REFRACTIVE INDEX OF THE PROTOPLASM IN SEA URCHIN EGGS*

The distribution of the refractive index (RI) of the protoplasm in sea urchin eggs was determined from the optical path differences at various regions of the cell measured by interference microscopy assuming that the cell structure is symmetrical about the line passing through the center of the cell and that of the nucleus in unfertilized eggs and about the spindle axis in fertilized eggs during mitosis and cleavage. The RI of the cytoplasm in the unfertilized egg was uniform except for the cortical region, which had the RI higher than that of the underlying endoplasm. The RI of the cortex was generally higher than that of the underlying endoplasm, which did not appreciably change during mitosis and cleavage. The RI of the nucleus was lower than that of the cytoplasm. The RI of the mitotic apparatus was lower than that of the surrounding cytoplasm. The fertilization membrane had a thickness of about 0.6 μm in hydrated state and about 25 nm in dried state (mean values). The RI of the perivitelline space was about 0.00015 higher than that of seawater, equivalent to 0.08 g/100 ml of contents.     

The impact of predation by burbot (<Emphasis Type="Italic">Lota lota</Emphasis> L.) on the macroinvertebrate community in the littoral zone of a large lake

We studied the effects of predation by juvenile burbot (Lota lota) on the macroinvertebrate community in mesocosm experiments in the stony littoral zone of Lake Constance, a large prealpine lake in Central Europe. Although the growth data of the burbot suggest that the benthivorous fish exerts strong predation pressure on the invertebrate community, the predicted level of consumption is only poorly reflected by changes in biomass and abundance of most invertebrate prey taxa. High exchange rates of the prey between the mesocosms and the ambient littoral environment apparently masked the true predation effects of fish. Also, life-history events such as hatching or synchronised emergence of larvae led to temporal effects that obscured the impact of predation. However, for the dominant prey organism, the freshwater amphipod Gammarus roeseli, direct lethal effects appeared when its migration was limited. When exchange with the ambient littoral zone was possible, abundance and biomass of G. roeseli were unexpectedly high in the mesocosms stocked with burbot, indicating behavioural responses to fish presence. During the experiment, the burbot gained ca. 15% in length and about 60% in body mass. According to stomach content analyses at the end of the experiment, the ingested prey consisted mainly of relatively large and abundant invertebrates. Our study indicates that predation by juvenile burbot should be an important factor in structuring the benthic invertebrate community in terms of qualitative and quantitative effects.     

Mass mortality of the invasive bivalve <Emphasis Type="Italic">Corbicula fluminea</Emphasis> induced by a severe low-water event and associated low water temperatures

The Asian clam Corbicula fluminea, originating from Southeast Asia, was first recorded in Lake Constance in 2003 and developed local mass occurrences afterwards. Effects of harsh winter conditions in 2005/2006 associated with a strongly decreasing water level were studied at three different depths at and below the mean low water level (MLL, MLL ?1 m and MLL ?3 m). Low winter temperatures produced a massive die-off of the C. fluminea population. The mortality of the clams was size class and depth dependent. At the mean low water level (MLL), all clams died because of lying dry. However, at MLL ?1 m and at MLL ?3 m, mortality was a consequence of water temperatures around 2°C for nearly 3 months. At MLL ?3 m, clams >5 mm died later than young clams <5 mm and later than clams of all sizes at MLL ?1 m. But in late spring even the clams >5 mm at MLL ?3 m were dead and only about 1% of the overall population of C. fluminea survived the winter conditions until spring 2006.Lethal effects of low water temperatures on C. fluminea, which may become effective only after a time lag, were corroborated in an outdoor mesocosm experiment with constant water level and without predation.     

Effects of water-depth and water-level fluctuations on the macroinvertebrate community structure in the littoral zone of Lake Constance

This article presents an evaluation of two sampling methods for assessing the biodiversity of heavily vegetated wetlands. The aim was to establish an effective sampling regime to maximise total taxon richness and minimise sampling effort. Three Integrated Constructed Wetland (ICW) systems in Annetown Valley, Co. Waterford, SE of the Republic of Ireland, were sampled during spring and summer 2005. The two methods that were evaluated were pond netting and two types of horizontal activity traps, namely “horizontal activity traps” (HATs) and modified “horizontal activity traps” (modified HATs). The activity traps provided a one-way funnel system and were constructed from 2 l plastic bottles, allowing for the passive collection of taxa. HATs were designed to capture macroinvertebrates in open water and modified HATs, which were designed specifically for this study, were used to sample within stands of dense emergent vegetation. Results show that a combination of pond netting and activity traps will yield a more complete estimate of taxon richness. The performance of Modified HATs was not significantly different from that of the HATs in dense vegetation. Tests on the sampling effort required for each method are also discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: R. Céréghino, J. Biggs, B. Oertli and S. Declerck The ecology of European ponds: defining the characteristics of a neglected freshwater habitat     

