Amphora margalefii Tomàs var. lacustris P. Sánchez var. nova,a new brackish water diatom

The rotifer Brachionus plicatilis can grow in a wide range of salinities and temperatures, but rapid shifts in both salinity and temperature may result in immobilized, non-swimming rotifers. The goal of this study was to examine the effect of perturbations in temperature and salinity on the swimming pattern of the rotifer.Only slight changes in mobility were observed when rotifers were exposed to changes in temperature (from 20 °C to 8–30 °C) and to an increase in salinity (from 20% to 30%). When the salinity was reduced to 15% and 5%, the proportion of mobile rotifers was reduced to 50% and 5%, respectively. The rotifers were throughout more resistant to perturbations in temperature than to those of salinity.Combined temperature and salinity perturbations compared to perturbations in each factor separately suggested a synergetic effect of temperature and salinity on the rotifers locomotion. Transfer from cultivation conditions to low salinity (5%) and high temperature (28 °C) resulted in very low percent of mobile rotifers (0–10%). However, if the temperature was reduced to 8 °C concomitant with the changes in salinity, the percent of mobile rotifers was 85%.Rotifers use a high share of their metabolic energy for locomotion, and it is therefore not surprising that perturbations in salinity and temperature may result in partial or complete immobilization.     

石油降解酶的筛选、固定化及添加表面活性剂强化其对石油的降解

在石油开采和加工过程中产生的含油污泥是石油化工行业中主要的污染源,会对周边生态环境造成持续性的危害。生物法降解被认为是一种有效且可持续的技术而备受关注。目前的研究多聚焦在微生物法降解石油,而酶法降解鲜有报道。【目的】本研究旨在优选石油高效降解酶,并深入分析其降解特性,以期构建一套酶法降解石油的体系。【方法】基于分子对接模拟技术,分析酶与石油类常见底物的结合可能性及结合模式,通过石油降解实验优选降解酶;通过添加表面活性剂及酶固定化进一步提高酶对石油的降解效果。【结果】利用分子对接模拟及实验验证获得5种具有石油降解能力的生物酶,其中枯草芽孢杆菌(Bacillus subtilis)来源的漆酶BsLac对石油的降解率最高,72 h可降解34.1%的石油;进一步研究发现,表面活性剂的添加均可促进BsLac对石油的降解,其中槐糖脂的促进作用尤为显著。当槐糖脂终浓度为1 000 mg/L时,石油的降解率最高,为46.3%;然而,2,2''-联氮-双(3-乙基苯并噻唑啉-6-磺酸)二铵盐的添加对BsLac降解石油的促进作用并不明显;固定化结果表明,以花生壳为吸附载体固定化的BsLac对石油的降解率最高,为56.3%。【结论】本研究基于分子对接模拟和实验筛选获得了石油降解酶BsLac,固定化的BsLac可实现石油的高效降解,为生物酶法降解石油的进一步探索奠定了实验和理论基础。     

Seasonal changes in the body size of two rotifer species living in activated sludge follow the Temperature‐Size Rule

Temperature‐Size Rule (TSR) is a phenotypic body size response of ectotherms to changing temperature. It is known from the laboratory studies, but seasonal patterns in the field were not studied so far. We examined the body size changes in time of rotifers inhabiting activated sludge. We hypothesize that temperature is the most influencing parameter in sludge environment, leading sludge rotifers to seasonally change their body size according to TSR, and that oxygen content also induces the size response. The presence of TSR in Lecane inermis rotifer was tested in a laboratory study with two temperature and two food‐type treatments. The effect of interaction between temperature and food was significant; L. inermis followed TSR in one food type only. The seasonal variability in the body sizes of the rotifers L. inermis and Cephalodella gracilis was estimated by monthly sampling and analyzed by multiple regression, in relation to the sludge parameters selected as the most influential by multivariate analysis, and predicted to alter rotifer body size (temperature and oxygen). L. inermis varied significantly in size throughout the year, and this variability is explained by temperature as predicted by the TSR, but not by oxygen availability. C. gracilis also varied in size, though this variability was explained by both temperature and oxygen. We suggest that sludge age acts as a mortality factor in activated sludge. It may have a seasonal effect on the body size of L. inermis and modify a possible effect of oxygen. Activated sludge habitat is driven by both biological processes and human regulation, yet its resident organisms follow general evolutionary rule as they do in other biological systems. The interspecific response patterns differ, revealing the importance of taking species‐specific properties into account. Our findings are applicable to sludge properties enhancement through optimizing the conditions for its biological component.     

