The choice and implementation of environmental bioassays

Bioassays play a crucial role in assessing the actual or potential impacts of anthropogenic agents on the natural environment. They can be used to probe the extent to which an ecosystem is being or has been polluted, and to predict the ecological impact of agents before release. Problems associated with the choice and implementation of bioassays in both these modes are discussed. A major general conclusion is that the experimental method should play a prominent part in addressing the problems.     

Energy allocation in the cladoceran Daphnia magna Straus,under starvation and refeeding

Summary Models incorporating the energetics of individual daphnids (Cladocera) have been developed to predict the effect of environmental variables, particularly food availability, on population dynamics. One of them, that of Kooijman (1986), assumes that all assimilated energy enters a storage compartment prior to use in production and metabolism, and that under starvation the stores are used to support maintenance, reproduction and somatic growth, in that order of priority. This predicts that, under starvation, reproduction and growth will continue for a time, and that after they cease death will be immediate. Another model, that of McCauley et al. (1990), assumes that assimilated energy is used directly for maintenance and production, and that stores are accumulated to support maintenance metabolism under starvation. This predicts that growth and reproduction should cease immediately upon starvation and that death will not be immediate. We have carried out laboratory experiments, manipulating starvation time, on Daphnia magna to distinguish between these two models. The results support features of both models in that reproduction, but not growth, ceases upon starvation. We therefore developed a third model in which both maintenance and growth are supported from stores under starvation, with maintenance taking priority over growth under these conditions.     

Importance of fungi in the diet of Gammarus pulex and Asellus aquaticus

An important component of the interaction between macroinvertebrates and leaf litter in streams in the extent to which consumers can differentiate between undecomposed and decomposing leaves. The detritivores Gammarus pulex and Asellus aquaticus fed preferentially on conditioned rather on unconditioned leaf material. Growth in A. aquaticus was significantly reduced when unconditioned leaves were provided, but in G. pulex no significant effect of conditioning on growth was observed. The capacity of G. pulex to tolerate reductions in food quality seems to be a consequence of a compensatory system in which respiration rates change to compensate for reductions in food quality. In this way a constant growth rate is maintained. Increases in ingestion rates to compensate for low quality food were not observed.     

On the regulatory nature of individual growth: some observations from freshwater snails

Various types of growth models are assessed. Results derived from observations made on growth in the freshwater snail, Planorbis contortus , suggest that in certain instances models based on a mechano-reductionist philosophy are acceptable whereas in others they are untenable. In the latter case a holistic representation is more biologically satisfying. Q10 values provide a useful assessment and summary of an individuals regulatory capacity with respect to growth when temperature disturbances are experienced. Another parameter, k corr is developed to carry out this function when other, particularly food supply, disturbances are involved.     

Ontogenetic patterns and phylogenetic trends in freshwater flatworms (Tricladida); constraint or selection?

Semelparity and iteroparity are unevenly distributed among the families of the Paludicola and this implies that there have been taxonomic restrictions on life-history evolution. Species differ in their investments in reproduction and high levels can be related, causally, to reduced life-spans; i.e. semelparous species invest more in reproduction than iteroparous species. However, there does not appear to be any fundamental reason why the extent and timing of these investments should not be open to modification by natural selection.A major morphological difference between the predominantly semelparous Dendrocoelidae and the predominantly iteroparous Dugesiidae and Planariidae is the presence of an anterior adhesive organ; dendrocoelids have one, but members of the other families do not. A plausible scenario can be formulated relating this structure, causally, to enhanced juvenile survivorship which, in turn, favors the semelparous life history.     

Locomotory strategies in freshwater triclads and their effects on the energetics of degrowth

Summary Dendrocoelum lacteum, feeding on active prey, adopted a sit and wait strategy upon starvation. During this time metabolic rate fell by a factor of 0.65. About 80% of the energy made available from the catabolism of tissue was lost as heat with the rest being lost as mucus. P. tenuis, feeding on inactive carrion, adopted a search out strategy upon starvation. Metabolic rate fell during this time but only by a factor of 0.72. About 50% of the energy from catabolism was lost as heat and about 50% as mucus. Hence the locomotory strategies adopted by each species during starvation depended on the mobility of the prey and lead to marked differences in the energy balance of the worms.