Abstract: In a 12 month study, the ecology of freshwater snails in Opa Reservoir (OR) and Research Farm Ponds (RFPs), in Obafemi Awolowo University, Ile-Ife, Nigeria, was studied. During each monthly visit, eleven sites (seven sites in OR and four sites in RFPs) were sampled for relative snail density, vegetation cover and physicochemical properties of the water bodies. The data were analysed using non-parametric statistical methods. A total of 7,524 snails (4081 in OR and 3443 in RFPs) belonging to ten species were collected. The species and their overall relative proportions are: Bulinus globosus (1.5%), B. truncatus (1.7%), B. forskalii (0.2%), Biomphalaria pfeifferi (11.0%) Potadoma moerchi (36.0%), P. freethi (6.9%), Melanoides tuberculata (40.0%), Lanistes libycus (0.9%), Lymnaea natalensis (0.7%) and Gyraulus costulatus (1.1%). The density of the snails varied seasonally and spatially according to a pattern common to most species. The seasonal pattern was basically unimodal with peak in either November/December or March/April for most species. Factors identified as affecting focal and seasonal distribution of snails were rainfall, pH, oxygen concentration, conductivity and presence or absence of macrophytes. Out of a total of 11 dominant macrophytes identified, only Ludwigia erecta served as indicator species for the presence of B. globosus, which is the established intermediate host of urinary schistosomiasis in Ile-Ife area. Fuirena umbellata and Commelina africana suspected to have some molluscicidal effect correlated negatively with B. truncatus (in RFPs) and Lanistes libycus (in OR), respectively. The likelihood of M. tuberculata as competitor snail against B. pfeifferi is discussed.
INTRODUCTION
The various economic and social benefits derived from the damming of rivers cannot be over emphasized. These include irrigation, fishing, sporting, recreation, navigation and hydroelectric power generation. No wonder in Nigeria, for example more than 300 reservoirs were established since 1973 following the devastating effect of the Sahel drought (Ofoezie, 2002).
However, damming of rivers and streams to create reservoirs gives rise to significant alterations in the natural ecology of the original water bodies. For instance, it creates new biotopes which are more conducive than hitherto for the breeding of freshwater snails including the intermediate host species and vectors of important human and animal diseases such as malaria, trypanosomiasis, schistosomiasis etc. (Waddy, 1975; Gilles, 1980; Hunter et al., 1993; Jewsbury and Imevbore, 1988; Ofoezie, 2002). Sometimes, these changes occur in areas where some of these diseases especially schistosomiasis were previously unknown (Ofoezie et al., 1991, 1997).
Several factors are considered as affecting the ecology of snails and other intermediate hosts of diseases, hence their focal and seasonal distributions. These include physical factors such as water current, temperature, turbidity, transparency and distribution of suspended solids, chemical factors such as ion concentration and dissolve gases in water as well as biological factors such as availability of food, competition and predator-prey interactions (Williams, 1970; Ofoezie, 1999).
Besides, aquatic macrophytes have been shown to play vital roles in the distribution of snails in different parts of Africa (Ofoezie, 1999). The macrophytes are believed to provide both food and shade and to provide breeding sites for the snails. However, the importance of different ecological factors vary significantly from one ecological zone to the other and even from one water body to the other, suggesting local investigations to identify important factors in each zone or water bodies (Dazo et al., 1966; Klumpp and Chu, 1977; 1980; Imevbore et al., 1988; Ofoezie, 1999).
This study was therefore designed and executed to provide such important information on the ecology of freshwater snails in Opa Reservoir and Research farm ponds. Both the reservoir and the ponds are located within the Obafemi Awolowo University (OAU) where they serve multipurpose needs including water supply, recreation, fishing and agricultural production. It is hoped that knowledge of the ecology of the snails in the water bodies will contribute significantly to future control strategies against disease transmitting species.