Anhydrobiosis of <Emphasis Type="Italic">Adineta ricciae:</Emphasis> Costs and Benefits

To study the effect of anhydrobiosis on the rotifer life cycle, we dried a bdelloid species, Adineta ricciae, and determined the life-history traits of 1) the rehydrated animals and 2) the offspring produced after a period of dormancy. In the first experiment, a cohort was dried when 8-days-old, kept dry for 7 days and then rehydrated. Recovery was about 75%. The recovered rotifers had similar longevity and significantly higher fecundity than the hydrated controls. The time spent dry was completely disregarded by the anhydrobiotic rotifers, as predicted by the ‘Sleeping Beauty’ model. In the second experiment, we recorded the life-history traits of the orthoclones produced by recovered mothers aged 11 days, and 18 days. These orthoclones were coupled to controls that had been established from hydrated mothers aged 11 days and 18 days. The orthoclones produced after dormancy had significantly higher fecundity and longevity than the controls of same maternal age. Maternal age had a marked effect on the life-history traits of the orthoclones of both lines, causing the same loss of fitness in both.     

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.     

Enriching Rotifers with “Premium” Microalgae. Nannochloropsis gaditana

The nutritive quality of Nannochloropsis gaditana cultured semicontinuously with different daily renewal rates was tested as a diet for short-term enrichment of the rotifer Brachionus plicatilis. After 24 h, dramatic differences in the survival, dry weight, and biochemical composition of the rotifers depending on the renewal rate of microalgal cultures were observed. Survival after the feeding period increased with increasing renewal rates. Rotifers fed microalgae from low renewal rate, nutrient-deficient cultures showed low dry weight and organic contents very similar to those of the initial rotifers that were starved for 12 h before the start of the feeding period. On the contrary, rotifers fed nutrient-sufficient microalgal cells underwent up to twofold increases of dry weight and protein, lipid, and carbohydrate contents with regard to rotifers fed nutrient-depleted N. gaditana. Consequently, feed conversion rate decreased in these conditions, indicating a better assimilation of the microalgal biomass obtained at high renewal rates. No single microalgal biochemical parameter among those studied can explain the response of the filter feeder. Similarly to gross composition, EPA and n-3 contents in rotifers fed microalgae from nutrient-sufficient cultures were double than the contents found in rotifers fed nutrient-limited microalgae. In addition, very high positive correlations between the contents of EPA and n-3 in N. gaditana and B. plicatilis were observed. These results demonstrate that selecting the appropriate conditions of semicontinuous culture can strongly enhance the nutritional value of microalgae that is reflected in the growth and biochemical composition of the filter-feeder even in short exposure periods.     

Lipid classes and fatty acid composition of rotifers (Brachionus plicatilis) fed two algal diets

