On temporal variation in the rotifer Keratella cochlearis (Gosse): the question of “Lauterborn-cycles”

The existence of separated forms within the population of Keratella cochlearis (Gosse) from Lake Plußsee was demonstrated on the basis of a biometric analysis. The morphs cochlearis, hispida and tecta were definitely not connected by transitional forms. Micracantha and tecta were not links of a macracantha-micracantha-tecta-series. Hence, the general validity of Lauterborn-cycles is questioned. In this connection, the taxonomy of this species is also involved, because it is derived from the idea of phenotypic cycles in the sense of Lauterborn.     

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.     

In situ clearance rates of planktonic rotifers

The in situ clearance rates of several rotifer species from a small, temperate eutrophic lake were measured using three radioactive tracer cell-types, a bacterium (Aerobacter), a yeast (Rhodoto-rula), and alga (Chlamydomonas). Rates were below 10 µl/anim/h but varied significantly among species. Keratella cochlearis, Kellicottia bostoniensis, and Conochilus dossuarius ingested all three tracer cells but rates varied substantially with tracer cell-type. Polyarthra dolichoptera and P. euryptera ingested only the algal cells. Co-occurring forms of K. cochlearis and species of Polyarthra differed markedly in size and in tracer cell utilization, indicating niche diversification in food resources.     

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.     

Rotifers as indicators of lake types in Estonia

Data on the pelagic rotifer fauna in 10 Estonian lakes accumulated for 20 years, as well as published data are analysed. It is possible to distinguish three main indicator groups among rotifers: (1) for oligo- and mesotrophic lakes (Ploesoma hudsoni, Keratella serrulata, Synchaeta grandis, Asplanchna herricki, Ascomorpha ovalis, Gastropus stylifer, Conochilus hippocrepsis); (2) for meso-and eutrophic lakes (Trichocerca capucina, Filinia longiseta, F. limnetica, Keratella quadrata, K. cochlearis tecta, K.c. hispida, Polyarthra euryptera, Keratella hiemalis, Trichocerca porcellus, T. pusilla); (3) for eutrophic lakes (Brachionus spp., Anuraeopsis fissa, Pompholyx sulcata, P. complanata, Trichocerca cylindrica, Hexarthra mira).     

An Experimental Analysis of the Acid Sensitivity of the Common Planktonic Rotifer Keratella cochlearis

An in situ mesocosm experiment was performed at neutral pH Lake O'Woods, West Virginia, to assess the impacts of acidification on the common planktonic rotifer Keratella cochlearis. This rotifer is typically replaced by K. taurocephala during the acidification of North American lakes. Despite a rapid pH reduction in this experiment (from 7.0 to 4.8 in 14 days), the abundance, mean body length and egg ratio of K. cochlearis did not decline in the acid treatment as compared to the untreated control. These results support the hypothesis that K. cochlearis is acid-tolerant, and suggest that its disappearance from acid lakes is the result of biotic interactions within the plankton, rather than the result of toxic effects of altered water chemistry.     

Morphological structure and functional patterns of Keratella cochlearis (Gosse) populations in stratified lakes

Vertical distribution of the rotifer Keratella cochlearis in stratified water columns of mesotrophic and eutrophic lakes during summer stagnation has been studied. Coexisting morphs K. cochlearis hispida (Lauterborn, 1898), K. c. tecta (Gosse, 1851) and K. c. cochlearis (Gosse, 1851) inhabit different layers in the water column and are vertically subdivided. The distribution of morph abundance and reproductive potential indicate that substitution of morphs within the vertical water column may be due to trophic conditions. The maximum population productivity is observed at the epi-metalimnion border. The maximum density zone lies below the zone of the highest productivity. The principle of sliced functioning is used to explain the adaptive significance of the morphological structure of populations under heterogeneous environmental conditions.     

