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ABSTRACT

Macrodistribution of gastropods in freshwater systems is a subject of many
surveys. Opinions vary on the effect of certain factors favouring the presence of snails in
aquatic habitats. No such investigation has been done in Obutu lake. Gastropods are very
efficient transmitters of helminth parasites, but how they do this within snail species and
between species is not well understood. Habitat overlap and biotic interactions may
explain the role of gastropods in the overall ecology of freshwater systems. This study
aims at providing a checklist of gastropod species with emphasis on the ecological factors
affecting their distribution in Obutu lake. The study was conducted between October
2000, and September 2004. The samples were collected fortnightly using scoopnet. The
snails were identified and screened for cercariae. To do this, each snail from each
demarcated area was put in a 300ml beaker filled halfway with filtered tap water and
placed in sunlight for 2h. At the end of the screening, snails from each sampled area were
randomly selected and dissected to search for any developing larvae that could not
emerge. Factors that affect gastropod bionomics, other biological interactions within the
lake and other indexes such. ,, a, s .r pH, , t~mperature, dissolved oxygen, alkalinity and
turbidity, were assessed using standard procedures. The aquatic microphytes and
macrophytes that support growth were also investigated by direct observations. The foods
and feeding habits of the different gastropods were studied using stomach content
analyses. Simpson’s Index for the whole species collected was calculated with a view to
determining the degree of dominance of the species. Other statistical analyses performed
included finding the level of association of helminth parasites between snails, the
frequency of occurrence as well as mean levels of infection using analysis of variance.
Altogether, eleven species of gastropods were collected and identified. These were made
up of two pulmonates (Lymnaea natalensis and Gyraulus costulantus) and nine
prosobranchs (Lanistes lybicus, Lanistes ovum. Pila wernei, Pila ovata, Potadoma
freethii, Bythinia sp. Orthalicus sp, Dvymaeus nurltilineatus and Bulimulus sp). The last
three species were arboreal gastropods and amphibious in character. Lanistes ovum was
the dominant species and it was circumboreal in distribution and dispersed over 60% of
the total area of the lake. There was a close association between these snails and aquatic
macrovegetation, macrophytes and microphytes. The microphytes were made up of
diatoms, desmids, filamentous algae and fungi. Six trematode larval stages were isolated.
Infected snails showed a form of nestedness in their distribution. Analysis of association
using Chi-squared showed no significant differences (P>0.05) between prosobranchs and
the macrophytes. Similar values for pulmonates showed significant association (P<0.05).
Aquatic macroinvertebrates and vegetations were found to be main bulk of foods of these
gastropods. Stomach content analyses suggested that organisms of wide size ranges and
of various shapes were used as foods by the snails. Other factors like consumption time,
searching time and the ration co.,m ,, b. inations .-I. ., , .,+ were found to overlap, indicating some level
of co-existence. Physico-chemical factors investigated did not significantly influence
(P>0.05) the abundance of gastropods.
I!

 

 

TABLE OF CONTENTS

Title Page
Certification
Contents
List of Tables
List of iigures
List of plates
Dedication
Acknowledgement
Abstract
CHAPTER ONE: INTRODUCTION AND LITERATURE REVIEW
Introduction
Aims and objectives of the study
Literature review
Physico – che.m ,i c.a l. g ro,p ,$ies of lakes
Abundance and distribution of gastropods in lakes
Foods and feeding habits of snails
Parasitic Helminths of ~astropods
CHAPTER TWO: MATERIALS AND METHODS
2.1 StudyofArea
2.2 Area of the Lake

2.2.1 Using ordinates
a. Trapezoidal rule
b. Simpson’s rule
2.2.2 Direct calculation
2.3 Physico – chemical properties
2.3.1 Rainfall
2.3.2 Temperature
2.3.3 Turbidity
2.3.4 Water depth
2.3.5 Water velocity
pH
Water density
Alkalinity
Dissolved Oxygen
Macrovegetation, macro -microphytes and Plankton Sampling.
Macroivertebrates., ,,a..n+Id.h ,m ,a,c .r,,o invertebratess ampling
Snails sampling programme
identification of snails
~;eservation of the snails It
Distribution and abundance of snails
Stomach content analysis
Trematode infections of snails
CHAPTER THREE: RESULTS
Area of Obutu lake
Trapezoidal rule
Simpson’s rule
Direct calculation
Physico -chemical properties of Obutu lake
Rainfall
Temperature
Turbidity
Dissolved oxygen
Macrovegetation, macro – microphytes and planktons
of Obutu lake
Macroinvertebrates and microinvertebrates of Obutu lake
Gastropod snails of Obutu lake
Comparative characteristics of pilid snails of Obutu lake
Arboreal snails o.cf, , O+b-uI- t.u, ,l a$.k,*e .