MATERIALS AND METHODS
Study areas: This study was carried out in Opa Reservoir (OR) and Research farm ponds (RFPs) both in O.A.U Ile-Ife located between Lat 7° 301-7° 351 N and Long 4° 301-4° 35E1. According to Akinbuwa and Adeniyi (1996), Opa Reservoir (Fig. 1) was established in 1978 by the impoundment of Opa River which took its source from Oke Opa Hills. It has a catchment area of about 116 km2 which is characterized by a rainy season of about eight months (March-October) and a dry season of about four months (November-February), a mean annual precipitation of 1000-1250 mm (Oguntoyinbo, 1982) and a mean annual temperature of about 27°C (Ndifon and Ukoli, 1989). The reservoir was primarily created to supply potable water to the university community for which reason fishing activities were permitted only for recreational and research purposes.
The RFPs are located at the University Teaching and Research farm, about 4 km from the central campus (Fig. 2). There are two ponds in the farm, here identified as Ponds A and B. Pond A was formed by the impoundment of Elerin and Omifunfun streams in 1967 (Adeniyi, 1971) while Pond B was established downstream of Pond A in 1981. The primary purpose of the ponds was to supply water for agricultural activities of the farm such as irrigation, fisheries, poultry and cattle watering.
Methods
Sampling stations: A total of eleven sampling stations (1-4) in the
RFPs and seven (5-11) in OR were selected for this study on the basis of accessibility
and variations in site ecology. Each sampling station covered about 25 m along
the shoreline and 2 m in-shore. Stations 1, 2, 5, 6, 7, 8 and 11 were human
contact sites and were therefore easily assessable while 3, 4, 9, 10 though
not easily accessible varied in ecological characteristics including macrophyte
coverage, presence of dead decaying organic materials, characteristics of the
substratum etc.
Snail sampling: Each of the eleven sites was sampled once every month for twelve months (July 2002-June 2003). Sampling in each site involved 30 passes of kitchen scoops supplemented by a manual search for 30 person-minute (WHO, 1985; Ofoezie, 1999). The scoop, attached to a 2 m pole, was usually dragged from the 2 m mark towards the researcher. The 30 passes were systematically thrown at about 1m distance along the 25 m stretch of each site. Each scoop was thoroughly searched and all snails present collected and kept in pre-labelled plastic containers containing damp and decaying leaves. The containers were later covered with perforated lids and returned to the laboratory where they were sorted, identified to species according to Brown and Kristensen (1993) and Brown (1994) and counted to determine the number of each snail species collected per site per month.
Physicochemical characterization of water: Surface water samples were collected once per month in each of the eleven stations in 2 L plastic containers by the simple dip method. Dissolved oxygen samples were collected in glass stoppered light reagent bottles and immediately fixed with Winklers reagent (KI and MnCl2) in the field and titrated in 0.0125 N sodium thiosulphate (APHA et al., 1992). Samples for Biochemical Oxygen Demand (BOD5) were determined in the same way except that it was collected in dark reagent bottles and fixed after incubation in the dark for 5 days. The atmospheric temperature and temperature of the surface water in each site were measured directly with ordinary mercury in glass thermometer. Transparency and depth were determined with the aid of a secci disc attached to a calibrated cord. pH and conductivity were determined using a pH comparator (Lovibond comparator) and an electric conductivity meter (Model 7020), respectively. Total alkalinity was determined by titration with standard sulphuric acid (N/50 H2SO4) using mixed indicators (methyl red and bromocresol green).
Aquatic macrophytes: Aquatic macrophytes were sampled as described by Ofoezie (1999). All types of marginal, emergent and floating macrophytes found in each site were collected, properly labelled and identified to species level at the O.A.U. herbarium. Subsequently, each vegetation type was monitored every month in each site for presence, coverage and relative association with snails. Coverage was determined by a simple estimation of the proportion of a site covered by each species and scored as 1 for < 5%, 2 for 6-25%, 3 for 26-50%, 4 for 51-75% and 5 for > 75%.
| Fig. 1: | Opa reservoir, Obafemi Awolowo University, Ile-Ife |
| Fig. 2: | Teaching and research farm showing the ponds and sampling stations |
Statistical analyses: All statistical analyses were performed using non parametric tests since none conformed to normal distribution even after transformations (Siegel and Castellam, 1988). Differences in snail density among stations were determined using the Friedman two-way analysis of variance and between OR and RFPs by the Wilcoxon matched pairs sign ranked test. The strength of correlation between snail species and between snail density and environmental parameters were determined using spearman ranked order correlation coefficient. All analyses were performed using SPSS version 10.
