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.     

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.     

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.     

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.     

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.     

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.     

Chronic toxicity of methylparathion to the rotifer Brachionus calyciflorus fed on Nannochloris oculata and Chlorella pyrenoidosa

The effect of sublethal levels of methylparathion (0, 1, 3, 5, 7 mg l^–1) on the freshwater rotifer, Brachionus calyciflorus, during their entire life cycle was studied. Rotifers were fed on two species of unicellular algae: Nannochloris oculata and Chlorella pyrenoidosa; both algal concentrations were 5 × 10⁵ cell ml^–1.The parameters used to determine the toxicity of this compound were survival, fecundity, net reproductive rate (R)_o, generation time (T), intrinsic rate of natural increase (r), reproductive value (V _x/V_o) and life expectancy at hatching (eo). All the demographic parameters studied were affected by methyl-parathion exposure on rotifers fed on both species of algae, but the toxic effect was larger when animals were fed on Chlorella pyrenoidosa; in this case, animals showed a decreased in fertility and also a delayed first reproduction. Sublethal methylparathion levels produced a reduction in most of the parameters selected, especially after exposure to 7 mg l^–1, where the animals died before reproducing.     

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.     

Life history characteristics of Brachionus plicatilis (rotifera) fed different algae

A detailed study of the life history of the rotifer Brachionus plicatilis was done at 20 °C, 20 ppt salinity, and 90 mg C 1^–1 food concentration. Rotifers were grown individually in culture plate wells (150 µl culture volume) and fed Isochrysis galbana Tahiti, Tetraselmis sp., Nannochloris atomus, or a l : 1 mixture (weight) of two of the algae. Observations were made every 2–8 hr and rotifers were sized and transferred to new food daily. A total of 19 different parameters were compared. Rotifers fed Isochrysis averaged 21 offspring per female, a 6.7 day reproductive period, a lifespan of 10.5 days and a mean length of 234 µm. After Isochrysis, the foods giving the highest growth, survival, and reproduction in decreasing order were Isochrysis + Nannochloris, Nannochloris, Isochrysis + Tetraselmis, Tetraselmis + Nannochloris, and Tetraselmis. Although the small volume culture system used in this study seems appropriate for studying life history of B. plicatilis, the results cannot always be directly applied to larger cultures.     

Demographic parameters of Brachionus calyciflorus Pallas (Rotifers) exposed to sublethal endosulfan concentrations

The effects of sublethal levels of endosulfan (0, 1, 1.5, 2.5 and 3.3 mg 1^–1) on the demography of the rotifer Brachionus calyciorus were studied. Life expectancy at birth (e _o), net reproductive rate (Ro), generation time (T) and intrinsic rate of natural increase (r) were significant differentes between blank controls and controls with acetone. The effective endosulfan concentration at which a given parameter value was reduced to 50% of the controls (EC50) was calculated for life expectancy.     

Seasonal changes in zooplankton abundance in the lower Rhine during 1987–1991

Zooplankton in the River Rhine was surveyed for five years at the Dutch sampling stations, Lobith (German/Dutch border) and Maassluis (at the point of discharge of the river into the North Sea). The zooplankton abundance showed an apparent seasonal pattern at both stations, characterized by low densities during the winter period, and higher densities during the summer period, with a spring peak. Zooplankton was dominated by rotifers at both stations, although during the winter periods the contribution of copepods was considerable. The rotifers were dominated byBrachionus angularis, B. calyciflorus, Keratella cochlearis andK. quadrata; the copepods by cyclopoid nauplii; the cladocerans by small-sized species mainly belonging toBosmina. At Maassluis the relative contribution of copepods was higher than at Lobith. Furthermore, the zooplankton at Maassluis included the speciesEurytemora affinis, characteristic for estuarine conditions. In spring, the rotifer density and water temperature and rotifer density and chlorophylla concentration were positively correlated. Furthermore, both rotifer density and chlorophylla were inversely correlated with discharge. The possible role of environmental factors (water temperature, chlorophyll content, discharge and biotic factors) controlling the river zooplankton dynamics is indicated.     

