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SIDDIES ON THE ECOLOO.Y OF LYlmAEA TOMEN.rOSA PFEIFPER. 1 855 
AND ?. COLUMELLA SAY 1 81 7 (MOLWSCA : GAS'IROPODA) , INTER.MEDIATE 
HOSTS OF THE COMMON LIVER. FLUKE FASCIOLA HEPATICA LINNAEUS 1 758 
IN NEW ZEALAND 
A thesis presented in partial 
fulfilment of the requirements for the degree 
of Doctor of Philosopny in 
Veterinar,y Science at 
Massey Universi? 
ROBERT ERIC HARRIS 
December 1 974. 
.. 
l 1 1 
ABSTRACT 
Studies on the ecology of Lymnaea tomentosa and ?. columella 
were carried out to provide a basis for further studies on the ecology 
of Fasciola hepatica infections in New Zealand. 
Intermediate hosts of !? hepatica in New Zealand are the 
native Australasian species ?. tomentosa, an introduced American snail 
1,. columella, and the European host ?. trungatula. The literature on 
the systematics, geographical distribution, general biology and ecology 
of these snails, together with their relationships with!? hepatica, is 
reviewed. 
Field observations on snail habitats indicated that ?. columella 
occupied ponds and spring fed marshes which remained wet even in dr,y 
seasons. ?. tomentosa was found in ?imilar marsh habitats. Stability 
appeared to be an important quality of the habitats of both species; 
factors unfavourable to the snails included rapid? flowing water, 
i 
marked seasonal fluctuations in water level, and shade from tall vegetation. 
The calcium content of water in habitats ranged from 4 to 70 ppm. Snails 
were more often found on flocculent than on firm mud and this preference 
was more marked in the case of ?. tomentosa. 
Snail population dynamics were examined on one pond habitat 
and three marsh habitats of ?. columella and a marsh habitat of ?. 
tomentosa. Age structure and densit.y of populations fluctuated wide? 
between and within each of the five years of the study. These changes 
were greater in populations of ?. columella and not directly related to 
rainfall or temperature variations. Field and laboratory evidence showed 
that both species could breed throughout the year even when temperatures 
were as low as 5?C but populations were much larger in summer and ear? 
autumn. 
Studies of the fecundity of the two snail species and the 
relationship between temperature and rate of development of eggs showed 
that whilst ?. columella has a higher adult mortality rate it also has 
a much greater reproductive potential than ?. tomentosa. Eggs of 
1,. columella develop over a wider range of temperatures, hatching up to 
0 0 
34.5 C whereas temperatures above 30 C are lethal to eggs of ?. tomentosa. 
Eggs of both species underwent some development at temperatures between 
5 and 1 0
?
C but the proportion that hatched at low temperatures was very 
small, particularly in the case of !!,. tomentosa. 
Both species showed similar responses to desiccation on 
filter paper at 16.5?C and 80 to 9oft relative humidity; survival time 
was closely related to shell length, with large snails considerably 
more resistant than smaller specimens. When snails were subjected 
to desiccation on mud results were much less predictable and the mean 
shell length of survivors was often less than that of snails which 
died. 
A biometric analysis of shell shape showed distinct differences 
between large specimens of ?. tomentosa and ?. columella but shell 
dimension ratios were not diagnostic in specimens under 4 mm in length. 
Examination of a pond and a marsh population of ?. columella showed no 
differences in shell shape attributable to habitats. 