Growth-rate dependent feeding rates in Daphnia pulicaria and Brachionus rubens: adaptation to intermediate time-scale variations in food abundance

The hypothesis was tested that zooplankton can adapt to fluctuationsof their food resources at intermediate time scales in orderto maximize their energy input. The cladoceran Daphnia pulicariaand the rotifer Brachionus rubens were cultured at a range offixed growth rates and then offered radioactively labelled algaeat limiting and non-limiting concentrations to determine theirfunctional response. Rotifers and cladocerans showed a remarkablysimilar response. Animals growing at low growth rates had highermaximum filtering rates, i.e. were better adapted to low foodconcentrations than those growing at high rates. The growthrate did not affect the maximum ingestion rate in any of thespecies, thus indicating that adaptation to the prevailing foodconditions was based on the process of food collection. Bothspecies were heavier when cultured at high growth rates. Plottingthe weight-specific maximum filtering rate versus the growthrate resulted in a pronounced negative slope. The slope forthe weight-specific maximum ingestion rate was also negative.Hence filtering and ingestion rates seem to be determined bythe size of the zooplankton, not by their weight.     

Seasonal bat activity related to insect emergence at three temperate lakes

Knowledge of aquatic food resources entering terrestrial systems is important for food web studies and conservation planning. Bats, among other terrestrial consumers, often profit from aquatic insect emergence and their activity might be closely related to such events. However, there is a lack of studies which monitor bat activity simultaneously with aquatic insect emergence, especially from lakes. Thus, our aim was to understand the relationship between insect emergence and bat activity, and investigate whether there is a general spatial or seasonal pattern at lakeshores. We assessed whole‐night bat activity using acoustic monitoring and caught emerging and aerial flying insects at three different lakes through three seasons. We predicted that insect availability and seasonality explain the variation in bat activity, independent of the lake size and characteristics. Spatial (between lakes) differences of bat activity were stronger than temporal (seasonal) differences. Bat activity did not always correlate to insect emergence, probably because other factors, such as habitat characteristics, or bats’ energy requirements, play an important role as well. Aerial flying insects explained bat activity better than the emerged aquatic insects in the lake with lowest insect emergence. Bats were active throughout the night with some activity peaks, and the pattern of their activity also differed among lakes and seasons. Lakes are important habitats for bats, as they support diverse bat communities and activity throughout the night and the year when bats are active. Our study highlights that there are spatial and temporal differences in bat activity and its hourly nocturnal pattern, that should be considered when investigating aquatic–terrestrial interactions or designing conservation and monitoring plans.     

Effect of Cullural Conditions on the Growth of Agrostemma githago Cells in Suspension Culture and the Concomitant Production of an Anti-Plant Virus Substance

An extract of cultured Agroxieinma githago L. cells was found to show potent inhibitory activity against plans virus infection. The effects of cultural conditions on the growth of the cell suspension and on the production of the inhibitor were examined. Since the production of substance was dependent on growth. experiments were made to improve growth. The optimum temperature was 26 to 30°C and optimum pH of the medium before autoclaving was between 5 and 7. In a medium of higher osmotic pressure, the water content of the cultured cells was lowered markedly. The growth rate in a small volume of the medium was higher than that in a larger volume at an early stage of the cultivation, but it was not changed by different inoculum sizes. The cells required thiamine and 2,4-D for growth but no other vitamins or growth regulators. The optimum level of 2,4-D was 0.1 mg/l. Higher sucrose concentration in the medium gave higher production of cell mass and of the inhibitor. However, 3% of sucrose was selected as the most economical concentration. For normal cell growth, the presence of both NH₄NO₃ and KNO₃ as nitrogen sources was required. The use of a single nitrogen source caused a long lag period or inhibition of the cell growth. KH₂PO₄ stimulated the growth when in was used in the level of 2.5 to 5 mM. The cell adhesion on the surface of the fermentor sometimes causes trouble in a large-scale cultivation. It was found that reducing the Ca²⁺ level in the medium prevented the cell adhesion and foaming remarkably. Based on the results obtained, a modified medium was established which was excellent for shortening the culture period and for efficient production of the anti-plant virus inhibitor.