Rotifers (Brachionus plicatilis), maintained on baker's yeast, were fed for 24h upon two algal diets, Isochrysis galbana (diet A) and Isochrysis galbana + Nannochloropsis gaditana (diet B). (These algal diets were selected for their potential use as essential fatty acid (EFA) boosters, taking into account the requirements of fish larvae). The effect of these algal diets on total lipid content, lipid classes and fatty acid composition was studied. The total lipid content increased after feeding upon both diets but no significant differences were found between the two types. Neutral lipid and polar lipid contents increased and a positive correlation was observed between the neutral lipids content of rotifers and that of the food supplied. However, the content of polar lipids in rotifers did not depend upon that of the diet. The increase in neutral lipid content was found to be higher in rotifers fed upon diet B, compared to diet A which increased the phospholipid content. Non-enriched rotifers contained only small amounts of polyenoic fatty acids, i.e. 18:3n-6, 18:3n-3, 20:4n-6, 20:5n-3 and 22:6n-3, the contents of which increased significantly by feeding both diets. The EFA composition (20:4n-6, 20:5n-3 and 22:6n-3) of neutral lipids and phopholipids in rotifers reflected the EFA composition of each diet. Diet B-fed rotifers had the highest content in 20:4n-6 and 20:5n-3, whereas rotifers fed diet A and the highest 22:6n-3 content. The mixed diet I. galbana + N. gaditana enhanced substantially the composition of lipid classes i.e. neutral lipids and of n-3 PUFA of rotifers in comparison with Isochrysis or yeast diets.     

Physiological adaptations in the survival of rotifers (Brachionus plicatilis,O. F. Müller) at low temperatures

The biology of the rotifer Brachionus plicatilis has been studied extensively in recent years, due to its importance in aquaculture. Today, the culture of several marine fish species relies completely on the daily production of live rotifers.In the present paper we explore the conditions that facilitate maintaining live rotifers for extensive periods at low temperatures. In addition to its possible contribution in providing reserve stocks for commercial application, these studies may be of ecological importance. They could explain some of the physiological adaptive mechanisms that are involved in the survival of rotifers under adverse environmental conditions.Experimental results showed relatively high survival rates (82–85%) in rotifers that were cultured at 25 °C and exposed later to -1 °C for 12–14 days. During this period, rotifers were kept without food and their media were not changed. The survival was found to depend on the rotifer culture conditions, prior to exposure to -1 °C. These included the type of food fed to rotifers (yeast or algae), the salinity in which they were cultured, and an essential acclimation period of 2–6 day at 4 °C or 10 °C. The acclimation period was associated with the synthesis of at least one specific protein and accumulation of lipids. Profiles of protein synthesis in rotifers incubated at 10 °C revealed a 94 kD protein, which did not appear in rotifers cultured at 25 or 37 °C. Immunoisolation, using a polyclonal antibody that was prepared against HSP60, revealed that this protein was synthesized in rotifers kept at 10, 25 or 37 °C. However, this antibody did not react with the 94 kD peptide.In addition, rotifers kept at 10 °C accumulated substantial amounts of lipids, including eicosapentaenoic acid (EPA), which is found in the algae fed to them. These results support the hypothesis of specific adaptations to survival at low temperatures during an acclimation period.     

Schistosoma mansoni: Effect of rotifers on cercarial output,motility and infect1vity

Spontaneous colonization of rotifers on the shells of schistosome-infected Biomphalaria glabrata reduced the cercarial output of these snails and the motility and infectivity of the cercariae which did emerge. The rotifer effect was observed not only in the actual presence of rotifers, but also in rotifer-conditioned water from which the rotifers themselves had been removed by nitration. The significance of the rotifer effect to laboratory maintenance of S. mansoni is considerable. It may be one of many factors which reduce the infective index under field conditions.     

氧化胁迫诱导苍白杆菌JP1形成VBNC状态及其特征

石油降解菌在各种有害环境因素作用下会进入活的非可培养(viable but non-culturable, VBNC)状态，从而影响其生长及石油降解率。为了研究有害环境因素对石油降解菌生长及石油降解率的影响，采用分光光度法、荧光染色-激光共聚焦显微镜观测H₂O₂胁迫下苍白杆菌(Ochrobactrum sp.)JP1细胞的生长及VBNC状态形成情况。结果表明，不同浓度H₂O₂对其生长有一定抑制作用，当培养液中H₂O₂浓度为75.0 mmol/L时，可有效抑制苍白杆菌JP1生长，处理12 h后苍白杆菌JP1进入VBNC状态。VBNC状态的苍白杆菌JP1细胞缩小变成球体，周质间隙增大；在适宜条件下，VBNC状态苍白杆菌JP1能够复苏为可培养状态，添加丙酮酸钠能够促进VBNC状态细菌细胞的复苏。复苏后的苍白杆菌RJP1具有良好的环境适应性和石油降解能力，为石油污染生物修复的菌种筛选及应用提供了新的策略。     