Over a 5 year period there was no detectable relationship 
between the intensity of !? hepatica transmission from a ?. columella 
habitat and snail population density. The on? correlation between 
!? hepatica transmission and rainfall was a possible inverse relationship 
between uptake in tracer sheep and December/January (mid-summer) 
rainfall. There was no evidence of any overwintering infection of 
?. hepatica infection in ?. columella. Uptake of flukes by tracer 
sheep was almost totally confined to the period between mid-summer and 
mid-winter. Both experimental and circumstantial evidence indicated 
that pugging by cattle rendered marsh habitats more suitable for snails, 
although exclusion of cattle from the experimental area coincided with 
an increase in the uptake of !? hepatica by tracer sheep. 
ii 
PREFACE 
Fasciola hepatica Linnaeus 1758, the common liver fluke, 
is a common parasite of grazing animals in IllB.l'\Y parts of the world; 
its wide range of mammalian hosts includes man. It is of considerable 
economic importance due to its own pathogenicity and as a precursor of 
"black disease" in association with the bacterium Clostridium nowi. 
iii 
The liver fluke is also of great biological and pedagogic interest as a 
model of a parasite vdth a succession of free-living and parasitic stages, 
as testified by the space devoted to descriptions of its life-cycle in 
textbooks of zoology and parasitology. 
There has been a recent revival of interest in fascioliasis in 
New Zealand due to an increase in the prevalence and distribution of the 
disease. The aim of the present study of the ecology of two of the 
intermediate hosts of ?:. hepatica, Lymnaea tomentosa Pfeiffer 18SS and 
1,. columella S?zy 1817, is to provide some basis for fUrther studies on 
the ecology, epidemiology and control of fascioliasis in New Zealand. 
Whenever possible the information obtained in this study was 
quantified and analysed statistically. The maximum level of probability 
that a result could have arisen by chance regarded as "statistically 
significant" is .os. The use of one, two or three asterisks a:f'ter the 
result of an analysis implies a probability of less than .OS, .? and 
.? respectively, and is equivalent to the terms p"'-.OS, pL.? and 
p?.001. 
.ACKNOWLEDGEMENTS 
Many people helped both directly and indirectly in the 
production of this thesis. Among the latter, who will not receive 
individual mention, were those friends whose casual enquiries acted 
as a stimulus to greater effort. More direct assistance was given 
by the following people : 
? supervisors, Dr. W.A.G. Charleston of the Veterinar,y 
Faculty of this university and Dr. L.K. Whitten from the Veterinary 
Research Division of the Ministry of Agriculture and Fisheries were 
unfailingly patient and help:ful. Dr. Charleston, being on hand, bore 
the greater burden but Dr. Whitten gave much useful advice, usually at 
short notice and often at inconvenient times. Professor R.N. Munford 
of the Veterinary Faculty, Mr. G.C. Arnold of the Mathematics Department, 
and Mr. c.s. Clarke of the Department of Food Technology all assisted 
with advice on statistical analyses but since they did not act as proof 
readers aey remaining errors are II\Y own. 
I had some stimulating discussions with Mr. N.B. Pullan in 
the early stages of the study, often during the course of a visit to a 
snail habitat one of us thought would be of interest to the other. 
Dr. M.J. Winterbourn of the Zoology Department, Canterbury University, 
identified the Ph.ysa found on the Pohangina farm? Others to whom nty 
thanks are due include Mr. D .K. Signal, Pohangina, and Mr. J. M. R,yder, 
Tokomaru, the farmers on whose property the snail population studies 
were conducted; both were totally co-operative at all times. The same 
is true of the management and staff of the c.w.s. Ltd. Meat Export Works, 
Longburn and the veterinary and lay staff of the Meat Division of the 
Ministry of Agriculture and Fisheries. Other Massey University staff 
whose help was invaluable were Mr. T.G. Law of the central photographic 
unit, the university printer Mr. P.J. Herbert, Mrs. L.C. Battye of the 
library, and the sheep farm supervisor Mr. P.H. Whitehead. I also 
acknowledge gratefully the grant from the Veterinary Faculty research 
fund which paid for sheep, fencing materials and other equipment. 