RESULTS
General summary of findings: The general summary of important findings of this study is presented in Table 1. A total of ten snail species comprising two subclasses, four families and seven genera were collected in the reservoir and ponds during the study period. These were Bulinus globosus (Morelet, 1866), B. truncatus (Audouin, 1827), B. forskalii (Ehrenberg, 1831), Biomphalaria pfeifferi (Krauss, 1848) Potadoma moerchi (Reeve, 1859) P. freethi (Gray, 1831) Melanoides tuberculata (Reeve, 1859), Lanistes libycus (Morelet, 1848) Lymnaea natalensis (Krauss, 1848) and Gyraulus costulatus (Krauss, 1848). Four (B. truncatus, B. forskalii, B. pfeifferi and P. moerchi) were found in RFPs while all except P. moerchi were found in OR.
Seasonal and spatial variation in snail density: Table 2 presents the total number of snails of each individual species collected from each of the eleven stations over the study period of 12 months. Three of the four snail species found in RFPs were collected from each station although station 1 appear the most preferred. In contrast, all snail species in OR focally distributed except M. tuberculata which was found in all stations. However, statistical analysis showed that only M. tuberculata was significantly distributed in RFPs while all species were significantly distributed in OR except P. freethi (Table 2). Bulinus globosus, the local intermediate host of Schistosoma haematobium were found only in stations S5, S6, S7 and S8 which are small tributary streams of the reservoir. Thus, they were not found in either the reservoir or in the ponds.
Figure 3 showed the monthly variations in the mean number of each snail species collected in the water bodies. The figure reveals a pattern of temporal variation, which is common for most species. Thus, all the species showed unimodal distribution with peak in November/December for B. globosus, B. truncatus and G. costulatus; March/April for B. pfeifferi, P. moerchi and P. freethi, June for M. tuberculata and L. natalensis while B. forskalii showed no discernible pattern but with peak in June. No relationship was seen between temperature and any of the snail species while rainfall was seen to affect the density of B. pfeifferi but not the other snail species.
Snail distribution in relation to physicochemical factors: The mean and range values of physicochemical properties of water in the sampling stations are shown in Table 3. Generally, water in the RFPs stations is slightly acidic while that in OR is slightly alkaline. Conductivity and alkalinity are lower in the RFPs than in OR.
Table 4 presents the result of the statistical analysis of the relationship between the physicochemical characteristics of the stations on one hand and between snail distribution (seasonal and spatial) and the physicochemical parameters on the other hand. In general, all the parameters investigated varied significantly among the four sites on RFPs and among the seven sites on OR except BOD and pH which were comparable in RFPs and OR stations, respectively.
| Table 1: | The general summary of snail and plant species found in Opa Reservoir and Research Farm Ponds, (July 2002-June 2003) |
| x-means present | |
However, a comparison of these properties using their overall means at the four sites in RFPs and the seven sites in OR and also the eleven sites put together showed that all but DO differed significantly between OR and RFPs.