Distribution and Abundance of Gastropod snail species of
Obutu lake
Gastropod snail abundance and aquatic macrophytes
Relationships between physico- chemical parameter and gastropod
snail abundance in Obutu lake 83
Natural foods and feeding habits of gastropod snails of
Obutu lake 83
3.7 Digenean Trematodes of gastropod snail of Obutu lake
3.7.1 Nestedness matrix of gastropods in Obutu lake
3.7.2 Distribution of infections by sites in Obutu lake
3.7.3 Infections of freshwater prosobranchs in the sites in Obutu lake
3.7.4 Infection of freshwater pulmonates in the sites in Obutu lake
DISCUSSION
References

 

 

CHAPTER ONE

INTRODUCTION AND LITERATURE REVIEW
INTRODUCTION
As with their terrestrial counterparts, aquatic ecosystems in tropical and subtropical
environments leave much to be investigated, especially in view of their diverse
and unique fauna and flora. Among aquatic ecosystems, lakes have attracted a lot of
attention both as natural habitats and as impoundment created by man in his quest for
development.
Snail-borne infections move back and forth through freshwater bodies between
man and snails in the course of the life cycles of the parasites (Sturrock, 1993). The snail
is the passive bystander in the whole process and, therefore, it is man that is the vector of
most snail-borne infections. Human activities and human behaviour relating to the water
bodies have played major roles in modifying the habitats and transn~issionp attems of the
various snail-borne diseases. Human and livestock contact with water relevant to snailborne
infections involve two processes: –
(a) Contamination of the habitat with parasite ova.
(b) Exposure of man to the infective stages of the parasites
1..I) ‘
In most cases, people or animals contaminating a habitat are different from those
that are later exposed to the infection. But those contaminating can also be those infected.
The two scenarios lead to two processes also: – The first leads to the spread of the snail-
11 . ..
borne diseases, and the other leads to super-infection which does not extend transmission
but increases morbidity.
A number of studies show that the prevalence, intensity and incidence of snailborne
infections diminish with distance from a known transmission site, usually a
freshwater system (Jordan, 1985). These sites are usually many in any given freshwater,
and so, catchments areas of transmission sites overlap, presenting a real problem to
parasitologists and public health planners, especially where disease prevalences are to be
plotted on a map to guide control teams. This allows planners to fall back to studying the
ecology, host-parasite relationships and transmission dynamics, to avoid a situation where
known diseases in one site can be controlled while adjoining areas are left out, thus
prolonging transmission and increasing prevalence.
Ecological studies concerning snails and disease transmission have shown that
there is a relationship in a given snail-parasite combination, that is expressed as
compatibility. This is measured as the size of infective stages produced by a given number
of exposed snails throughout the lifespan of the snail population or as the infection rate in
a population of exposed snails.
In compatible snail-parasite combinations, known results of studies show that
highly compatible relationships are characterized by a high production of infective stages,
being the result of a high infection rate, a low mortality among the infected snails and a
high daily production of parasite infective larvae per infected snail. The demonstration of
naturally-acquired infections in snails has been the principal sure way of incriininating a
snail population as hosts for a particular parasite population in a given particular habitat.
These realizations have led to in-depth studies of the ecology of snail habitats, the
distribution of snails, and their natural infections in given habitats. Furthennore, using
., ,,. ,”I. 3,’ , .,a ‘
experimentations involving both sympatric and allopatric snail-parasite combinations,
significant contributions have been made in elucidating the snail species strains and
possible hybridization characteristics of parasites within given snail-parasite combinations
11 . ..
in specific environments. These have also yielded a lot of infomation on the taxonomy of
many freshwater gastropods, thereby extending our knowledge in the direction of which
gastropod (or snail) is responsible for the transmission of which parasite; on the subjects
such as transmission seasons; and transmission potentials of. the various habitats (Okafor,
Transmission of snail-borne infections is an area that is rarely continuous except in
few cases. Usually, changes in climate affect both the humans or animals and the snail
3
populations (Chandiwana et al., 1987). Despite this known seasonality of transmission of
snail-borne infections due to climate, there appears to be a circadian rhythm of discharge
of parasite eggs and peak abundance of infective larvae in the habitats at given times
(Wright, 197 1).
The biological advantage of these rhythms is that they are means of facilitating
transmission. The other implication is that they show that the snails are readily available
for the iniracidia hatching out of trematode eggs within defined periods that support
abundance of snails. It thus becomes imperative that for any identified snail habitat, a full
understanding of the snail population dynamics, the bionomics and other biotic
interactions are important, in the overall assessment of the habitat with respect to its role
in snail-borne disease transmission.
Macrodistribution of gastropods in freshwater systems has become the subject of
inany surveys. In most habitats, it has been found to be limited by temperature, rainfall
and drought. For this reason, species will probably fail to colonize some habitats, while in
some other habitats they are abundant. It is, therefore, very important that factors of the
environment that affect the macrodistribution of gastropods in any environment are
investigated and used to understand the biological interactions within the habitats.
., $4. . *T. .,’ , , .>, ‘
While serious attention is paid to the macrodistribution of snail genera and species,
a lot more attention needs to be paid on the microdistribution parameters of the species.
For example, it will be highly instructive to understand the factors that support
11 …
oviposition, egg hatching and survival of the young snails in their preferred habitats. In his
studies, Boag (1985) found that thermic stability appeared to determine substrate selection
in adult gastropods and it would be interesting to find out if the young snails are similarly
affected. According to Banientos (1998) the number of eggs and juveniles increase with
the increase of temperature during the morning (from 6.30 am), and with decrease of
temperature later in the morning (around 10.00 am). This finding suggests the need to
study the roles of aquatic macrophytes in the freshwater habitats as agents of thermic
stability, beyond their traditionally known roles of serving as oviposition sites and food for
the snails.
