养鱼池轮虫休眠卵分布和萌发的研究

东北地区部分鱼场养鱼池底泥中轮虫的休眠卵,表层(0—5厘米)数量每平方米为1.2—503万个,个别池塘高达1,573万个,其中完全暴露于泥表面的数量约占1—2％。休眠卵数量差别和池塘环境条件关系密切;各泥层中的轮虫休眠卵数量呈“V”形垂直分布的趋势。萼花臂尾轮虫和角突臂尾轮虫的休眠卵在水温10—40℃;pH4.5—11.5;溶氧0.3毫克/升以上和盐度8.5％以下的条件下可以萌发。10℃为其发育的生物学零度。根据轮虫休眠卵分布和它的环境条件的关系,以及萌发的生态条件,阐述了在养鱼池中增殖轮虫所应采取的措施及其理论根据。     

Ecological problems of Lake Ladoga: causes and solutions

We studied the outcome of competition between a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer species at five algal (Scenedesmus acutus) concentrations (0.5 × 10⁶ to 40.5 × 10⁶ cells ml^–1) and with varying initial densities in mixed populations (100 to 0% of B. calcyciflorus or A. fissa), the combined initial biomass being 0.2 µg ml^–1 in all test jars. Experiments were conducted at 28 ± 1 °C.Regardless of food concentration, B. calcyciflorus showed a greater increase in biomass than A. fissa, peak densities (mean ± standard error) at the lowest food concentration in the controls being 1.34 ± 0.31 µg dry weight ml^–1 and 0.82 ± 0.08 dry weight ml^–1, respectively. At the lower food concentrations, A. fissa displaced B. calyciflorus and vice versa at the higher food concentrations. At the intermediate food concentrations of 4.5 × 10⁶ cells ml^–1, B. calyciflorus outcompeted A. fissa only if its initial population density was three times higher. The rates of population growth in controls varied from 0.792 ± 0.06 d^–1 to 1.492 ± 0.13 d^–1 for B. calyciflorus and 0.445 ± 0.04 to 0.885 ± 0.01 for A. fissa depending on food level. When both species were introduced together, low food levels favoured higher abundance of A. fissa than B. calyciflorus, suggesting, in nature, it is likely that small Anuraeopsis colonize oligotrophic water bodies more successfully than larger Brachionus. The results also suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.     

Comparative studies of the population genetic structure of the Brachionus calyciflorus species complex from four inland lakes in Wuhu,China

To study how the population genetic structure in zooplankton respond to environmental conditions, using comparative limnology, the genetic diversity and genetic differentiation of the B. calyciflorus complex collected from four inland lakes in Wuhu City, China were investigated based on the 16S rRNA gene and nuDNA ITS sequences. The results displayed a high genetic diversity, and the nucleotide diversity of the 16S rRNA gene was higher than that of the ITS sequence. The phylogenetic analyses grouped the four populations into two cryptic species (Bc-JT and Bc-FL) with strong support. The two cryptic species were found in lakes with different trophic levels, demonstrating significant ecological specialization. The origins of clone TW12 were not consistent in the phylogenetic trees between two genetic markers, which might be attributed to the effects of male-mediated gene flow on the phylogenetic relationships of rotifers. The nucleotide diversity of the cryptic species Bc-JT was higher than that of Bc-FL, indicating that eutrophication might decrease the genetic diversity of cryptic species. The total phosphorus concentration in water bodies might be the most important factor affecting the genetic diversity of species.     

The effects of thin layers on the vertical distribution of the rotifer, Brachionus plicatilis

Microscale patches of resources occur in both the horizontal and vertical dimensions, and in the latter case are referred to as thin layers. These layers may affect ecological processes like behavior, predation, growth and reproduction in phytoplankton and zooplankton. The objective of this study was to determine possible effects of physical and biological thin layers on the vertical distribution and diel vertical migration of the rotifer Brachionus plicatilis. We used four, 2 m tall, experimental tanks fitted with video cameras which panned the vertical extent of each tank and enabled us to sample on the centimeter scale. The experimental tanks consisted of a thin layer (25 cm thick) of Nannochloropsis oculata, whereas control tanks consisted of homogeneously distributed algae. Rotifers aggregated in the thin layers of N. oculata, and dispersed (becoming evenly distributed) after depleting the algae within the thin layer (ca. 6 h). In contrast, rotifer aggregation in the physical thin layer of control tanks was shorter in duration (ca. 2.5 h) and rotifers were homogenously distributed for the remainder of the experiment despite persistent salinity stratification. No signs of diel vertical migration were noted in either experimental or control tanks. A second set of experiments was run to examine the response of rotifers to a choice of food, i.e., thin layers of the diatom, Skeletonema costatum versus the eustigmatophyte, N. oculata. For the choice experiments, two thin layers were created, one with each food option. Our results suggest that B. plicatilis aggregates and feeds preferentially on N. oculata over S. costatum. In both types of experiments rotifers responded, in terms of distribution, to thin layers of algae within the first half-hour of introduction and remained in the thin layers until the food source was depleted. Our results suggest that rotifers may be important grazers on thin layers because of their ability to quickly locate and take advantage of ephemeral food patches.     