| Fig. 3: | Mean monthly variation in density of snail collected from
Opa Reservoir (OR) and Research Farm Ponds (RFP) in relation to rainfall
and temperature at Obafemi Awolowo University, Ile Ife (July 2000-June 2003) |
| Table 2: | Distribution and density of snails collected from eleven sites on Opa Reservoir and Research Farm ponds, O.A.U., Ile-Ife (July 2002-June 2003) |
| + Friedman two way analysis of variance for differences in monthly snail counts. * Statistics cannot be computed because one of the variables is constant | |
| Table 3: | Median and range values of the physicochemical characteristics of water collected from seven sites from Opa reservoir and four sites from Research Farm (July 2002-June 2003) |
| Values in parenthesis represent the range of the parameters | |
| Table 4: | The result of statistical analysis of temporal variation in some physicochemical and biological (relative snail density) parameter measured at four sites in Research Farm Ponds and seven sites in Opa Reservoir, O.A.U. Ile-Ife (July 2002-June 2003) |
| + Friedman analysis of variance, * Wilcoxon signed pair rank test, X-Kendall’s coefficient of concordance, ns = not significant | |
| Table 5: | Correlation coefficient matrix between snail species and physicochemical properties of water in Opa reservoir and Research farm ponds, O.A.U., Ile-Ife (July 2002-June 2003) |
| * Correlation is significant at the 0.05 level, **Correlation is significant at the 0.01 level, a- Cannot be computed because at least one of the variable is constant | |
| Table 6: | Correlation coefficient matrix between snail species pairs in Opa reservoir and Research farm ponds (July 2002-June 2003) |
| * Correlation is significant at the 0.05 level, ** Correlation is significant at the 0.01 level, a- cannot be computed because at least one of the variable is constant | |
The result of the correlation analysis between physicochemical parameters and the snail species densities are presented in Table 5. In RFPs, all the snail species correlated positively with alkalinity and conductivity, negatively with air temperature but only P. moerchi and air temperature correlated to a statistically significant level (p<0.01). In OR, of the 81 correlations performed, 38 were positive and 43 were negative although only 4 were of statistical significance. These include P. freethi which correlated positively and significantly with both pH and conductivity and B. truncatus and B. pfeifferi each of which correlated negatively and significantly with BOD5 .
Table 6 presents the correlation matrix between the snail species. In the RFPs, all the snail species were negatively correlated with one another except P. moerchi and B. pfeifferi, which correlated positively. None however correlated to a statistically significant level. In OR, of the 22 correlation values, 12 were negative while 10 were positive. However, none of the negative correlations were significant except that between M. tuberculata and B. pfeifferi. Of the 10 positive correlations, 5 were significant while the rest were not. The 5 significant correlations were B. pfeifferi and L. libycus (r = 1.00, p<0.01, B. pfeifferi and B. truncatus, (r = 0.427, p<0.05); B. pfeifferi and G. costulatus (r = 0.594, p<0.05), G. costulatus and L. natalensis (r = 0.714, p<0.05).G. costulatus and B. truncartus (r = 0.726, p<0.01).
| Table 7: | The frequency and relative coverage (represented as scores) of aquatic macrophytes at eleven sites in OR and RFPs (July 2002-June 2003) |
| N = Frequency of occurrence, CO = Coverage, * = Dominant macrophyte based on a cutoff coverage of 7.0) | |
| Table 8: | Correlation coefficient matrix between snail species and dominant aquatic macrophyte in Opa reservoir and Research Farm Ponds, O.A.U., Ile-Ife (July 2002-2003) |
| a- cannot be computed because one of the variable is constant, * Correlation is significant at the 0.05 level, ** Correlation is significant at the 0.01 level | |
Snail distribution in relation to aquatic vegetation: The frequency of occurrence and relative coverage by site of the aquatic macrophytes in RFPs and OR are presented in Table 7. Ludwigia erecta and Acroceras zizanioides are the most frequently encountered but only A. zizanioides stands out as the densest. Based on a cut off point mean relative coverage of 7.0, eleven macrophytes were considered as dominant in the two water bodies. These plants which include Cynodon dactylon, Fuirena umbellata, Pentodon pentadrus, L. erecta, A. zizanioides, Commelina africana, Polygorum lanigerum, Paspalum polystachyum, Azolla africana, Panicum maximum and Typha australis were also correlated with snail species (Table 8). In RFPs, of the 17 correlations computed only 5 were positive and 4 statistically significant. They include P. moerchi which correlated positively with two macrophytes (L. erecta and C. africana) and negatively with F. umbellata, as well as B. truncatus which correlated negatively and significantly with F. umbellata.