一株耐碱高效长链烷烃降解菌C24MT1的筛选及其降解特性

【背景】石油被称为“液体黄金”,人类的工业生产活动在利用其创造巨大社会价值的同时,也对自然环境造成了严重的污染。微生物修复技术是现阶段治理石油类污染有效的手段之一,具有经济、高效、无二次污染等优点。【目的】从受石油污染的土壤中分离高效降解长链烷烃正二十四烷的菌株,探究其降解特性及在微生物修复中的应用前景。【方法】通过形态学及16S rRNA基因测序进行菌株鉴定,采用气相色谱法检测菌株对正二十四烷的降解效果,并结合气相色谱-质谱(gas chromatography-mass spectrometer, GC-MS)分析降解中间产物以推测其潜在代谢途径。【结果】筛选到一株可高效降解正二十四烷的菌株C24MT1,经鉴定为不动杆菌属(Acinetobacter)。该菌株最适降解条件为30 °C、pH 9.0、盐度2 g/L,该条件下生长7 d对9 g/L正二十四烷的降解率高达86.63%;与此同时,菌株在强碱性环境(pH 11.0)中生长良好(OD₆₀₀为0.39)并保持较高烷烃降解率(75.38%),对极端环境具备较强的耐受能力;对降解中间产物进行分析,推断菌株代谢长链烷烃正二十四烷的途径可能包括末端氧化及次末端氧化。【结论】不动杆菌C24MT1具有良好的环境适应能力及烷烃降解能力,在后续微生物菌剂开发和石油类污染土壤的环境修复领域具有巨大的应用前景。本研究可为盐碱地区高浓度石油类污染土壤的修复提供优良菌种,并进一步丰富石油烃类生物降解的菌种资源库。     

Effectiveness of the cleaning performance of the bacterial strain <Emphasis Type="Italic">Acinetobacter</Emphasis> sp. SG06-02 in the petroleum hydrocarbon-polluted intestine of clams

Summary A petroleum-degrading bacterial strain Acinetobacter sp. SG06-02 in the oil-polluted intestine of shellfish clam was previously isolated in our laboratory. In this study, the effectiveness of using its cleaning performance on petroleum pollution was investigated. The accumulation of petroleum in the intestine of Ruditapes philippinarum was determined by a UV spectrophotometer. The survival and growth of the SG06-02 strain in the intestine of Ruditapes philippinarum after oil pollution were examined. The concentration of oil accumulated in the Ruditapes philippinarum intestine quickly rises to 254.97 mg/kg within 12 h after placing the clam in the petroleum-polluted seawater tank. After decontaminating using Acinetaobacter we found that the concentration of petroleum inside the Ruditapes philippinarum declined significantly. The results of the cleaning experiments showed that the degradation activity by using bioremediation method of the strain SG06-02 increased 12.8∼30.2% compared to using clean seawater in 4 days. This research indicated that the petroleum degrading bacteria could survive and was effective in cleaning oily pollutants in the seashell.     

The use of marine yeast (Candida sp.) and bakers' yeast (Saccharomyces cerevisiae) in combination with Chlorella sp. for mass culture of the rotifer Brachionus plicatilis

Use of marine yeast and bakers' yeast in combination with Chlorella sp. for the large-scale production of the rotifer Brachionus plicatilis was investigated. The culture density of marine yeast fed rotifers was significantly higher than rotifers fed bakers' yeast. Rotifer production was significantly higher and the doubling time was lower for marine yeast fed rotifers than for bakers' yeast fed rotifers. It appears that the addition of marine yeast to the feed enhances the birth rate and overall production of rotifers in the culture system. The nutritional quality of rotifers is discussed.     

