4.0 DISCUSSION
In this study the most abundant and dominant species was M. tuberculata accounting for 76.6% followed by B. unicolar14.9%, then C. bulimoides 3.3% and the least were B. tenuissimus and B. globosus with 0.6% each. The dominance of M. tuberculata in all sampling sites over other species in the water body in Warwade dam did not agree with findings of Dogara et al.(2019) that reported it as the second most abundant and dominant species. However, other works carried out in similar water bodies by Rader et al. (2003), Owojori et al. (2006), Abdulkadiret al. (2017) and Almeida et al. (2018) reported M. tuberculata to be the most abundant and dominant species.
The abundance and dominance of M. tuberculata in this study compared to other species was quite glaring and B. globosus which was the most dominant had now been pushed to the rear. This could be explained by the fact that M. tuberculata unlike other snail species such as B. globosus and L. natalensis , is known to thrive only in standing or slow flowing water bodies (Rader et al., 2003) and that it is an invasive species, which competes well withB. globosus and Biomphalaria species (Alexandre et al., 2005). The relatively high percentage observed of M. tuberculata reflect their superior natural adaptation such as the presence of their hard protective shell that can withstand harsh environmental conditions and also protect them against predators and drought (Supian and Ikhwannuddin, 2002 and Sharma et al., 2013).
In this study, it appeared M. tuberculata had out competed all the encountered fresh water snail species in Warwade dam. The greater abundance of M. tuberculata is due to its capacity to explore different environments and tolerance to limnological variation and different types of substrates (Freitas et al., 1987). M. tuberculata, an invasive species is a great competitor and has successfully reduced populations of species of the genusBiomphalaria , which are intermediate hosts of schistosomiasis (Lima et al., 2013). Perhaps that is why according to Oloyedeet al. (2016 in press) it is now monitored worldwide because of its fast growing distribution and its economic importance. This also gives the basis for the introduction of M. tuberculata in areas where schistosomiasis was prevalent in the hopes of reducing populations of vector snails such as B. glabrata and Biomphalaria straminea , and thereby reducing the incidence of the disease(Pointer and Guvard, 1992).
Comparison test indicated that in terms of abundance and distribution there were more freshwater snails at site with human activities when compared to lentic water, lotic water and vegetation cover. The observed trend may be due to decreasing favourable conditions required for the survival of the fresh water snails. Incidentally M. tuberculata, B. globosus, B. tenuissimus and Lymnaea natalensis were also the most abundant in the site with most human activities. This is consistent with scientific explanation that vectors as transmitters of diseases position themselves where they are likely to come into contact with their suitable hosts. Moreover, B. globosus and B. tenuissimusthat transmit schistosomiasis were prevalent in the study area. L. natalensis was also collected throughout the months of sampling; its presence is also of great public health importance as is known to transmit F. gigantica or F. hepatica that causes fascioliasis or liver rot disease in ruminant.
Fresh water snails’ distribution and abundance varied significantly across the four sites within the same period and duration of sampling (April to October, 2019). Freshwater snails were more abundant in the months of July and August with exception of lotic site where the highest number was in the month of May because during raining season the water was contaminated by organic and inorganic substances such as fertilizers from surrounding farmlands, the alkalinity of the water were low and as from September to October the distribution of freshwater snail decline due to low rainfall. The highest abundance of fresh water snails recorded in the month of May, 2019 in the lotic site could be as a result of increase in volume of rain and physico-chemical factors that favour the rapid growth and multiplication of freshwater snails in site. However, the mean snail abundance at sites with vegetation cover (M = 68.9) did not significantly differ from either site with human activities, or from lentic or lotic water sites. This could be attributed to environmental changes especially rainfall and some important physico-chemical factors such as temperature, pH, turbidity and phosphate that are required for the growth and survival of the fresh water snails. Rainfall began to increase from April, peaked in August/September before it ceases at the end of October. The onset of rainfall changes the stability of the dam ecosystem prevalent during the dry season which even destroys some microhabitats of the freshwater snails. However, over time, increase in rainfall washes from surrounding farms and lands organic and inorganic materials that promote the growth, survival and fecundity of freshwater snails thus, leading to a peak population in August except for the lotic site that remained more or less undisturbed by the changes in the volume and intensity of rainfall.
The temperature and pH ranges recorded were within the limits suitable for the survival of freshwater snails in all the sampling sites. The lower values of turbidity recorded in sites with human activities and lentic compared to vegetation cover and lotic might have accounted for the relative freshwater snails’ abundance in the different sites. The lack of remarkable variation in all the sampling sites of dissolved solids, dissolved oxygen, colour, depth of the water body, concentration of calcium, nitrates and phosphate goes to explain the relative abundance of freshwater snails in all the sampling sites. Of the twelve physico-chemical factors measured only three; pH, current velocity and magnesium were found to significantly vary across the four sampling sites of the dam. This may be due to relative stability of conditions in the dam ecosystem that favours the survival of freshwater snails. Two factors pH and current velocity are known to be critical for the existence and survival of freshwater snails, therefore their variation over time appear to account for the difference in distribution and abundance across the four sampling sites. However, both of them were within the limits required for the growth and survival of freshwater snails. The pH recorded from all the sampling sites was slightly alkaline which favours the existence of freshwater snails. The maximum velocity recorded from all the sampling sites was less than 40cm/s which according to Jones, (1993) is suitable for the survival of freshwater snails. Snail intermediate hosts of schistosoma are intolerant of strong current and are not found in flowing water bodies, they are usually found in areas where the velocity is low (Strong et al., 2008). This explained why their abundance and distribution was low in sites of human activity and lentic. Only calcium was found to be significantly associated with the abundance and distribution of fresh water snails. This is because calcium is an important element required in the formation of freshwater snails.
Literature showed that leaves of floating plants protect snails from bright sunlight and high temperature (Heineman, 2009), thus providing a breeding site for the freshwater snails. Also according to Adewole (2001), oviposition is stimulated by the oxygen produced by the plant. In this study typha grass and other plants around served as an indicator plant for the presence of B. globosus in Warwade dam which is the established intermediate host for the urinary schistosomiasis.