In OR, of the 39 correlations computed, 13 were positive but only one (B. globosus and L. erecta) was significant (r = 1.000, p<0.01). There was also a highly significant negative correlation between C. africana and L. libycus (r = -0.97, p<0.001).
DISCUSSION
This study has identified a total of ten snail species namely B. globosus, B. truncatus, B. forskalii, Biomphalaria pfeifferi, Potadoma moerchi, P. freethi, M. tuberculata, L. natalensis, L. libycus and G. costulatus in Opa Reservoir and Research farm ponds. According to Barclay, (1958), this is an indication of stable coexistence found only in habitats, which are capable of supporting mutually exclusive and conducive niches for different species population. Out of the 10 snail species collected in the reservoir and ponds, 6 are pulmonate snails while 4 are prosobranch snails. Out of the 6 pulmonate snails; 5 are known intermediate hosts of animal and human diseases. L. natalensis is host to Fasciola gigantica in many parts of Africa (Brown, 1994), B. forskalii transmits Schistosoma intercalatum in Nigeria (Arene et al., 1989) Cameroun (Wright et al., 1972) and Gabon (Brown et al., 1984) and B. pfeifferi is host of S. mansoni in Nigeria (Cowper, 1973; Adewunmi et al., 1990) B. globosus and B. truncatus are known intermediate hosts of S. haematobium in Nigeria (Adewunmi et al., 1990) and other parts of Africa (Doumenge et al., 1987). However, B. globosus is the major host of S. haematobium in the area south of 9° north including the present study area (Sellin et al., 1980).
The distribution of B. globosus is patchy and found only in small streams tributaries of the reservoir and not the pool itself. Fagbola (1989) had demonstrated the invasion of the dam by B. truncatus and B. pfeifferi since 1989. This situation has remained so even after 14 years. Moreover, a complimentary investigation had shown that only B. globosus shed the human schistosome cercariae in the tributary streams (Owojori, 2004) and this is an indication of imminent risk of schistosomiasis transmission in the reservoir. The observed relative abundance of B. pfeifferi in the RFPs is also very worrying in the light of recent southward movement of the species and risk of S. mansoni transmission previously restricted above 9°N (Sellin et al., 1980). Most of the snails in the ponds and reservoir showed marked seasonal variation in density, the pattern of which is common in all species. In spite of this, peak abundance of the snails were recorded in November/December for B. globosus, B. truncatus and G. costulatus, March/April for P. moerchi, P. freethi and B. pfeifferi, May/June for B. forskalii and M. tuberculata and September for L. libycus. This trend agrees with the report of several workers from the southwest Nigeria (Hira, 1969a; Hira and Muller, 1966) and Igwun, eastern Nigeria (Udonsi, 1990). It however contrasts with bimodal pattern seen in Yola, northern Nigeria (Akogun and Okin, 1993) Oyan, southwest Nigeria (Ofoezie, 1999), Lake volta, Ghana (McCullough, 1957; Klumpp and Chu, 1977) and lake Kariba, Zambia (Hira, 1969b). According to these reports, snail maxima are determined by local physicochemical and biological conditions of particular ecosystems. In this study area, it appears the overriding factors include rainfall, pH, oxygen concentration, electrical conductivity and presence or absence of aquatic macrophytes.
On the basis of significant correlations, it can be inferred that in OR, there is close positive association between B. pfeifferi and the trio of L. libycus, B. truncatus and G. costulatus as well as between G. costulatus and the duo of B. truncatus and L. natalensis. It should however be noted that these associations were not supported by field observations. In spite of this, the observed statistical correlations agree with the findings of Thomas and Tait (1984) in Ibadan where L. libycus coexisted with B. globosus and B. pfeifferi and the works of Madsen (1985) both of which support the likelihood of species overlap.