Observations on the susceptibility of some protists and rotifers to predation by asplanchna girodi

Direct observations of behavioral interactions show that the predator A. girodi: 1) easily ingested Synchaeta pectinata, two forms of Keratella cochlearis cochlearis, and individuals of Conochilus unicornis and C. dossuarius enzymatically dissociated from their matrix; 2) rarely, if ever, captured Kellicottia bostoniensis and intact Conochilus; and 3) generally rejected the peritrich ciliate Rhabdostyla sp. and the dinoflagellate Peridinium sp. Coloniality and secretion of a gelatinous matrix in Conochilus can be viewed as adaptations to limit mortality from invertebrate predation. Intraspecific variability in the feeding responses of A. girodi is considered.     

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.     

Daily timing of hatching of a rotifer,Keratella cochlearis

Groups of Keratella cochlearis f. tecta were collected over a complete diel period from a shallow pool. Observed in the laboratory, the hatchings of the field-produced eggs were not significantly periodic.     

Spine development in the rotifer Keratella cochlearis: induction by cyclopoid copepods and Asplanchna

SUMMARY. 1. Spine development in a clone of unspined Keratella cochlearis was promoted by a water soluble factor released by the copepods Tropocyclops prasinus and Mesocyclops edax and by the predatory rotifer Asplanchna priodonta. Between 9 and 55% of K. cochlearis populations cultured in predator-conditioned media responded to the inducing chemical.
2. The K. cochlearis form possessing a posterior spine of medium length was much less susceptible to predation by small Tropocylops and Asplanchna than the form lacking the posterior spine. These predators consistently selected the unspined form over the spined form when offered equal densities of each. However, both spined and unspined forms were equally susceptible to predation by large Mesocyclops.
3. The phenotypic variation of offspring produced from induced unspined Keratella females encompassed much of the variation reported for the taxon in North America. These morphotypes are similar to the variation of forms reported for the K. cochlearis tecta series known from Europe. We suggest that the North American morphotypes be identified in studies of this species because the presence of the posterior spine can greatly affect predator selectivity.     

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.     

Seasonal fluctuation of planktonic rotifers in Azibo reservoir (Portugal)

The rotifers of the recently-commissioned Azibo reservoir were studied during two years. The community was similar to that of other portuguese reservoirs with Keratella cochlearis, Polyarthra spp., Asplanchna priodonta and Collotheca pelagica dominant, although densities varied strongly. Maximum rotifer density was 640 ind l^–1 in 1987 and 315 ind l^–1 in 1988. During the first year two density peaks were observed, while in the second year fluctuations were irregular. Phytoplankton availability was considered the main reason for the fluctuation of rotifer density, although temperature, dissolved oxygen and cladoceran interference may also have played a role. A high density of Anabaena flos-aquae might have facilitated the dominance of K. cochlearis in 1988.     

美洲龟甲轮虫的分类特征及与我国热带地区常见龟甲轮虫的比较

美洲龟甲轮虫Keratella americana Carlin,1943年被认为主要分布在美洲地区,但陆续报告在其他地区有出现。2015年在对华南地区浮游动物调查时发现,星云湖、抚仙湖、百色水库及飞来峡水库中均出现该种类。检测了广西百色水库中龟甲轮虫的COⅠ序列,与已知美洲龟甲轮虫序列相似度高达98%。中国该龟甲轮虫种群与墨西哥美洲龟甲轮虫种群间差异度为2%,进一步确定为美洲龟甲轮虫。文章描述了美洲龟甲轮虫形态及生境,使用扫描电镜法比较了美洲龟甲轮虫、螺形龟甲轮虫Keratella cochlearis（Gosse,1851）、热带龟甲轮虫Keratella tropica（Apstein,1907）及无棘龟甲轮虫Keratella tecta（Gosse,1851）的咀嚼器。美洲龟甲轮虫主要特点为:背甲长度远大于宽度,侧中棘刺短于侧棘刺,背甲中央最后一个板块未封闭,向后棘刺延伸。4种龟甲轮虫咀嚼器主要区别在于砧基的形状,美洲龟甲轮虫砧基比其他3种轮虫发达,螺形龟甲轮虫砧基似“T”形,热带龟甲轮虫砧基为扇形,无棘龟甲轮虫平直。种间的砧基存在明显差异,可作为热带地区龟甲轮属的分类依据。     