Predatory and toxic effects of the turbellarian (Stenostomum cf leucops) on the population dynamics of Euchlanis dilatata,Plationus patulus (Rotifera) and Moina macrocopa (Cladocera)

Catenulid turbellarians, common in shallow, tropical ponds, affect their rotifer prey via the production of toxins. There is, however, no quantitative information on their effect on the demography of their prey. Here, we test the impact of Stenostomum cf leucops on the population dynamics of the rotifers Euchlanis dilatata and Plationus patulus, and the cladoceran Moina macrocopa. Experiments were initiated with rotifers at 0.5 ind. ml⁻¹ and the cladoceran at 0.2 ind. ml⁻¹; growth patterns were compared in the absence and presence of worms (2 Stenostomum ind. per 50 ml). Results revealed that brachionids were most adversely affected: there was a lower growth rate of the rotifers in the presence of worms (P < 0.01, repeated measures ANOVA), although at the densities applied, the predator did not wipe out its prey. These littoral predators may therefore regulate rotifer prey in natural conditions. In Moina, the population evolved differently; initially, we found no difference between control and treatment, but after about 10 days, the population collapsed, irrespective of a direct or indirect contact with the predator. This delayed effect deserves more study, as it could represent flatworm toxin accumulation by the cladoceran.     

Vertical niche separation of two consumers (Rotatoria) in an extreme habitat

Herbivore populations are commonly restricted by resource limitation, by predation or a combination of the two. Food supplement experiments are suitable for investigating the extent of food limitation at any given time. The main part of this study was performed in an extremely acidic lake (pH 2.7) where the food web consists of only a few components and potential food sources for herbivores are restricted to two flagellates. Life table experiments proved that Chlamydomonas was a suitable food source whereas Ochromonas was an unsuitable food source. The two flagellates and the two rotifers exhibit a pronounced vertical distribution pattern. In this study, a series of food supplement experiments were performed in order to: (1) quantify and compare potential resource limitation of two primary consumers (Cephalodella hoodi and Elosa worallii, Rotatoria) over time, (2) compare their response at different temperatures, (3) evaluate the effect of having an unsuitable food source alongside a valuable one, (4) estimate the effect of predation on rotifers by Heliozoa, and (5) compare the results with those from other acidic lakes. Additionally, the spatio-temporal population dynamics of both species were observed. The field data confirmed a vertical separation of the two species with E. worallii dominating in the upper water layers, and C. hoodi in the deeper, cooler water layers. The results from the food supplement experiments in which Chlamydomonas served as the supplemented suitable food source showed that the two rotifers were food limited in the epilimnion throughout the season to different extents, with Cephalodella being more severely food limited than Elosa. The experiments at different temperatures provided evidence that Elosa had a higher optimum temperature for growth than Cephalodella. When the unsuitable food algae Ochromonas was added alongside the suitable food source Chlamydomonas, C. hoodi was unaffected but E. worallii was negatively affected. Predation of Heliozoa on rotifers was observed but the total effect on the rotifer dynamics is probably low. The comparison with other lakes showed that resource limitation also occurred in one other lake, although to a lesser extent. Overall, the vertical separation of the two rotifers could be explained by both their differential extent of resource limitation and differential response to temperature.     

The composition and structure of rotiferan and crustacean communities of the lower rio nhamundá, amazonas,Brazil

The crustaceans and rotifers of 17 samples from the plankton and 3 from the littoral from lakes of the lower Rio Nhamundá, an affluent of the Amazon were studied.

145 taxa of rotifers and 46 of crustaceans were found; two rotifers and three crustaceans are new to science: Keratella americana nhamundaiensis, Euchlanis triquetra var. nhamudaiensis, Echinisca superaculeata, E. sioli, and E. mira. In the taxonomical part 24 rotiferan and 22 crustacean taxa are described in more detail, drawings of them given, and partly remarks made about the geographical distribution.