I am especially grateful to a most efficient and enthusiastic 
typist, Mrs. Joy Pearce. Almost none of the problems alleged to be 
inevitable in producing type drafts of PhD theses arose, and most of 
the minor errors discovered in the final draft were due to nty inadequate 
editing at an earlier stage. 
iv 
TABLE OF CONTENTS 
Page 
ABSTRACT i 
PREFACE iii 
ACKNOWLEDGEMENTS iv 
1 ? INTRODUCTION AND LITERATURE SURVEY 1 
1 .1 Systematics of the New Zealand Lymnaeidae 1 
1 .1 .1 Some morphological features of the 
New Zealand Lymnaeidae 2 
1 .2 Geographical distribution of the snail hosts of 
Fasciola hepatica 5 
1 ? 2.1 Lymnaea truncatula 
1 .2.2 Lymnaea tomentosa 
1 .2.3 Lymnaea columella 
1 .2.4 Other snail hosts 
1 .3 General biology of the snail hosts of Fasciola 
hepatic a 
1 .3 .1 Growth 
1 .3.2 Circulate? system 
1 .3.3 Respiration 
1 .3.4 Feeding and digestion 
1 .3.5 Reproduction 
5 
7 
7 
8 
8 
8 
10 
10 
11 
12 
1 .4 Ecology of the snail hosts of Fasciola hepatica 14 
1 .4.1 Habitats 14 
1 .4.2 Food 18 
1 .4.3 Effects of temperature 19 
1 .4.4 Aestivation 21 
1.4.5 Light 22 
1 .4.6 Behaviour 
1 .4 .7 Intra-specific relationships 
1 .4.8 Inter-specific relationships 
1 .4.9 Life cycles in the field 
23 
23 
25 
28 
V 
2. 
Page 
1 .5 The relationship between Fasciola hepatica and 
its snail hosts 29 
29 
31 
1 .5.1 Miracidia and snails 
1 .5.2 The intra-molluscan stages 
1.6 The epidemiology of Fasciola hepatica 
1 .6 .1 Prevalence 
33 
33 
1 .6 .2 The epidemiology of Fasciola hepatica 
infections transmitted by Lymnaea truncatula 34 
1 .6 .3 The epidemiology of Fasciola hepatica 
infections transmitted by Lymnaea tomentosa 35 
1 .6.4 The epidemiology of Fasciola hepatica 
infections transmitted by Lymnaea columella 37 
HABITATS 38 
38 
38 
44 
44 
48 
48 
2.1 
2.2 
2.3 
2.4 
2.5 
Terminology 
Macrohabi tats 
Microhabitats 
2.3 .1 Sampling methods 
2.3.2 Analytical method 
2.3.3 Results 
Discussion 
Conclusions 
51 
55 
3. POPULATION STUDIES 
3.1 Habitats 
57 
57 
62 
62 
67 
3 .2 Methods 
3 .2.1 Sampling 
3.2.2 Temperature and rainfall 
3 .3 Results 
3.3.1 
3.3 .2 
3 .3 .3 
3 .3 .4 
Population densi? 
Shell lengths and population structure 
The effects of pugging 
Temperature and rainfall 
3 .4 Discussion 
3.4.1 
3 .4.2 
3.4 .3 
Sampling methods 
Snail populations 
The effects of cattle upon marsh habitats 
67 
67 
69 
74 
77 
84. 
84. 
85 
87 
vi 
Page 
4. THE EPIDEMIOLOGY OF FASCIOLA HEPATICA INFECTIONS m SHEEP 
GRAZING A MARSH HABITAT OF LYMNAEA COLUMELLA 89 
89 
90 
91 
96 
5 .  
6. 