It is also known that certain habitat factors may clearly correlate differently
between eggs, juveniles and adults. For example, in land gastropods, it has been noted that
humidity of litters and soil and season correlate mainly more for eggs than other stages
(Okafor, 1990a). In similar studies, earlier, others found correlation to be higldy stage
specific. They also found that high relative humidity is also an important factor for snail
and egg abundance which was an expected result due to the hygroscopic nature of such
snail eggs and to the snails’ slime trail production. The influence of some factors on egg
oviposition site selection, and choice of snail microhabitats, will be investigated. Findings
of such investigations suggest that certain factors definitely favour the presence of snails
even when the majority of other studies suggest an integrative role of the various
environn~entalf actors.
Brown and Lodge (1993) stressed the relationship between abundance and habitat
structural complexity, the latter increasing the area that can be colonized by the snails.
Abramsky et al. (1990, 1992) and Brown and Lodge (1993) showed that habitat depths are
correlated also with snail abundance, as substrates further down are used to lay eggs and
., ,, . . “f. .,’ . .3, .
hide from predators. Whether this also occurs in aquatic habitats needs to be investigated.
Other studies indicate new interests and the roles of factors like phospliorus and pH
values on snail oviposition (Barrientos, 1998), on feeding, oviposition (Barrientos, 1996,
,I . .
1998, Boag and Wishart, 1982, who also recorded that in high density populations, the
most frequent food plants were usually the common ones in the plant community, in a
more opportunistic way. Chatfield (1975) showed that more diverse habitats make
specializations less desirable and may lead to high diversity.
It is common in many snail habitats, to see live snails occurring side by side with
empty shells. The ecological significance of this has been investigated in very few
habitats. In those studied, it was noted that the different results of the analysis of living
5
snails and shells’ tendencies suggest that those categories should not be considered
together as some of the scientists do (Coppois, 1984). Shell occurrence can be the result of
factors that operated in the past but that may be currently absent. Such past events inust be
investigated for the prevention of future occun-ence of its mortality cycle.
Much of the evidence available now on parasite community structure suggests a
non random pattern in parasite species distribution among hosts. Therefore, there is the
potential for spatial and temporal variation in community structure and thus the detection
of inlportant short term or very local processes. Results from field surveys suggest that
even when there are departures from random assembly (expressed as positive associations
between the most prevalent parasites, species or nested patterns) no repeatability in
parasite community structure existed in time or space. The study of snail ecology has
thrown up several interesting results: the influence of snail size differences within each
locality; the presence of positive correlation in most of the painvise comparisons; the
nature of subgroups exhibiting nestedness, and the influence of distance between localities
on the likelihood of finding nestedness.
One of the most influential variables is the size of the individual. In fish species,
localities within nestedness in any subgroup also had a significant correlation between fish
., ,, . .”f. .?’ . .3?
size and the logarithm of the total number of parasites per fish (Poulin, 2000; Poulin and
Valtonen, 2001) for the internal parasite communities of 23 populations of fish. Whether
this is true of aquatic freshwater snails needs to be ascertained.
11 . ..
Concerns for the role of gastropods in disease transmission have prompted
increased quantitative monitoring of snails and their habitats, providing evidence that
some populations are undergoing cyclical changes in population density following
changes in physico-chemical changes in their habitats. Furthermore, parasites represent an
increasing threat to natural populations and recently, attention was focused on examining
methods by which disease threats can be managed in free-ranging wildlife (Lafferty and
Gerber, 2002). To assess and manage snail-borne infection risks effectively, baseline
6
information on patterns of infection in natural populations is critical. Many field studies
have examined within population infection patterns by focusing on the effects of factors
such as host behaviour (Rubenstein and Holin~ann, 1989); demography (Halvorsen, 1986)
and genetics, on infection rates.
Very few studies tried to look at the effects of community level interactions on
infection rates despite the fact that many parasites infect and are transmitted by n~ultiple
host species within the same community. To understand the dynamics of community level
interactions on infection rates, a community level approach is needed because both interspecific
as well as intra-specific processes influence transn~ission. This is particularly true
for parasites like trematodes whose cercariae accumulate in the external enviroiment and
in which spatial overlap between sympatric hosts is sufficient for cross-species parasite
transn~issionto occur.
For parasites shared across host species, it is expected that increasing overlap
among species will correlate with increased rates of infection. increased habitat overlap
among species is also expected to be positively correlated with parasite taxa richness.
Aims and Objectives ofthe Study: The aim of this study is to study the ecology of
gastropods and their trematode infections in Obutu Lake, an inland lake in the Orumba
. ,’. ..I. .,. . .I>
North Local Government Area of Anambra State. The lake has not been characterized by
anybody in the past, and the present study of the ecology of its gastropod mollusks
presents a good opportunity to do so.
I . .
The objectives of this study are to: –
1. Characterize some of the physico-chemical variables in the lake.
2. Investigate the interactions and associations between these variables.
3. Investigate the roles of the investigated variables on the observed abundance and
distribution of the gastropods in the lake.
4. Studies on abundance and frequency of occurrence of gastropod snails in these
habitats.