In the biocenotical part the conditions of dominances were studied: Brachionus zahniseri and Keratella americana within the rotifers and Bosminopsis deitersi and Oithona amazonica within the crustaceans are the dominant species in the pelagial. In the littorial Synchaeta stylata and Stre‐blocerus pygmaeus were found to be dominant.

The species diversity and also the evenness is higher within the rotifers than in the crustaceans. By means of an index of similarity dendrograms were drawn where groups of lakes can be differentiated by using the similarity between the crustaceans. The proportion of neotropical species in relation to pantropical and cosmopolitical species is lowest in the rotifers, higher in the cladocerans and highest in the copepods at 100%.     

石油污染生态系统中细菌群落结构及其代谢机制研究进展

石油化工产品的不合理处置与泄漏导致石油及其衍生物大量释放到环境中,由此造成的环境污染问题日益严重,石油污染已成为全球性公害之一。微生物修复技术凭借其成本低、环境友好等优势,广泛应用于石油污染的治理。大量研究表明功能微生物群落在石油污染生态系统的修复体系中发挥了重要的作用。其中,细菌是最主要、最活跃的石油降解微生物。然而,在原位/异位生物修复过程中,存在功能菌群在污染体系中难维持、易失调及石油烃降解途径不明晰等问题。因此,本文总结了石油污染自然生态系统和微宇宙实验体系中的细菌群落结构、石油烃代谢机制及相关功能基因,并对微生物法处理石油污染的未来研究方向提出展望,为石油污染场地生物修复方案的制定提供理论参考。     

盐酸四环素浓度和食物密度对萼花臂尾轮虫生活史特征的综合影响

近年来,抗生素的环境污染问题已引起人们的广泛关注,其在环境中的残留可对水生态系统的结构和功能产生重要影响。迄今,盐酸四环素浓度和食物密度对轮虫生活史特征的影响研究尚未见报道。以萼花臂尾轮虫(Brachionus calyciflorus)为受试动物,研究了在不同斜生栅藻(Scenedesmus obliquus)密度下,不同浓度的盐酸四环素(Tetracycline hydrochloride)对萼花臂尾轮虫生活史特征的影响。结果表明,食物密度和盐酸四环素浓度对轮虫的生命期望、净生殖率、世代时间、内禀增长率、平均寿命和后代混交率,以及食物密度和盐酸四环素浓度间交互作用对除了内禀增长率外的其他5个统计学参数均具有显著影响。在各食物密度下,轮虫的特定年龄繁殖率高峰值随着盐酸四环素浓度的增加呈现先增加后降低的趋势;盐酸四环素对轮虫生长和繁殖参数的影响呈现"低促高抑"的特点。3个食物密度下,高浓度的盐酸四环素显著提高了萼花臂尾轮虫的后代混交率,且在1.0×10~6个/mL藻密度下毒物浓度与轮虫后代混交率间具有显著的剂量-效应关系。食物密度的高低显著影响盐酸四环素对萼花臂尾轮虫的毒性效应。     

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.     

Environmental factors influencing the vertical migration of planktonic rotifers in a hypereutrophic tarn

The diel vertical migration of planktonic rotifers in a small, hypereutrophic tarn was investigated on four occasions in 1983. When the tarn was isothermal the rotifers were distributed throughout the water column. After stratification, the rotifers were confined to the top 1–2 m of oxygenated water. On all four dates the rotifers were aggregated at specific depths in the water column. On some occasions, the pattern of aggregation changed as the animals performed distinct diurnal migrations. Keratella cochlearis, K. quadrata and Polyarthra vulgaris usually followed the reverse migrations of the phytoplankton. In contrast, the movements of Anuraeopsis fissa were less pronounced and were associated with variations in the depth of the oxycline.