Knowledge of the distribution and habitat preference of freshwater snail intermediate hosts can provide information to initiate and set-up effective snail control programmes. However, there is limited research conducted on the factors driving the occurrence and abundance of freshwater snail intermediate hosts in the Ethiopian Rift Valley. Hence, in this study, we investigated how environmental and biotic factors influence the occurrence and abundance of the snail intermediate hosts in Ethiopian Rift Valley region. Data on freshwater snails, physico-chemical water quality parameters, physical characteristics of habitat, predators and competitors, and anthropogenic activity variables were collected from 174 sampling sites during the wet season of 2017 and 2018. Generalized linear models were used to identify the main environmental and biotic factors affecting the occurrence and abundance of the snail species. It was found that Bulinus globosus (31.7%) was the most abundant snail species followed by Lymnaea natalensis (21.6%), Lymnaea truncatula (15.1%) and Biomphalaria pfeifferi (14.6%). Generalized linear models indicated that physico-chemical parameters (water temperature, turbidity, chlorophyll-a, dissolved oxygen, chemical oxygen demand, alkalinity, calcium, magnesium, nitrate and ammonia), physical habitat characteristics (water depth, canopy cover, macrophyte cover and substrate type) and biotic factors (abundance of predators and competitors) were found to be the main variables determining the occurrence and abundance of snail species in the Ethiopian Rift Valley region. In terms of anthropogenic activities, human settlement, farming, bathing and swimming, clothes washing, grazing, drainage of land, car washing, boating, fishing and silviculture were also important variables determining the occurrence and abundance of snail species in the region. The findings reported herein suggest that integrated snail control strategies should be considered to control snails via protection of water bodies from disturbance by anthropogenic activities. In this way, it is possible to reduce the concentration of organic matter and dissolved ions in aquatic ecosystems which are conducive for the presence of snails.

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Environmental and biotic factors affecting freshwater snail intermediate hosts in the Ethiopian Rift Valley region

Differentiation between Bulinus truncatus and Bulinus hexaploidus by morphological characters, chromosomal study and compatibility with Schistosoma haematobium.

Freshwater snails of the genus Bulinus, mostly Bulinus truncatus/tropicus complex occur in Ethiopia in various ploidy levels (diploid, tetraploid, hexaploid and octoploid) which are otherwise rare in other animal phyla. This study provides data on the occurrence and distribution of various ploidy levels of B. truncatus/tropicus complex in Ethiopia based on meiotic bivalent chromosome counts. Emphasis was made on the role of shell morphology and chromosome number in species identification of B.truncatus /tropicus group. Specimens were collected from fourteen different localities in Ethiopia. Chromosome preparation was made from gonad tissue (ovotestis). The result showed that diploid species are mainly associated with low altitude, tetraploid with both low and medium altitudes whereas hexaploids and octoploids with high altitudes. Unexpected result was found in Lake Hora, where both diploid and tetraploid populations were occurring together in the same microhabitat which is in contrary to suggestions of some authors that two cytotypes of Bulinus truncatus/ tropicus complex do not occur in the same water body in Ethiopia. It was recommended that thorough investigation of each water body where snails are occurring, should be undertaken using cytological and molecular approaches and that the possibility of hexaploid Bulinussnails as potential host of human schistosome parasites in the highland of Ethiopia.

Polyploidy in Bulinid Snails, with Emphasis on Bulinus truncatus/tropicus Complex (Planorbidae: Pulmonate Mollusks) from Various Localities in Ethiopia

Ogheneochuko Siemuri

Papers

Polyploidy in Bulinid Snails, with Emphasis on Bulinus truncatus/tropicus Complex (Planorbidae: Pulmonate Mollusks) from Various Localities in Ethiopia