5. Studies on biotic interactions within the lake.
6. Studies on transmission of parasites through their snail hosts.
7. An assessment of habitat overlap, patterns of infections across snail host species,
and, comparative studies on habitat nestedness, overlap and parasite prevalence
and richness.
It is believed that a good knowledge of the ecology of these gastropods would have
been gained, for us to be able to predict the status of snail-borne infections of man, fish
and other livestock in the area. All these will be used to determine health risks; make
environmental assessments; and for rapid assessnlent records (e.g. use of key informants)
in the area.
8
LITERATURE REVIEW
Using the principle of competitive exclusion, which states that two species
requiring the same resources cannot co-exist, one will assume that in nature, the
competing species and their biotic environment remain genetically constant (Pimentel et
al., 1965). From what is known about gastropods, this may be a broad assuinption, as
various studies show that a habitat may contain as many as 17 gastropod species, each
competing with one another and yet co-existing. Ecology of gastropods in freshwater
systems is a study on interspecific competition, co-existence and sharing of resources in
the ecosystem, be it food, space, or other necessary resources.
Observations in nature show that molluscs generally are abundant and species are
evenly distributed in favourable habitats with focal dominance of species as a result of
environmental requirements and adaptation (Okafor and Obiezue, 2004). Field studies on
distribution and abundance of gastropods suggest that stronger species are increasing in
number while weaker species are sparse. Strength can be judged within two very
important processes namely:-
1. The ability to show adaptive plasticity and
2. Possession of a high biotic potential. Both attributes confer strength to a
.,. , . .w 3. 9,. ?.> ‘3, ‘
species and, therefore, determine abundance and frequency within and between habitats.
Evolutionary adjustinents between species of gastropods and their environment
would function in either a non-random or random distribution. The sparse species, 11011-
I* . .
randomly distributed in scattered small colonies would have an advantage because this
distribution provides the most favourable opportunity for evolutionary advance which
Wright (1937) suggested was important for any species survival. Furthemlore, it has been
seen-that more space and larger populations should provide the necessary time for
developing adaptations needed for co-existence among the species sharing resources in the
same ecological niche. It is also known that as a competing species becomes the dominant
species, it is at an evolutionary disadvantage because intraspecific competition is the main
9
selective force acting on it. The sparse species, because it is under selective pressure from
interspecific competition, has an evolutionary advantage. Given sufficient time, a genetic
adjustment between the competing species should result (Pimentel, 1965). Space-time
structure of the environment and the survival of animal species have been well studied
(Pimentel et al. 1963; Learner and Ho, 1961). They showed that to keep two or more
competing species co-existing would require the manipulation of a portion of the
genotypes making up each species population with competitive ability being selected for.
In populations with dominant species, intraspecific competition selection
predominates because there is a greater chance for individuals of this species to interact
with their own kind than with individuals of the sparse species. This principle might be
playing itself out in gastropod populations in freshwater systems. Looking at the gastropod
lists found in malacological surveys shows that this is the case. One other feature of
gastropods is that dominance amongst them shifts between species and individual habitats.
As a result, the gastropod dominant in habitat A is always different from those dominant
in habitant B. It is, therefore, very important and makes ecological sense to study the
frequency, abundance, distribution and associations between gastropods in any given
habitat, in order to put on record, the character of the habitat and the nature of species
..,,..rl..?’ ,.st ‘
interaction at work therein.
Various aspects of feeding in pulmonate gastropods have been described, in
particular, the relationship between snails and vegetation; food selection; feeding rates and
I . ..
assin~ilation (Chatfield, 1975); feeding and growth; and feeding behaviour and
distribution.
There is paucity of information on diets of other freshwater gastropod species in
nature. Further, little information seems to exist on the types and amounts of nutrients
required by them for normal growth. Many of the available studies show that because
gastropods live in habitats that support rich microflora and microfauna, they mostly feed
on these and other organic matter (Watson, 1958; WHO, 1965).
It was also noted that of these species and genera, the food items vary from habitat
to habitat (Ndifon and Ukoli, 1989). Calow (1973) demonstrated that Planorbis contortus
selectively ingests the detritus materials which adhere to the surfaces of submerged stones,
rather than on the encrusting algae.
Following from these, it is apparent that there is an important need that for each
habitat, the items of diet should be known in addition to the other biological interactions
between species growth and distribution.
There are relatively few adequate field studies on molluscan growth rates and life
cycles (Russell-Hunter, 1964, Okafor and Anya, 1991). For many species of freshwater
snails, the results are based on studies of the linear measurements of shell dimensions.
Studies on the timing of egg-laying indicate that there is a high correlation between
this and humidity of the environment (Barrientos, 1998). During the periods of low
humidity, gastropods prepare themselves physiologically while aestivating, to lay eggs at
the beginning of rainy season (Hodgson and Shacl~ak, 1957). Other factors that adversely
affect the number of eggs that eventually hatch are noted to be predators, pH changes and
drying as a result of desiccation.
