Keratella distribution in North Patagonian lakes (Argentina)

The distribution of Keratella species from 15 different lakes in North Patagonia (Argentina) was analysed. The genus was not present at altitudes above 1000 m. K. tropica was restricted to Patagonian Plateau lakes with a comparatively high conductivity. A morphometric analysis of the widely distributed K. cochlearis was performed. Results showed three groups of K. cochlearis corresponding to Andean lakes, Patagonian Plateau lakes and a Patagonian Reservoir.     

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.     

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.     

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.     

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.     

Changes in the population dynamics of Keratella cochlearis (Gosse), Kellicottia longispina (Gosse) and Polyarthra vulgaris Carlin in a fertilized enclosure

The population dynamics of phytoplankton and zooplankton in a fertilized enclosure were studied from April 1977 to June 1978. During the first spring period, the rotifers Keratella cochlearis, Kellicottia longispina, and Polyarthra vulgaris were scarce. During the following spring, all three species were abundant. An attempt is made to explain these differences. Food resource competition from cladocerans, and the low food quality and possible inhibitory effects of Coccomyxa sp. were found to be the most likely factors limiting the growth of these rotifers during the first spring period.     

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).     

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.     

Horizontal distribution of the plankton rotifers Keratella cochlearis (Bory de St Vincent) and Polyarthra vulgaris (Carlin) in a small eutrophic lake

The plankton rotifers Keratella cochlearis and Polyarthra vulgaris were sampled at 10 cm below the surface at different distances from two dissimilar shores and in the centre of a small eutrophic lake. Light and depth were measured at each sampling point. In each case the numbers of rotifers per liter increased with distance from the shore. There was a significant correlation between the numbers for the two species for the two shores, but none in the centre. In the case of one shore there was a strong correlation between rotifer numbers and supra-surface ambient light.     

On morphological variation in Keratella cochlearis populations from Holstein lakes (Northern Germany)

Keratella cochlearis occurs in many Holstein lakes (northern Germany) as three well defined and separated forms: cochlearis, hispida, and tecta, each showing very little variation between the lakes. The present data show that the tecta form did not originate from a Lauterborn cycle.     

Experimental studies on the seasonal variation of the rotifer Keratella cochlearis (Gosse)

A culture containing the rotifer Keratella cochlearis was divided into two aliquots, one reared at + 20°C and the second at + 5°C, all other conditions being identical. The experimentl lasted 74 days. At 20°C the animals did not undergo any noticeable morphological changes, while at 5°C the posterior spines grew longer, a development which had nearly reached its maximum as early as 30 days after the experiment began, i.e. after very few generations. The experiment shows that the temperature (and its direct consequences, such as slower growth) plays an important role in causing seasonal morphological variation in this rotifer. The results are in accordance with field experience as well as with culture experiments on other brachionid rotifers.     

A method for the laboratory culture of the planktonic rotifer Keratella cochlearis (Gosse)

Procedures for the continuous laboratory culture of Keratella cochlearis in a defined medium and upon an algal food are described. Culturing success appears to be a function of food availability as well as composition. This availability requirement is satisfied by the use of test tubes and inverted titration plate concavities as culture vessels. The satisfactory culture medium contains an ammonia compound as a nitrogen source.     

Effects of Microcystis aeruginosa on interference competition between Daphnia pulex and Keratella cochlearis

Laboratory experiments tested the hypothesis that a toxic strain of Microcystis aeruginosa decreases the ability of Daphnia pulex to interfere with Keratella cochlearis. To test a variety of conditions, juvenile and adult Daphnia were exposed to the cyanobacterium for different time periods prior to, and during the experiments. Adult Daphnia not only suppressed rotifers over successive two-day intervals, but also had a significant impact within a 24-hour period. However, the presence of Microcystis (5 × 105 cells ml^–1) decreased the Daphnia effect in both experiments. Although juvenile Daphnia also significantly suppressed Keratella population growth, the presence of Microcystis (10⁵ and 5 × 10⁵ cells ml^–1) caused a significant reduction in daphniid body size and decreased the ability of both nonacclimated and acclimated daphniids to suppress rotifers. Keratella inhalation and mortality are positively correlated with filtering rates and body size of Daphnia. Therefore, the feeding rates and size structure of a Daphnia population will determine its potential to interfere with vulnerable rotifers. In all experiments the presence of Microcystis significantly decreased the ability of Daphnia to interfere with this rotifer despite the fact that Keratella was also inhibited. In the field this effect might be augmented if Microcystis colonies are more easily ingested by cladocerans than by the rotifers.     

