The Taxonomy, ecology and trematode infections of gastropod snails on the northern shoreline of Lake Kariba, Zimbabwe.

Abstract

Freshwater gastropods make up about 75% of all freshwater molluscs in the African continent. Their classification and identification have been largely based on morphological characteristics, with the main focus on shell morphology, which usually causes difficulties in distinguishing between some species belonging to the same genus, and between genera belonging to the same family. However, molecular barcoding has been shown to be more accurate in identifying gastropods to species level. Freshwater gastropods play an essential role in the transmission of digenean trematodes of both human health and veterinary importance, serving as intermediate hosts of these parasites. Accurate information on the species of gastropod in water bodies as well as their association with trematodes is essential in the development of strategies to prevent these health problems and economic losses linked to them. Studying the diversity of gastropod species in the country's fresh water bodies could also be an integral component of the assessment of the water physico-chemical parameters and impact of waste water. The identification of freshwater gastropods in Zimbabwe, as well as the trematodes they transmit, has largely been based on morphological characterisation. Recently documented gastropods in Lake Kariba comprised Bulinus globosus and Biomphalaria pfeifferi only, which are the intermediate gastropod hosts of Schistosoma haematobium and Schistosoma mansoni, respectively. A one-year investigation into the gastropod fauna of Lake Kariba as well as the trematode fauna of each gastropod species was undertaken along the northern shore. Sixteen sites, adapted from a similar study of 2003, were sampled monthly from May 2017 to April 2018 for gastropods and water for quality testing. Morphological and molecular identification of gastropods was carried out for all gastropods collected. Morphological identification was done with standard keys which use shell morphology before they underwent molecular identification. Molecular analysis was conducted using simplex PCR targeting specific gastropod markers using primers COI_gastropod_F and COI_gastropod_R. Morphological analysis of specimens yielded 9 species belonging to 5 genera. The identity of Melanoides tuberculata was solely morphological, while the other 8 morphotypes were subjected to molecular barcoding and yielded 8 species belonging to 7 genera. Molecular barcoding results were used to conclude the identity of the obtained gastropods. It was concluded that there were at least nine species of freshwater gastropods in Lake Kariba, including Melanoides tuberculata whose identity was solely morphological. Six previously undocumented gastropod species were identified in this study; namely, Bulinus truncatus, Bulinus forskalii, Gyraulus sp., Pseudosuccinea columella, Radix sp. and Succinea sp. The study revealed misidentifications which often occur in morphological identification, due to similarities in closely related gastropods of the same family. Samples for water physico-chemical parameters were analysed for selected parameters, including pH, dissolved oxygen content and conductivity measured in situ. Gastropod abundance was recorded per site per species for every sampling month and a non–metric dimensional scaling analysis was carried out to show gastropod community composition Overall, gastropods were most abundant in the hot, wet season and least abundant in the wet season. It was concluded that planorbids had a preference for water with high nutrient content and low oxygen levels, while lymnaeids prefer habitats with fairly low nutrient content and higher oxygen levels. The study also drew the inference that the correlation of B. truncatus with high levels of nutrient could be used as an indicator for water pollution with regards to sewage and biological waste from industrial plants. Gastropod infections with trematodes were investigated using two approaches; that is cercariae–shedding experiments and the multiplex–PCR for detection of pre–patent infections. Shedding experiments yielded less infected gastropods (0.37%) than molecular barcoding (32.24%). Six trematodes were detected and identified only to family level; that is Notocotylidae, Psilostomatidae, Paramphistomoidae, Fasciolidae, Diplostomatidae and Leucochloridiidae. These trematodes were found in five gastropod species, P. columella, Radix sp., and B. forskalii which were each infected by at least one trematode species, and B. truncatus being infected by at least three species. Succinea sp. was infected with a species of the Leucochloridiidae family. It was concluded that the trematodes detected could potentially cause health problems in both aquaculture and wildlife in the studied area. Additionally, the occurrence of new gastropod species, such as P. columella and B. truncatus in the lake introduce the possibility of introducing new species of both the Fasciola and Schistosoma genera. It was recommended that further investigation into the particular species of trematodes belonging to each family reported in the present study is essential in the development of effective strategies to prevent public, veterinary and aquaculture health problems, as well as curb economic losses in Kariba.