4.1 Introduction 
4.2 Methods 
4.3 Results 
4.4 Discussion 
REARING SNAILS m THE LABORATORY 101 
5.1 Introduction 101 
5.2 Experiments 101 
5 .3 Summary of results 119 
5.4 Discussion 120 
OVIPOSITION 123 
123 
123 
125 
6.1 
6.2 
6.3 
6.4 
Introduction 
Methods 
Results 
6.3.1 
6.3.2 
6.3.3 
6.3.4 
6.3.5 
6.3.6 
6.3. 7 
6.3.8 
Comparative fecundity of ?? tomentosa and 
?. columella 
Shell length and :fecundity 
Growth rate and fecundity 
Eggs per capsule and the stage of oviposition 
The effect of water changes upon fecundity 
The relationship between capsule production 
and egg production 
Oviposition at low temperatures 
Summary of results 
125 
127 
132 
132 
137 
140 
142 
142 
Discussion 142 
6.4.1 Relative fecundity of L. tomentosa and 
f!. columella 
-
142 
6.4.2 Shell length and :fecundity 143 
6.4.3 Factors influencing the number of eggs per capsule 145 
6.4.4 Water changes and :fecundity 147 
6.4.5 The relationship between capsule production and 
egg production 1 47 
6.4.6 The significance of oviposition at low 
temperatures 1 48  
vii 
7. 
8. 
9. 
THE EFFECTS OF TEMPER.A'IURE .AND DESICCATION UPON THE 
DEVELOPMENT OF THE EGGS OF LYMNAEA TOMENTOSA AND 
J!. COLUMELLA AND OBSERVATIONS ON THE RESPONSE OF 
THE SNAILS TO HEAT STRESS 
7.1 
7.2 
7.3 
Introduction 
Experiments 
7.2 .1 
7 .2.2 
7.2.3 
7.2.4 
7.2.5 
Comparison of the oviposition to hatching 
time o f'  eggs from different snails of the 
same species 
The ef'f'ect of temperature upon the rate of' 
development from oviposition to hatching 
Viabilit,y of' the eggs of'?. tomentosa and 
J!. columella af'ter holding at 4oc 
The relative susceptibility of' the 
L. tomentosa and L. columella to 
desiccation 
-
The resistance of' J!. tomentosa and 
.!!- columella to heat stress 
eggs of 
Discussion 
THE EFFECTS UPON LYMNAEA TOMENTOSA AND L. COLUMELLA OF 
DESICCATION IN AIR -
8.1 Introduction 
8.2 Method 
8.3 Results 
8.4 Discussion 
DESICCATION ON MUD 
9.1 Introduction 
9.2 Methods 
9.3 Results 
9.4 Discussion 
Page 
149 
149 
149 
149 
150 
153 
158 
159 
160 
164 
164 
164 
166 
170 
174 
174 
174 
179 
188 
1 0. SHELL SHAPE 191 
1 0.1 Introduction 
10.2 Methods 
10.3 Results 
191 
191 
194 
1 0.3 .1 Comparison of' li,. tomentosa and &. polumella 1 94 ? 
viii 
10.3 .2 Changes in shell shape associated with 
size 
10.3 .3 Changes in the shell shape of ?. columella 
associated with habitats and collection 
Page 
195 
dates 198 
10.4 Discussion 202 
11 ? GENERAL DISCUSSION 204. 
REFERENCES 209 
APPENDICES 221 
ix 
Table 
1 .1 
2.1 
2.2 
3.1 
3 .2a 
3 .2b 
3.3 
3.4 
3.5 
3.6 
4.1 
4.2 
5.1 
5.2 
LIST OF TABLES 
Reported oviposition to hatching times for the 
eggs of &? truncatula and k_. tomentosa 
Chemical values of water from some selected habitats 
of &. tomentosa and k.? columella 
Associations between Lymnaea tomentosa or Lymnaea 
columella and some habitat factors 
Habitats whose Lymnaea populations were studied 
Numbers of positive samples (one or more snails) in 
pugged and control quadrats from habitats no. 2 
and 3 
Numbers of Lymnaeid snails collected from habitats 
2 and 3, pugged and control quadrats 
Monthly rainfall (mm) Pohangina Domain 
Monthly rainfall (mm) Tiritea 
Mean monthly temperatures (?C) recorded at D.S .I.R., 
Palmerston North 
Annual rainfall and Lymnaea population density 
Percentages of k.? columella 9 mm in shell length 
in pooled annual collections from the marsh 
habitat, and the uptake of flukes in tracer sheep 
Deviations from a 21 year mean rainfall (mm) in 
Pohangina Domain 
Summary of results of experiment 5.2 