Studies on parasitic infections in field populations of gastropods show that
.& ‘, . 4 -1. 7,. , .,> .
transmission is purely vertical and is predicted to cause low pathogenicity in their hosts
(Kelly et al., 2003). While parasites do not have any effects on host gastropod survival,
they cause a reduction in juvenile growth. This effect is also modulated by resource
. 11 . ..
availability and intra-specific competition, with infected individuals (Washburn et al.,
1991). Parasites are also known to be transmitted horizontally between unrelated or related
hosts of the same or different generations. Studies of Agnew and Koella (1997) showed
that selection for successful horizontal transmission favours a high parasite burden that is
often associated with high levels of pathogenicity in the hosts. Dunn et nl. (2001) were
also of this view. They showed that horizontal transmission contrasts with vertical
transmission as in the latter, the parasites are transmitted from parents to offspring through
11
successive host generations. It was also noted from the studies of Ewald (1 987), Smith and
Dunn (1991), Hurst and Majerus (1 993) and Dunn ef al. (1 995) that vertically transnlitted
parasites depend on host reproduction for successful transmission: It is also noted that
vertical traixmission is often associated with low pathogenicity and burden (Herre, 1993;
Ewald, 1994; Dunn and Smith, 200 1) and causes a reduction in the growth of hosts during
development from hatching to adult, as well as reduction in weight of the infected host as
much as 20 % in comparison with uninfected hosts. These findings are in keeping with
known patterns of host-parasite relationships in many animal groups. It is, therefore,
believed that parasite-induced reduction in growth rate is likely to lead to decrease in host
fecundity and a concomitant decrease in opportunities for vertical transmission of parasites
to hosts of subsequent generations.
Several studies have been carried out on the compatibility of different trematode
species to their different intermediate molluscan hosts, to evaluate if the interniediate hostparasite
relationslip influences the prevalence of the snail-borne diseases or transmission
of the parasites (Taylor, 1970; Tchuem-Tchuente et al., 1999). There has also been
interesting comparisons of the cercarial output of each parasite species from the
intermediate host snails especially between pure strains and hybrids, which showed that
. ,,…*I..? . , ,.I> ‘
cercaria production was lower in hybrids. (Webster and Southgate, 2003)
Field studies also show that the distribution of parasites and their prevalence were
historically restricted to a small number of foci, in spite of the fact that the gastropod
It . .
intermediate hosts are very widely distributed. It has been suggested also that there exist,
many interactions of parasite species, environmental changes or ecology, and host
susceptibility to parasites, that affect the distribution of parasites within snail populations
(Mutani et al., 1985, and Southgate et al., 1998).
Studies on the survival of the life cycle stages show that environmental parameters
like high water temperatures, low pH, salinity, and hardness of water all induced mortality
to the life cycle stages. Most effects are seen on the physiology of the stages e.g. inhibition
P’
12
of the utilization of glycogen reserves and antagonistic actions of a mixture of chenlical
elements that affect survival of the stages. Majority of these survival studies suggest that
the different stages of development of the parasite are as susceptible to enviro~~~~~ental
effects as have been demonstrated for other aquatic invertebrates.
It is well hown that cercariae possess a period of maximum infectivity which is
much shorter than their life span, but there have been few attempts to analyze their
mortality over this period. Working with Fasciola gigantica cercariae, Okafor and
Igbinosa (1988) found that activity ceases after 9 hours of active swimming.
Available literatures indicate that only acute concentrations of substances can
induce any effect on cercariae during their initial crucial period of activity. They showed
that it is not only infectivity of the cercariae that was affected by environmental
parameters, but that also their tail loss was affected in addition to the decaudised cercarial
life span. They found that tail loss increased in parallel with a decrease in survival of the
cercariae.
Studies on metacercarial survival have been limited to metacercariae residing in
protective multilayered cysts in invertebrate intermediate hosts. Studying
Echinoparyphium recurvatunl within the second intermediate host Lyinnaea peregra has
.<,’. . Wl. .I. , ‘3+ ‘
shown that a five-day exposure to certain environmental toxicants did not significantly
affect metacercarial viability but that it reduced mcan survival to about 74.3 %. This
showed that there is a high tolerance of metacercariae to environ~nental parameters and
(1 . ..
pollutants. Studying the functional biology of the cycle stages, a range of polluting
materials and conditions have been demonstrated to have toxic effects on the
embryonation and hatching of miracidia from the egg. The materials and conditions
include heavy metals, pesticides, fertilizers and acidification. Survival of developing
embryos was reduced in the presence of high toxicant concentrations (Guttowa and
Borriecka, 1975; Guttowa, 1976); while low concentratio~ls reduced the activity of the
reduction- oxidation enzymes in the embryos of Fasciola hepatica (Guttowa, 1975).
Extending this investigation further, it was found that during acute short duratioil
exposure, there was a reduction in penetration ability of the cercariae. It was also reported
that different species of target hosts may have differing susceptibilities to toxicantexposed
cercariae. Furthenore, it was revealed that the exposure of the target hosts to
toxicants can induce an increase or decrease in susceptibility to cercariae, probably via
immuosuppression or immunostimulation.
There are inlierent complexities in studying parasite transmission dynamics in
natural multi-host systems. These have limited our understanding of host-parasite or liostcommunity
environment ecology. Despite the difficulties, attempts are being made to
understand these systems. In communities comprising closely-related species, crossspecies
contact is an important determinant of generalist parasite infection risk and should
be considered even when evaluating patterns of infection even within single host
populations. It is also known that hosts living in diverse habitats may be more susceptible
to parasite “spillover” than other hosts (Begon and Bowers, 1995). Adequately quantifying
cross-species contact rates in natural ecosysten~sm ay be instructive.