Distribution and abundance of zooplankton populations in Crater Lake,Oregon

The zooplankton assemblages in Crater Lake exhibited consistency in species richness and general taxonomic composition, but varied in density and biomass during the period between 1988 and 2000. Collectively, the assemblages included 2 cladoceran taxa and 10 rotifer taxa (excluding rare taxa). Vertical habitat partitioning of the water column to a depth of 200 m was observed for most species with similar food habits and/or feeding mechanisms. No congeneric replacement was observed. The dominant species in the assemblages were variable, switching primarily between periods of dominance of Polyarthra-Keratella cochlearis and Daphnia. The unexpected occurrence and dominance of Asplanchna in 1991 and 1992 resulted in a major change in this typical temporal shift between Polyarthra-K. cochlearis and Daphnia. Following a collapse of the zooplankton biomass in 1993 that was probably caused by predation from Asplanchna, Kellicottia dominated the zooplankton assemblage biomass between 1994 and 1997. The decline in biomass of Kellicottia by 1998 coincided with a dramatic increase in Daphnia biomass. When Daphnia biomass declined by 2000, Keratella biomass increased again. Thus, by 1998 the assemblage returned to the typical shift between Keratella-Polyarthra and Daphnia. Although these observations provided considerable insight about the interannual variability of the zooplankton assemblages in Crater Lake, little was discovered about mechanisms behind the variability. When abundant, kokanee salmon may have played an important role in the disappearance of Daphnia in 1990 and 2000 either through predation, inducing diapause, or both. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Zooplankton feeding ecology: bacterivory by metazoan microzooplankton

Bacterivory by the rotifer Brachionus plicatilis Müller, nauplii and copepodites of the copepods Centropages Krøyer sp. and Acartia tonsa Dana, and the tintinnid Favella panamensis Kofoid & Campbell was examined using fluorescently labelled bacteria (FLB) and epifluorescence microscopy. FLB were < 1 μm in diameter, and were offered at environmental concentrations (1.47−9.08 × 10⁶ cells·ml⁻¹). FLB were visible within rotifers, nauplii, copepodites, and tintinnids, confirming ingestion. Rotifer clearance rates (32–418 μl·animal⁻¹·h⁻¹) exhibited no relation with FLB concentration. In some cases rates of clearance of FLB by rotifers were different with alternative phytoplankton food (Nanochloris Naumann sp.) than in replicates with FLB alone, whereas in other cases presence of alternative food exhibited no clear effects on rates of ingestion of FLB. Clearance rates for all six naupliar stages of A. tonsa nauplii (0–320 μl·animal⁻¹·h⁻¹) were stage-related, with higher rates by NIII-VI nauplii than NI-II nauplii. Nauplii had higher rates of clearance of FLB in the absence of alternative phytoplankton food (Isochrysis Parke sp.). Clearance rates of FLB by a single stage of Centropages sp. nauplii, A. tonsa CI copepodites and F. panamensis (each obtained at only a single food concentration of either 1.5 or 5.0 × 10⁶ cells·ml⁻¹) were within the range of 85–142 μl·animal⁻¹·h⁻¹. These ranges were similar to those of rotifers and A. tonsa nauplii. This is the first report of FLB ingestion by metazoan marine microzooplankton. Although rotifers and ciliates might be expected to ingest small particles such as FLB using ciliary induced feeding currents, the means by which nauplii and copepodites eat FLB is less clear. We propose that they may “eat” bacteria as they “drink” to osmoregulate.     

Effect of diets containing γ-linolenic acid on n-6 highly unsaturated fatty acid content of rotifer (Brachionus plicatilis)

The effect of various diets containing linoleic and/or -linolenic acids was studied on n-6 fatty acid composition of the rotifer Brachionus plicatilis. The rotifer's abilities for transformations of n-6 fatty acids were evaluated. Diets containing only linolenic acid as n-6 fatty acid induced low levels of other n-6 fatty acids in rotifers while a diet containing also -linolenic acid led to substantial amounts of di homo -linolenic acid in the rotifers through elongation. Desaturation of -linoleic acid to gamma linolenic appears to be the limiting factor of n-6 highly unsaturated fatty acid biosynthesis by the rotifer. Two sets of experiments were compared using different techniques and different sources of -linolenic acid: Spirulina in inert food or borage oil in emulsion with baker's yeast. Rotifers fed with inert diet with Spirulina contained arachidonic acid while those fed with borage oil had very low arachidonic content. High level of n-3 fatty acids incorporated into the diets seemed to exert inhibitory effects on n-6 transformation rate.     

Gravel pits support waterbird diversity in an urban landscape

Few studies exist documenting changes in rotifer communities over long time intervals. Here, we explore seasonal and long-term variation in rotifer communities in four Polish lakes sampled in 1976 and again in 1997. Rarefied, asymptotic species richness did not differ significantly across study years, although values in 1997 tended to be higher. Simpson’s and Shannon–Wiener diversity measures provided inconsistent temporal results, with only the former indicating significantly higher richness in 1997. Sorensen’s coefficient of community similarity was as high among lakes in 1976 (0.81) and in 1997 (0.76) as within lakes across the 21-year span (0.77). Nonlinear redundancy analysis of species’ abundances revealed large, consistent seasonal changes across lakes, smaller consistent shifts between sampling periods, and small differences between lakes. Collectively, these metrics indicate that species composition was relatively stable among lakes within years and within lakes between years, while species’ abundance patterns were far more variable and most affected by season. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: S. I. Dodson     

Studies on the ultrastructure of the rotifer Habrotrocha rosa Donner (Aschelminthes)

The alimentary system of Habrotrocha rosa is composed of the alimentary channel, five digestive glands and another gland which is in close relation to the intestinal syncytium. After the present investigation, the alimentary channel can be divided into sections which clearly differ from each other, these are: oral cavity, pharynx, oesophagus, mastax, stomach-hose, syncytium of intestine, and terminal intestine.