Effects of :food and water upon mean growth rates of 
k.? columella during experiment 5.2 
X 
Page 
20 
43 
49 
62 
78 
79 
80 
81 
82 
83 
95 
95 
106 
107 
Table 
5.3 
5.4 
5.5 
5.6 
5.7 
5 .8 
5 .9 
5 .10 
5.11 
5.12 
6.1 
6.2 
6.3 
6.4 
6.5 
6.6 
6.7 
Deaths of both species in experiments 5.2 and 5.3 
Mean increases in she 11 length during experiments 
5.3 ani 5.5 
Effects of food and water upon mean growt.l-J. rates of 
?. columella during experiment 5.3 
Effects of food and water upon mean growth rates of 
?. tomentosa during experiment 5.3 
Deaths of & .  tomentosa dltt'ing experiment 5.4 
The effects of meat and vegetable supplements on the 
production of egg capsules by &. tomentosa in 
experiment 5.4 
The effects of meat and vegetable supplements on the 
number of young snails produced by L. tomentosa 
in experiment 5.4 
-
Survival times of immature L. tomentosa and ?. columella 
in experiment 5.5 -
?rowth rates of L. columella and &? tomentosa in 
experiment 5.5-
Second generation snails at the end of experiment 5.5 
Fecundity of mature snails under different experimental 
conditions 
Comparison of fecundity of three size groups of snails 
from experiment 6.1 (?. tomentosa ) 
Analysis of variance of differences in egg production 
between the three size groups of Table 6.2 
(?. tomentosa ) 
Differences in eggs per capsule between the three size 
groups of ?. tomentosa in experiment 6.1 
Two level nested ana?sis of variance of results shown 
in Table 6.4 
Regression of egg production for the f'irst period of 
experiment 6 .1 on shell length of ?. tomentosa 
Regression of capsule production on the increase in shell 
length of &? columella during experiment 6.3 
xi 
Page 
108 
109 
110 
111 
113 
114 
11 5 
117 
118 
119 
126 
127 
128 
128 
129 
129 
132 
Table 
6.8 
6.9 
6.10 
6.11 
6.12 
6.13 
6.14 
6.15 
6.16 
6.17 
7.1 
7.2 
7.3 
7.4 
Regression o:f eggs per capsule on increase in shell 
length o:f ?. columella during experiment 6.3 
Di:f:ferenoes in eggs per capsule produced by ?? tomentosa 
during the :first period at 24?C in experinent 6.1 
comparing (a) smils which died with (b) snails which 
survived 
Di:f:ferences in eggs per capsule produced by ?. tomentosa 
during the second period at 24?C comparing (a) snails 
that stopped la.ying with (b) snails th at produced eggs 
throughout the experiment 
The ef:fect o:f water changes upon :fecundity of 
?. tonentosa in experiment 6.1 
The effect o:f water changes upon fecundity of 
?. columella in experiment 6.3 
Analysis of variance of capsule production by 
1? columella in experiment 6.3 to test the effect 
of water changes 
Analysis of variance of egg production by 
?. columella in experiment 6.3 to test the effect 
of water changes 
Correlations between total capsule production by groups 
o:f snails and their total egg production at each 
collection date 
Examples o:f some correlations between total capsule 
production and egg production by individual snails 
Mean eggs per capsule produced by ?. columella (Data 
from Baily, 1931) 
Comparison of the oviposition to hatching time of egg 
capsules from twenty-one ?. columella at 20?C 
Means and stardard deviations of oviposition to 
hatching times for ?. columella as assessed by (a) 
hatching of the first egg in each capsule, and (b) 
hatching of 5<$ of the eggs in each capsule 
Stepwise IID.lltiple regression of days to hatching of 
egg capsules on temperaillre 
Viabilit.y of the eggs of ?. tomentosa and ?. columella 
after storage at 4?C 
xii 
Page 
132 
136 
137 
138 
139 
139 
139 
