Response of Daphnia to substances released from crowded congeners and conspecifics

The effects of chemicals released from crowded congeners andconspecifics on life history parameters of the freshwater zooplanktersDaphnia cucullata and Daphnia pulex were examined. Length andage at maturity of D. pulex were affected by crowding chemicals.Reproduction was lower in crowded medium, and ephippia wereproduced. Newborn D. pulex in crowded medium were significantlylonger than the controls. The intrinsic rate of population increaseof D. pulex was 14 and 25% lower than the control when exposedto crowded medium from D. cucullata and D. pulex, respectively.Neither urea nor ammonia (at 1 mg l^-1) seemed to be responsiblefor these effects in D. pulex. In D. cucullata, no significanteffect of crowding infochemicals on length and age at maturitywas found. However, crowding chemicals reduced reproduction.No ephippia were produced in crowded medium, but up to 83% non-developingeggs were observed in D. cucullata. Newborns were similarlysized in crowded and standard medium. The intrinsic rate ofpopulation increase of D. cucullata was 44 and 96% lower thanthe control when exposed to crowded medium from D. cucullataand D. pulex, respectively. Clearance rates of D. pulex weresignificantly reduced in crowded media compared with standardmedium, which could partly explain why the animals exposed tocrowding chemicals reacted as if they were food limited.     

Behavioural response of Daphnia to olfactory cues from food, competitors and predators

The behavioural response of the freshwater zooplankter Daphniato chemicals from its food, the green alga Scenedesmus, to algalcells, to the green colour from chlorophyll, to chemicals releasedfrom congeners and conspecifics, and to chemicals released frominvertebrate (Chaoborus) and vertebrate predators (Leuciscus)was investigated in a Y-tube olfactometer. No preference wasobserved either for medium that had contained algae, or formedium with algal cells or with the green colour of algae, offeredas alternatives to clean medium. In contrast, swimming speedwas significantly reduced at high algal concentrations and inthe presence of green colour. Moreover, starved animals hadlost their rheotaxis. Neither Daphnia magna nor Daphnia pulexhad a preference for either clean medium or medium that hadcontained conspecifics, but D. magna significantly chose cleanmedium when medium which had been inhabited by D. pulex wasthe alternative. No avoidance of medium from Chaoborus cultureswas found, but D. magna significantly avoided medium that hadbeen inhabited by ides (Leuciscus idus L.). The responses observedcould result in aggregation of animals by reduced swimming speedat high algal densities and by avoidance of areas with predators.When these cues have become less important due to food depletionand decreased predation pressure, the interspecific-competitor-relatedcues might result in desegregation.     

A revised secondary structure model for the internal transcribed spacer 2 of the green algae Scenedesmus and Desmodesmus and its implication for the phylogeny of these algae

Secondary structure analysis of 34 internal transcribed spacer 2 (ITS-2) sequences showed that the current model for the green algae Scenedesmus and Desmodesmus is not accurate. In particular, helix I of the currently used model showed considerable deviations from our new model. The newly proposed model is supported by many two-sided compensated base pair changes and fully compensated insertions in all four helices. Phylogenetic analysis by maximum parsimony based on the new alignment confirmed the recent division of the old genus Scenedesmus into the new genera Scenedesmus and Desmodesmus. However, the analysis was not able to show phylogenetic relationships within these two genera. Hence, the ITS-2 region alone is not suitable for clarifying the phylogeny of Scenedesmus and Desmodesmus and new regions have to be found for future sequence analyses.     

Cyanobacteria blooms cannot be controlled by Effective Microorganisms (EM<Superscript>®</Superscript>) from mud- or Bokashi-balls

The aim of the present contribution is to assess whether the abundance of mollusk species in the Southern Pampas (Argentina), an environmentally homogeneous region, reflect the local conditions of water bodies. In order to test this hypothesis, a comprehensive study was conducted in 30 sites regularly distributed across the region. At each site, the abundance of mollusk species was determined, and a series of physico-chemical measurements taken. Principal Component Analysis (PCA) and Cluster Analysis (CA) were used for the ordination of sampling sites based on the measured environmental variables. In addition, Canonical Correspondence Analysis (CCA) was conducted to explore the relationships between environmental variables and mollusk abundances. Mollusk species were represented by the gastropods Biomphalaria peregrina, Chilina parchappii, Heleobia parchappii, Physa acuta, Pomacea canaliculata, Stenophysa marmorata, Uncancylus concentricus and the bivalve Musculium argentinum. Although aquatic vegetation cover, conductivity, and substrate were among the main parameters influencing mollusk distribution, their effect was insufficient to explain the spatial distribution pattern of the species in a regional scale. It is because the Southern Pampas is a very homogeneous area, and the ranges of these environmental conditions are within the range of ecological tolerance of most of the species represented. Yet, some species resulted good indicators of environmental conditions at local (microhabitat) scale, i.e., particular microhabitats that occur in different water bodies as well. In fact, even distributed in many different water bodies along the Southern Pampas C. parchappii is always linked to lotic environments, and U. concentricus is exclusively restricted to hard substrata. On the other hand, H. parchappii is the only species represented in mesohaline waters and P. acuta appeared to be a good indicator of pollution in the area.     

The effect of substances from different zooplankton species and fish on the induction of defensive morphology in the green alga Scenedesmus obliquus

The common green alga Scenedesmus obliquus may respond morphologicallyto the presence of natural enemies. Exposure to water-bornecues from the herbivorous zooplankton Ceriodaphnia reticulata,Daphnia galeata x hyalina, Daphnia magna and Daphnia pulicariastimulated the formation of protective colonies in S. obliquus.This response seemed strongly related to the amount of algaegrazed upon, because a highly significant correlation betweenthe amount of S. obliquus consumed and the induced colony formationwas found. However, when exposed to medium that had been inhabitedby the carnivorous zooplankton Bythotrephes longimanus and Leptodorakindtii, no colony formation occurred. A similar result wasobtained in two different experiments when S. obliquus was exposedto filtrate from cultures of the freshwater fish ide (Leuciscusidus) and perch (Perca fluviatilis). These results support thehypothesis that S. obliquus responds to a herbivorous zooplanktonchemical cue, rather than to a more general animal excretoryproduct. No support was obtained for the hypothesis that cuesfrom the enemy of their enemy could serve as signals to thealgae: despite the presence of filtrate from fish culture, filtratefrom Daphnia cultures induced the formation of colonies in S.obliquus. The biological activity seems to be linked to thealga–grazer interaction, which ensures a reliable cuethat evokes the morphological response of S. obliquus only whennecessary. The grazer-induced colony formation can be viewedas an adaptive reaction in habitats with variable grazing pressurefrom an assemblage of many different herbivores to pare downmortality through grazing.