Two types of experiments were conducted to test whether invertebrate predators would ingest the rotifer S.
socialis. The first type of experiment done was Microcosm experiments, where dragonfly nymphs, damselfly nymphs, notonectids, and Hydra were offered prey either singly or in combination in several separate experiments. The preys were S.
socialis; Epiphanes senta and Daphnia magna. In the absence of predators (control treatments), nearly all prey survived the 24-h feeding trials. The presence of predators greatly affected prey survival. The number of prey surviving in each microcosm was recorded at 12, 18, and 24 h at which point the experiments were terminated. The second type of experiment done was Paired-feeding experiments where predators were offered prey in a pairs again in separate experiments, wherein members of D.
magna were replaced with either a rotifer, S. socialis or E. senta. fter 24 h, 60–100% prey items of S. socialis survived the predators. Individuals of E.
senta (6–89%) and D. magna (o25%) showed a lower level of survival. When offered rotifers and individuals of D. magna simultaneously, predators tested consistently consumed more specimens of Daphnia.
Predators all consumed significantly more specimens of E. senta than S. socialis after 24 h. Majority of the four predators captured members of S. socialis, but these colonies were frequently released rather than ingested, although in some cases colony structure was seriously disrupted.In both the microcosm experiments and in the paired-feeding trials the insect predators tried to ingest colonies of S. socialis .
Only specimens of Hydra littoralis did not attack the colonies. Thus, colonies of S. socialis are potential prey for these three insects in natural habitats.
The results suggest that the unpalatable nature of members of S. socialis to certain fishes extends to several invertebrate predators.From the introduction of Whirling disease in 1958 it is now known to 21 states.
Before,it was assumed that it was not a problem in wild stocks, but recent experience in Colorado and Montana suggests that whirling disease plays a major role in the decline of up to 90% in rainbow trout in some rivers. One of the most important questions to arise from this discovery is the cause of the difference in susceptibility of rainbow trout in different regions.Whirling disease is caused by the myxozoan parasite Myxobolus ( Myxosoma) cerebralis and has been associated with serious declines in wild rainbow trout populations in the western United States in recent years . This has led to an increase in interest in the biology of aquatic oligochaetes that serve as alternate hosts for this and other fish parasites such as the cestodes Archigetes and Caryophyllaeus and the nematode Eusttvngylides.Very little is known on the role of oligochaete worms in relation to various myxozoans, and even less on other pathogens and parasites thus, it is interesting to note that an old concept that they are in fact Cnidaria has been supported by a recent investigation, thus Myxozoans are now recognized as members of the phylum Cnidaria.Markiw and Wolf have demonstrated that completion of the life history of the parasite required the presence of aquatic oligochaetes.
The specific identity of the oligochaetes remained somewhat questionable, as the Tubifex cultures used were obtained from a biological supply houseT. tubifex is one of the oldest named aquatic oligochaetes, it has been described as early as 1774. T. tubifex is not usually a common species. It can be found in marginal habitats that are both very unproductive, such as the bottom of Lake Superior, andhighly contaminated sites with high levels of organic matter and low oxygen tensions .It can respire anaerobically, in fact they can be found in polluted localities all over the world.They are generally conveyor belt feeders, continuously ingesting whole sediment,perhaps selected for particle size and organic content, and digesting at least some of thebacteria that are engaged in breaking down the organic matter present.The tubilicid worm population in an ecosystem may provide an infective reservoir as the disease persists for long periods in the worm.
There have been no recorded attempts to eradicate tubificids. Control of worm populations might be achieved by reducing organic inputs, lowering erosion, increasing Row and removing objects that accumulate pockets of silt. However, the worms serve to mineralize organic matter and convert it to worm tissue, which is in turn nutrition for many other taxa. Without them sediments might tend to anoxia.
A better approach to controlling worm populations that create a hazard to fish stocks such as trout is to eliminate the conditions that promote large populations of tolerant worm species. This may involve maintaining adequate flushing at intervals, minimizing organic loading, and limiting siltation from agricultural practices, construction, erosion, deforestation and other impacts on the whole river valley.