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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.

Abstract: The existence of separated forms within the population of Keratella cochlearis (Gosse) from Lake Plusee 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.

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.

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.

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.

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.

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).

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.

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.

This study adds 25 rotifer species to the fauna of India viz.Cyrtonia tuba(Ehrb.)Epiphanes macrourus(Barrois & Daday),Liliferotrocha subtilis(Rodewald),Microcodides chleana(Gosse),Brachionus dimidiatus(Bryce),Keratella ticinensisCarlin,Notholca labis(Gosse),Platyias leloupi(Gillard),Euchlanis incisaCarlin,Mytilina bisulcata(Lucks),Wolga spinifera(Western),Lecane(Lecane)althausiRudescu,L.(L.)doryssaHarring,L.(L.)elongataHarring & Myers,L.(Monostyla)bifurca(Bryce)L.(M.)lamellata thalera(Harring & Myers),L.(Hemimonostyla)blacheiBerzins,Cephalodella gigantheaRemane,Monommata arndtiRemane,Trichocerca(Trichocerca)pusilla(Lauterborn),Testudinella emarginula(Stenroos),Ptygura melicertaEhrb,P. tacitaEdmondson,Filinia cornuta(Weisse),Collotheca mutabilis(Hudson),C. ornata(Ehrb.) andC. trilobata(Collins).B. dimidiatusandP. leloupiare new records from Delhi Region.

Filinia terminalis kergueleniensis nov. spp. (Family Filiniidae Bartos 1959), from Lake Studer 2 is described and figured. Filinia terminalis kergueleniensis differs from the type in length of bristles and their insertion. Keratella sancta was found again; it is the third record after its discovery by Russel 1944 in New Zealand. Notholca cf. jugosa Gosse 1889 is also found in the plankton of the subantarctic lake. The new subspecies and Keratella sancta occur all year round, while Notholca cf. jugosa is sporadic. Population dynamics appear to be dependent on changes in temperature and chlorophyll content. We have estimated the relative contribution of these three species to community biomass from their numerical abundance and measured volumes. The major community biomass peak lasts from March to June with a secondary maximum between October and January.

Abstract: Two taxa of Anuraeopsis, A. fissa fissa (Gosse, 1851) and A. miraclei nov. spec. occur in meromictic and stratified karstic lakes in Spain. They coexist, but may also live separated in different strata with A. miraclei restricted to a zone near the chemocline, characterized by low temperatures and low oxygen concentrations. The two Anuraeopsis species differ from each other in size, shape, structure and thickness of the lorica, size of the trophi elements, shape of Weber's organ, and egg-structure.

The influence of temperature (t) upon rotifer embryonic development rate (De) has been analysed using data from the literature, and the author's own results from experimental and natural populations. For Keratella cochlearis (Gosse), within the temperature range of 1–28°C, this relationship is best expressed by the equation: 1/De = 0.002 + 0.00025t + 0.000065t2.

Scanning electron micrographs have been prepared of two species of rotifer, Floscularia ringens (Linnaeus, 1758) and Asplanchna priodonta Gosse, 1850, from specimens stored in the collections of the British Museum (Natural History) for over seventy years. It is believed that these include the first scanning electron micrographs of a sessile rotifer.

In the course of investigations on the systematics and zoogeography of rotifers, the author found that 48% of all taxa treated showed a limited distribution (most were periphytic or benthic periphytic species from shallow waters). The following species were limited to the Palaearctic Region: Marine: Synchaeta triophtalmaLauterborn, S. voraxRousselet, and S. curvataLie-Pettersen. Brackish water: Colurella dicentra(Gosse), C. halophilaWulfert, and C. unicaudaEriksen. Among Holarctic species, Notholca psammarinaBuchholz & Rühmann and the two semi-species N.(striata) bipalium(O. F. Müller) and N(striata) striata(O. F. Müller) are discussed. A series of warm-stenothermic species were found to be pantropical: Lecane leontinaTurner, L. monostyla(Daday), L. harringi(Ahlstrom), Lepadella latusinus(Hilgendorf), Trichocerca chattoni(de Beauchamp), and Platyias leloupiGillard, while Lecane plesiaMyers and L. punctata(Murray) seemed to be confined to the Neotropical Region. The data collected also suggest that:i)i) Keratella wirketissiKutikova, K. kamtchaticaKutikova, K. cruciformis(Thompson), and K. eichwaldi(Levander) might be good species.ii)ii) Brachionus forficulaWierzejski, an Old World species, is a geographical vicariant of both B. havanaensisRousselet and B. traheaMurray from the New World.iii)iii) Cases of ecological vicariance are found in Testidinella elliptica(Ehrb.) and T. clypeata(O. F. Müller) from fresh and brackish water respectively; Keratella cruciformis(Thompson) and K. eichwaldi(Levander) from the sea and brackish water respectively; Keratella quadrata (O. F. Müller), K. valga(Ehrb.), and K. tropica(Apstein), dominant in arctic to cold-moderate, in warm-temperate, and in subtropical-tropical climates respectively. Some rotifer species are presently in expansion: the cases of K. tropicaand of Brachionus falcatusZacharias in Europe are analysed. A related case is that of man-made faunas: the presence of Brachionus havanaensisin Sangchrist Lake, Ill., USA, and the Rotifer fauna of the River Loire (France) are discussed.