Morphological variation of Keratella cochlearis (Gosse) in Lake Biwa,Japan

The length of the lorica (LL) of Keratella cochlearis cochlearis and of K. cochlearis tecta and the length of posterior spine (PSL) of the latter morphotype were measured in the strongly eutrophic basin and also in the mesotrophic basin of Lake Biwa, Japan, from September to December, 1980. In the population from the mesotrophic basin, the individuals with longer PSL prevail and the tecta forms are extremely rare. The LL values of both morphotypes from one sample do not differ. In December the LL increased to 95 µm in both morphotypes from 80 µm observed in September, while the PSL values decreased abrubtly in both basins in the middle of this period. It is suggested that the observed increase of LL could be related to the thermic factor, i.e. a steady decrease of water temperature, and the changes of PSL are correlated with the increase of nannoplankton and detritus aggregates noted in November. In this month an increase in fecundity and in the total numbers of rotifers took place as well (Hillbricht-Ilkowska, in press).     

Acanthodiaptomus denticornis another omnivorous calanoid copepod: description of its mouth appendages and feeding experiments on animal prey

The mouthpart morphology of the freshwater calanoid copepod Acanthodiaptomus denticornis was examined with optical microscopy. The mouthparts have sharp teeth and stout appendages with clawlike setae, typical of omnivorous calanoid mouthpart morphology. Observation of the buccal aperture with Scanning Electron Microscopy shows a large opening permitting feeding on prey as large as Keratella cochlearis. These observations agree with our feeding experiments which show that A. denticornis feed on K. cochlearis.     

A centrifugation method for measuring the relative density (specific gravity) of planktonic rotifers (Rotifera), with values for the relative density of Polyarthra major Burckhardt and Keratella cochlearis (Gosse)

The relative density of Polyarthra major and Keratella cochlearis was measured by centrifugation in solutions of different densities. Ficoll was used as the solute. The mean values for the relative density of living specimens of P. major was found to be 1.0277 ± 0.0062. The value for preserved animals was lower at 1.0160 ± 0.0029. The relative density of fixed specimens of K. cochlearis was measured and found to be approximately 1.0450 ± 0.0175.     

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.     

Diel variation in the egg ratio of rotifers throughout the season (preliminary report)

For the rotifersKeratella cochlearis andKellicottia longispina diel variation in egg ratio was studied throughout the season. In April no variation was found, in July and early September diel variation was significant forK. cochlearis. The diel patterns are discussed in relation to the mean temperature experienced by the populations.     

Regulation of rotifer species by invertebrate predators in a hypertrophic lake: selective predation on egg-bearing females and induction of morphological defences

Temporal changes in the biomass of rotifer plankton were examinedin a shallow hyper trophic lake during 1 month (April/May),when the predatory rotifer Asplanchna brightwelli develops.The abundance of herbivorous rotifers was regulated by predationfrom A.brightwelli and from the copepod Acanthocyclops robustus.The densities and fecundity rates of Keratella cochlearis andKeratella quadrata were negatively related with the biomassof predators. Stomach analyses showed that Asplanchna fed selectivelyon reproductive females of K.cochlearis, reducing the fecundityof this species. Predators induced longer caudal spines in K.quadrata,which were negatively related to the fecundity of this rotifer,suggesting a reproductive cost associated with spine production.In contrast, spine length of K.cochlearis was not related topredators, but to temperature. These results showed that predatorscan reduce rotifer densities through increasing mortality andthrough decreasing rotifer fecundity rates regardless of phytoplanktonbiomass. We also show that morphological defences of K.cochlearisand K.quadrata are induced in different ways.