Different species do not interbreed because they are separatcd by barriers to
hybridization or isolating mechanisms. If the barrier is geographic or ecological, the
.c ,,. <*I. .*’ , .?8
species are allopatric. If two species have overlapping ecological ranges, they are
syinpatric and are separated by some type of reproductive or genetic isolation. Allopatric
species may or may not be genetically isolated, but sympatric species must be. As
11 . .
populations become isolated, they find ecological regions or niches where they can live
and to which they become adapted.
Allopatric and sympatric populations have been well studied in most animal
species. For example, in tree frogs it was found that the mating call, a behavioural
characteristic, is probably the key isolating mechanism (Littlejohn, 1966). In gulls, food
preferences seem to isolate two species. They pointed out that with a great abundance of
human refuse the barrier appears to be breaking down. Whether two similar species can
occupy the same niche at the same time has been debated. The subject was discussed with
relation to food preferences of grasshoppers. Using the Acridian species Me1arloplu.s
bivittatus, Melanoplus di$erentialis and Melanoplus lakinus. Littlejohn (1966) also
observed that they occurred in the same habitat, utilizing the same foods and suggested
that such coexistence show that the species were in the same niche and in competition for
a common food supply. Their food usages were investigated by offering samples of the
dominant and semi dominant vegetation from the common habitat to caged populations of
each species and then estimating the amount consumed. The overall usage of foods of each
species formed a preferential pattern sufficiently different from the patterns of the other
two species, to indicate that the three species populations occupied separate niches in the
community and were not in complete competition for food.
Variations within a population may occur in a series of gradual changes ranging
from one extreme to the other. Nelson (1967) described such a cline for two fonns of each
of two races of P~.unellav ulgaris and discussed their habitats. He showed in California by
two sequential conlmon garden studies, the existence of a first year flowering low
elevation race adapted to mild winters and a second year flowering montane race adapted
to severe winters. Characteristics of habitat and growth rates provide for the recognition of
. . . I
coastal and inland forms within the low elevation race and cascade and sierra11 forms
within the montane race. The distribution of these four racial fornls in California was
found to be correlated with climatological features. Clinal variation correlated with
11 . ..
latitude and elevation and can be demonstrated within each fonn. A sequence by which the
four fornls might have evolved was postulated from the study. Further analysis of the
Californian results and comparison with results from European studies suggested that
modes of intraspecific variation in California and Europe may be more comparable than
previous studies would indicate.
A large homogenous population may fragment into several populations occupying
different niches to which they become better adapted by natural selection. Such adaptive
15
radiations have been subject of various studies in animal species. Steeves (1 966) showed
that affinities and distribution followed adaptive radiation of three groups of troglobitic
asellids in the complex of limestone caves in Florida. The habitat exerts some control ovcr
aninla1 distribution by creating feedback loops on which natural selection acts on to
determine the kind of organism that will prevail in a certain ecological situation (Heslop-
Harrison, 1966).
The environmental control may be driven by regular seasonal alterations with some
environmentally modulated distribution, encouraging species diversification. The success
of animal species can only be understood in relation to these general ecological principles
and the kind of coinmunity to which they belong. To date, no serious studies in these
directions exist for gastropods. Most studies are limited to providing checklists of species
present in any habitat, the diets of few species, and their roles in disease transmission,
without showing why, for example, Pila wernei, Pila africana and Pila ovata will thrive
successfully in the same habitat, or the microdistributional forces controlling the
distribution of Lanistes nzoerchis and P. freethii in the same habitat. It will also be
instructive to investigate the food preferences of gastropods within their freshwater
habitats. Studies by Kamykowski (1978) using computer modeling, investigated the
…, ..I.,?. .1.b .
interactions between internal seiches and planktonic organisms that undergo vertical
diurnal migrations. He observed some form of patchiness. The character of the patchiness
was seen to be influenced by physical variables such as:
(1 . ..
(i) The sampling interval in the field.
(ii) The frictional damping of the basin, and
(iii) The phase angle between the internal seiche and the daylight cycle of vertical
migrations; and by biological variables such as (1) organism behaviour patterns and (2)
movement speed. The computer models yielded results that compared favourably with
some of the zooplankton distribution reported in the literature. Thus, the population
dynamics of a migrating prey (predator) is probably affected by occasional coincident
16
encounters with various bands of migrating predators (prey). Other similar studies
indicated that vertical migration of planktonic organisms was made possible by the ability
to move against the pl~ysical water movements such as upwelling and wind mixing. Thc
migration of macrophytes was found to be planktonic in nature, responding to the light
intensity at the water surface. Such will affect the spatial distribution patterns of the
various aquatic animal species. For gastropods, Todd (1978) working with Oldzidoris
nzuricata (Mollusca: nudibranchiata) found that recruitment of the bentl~icp hase of the life
cycle occurred in late Junelearly July with settlement and n~etamorphosis of the
planktotrophic veliger, the post metamorph juveniles exhibiting an overdispersed
distribution. This was interpreted as an expression of environillental heterogeneity. An
apparent change in behavioural response of the individuals has been detected by following
changes of “patchiness”, and mortality has been shown to be of an “all or none” type
operating on clumps of individuals, with the conclusion drawn that predators adopted the
strategy of seeking out the high density clumps of the nudibranchs. Mortality, in this case,
was viewed as a n~echanisin inadvertently easing the energetic stress placed upon the
individuals of the population in seeking resources, this stress arising from the aggregated
nature of their spatial .dispersion. It is yet to be verified if similar things happen with
. ,,..wl..?’ .rl
freshwater gastropods,
According to Krebs (1996t), habitat selection has been defined as the interactive
expression of responses which tend to mainfain the association of an animal with a
,I . ,.