An individual-based population model for the prediction of rotifer population dynamics and resting egg production

Most species of rotifers have a combination of sexual and asexual reproduction, with sexual reproduction resulting in resting eggs, which can lay dormant for long periods. The occurrence of sexual reproduction affects population dynamics through the temporary presence of male rotifers, and a reduction in the growth of the number of female rotifers. A previously published, individual-based model used dynamic energy budget theory to describe rotifer food intake, growth, egg production, and mortality, but assumed asexual reproduction only. In the current study, we have expanded the model to describe the entire reproductive cycle of the rotifers, making it usable for investigating relationships, such as those between the signal triggering mictic egg production, and the timing and number of resting eggs produced. The model is intended for use in predicting the specific future development of cultures, for instance, as a process model in rotifer or resting egg production for aquaculture. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research     

Intraspecific variability of Brachionus plicatilis

An extensive study of frontal margins of the lorica of Brachionus plicatilis was undertaken in an attempt to define its variability within this species. Specimens from mass cultures and from the natural environment were examined.     

Temporal and spatial distribution of rotifera in a Chilean reservoir: A possible effect of impoundment hydrodynamics

Rotifers were sampled at monthly intervals for a year at four monitoring stations in Rapel Reservoir (a hydroelectric impoundment) in Central Chile (34°02S; 71°35W). Fifteen species were identified, but only Keratella cochlearis was consistently found at each station, it usually was the most abundant (> 50% of total rotifers). Marked differences were found among sites; stations I and 2, those nearest to the dam, showed greatest rotifer densities in spring and autumn. Station 4, that nearest to the inflowing rivers, had its highest rotifer abundance in summer. Station 8 had the highest mean density but the least marked seasonal changes in rotifer numbers.The impoundment water dynamics and the relative locations within the lake (distance from the dam or from the rivers) are suggested as determinants for different rotifer assemblages and, probably, for most other planktonic organisms.     

Life table demography and population growth of Brachionus variabilis Hempel, 1896 in relation to Chlorella vulgaris densities

We studied the life history variables and population growth characteristics of Brachionus variabilis, which was recorded for the first time from Mexico. The animals were fed Chlorella, using five concentrations (0.25, 0.5, 1, 2 and 4 × 10⁶ cells ml^–1) at 25 °C. Food density was observed to have significant effect on life expectancy, average lifespan, gross reproductive rate, net reproductive rate, generation time and population growth rate. The average lifespan ranged from 3 to 6 days depending on the food density. The net reproductive rate ranged from 2 to 7 neonates female^–1 d^–1. The rate of population increase per day varied from 0.14 to 0.35. The highest net reproductive rate and average lifespan and life expectancy were recorded at Chlorella concentrations of 1 × 10⁶ and 2 × 10⁶ cells ml^–1.     

Biotechnology and aquaculture of rotifers

Biotechnology can be defined as any technology that involves living organisms or their derivatives. In applying this definition to rotifers, we focus on their contribution in culturing of early larval stages of marine fish. After almost four decades of marine fish culture in captivity, the success of this worldwide industry is still quite dependent on mass culture of the species Brachionus plicatilis and B. rotundiformis. In mass culture, the rotifers are continuously driven to reproduce at high rates, in relatively extreme environmental conditions of high population density and high loads of organic matter. Therefore, the success of mass cultures and future improvements in these systems relies on a close interaction between basic and applied studies of rotifers. In the present review, we will attempt to analyze why rotifers are suitable for early life stages of fish and to describe, in general, methodologies that have been devised for reliable supply of rotifers in large quantities. Problems associated with rotifer production, nutritional quality and effect on fish health and nutrition, will be discussed. Research on B. plicatilis and B. rotundiformis has increased enormously during the past three decades and these two species are the best-studied rotifers so far. While much of the research on these species is directed or devoted to the needs of aquaculture industry, they are also used as models for addressing basic biological questions, due to the relative ease of culture and their availability. Studies on feeding, pheromones, speciation in rotifers, the occurrence and putative hormones involved in sexual and asexual reproduction and production of resting eggs, are few examples of such studies. Rotifers will probably maintain their role as food organism for fish larvae, in spite of attempts to replace them with more accessible formulated food. Development of new culture methods that will improve the nutritional quality and production efficiency of rotifers may result in more diversified and flexible tasks for these organisms in aquaculture.     

