Ecology of Gastropods and their Trematode Infections in Obutu Lake Orumba North Local Government Area of Anambra State, Nigeria.

Abstract

Macrodistribution of gastropods in freshwater systems is a subject of many surveys. Opinions vary on the effect of certain factors favouring the presence of snails in aquatic habitats. No such investigation has been done in Obutu lake. Gastropods are very efficient transmitters of helminth parasites, but how they do this within snail species and between species is not well understood. Habitat overlap and biotic interactions may explain the role of gastropods in the overall ecology of freshwater systems. This study aims at providing a checklist of gastropod species with emphasis on the ecological factors affecting their distribution in Obutu lake. The study was conducted between October 2000, and September 2004. The samples were collected fortnightly using scoopnet. The snails were identified and screened for cercariae. To do this, each snail from each demarcated area was put in a 300ml beaker filled halfway with filtered tap water and placed in sunlight for 2h. At the end of the screening, snails from each sampled area were randomly selected and dissected to search for any developing larvae that could not emerge. Factors that affect gastropod bionomics, other biological interactions within the lake and other indexes such . ,, a, s .r pH, , t~mperature, dissolved oxygen, alkalinity and turbidity, were assessed using standard procedures. The aquatic microphytes and macrophytes that support growth were also investigated by direct observations. The foods and feeding habits of the different gastropods were studied using stomach content analyses. Simpson's Index for the whole species collected was calculated with a view to determining the degree of dominance of the species. Other statistical analyses performed included finding the level of association of helminth parasites between snails, the frequency of occurrence as well as mean levels of infection using analysis of variance. Altogether, eleven species of gastropods were collected and identified. These were made up of two pulmonates (Lymnaea natalensis and Gyraulus costulantus) and nine prosobranchs (Lanistes lybicus, Lanistes ovum. Pila wernei, Pila ovata, Potadoma freethii, Bythinia sp. Orthalicus sp, Dvymaeus nurltilineatus and Bulimulus sp). The last three species were arboreal gastropods and amphibious in character. Lanistes ovum was the dominant species and it was circumboreal in distribution and dispersed over 60% of the total area of the lake. There was a close association between these snails and aquatic macrovegetation, macrophytes and microphytes. The microphytes were made up of diatoms, desmids, filamentous algae and fungi. Six trematode larval stages were isolated. Infected snails showed a form of nestedness in their distribution. Analysis of association using Chi-squared showed no significant differences (P>0.05) between prosobranchs and the macrophytes. Similar values for pulmonates showed significant association (P<0.05). Aquatic macroinvertebrates and vegetations were found to be main bulk of foods of these gastropods. Stomach content analyses suggested that organisms of wide size ranges and of various shapes were used as foods by the snails. Other factors like consumption time, searching time and the ration co.,m ,, b. inations .-I. ., , .,+ were found to overlap, indicating some level of co-existence. Physico-chemical factors investigated did not significantly influence (P>0.05) the abundance of gastropods.