particular type of habitat. This is distinct from the establishment of home ranges where a
specific site is chosen within a favourable habitat. It is also speculated that habitat
selection is influenced by (1) habitat changes (2) invasion of a new habitat (3) changes in
climate (4) vegetation (5) structural alterations to the environment of the original habitat
(6) selection for behavioural, morpl~ological and physiological alternatives i.e. those
facilitati~g survival in and successful exploitation of new conditions (7) intraspecific
interactions (8) aggressive interactions between individuals of different sexes or sizes that
17
force segments of the population to occupy marginal habitation (9) selection within
marginal habitats to facilitate survival and resource utilization (10) interspecific
interactions (1 1) competition, by coalesciiig of previously separate ranges or by huniaii
inipact on the habitat (12) aggressive interactions that result in one or both having to
occupy new areas (13) selection for adaptation to survive in a new habitat (14)
reinforcement of habitat utilization by development of appropriate behavioural responses
to habitat features.
Although the population dynamics of aninial species has been studied extensively,
the factors that regulate tlieni remain controversial (Krebs, 1996). Populatioiis of animals
tend to fluctuate cyclically or regularly (Krebs, 1966, Boonstra et al., 1998) and these
periodic fluctuations may be caused by variations in their food supply, climatic changes
and other density-dependent factors. Thus, both seasonal and spatial variations in density
and biomass are quite high. Most studies, however, show that despite this, mean values are
comparatively low and disjunct distribution is coni~ilo-~i d istribution being controlled by
such other factors as hypoliinic oxygen depletion, food supply (especially chloropliyll – a
concentration habitat morphology and perhaps, by substrate composition in addition to the
already – mentioned predator – prey interactions.
. , . . “3. , .r, ‘
Drift rates have been measured in niost aquatic organisms but not the gastropods.
Results from insect studies show that there is a relationship between periodicallyoccurring
increases in drift and activity due to partly exogenous control and partly due to
11 . ..
endogenous rhythms, and that there is a gradation among behavioural groups in tlieir
tendency to leave the substrate and enter the water colunin (Corkum, 1978).
Physico-chemical Properties of Lakcs: Limnological studies of water bodies, as well as
the nature of the environnient, inipact seriously on the ~nalacological aspects of such
bodies of water. Lakes are known to be home to many snails, and numerous studies have
been carried out on as many lakes as are accessible. Studies on gastropod fauna of sucli
freshwater systems have aided in understanding the ecology of the bodies of water, as well
18
as the distributional relationship within the snail communities, and between them and the
character of their habitats. Such studies also open up hidden areas of the environmeiltal
biology, life cycles and biotic potential characteristics of the fauna.
Snails appear to exist in endemic foci (Solem, 1969; Breure, 1979). There is n
general assun~ptiont hat when snails are found in a given environment, they interact in
many ways with themselves and with their immediate environment. Pimentel (1 965) noted
that they both compete and yet co-exist. He further noted that interspecific competition
may change genetically so that species later co-exist. And when they do, they begin to
utilize the same resources in the ecosystem. In most cases, there is often a divide of
stronger and weaker species, each fluctuating in population depending on their responses
to environmental parameters and resources. This fluctuation leads to abundance of one
group and a decline in the population density of the other.
Many field surveys suggest that apart from the intrinsic intraspecific interactions,
certain factors of the environment like temperature, pH, alkalinity, dissolved oxygen aild
other climatic and edaphic factors, support and sustain oscillations in the population of
groups of species thus allowing for shifts in dominance of species even within the same
habitat. Co-existence, especially of apparent relatedness has been found to support the
., ,, . .*I. .?’ , <.,a ‘
establishment of comn~unitiesin suitable environments while in harsh areas no species can
thrive. The implication of this is that distribution of species is judged against the resource
gradients. This often gives an impression that the species pack in relation to abundailce or
11 . ..
lack of particular conlponents of the environment. Understanding the dynamics of this
relationship helps to resolve the underlying force determining the phenomenon of species
overlap on a particular resource gradient.
Obutu Lake is apart from being a favourable habitat for many species, also
provides water for human consuinption and allows for other human activities, including
fishing and processing of agricultural produce. It also has the potential of being developed
into a recreational facility. It is a near-closed water body with almost no hydrological
19
connection with either any sea or rivers. Its volun~e reflects interactions with direct
precipitation and recharge from rivulets and run-off water from the adjoining farm lands
and forests. This situation makes characterization of the lake imperative not only for its
gastropod fauna but also to know its physico-chemical composition and hence, the
character of the lake. Quantitative characterization of environmental variables playing in
the lake is, therefore, of very important value (Arie and Boyd, 1980).
