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.