. 3. The main drawback of the FLA technique was that only a little more than one-third of the total amount of algal cells of both C, vulgaris and S. bacillaris were well stained with the dye (DTAF), despite the use of a higher concentration of dye and a longer staining period than recommended in the literature. 4. The FLA method can be successfully applied in grazing studies involving size selection and competition for food among zooplankton. The method complements existing techniques for measuring the clearance and ingestion rates of filter-feeders.     

The Relative Role of Interference Competition in Regulation of a Rotifer Community during Spring Development in a Eutrophic Reservoir

The relative importance of interference and exploitative competition from Daphnia was tested in contrast to known role of Cyclops vicinus predation, influencing the rotifer community in Římov Reservoir during spring period. The abundances of five dominant rotifer species and their birth and death rates were analysed, together with changes in chl‐a concentration, HNF, bacteria biomass and the abundances of predator and competitor species, from high frequency sampling. Development of loricated species (Keratella cochlearis, K. quadrata and Kellicottia longispina) was driven predominantly by food availability. The dominant soft‐bodied species Polyarthra spp. and Synchaeta lakowitziana were affected predominantly through C. vicinus predation. The role of mechanical interference from D. galeata, although may be important in a limited periods was overclouded by predation and exploitative competition in general. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Seasonality,population dynamics and production of planktonic rotifers in Lough Neagh,Northern Ireland

The abundances, population dynamics and production of the rotifer community of Lough Neagh were examined for a three year period. Keratella cochlearis was the most abundant species accounting for over 40% of biomass followed by Polyarthra dolichoptera and Notholca acuminata. The mean standing crop for the rotiferan zooplankton increased in successive years (41, 51, 75 mg dwt m^–2) as did production (1037, 1322, 1417 mg dwt m^–2 y^–1). The seasonal pattern of biomass expression and production varies markedly in different years. Instantaneous birth rates tend to be lower but more consistent for the more abundant species, instantaneous death rates show periods of negative mortality indicating an inadequacy of the model employed but explicable as hatching of resting eggs. K. cochlearis as the most successful species is explained as its perennial appearance and adaptation to the low annual temperature cycle found in the lough. The population succession and the annual occurrence of species differs in each year.     

Prey selection by Asplanchna girodi (Rotifera): the importance of prey defence mechanisms

1. Predator (laboratory-grown Asplanchna girodi and prey (several field-collected non-carnivorous rotifer species) interactions were studied in the laboratory as a function of both predator and prey densities. The clearance rates of A, girodi decreased with increasing prey density. Predator density had no effect on the feeding of A. girodi. 2. Asplanchna girodi selectively fed on Keratella cochlearis. Its clearance rates for K. cochlearis were much higher than those for Polyarthra and Brachionus. Short-spined forms (spine length less than 25 μm) showed a significantly higher susceptibility to predation than either the non-spined or the long-spined forms. 3. Large Asplanchna individuals fed selectively on reproductive females of K. cochlearis, thereby reducing the fecundity of their prey.     

Urban rotifers: structure and densities of rotifer communities in water bodies of the Poznań agglomeration area (western Poland)

The rotifer fauna of 19 mostly small water bodies (natural and artificial ponds, clay-pits and pools) in Pozna was studied on four occasions during 1996–98 to determine the suitability of urban areas for rotifer habitats. Rotifers were present in all the water bodies studied, with 114 species in 39 genera found, representing ca. 25% of all rotifers recorded from Poland. Mean diversity was 10 spp (range 1–36). Most common were: Brachionus angularis and Keratella cochlearis (spring), Colurella uncinata, Lecane closterocercaand Lepadella patella (summer) and L. closterocerca and K. cochlearis (autumn). Rotifer densities (1-1503 ind l^–1), Shannon's diversity (H 0.00-3.71) and dominant species differed in different water-bodies. The index of percentage similarity of community showed strong differences in qualitative structure of rotifer assemblages. The different types of water habitats, both the existing or the newly created in towns, may explain the relatively high diversity of rotifer communities observed in the urban areas studied.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

The evolution of sex-change mechanisms in fishes

Synopsis Five distinct sex-change mechanisms are identified among sequentially hermaphroditic fishes based on socio-ecological characteristics. The primary determinants of the sex-change mechanisms appear to be social organization and mating system, which in turn depend on resource distribution in space and time. The ability of a single individual to control all mating in the social unit, which is related to the size of the social unit, differentiates three suppression mechanisms from two induction mechanisms. Sex-change suppression, which is characteristic of species with small group size and rigid dominance hierarchies, refers to inevitable sex change in the absence of group dominance. Ability to migrate between resource patches differentiates protogynous suppression (e.g. inLabroides dimidiatus) from protandrous suppression (e.g. inAmphiprion spp.). Early sex change appears to have evolved from protogynous suppression under special conditions involving the loss of mating control by a single dominant individual in certain species (e.g.Centropyge spp. ). Sex-change induction, which is characteristic of species with large social groups lacking rigid dominance hierarchies, refers to the requirement that sex change must be induced by specific characteristics of (or changes in) the social group, regardless of dominance status. Ability to distinguish sex, or its importance, differentiates sex-ratio induction (e.g.Anthias squamipinnis) from size-ratio induction (e.g.Thalassoma spp.). Alternative models account for the possibility that all cases of sex change require stimulation from smaller conspecifics (universal induction-inhibition model) or that all fish have the genetic capacity to switch mechanisms, depending on changing ecological conditions and resulting changes in mating system (behavioral-scaling model). Neurophysiological models suggest that induction mechanisms, which require at least two categories of environmental stimuli, may have evolved from the simpler suppression mechanisms, which require only one kind of input from the environment.     

Effects of interspecific gene flow on the phenotypic variance–covariance matrix in Lake Victoria Cichlids

Rotifers are ubiquitous freshwater animals for which many complexes of cryptic species (i.e. distinct species that are morphologically difficult to distinguish) are described. Keratella cochlearis occurs globally and shows a wide phenotypic diversity indicating the potential presence of a species complex. We sampled lakes of the Trentino-South Tyrol region (Italy) and investigated mitochondrial genetic diversity in K. cochlearis in relation to detailed lorica measurements. We sequenced the mitochondrial cytochrome c oxidase subunit I and used the generalised mixed Yule coalescent approach, Poisson tree process model and automatic barcode gap discovery to delimit mitochondrial groups, associated with putative evolutionary significant units (ESUs). Based on 248 sequences, eight putative ESUs were indicated that could only partially be delimited by lorica morphology. Specifically, several morphological characteristics (i.e. spinelets, bended median ridge, and posterior spine) were found in specimens of different putative ESUs, and thus, these characters seem to be of poor discriminatory value. Furthermore, different putative ESUs of K. cochlearis were found in the same lake. We conclude that the high mitochondrial genetic diversity may be linked to tolerance of K. cochlearis to varying environmental conditions.