Studies such as those by Imevbore, 1970; Vannote et al., 1980; Vannote and
Sweeney, 1980; Rabeni and Minshall, 1977; Clifford et al., 1989; Clifford and Carney,
1991, have shown that environmental factors interact in assorted combinations, producing
their effects on the flora and fauna of the lakes or other similar freshwater bodies for that
matter.
2. Abundance and Distribution of Gastropods in lakes: Patterns of snail species
diversity and distribution have been the subject of diverse and varied studies in many
freshwater ecosysten~s. Examples abound both in the international studies and among the
local Nigerian scientists. Most of the studies point at habitat diversity as a feature
controlling the number and diversity of species in any given ecosystem. Thus, species
richness and/or similarity between species within a given habitat, are attributed to this
.*,,.: ..r. st.. .7, .
factor. In adverse habitat conditions, snail species abundance becomes nearest its
minimum sustainable population size and sometimes may not be found. The reverse is the
case when conditions in the habitat are favourable and so, snail abundance approaches the
I( . ..
maximum sustainable population size.
The aim of this study is to investigate the snail abundance and factors that control
fluctuations in the snail numbers and diversity of species.
3. Food and Feeding Habits of Snails: Feeding of gastropods had attracted a lot of
attention, bringing to the fore, the structural architecture of the snails, the mode of feeding
and the foods. For example, it is now known that snails may feed on many aquatic
macrophytes. Beyond these, it is also known that many species live on leaf n~ouldsu nder
20
or near rocks on beds of their habitats, while others feed on epiphytic growths such as
algae, fungi and lichens. Most importantly, because of the nature of their radulae (which
differ with genera) they feed mainly on glabulous materials (Okafor, 1 99Ob).
There is controversy over the actual roles of aquatic macrophytes relating to the
distribution of gastropods in freshwater habitats. Many scientists like Okafor (1984),
Ndifon and Ukoli (1989), showed the importance of aquatic macrophytes in freshwater
snail ecology, and some even worked out indices of association between them. Most also
suggested that snails depended on these plant species for shelter, eg oviposition sites eg
and food eg. Others claimed that there was no significance in the association between
freshwater gastropods and aquatic nlacrophytes. Thus, we iind ourselves in a state of flux.
Paucity of literature on the food of mollusks had led scientists to study the diets of
slugs in nature, stressing on the nature and types and amounts of nutrients required for
nornlal growth. Within the same period, Calow (1970) showed that there was close
relationship between diet and growth of fresh water gastropods.
Detailed study on pulmonates by Runham (1975) showed explicitly that diet is a
very important component of snail biology. In this work, qualitative studies were
conducted on the diets of pulmonates and prosobranchs found in Obutu Lake, through the
., ,, .. < -r. .,. , .7> .
examination of their stomach contents, in order to complen~entth e observations made in
the field.
4. Parasitic Helminth of Gastropods Freshwater invertebrates have been known to cause
It . .
severe medical, veterinary and economic problen~sT. hey may also have adverse effects on
other biota within the freshwater ecosystems. For gastropods, many species serve as
intermediate hosts of trematode parasites. The trematodes (i.e. flukes) are responsible for
large losses and considerable economic impact on the livestock industry in many
countries. The gastropods play very crucial roles in the transmission of the flukes because
the internlediate asexual stages of the parasites develop in them. The snails, because they
2 1
have high biotic potentials, spread rapidly and are often abundant in suitable habitats
subjected to human disturbances (Williams, 1970).
Trematode parasites are known to generally impose costs on their snail hosts in
terms of fitness, growth, reproduction and survival (Lehman, 1993; Jaenike et al., 1995).
Part of the effects on fitness can be modulated by availability of resources and
intraspecific interactions, but the infected individuals still suffer higher costs than the
uninfected (Washbum et al., 1991).
It is common for trematode parasites to be transmitted horizontally i.e. they are
transmitted between unrelated host species, and between related species of different or the
same generations. This is commonly found in parasites transmissible through feeding of
the infective stages or tissues. Any parasite that is transmitted horizontally favours high
parasite burden in snail communities (Agnew and Koella, 1997; Dum et al., 2001). In
nature, however, vertical transmission is the norm and most studies show that the parasites
are transmitted from parents to offspring through successive host generations. It is
believed that vertical transmission of parasites depends on host reproductive perfonnance
for its success (Smith and Dunn, 1991; Hurst and Majerus, 1993).
Empirical studies have shown that vertical transmission is often associated with
.. ,, . . “7. .?’ , >.>a
low burden in host communities (Herre, 1993; Ewald, 1994). However, most trematode
diseases have a wide geographical distribution because there is an extremely close host –
parasite relationship between the larval stages and their snail hosts.
It . ..
The distribution of trematodes is largely influenced by the distribution of their
intermediate snail hosts whose distribution is, in turn, dependent on environmental and
ecological factors. This relationship has generated a lot of interests among parasitologists,
leading to numerous studies on their susceptibility, environinental requirements, focality
and other transmission dynamics of parasites within snail communities.
22
The present study looks at the parasitic trematode infections of gastropods in
Obutu Lake. The data obtained were used to explain the association between the parasites,
their snail hosts and their environment.

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