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.     

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.     

Changes in growth and size of Keratella cochlearis (Gosse) in relation to some environmental factors in cultures

Keratella cochlearis (Gosse) was cultured non-axenically in Carefoot medium diluted with Erken water at 5 °C, 15 °C and 20 °C with Rhodomonas minuta (Skuja) as a food alga. The rotifer reached ca. 120 ind. ml^?1, having generation times of 2–7 days, a Q₁₀-value of ca. 2, and at the lowest temperature >20% longer posterior spines. When co-cultured with Chlorella sp., at 0–30 mg Ca l^?1 and 1.6 meq NaHCO₃ l^?1 in medium L 11 at 20 °C, the maximum generation time and individual numbers were 3–4 days and up to 100 ind. ml^?1, respectively. Animal numbers increased in relation to nutrient multiples, up to two multiples, of the culture medium L 16. Growth and length were reduced, although the width increased above two multiples of this culture medium. The trace metal tolerance was broad and increased additions of a metal mixture (L 11) slightly increased the length of the rotifers. No major changes in the length were observed when HCO₃ or Ca were varied in the culture medium (L 11), although a decrease in the length was noted in old cultures.     

Cyclomorphosis in Keratella tropica (Apstein) of Lake Asejire,Nigeria

Of one thousand, one hundred and four (1104), Keratella tropica collected with a 55 m mesh plankton net from the surface of Lake Asejire from February 1974 to February 1976 lorica length and width, and the lengths of the posterior spines were measured.Lake Asejire K. tropica are among the smallest in Africa. Variations in the patterns of spine length development were independent of season as individuals without left posterior spines and with relatively long left and right posterior spines occurred as commonly in the rainy as in the dry season months.Correlations between K. tropica dimensions and thirty environmental factors underscore the importance of physical environmental factors and point to biological and genetic factors as determinants of the patterns of cyclomorphosis in K. tropica.     

Keratella mexicana sp. nov., a new planktonic rotifer from Aguascalientes,Mexico

A new species of Keratella is described from a small reservoir in Aguascalientes, Mexico. The species appears related to K. slacki Brziss, 1963 and K. lenzi Hauer, 1953.     

The genusPicris (Asteraceae,Lactuceae) in Tropical Africa

Three species of the genusPicris L. are native in Tropical Africa:P. abyssinica Sch. Bip. (Ethiopia),P. xylopoda Lack, spec. nova (Nigeria, Ethiopia) andP. humilis DC. (Senegal, Mali). There are indications that the two perennial species,P. abyssinica andP. xylopoda, are related to and have evolved from a primitive Central Asiatic stock in a manner parallel to many African species ofCrepis L.P. humilis, on the other hand, is a small annual plant with a high number of derived characters. The introduced species of European origin growing south of the Sahara are briefly mentioned.     

Distribution patterns of the Diaptomidae (Calanoida: Copepoda) in southern Africa

All freshwater calanoids in southern Africa belong to the family Diaptomidae of which there are two subfamilies, the Paradiaptominae (Lovenula, Paradiaptomus and Metadiaptomus) and Diaptominae (Tropodiaptomus and Thermodiaptomus). The Paradiaptominae are endemic to Africa, while the Diaptominae have one endemic African genus Thermodiaptomus, and the other genus Tropodiaptomus, occurs in other parts of the world as well as Africa. Knowledge of distribution of the Diaptomidae has been hampered by incomplete taxonomic knowledge and lack of co-ordination of available information. Distribution patterns of 19 described species and 11 new species confirm that the majority of species have a limited distribution, and that the Paradiaptominae are arid-adapted and belong to temporary pool communities. Six species are endemic to coastal pans and vleis of the western and southern Cape Province. Only those species with a wider distribution have been recorded as colonising man-made lakes. The four Lovenula species are predators and often co-exist with a Metadiaptomus species as prey. Synapomorphies of widely separated species of the Paradiaptominae, provide interesting insights into how their evolution may have been governed by vicariance, dispersal and palaeoenvironments of the African continent.