Molecular genetics of rotifers: preliminary restriction napping of the mitochondrial genome of Brachionus plicatilis

Methods are presented to extract and purify mitochondrial DNA from the rotifer Brachionus plicatilis. The mtDNA obtained is of sufficient purity for digestion with restriction endonucleases. EcoR I restriction patterns are presented for 4 geographically separated clones. A restriction map based on digestion with 5 different restriction enzymes is included for one of these clones. Finally, use of mtDNA analysis for studies on the population structure and biogeography of rotifers is discussed.     

Demographic parameters of Brachionus patulus Muller (Rotifera) exposed to sublethal DDT concentrations at low and high food levels

The combined effects of sublethal levels of DDT (0, 15, 30, 45, and 60 µg·l^–1) and food (Chlorella at 1 and 3 × 10⁶ cells ml^–1) on the demography of the rotifer Brachionus patulus were studied. The average lifespan, life expectancy, net reproductive rate (R_o) and intrinsic rate of increase (r) were significantly greater at the higher food level, but declined with increasing DDT concentration. A significant DDT food interaction was manifested in the toxicity-mitigating action of the higher food density at higher DDT levels. The r values were negative at DDT levels 30 µg · l^–1 and above, only when food density was low. The effective DDT concentration at which a given parameter value was reduced to 50% of that in the controls (EC50) was lower at lower food levels for both survivorship and reproduction, and lower for reproduction than for survivorship at either food level.     

The influence of sampling strategy on the apparent population dynamics of planktonic rotifers

Rotifer composition and density, chlorophyll-a content, secchi depth and temperature were investigated with high sampling frequency (every third day in summer, weekly in winter) in the eutrophic lake Bielersee from April 1981 to April 1982. Long intervals between sampling, especially from spring to autumn, lead to contradictory interpretation of abundance dynamics of both total rotifer community and single species. Short intervals between sampling revealed a sequence of mass developments of the different species. As an example, alternating gradation phases are shown for Synchaeta grandis and Synchaeta stylata.     

Factors affecting low temperature preservation of the marine rotifer Brachionus rotundiformis Tschugunoff

Experiments were performed to determine suitable conditions for low temperature preservation of small S (Fukuoka) and ultra-small SS (Thai) strains of B. rotundiformis. For this, single rotifers (an adult bearing one egg or a 4-h neonate) were incubated for 10 days in 1 ml seawater (22 ppt salinity). The highest survival was achieved at 10 and 12 °C for S-strain and 12 °C for SS-strain. The effect of salinity, change of culture medium and feeding regime were further tested on rotifers (300 ind. ml^–1) cultured in vials containing 10 ml seawater and microalgae at 12 °C. Survival of S-strain was highest (55.5±0.8%) at 35 ppt, while SS-strain survived best (43.1±2.6%) at 17 ppt. Survival was suppressed by changing the culture medium every 4 days. Feeding rotifers every 2 days yielded better survival (66.2±6.6%: S-strains, cultured at 35 ppt and 81.8±5.2%, SS-strains cultured at 17 ppt) than feeding them only at the beginning of the experiment or at 4-day intervals. An acclimation at 20 °C for 24 h before transferring them from their usual culture temperature (28 °C) to 12 °C resulted in higher survival of SS-strain. For S-strain, however, no significant improvement resulted from acclimation. SS-strain was more susceptible to lower temperature and higher salinity than S-strain.