The only significant negative correlation observed in this study occurred between M. tuberculata and B. pfeifferi signifying probable interspecific competition between them. Inter specific competition among freshwater snails is well established and M. tuberculata has been suggested as an important agent for biological control of intermediate hosts especially Biomphalaria glabrata (Dupouy et al., 1980; Kawazoe et al., 1980; Brown, 1982; Pointier, 1986; 1993; Pointier et al., 1989; 1991; Pointier and Guyard, 1992; Schlegel et al., 1997). Although the relationship between this species and B. pfeifferi is not well documented, the negative association found in this study is a clear indication that B. pfeifferi probably does not thrive well in the presence of M. tuberculata. However, further studies may be required to substantiate the extent at which this species could be used to control B. pfeifferi in tropical Africa and elsewhere.
Apart from DO, all other physicochemical variables investigated by this study varied between OR and RFPs. This confirms the report that water chemistry varies among water bodies and does influence snail distribution at the regional and sectoral levels (Williams, 1970; Okwuosa and Ukoli, 1980; Ndifon and Ukoli, 1989). For instance, physicochemical factors were not correlated with snail density at RFPs while at OR, BOD5 was negatively correlated with the densities of B. truncatus and B. pfeifferi while pH and conductivity were positively correlated with P. freethi. Other workers have suggested that in Nigeria, snail density is not significantly affected by water turbidity, temperature, alkalinity, acidity and presence of major ions but is significantly limited by conductivity, DO and BOD5 (Okwuosa and Ukoli, 1980; Ofoezie, 1999). The present study while supporting the above did not show significant correlation between snail density and DO. Although this study did not include cations and anions, these parameters have been well studied in Opa Reservoir (Eludoyin et al., 2004). Most of these parameters were within limit except SO4 which was fairly high.
The strong snail-plant association observed in this study is consistent with reports from many parts of Africa. The most important aquatic macrophytes in RFPs and OR were Fuirena, umbellata, Ludwigia erecta and Commelina africana. In RFPs, the density of P. moerchi was positively correlated with Ludwigia erecta and Commelina africana. A similar trend was seen in OR where a strong association was found between B. globosus and L. erecta. Similar trends have been reported in many parts of Africa (Paperna, 1969; Odei, 1973; Klumpp and Chu, 1980; Hillali et al., 1985; Madsen et al., 1988; Ofoezie, 1999). For instance, in lake Volta, B. rohlfsi was found in association with Ceratophylum demersum (Paperna, 1969; Odei, 1973; Klumpp and Chu, 1977) in Egypt, Potamogeton crispus and Eichornia crassipes were positively correlated with B. truncatus and Bulinus alexandrina (Dazo et al., 1966) and in Oyan Reservoir, Impatiens irvingii significantly correlated with all snails species and especially Indoplanorbis exustus, a recently introduced species to that area (Ofoezie, 1999).
Some significant negative association were also recorded during this study especially between Fuirena umbellata and the duo of B. truncatus and P. moerchi in RFPs and C. africana with L. libycus in OR. Although, it is difficult to explain this negative association, studies have shown that where this situation exists, such plants could have some molluscicidal effect and could be used to reduce the density of the snails involved. This however requires further investigation.
CONCLUSIONS
This study has thus provided baseline data in fashioning out control strategies for vector snails in this ecological zone. It has also suggested that focal distribution of snails in OR and RFPs could be attributed to chemical factors of water such as pH, conductivity and BOD5 and to snail plant association. It is also recommended upon confirmation that M. tuberculata may be used in biological control of B. pfeifferi while L. erecta; a marginal plant may be cleared around water bodies in this ecological zone to reduce snail population.
ACKNOWLEDGMENTS
The authors are grateful to the Management of Opa Dam and Teaching and Research farm, O.A.U. Ile-Ife for permission to carry out this study. We also wish to thank Dr. I.F Adeniyi of Zoology Department, OAU, Ile Ife for allowing us his laboratory for chemical analyses of water and also the staff of the Obafemi Awolowo University herbarium for assistance rendered in plant identification.