141 
141 
146 
150 
1 51 
1 P3 
156 
Table 
7.5 
7.6 
7.7 
8.1 
8.2 
9.1 
9.2 
9.3 
9.4 
9.5 
9.6 
9.7 
9.8 
9.9 
9.10 
Statistical evaluation of some of the results shown 
in Table 7.3 
Resistance of egg capsules to desiccation 
The relationship between the log of days to hatching 
and temperatures above 25?C for egg capsules of 
?. tomentosa and ?? columella 
Relationships between shell length and LD50 
in 
1? columella and 1? tomentosa 
Comparison of the regression lines for log LD50 on log shell length in ?. columella and 
?. tomentosa by analysis of covariance 
Survival rates of snails recovere d at the end of 
experiman t 9 .1 
Survival rates of snails recovered at the end of 
experimant 9. 2 
Survival rates of snails recovered at the eoo of 
experiment 9.3 
Survival rates of snails recovered at the end of 
experiment 9.2 with regard to location of the 
snails 
Survival of 1? columella with regard to its location 
at the end of e::1<periment 9.2 
Survival of 1? tomentosa with regard to its location 
at the end of experiment 9.2 
Survival of .&,. columella with regard to its location 
at the end of experiment 9.3 
Survival of !!,. tomentosa with regard to its location 
at the end of experiment 9.3 
A comparison of survival rates among snails looated 
on Im.ld at the end of experiment 9.2 
A comparison of survival rates among snails located 
on mud at the end of experiment 9.3 
Page 
157 
158 
162 
167 
170 
182 
183 
183 
183 
184 
184 
184 
184 
185 
185 
xiii 
Table 
9.11 
9.12 
9.13 
9.14 
9.1 5 
9.16 
9.17 
9.18 
9.19 
9.20 
1 0.1 
10.2 
10.3 
1 0 .4 
A comparison of' survival rates among snails located 
on the sides of' pots at the end of' experiment 9.2 
A comparison of' survival rates among snails located 
on the sides of' pots at the end of' experiment 9.3 
Survival of snails placed directly on mud surface of' 
aquaria one and four in e::xperiment 9.3 
Survival of snails in pots whose water was allowed to 
recede gradually 
Survival of ?. columella on (a) mud and (b) in pots 
with a receding water line 
Survival of ?. tomentosa on (a) mud and (b) in pots 
with a recedi? water line 
Location of snails recovered at the end of 
experiment 9.2 
Location of snails recovered at the end of 
experiment 9.3 - pooled results 
Location of snails recovered at the end of experiment 
9.3(a ) : snails placed on mud 
Location of snails recovered at the end of experiment 
9.3(b ) : snails placed in pots with receding water 
lines 
Design of' the two level nested analysis of variance used 
to investigate the importance of' habitats and dates of' 
collection as factors influencing tre shell shape of' 
b columella 
Analysis of' variance to determine whether soroo shell 
proportions change significantly between snails in 
different shell length groups 
Results of two level nested analyses of' variance to 
determine whether the shell shape of ?. columella 
differed between habitats and collection dates 
Variabili? of' shell shape in k? tomentosa and 
,k. columella 
xiv 
Page 
185 
185 
186 
186 
186 
186 
187 
187 
187 
187 
194 
197 
200 
201 
Figure 
1 .1 
1 .2 
1 .3 
1 .4 
1 .5 
1 .6 
1 ? 7 
1 .8 
LIST OF F]GURES 
Terminology used in describing shells 
Terminology used in describing distal genitalia 
Lymnaea truncatula 
Lymnaea tomentosa 
Lymnaea columella 
Geographical distribution of k? truncatula 
Diagrammatic longitudinal section of gastropod head 
Diagram showing the main features of the alimentary 
canal of basommatophoran pulmonates 
1 .9 Diagram of the reproductive system of Lymnaea 
peregra 
3.1 Marsh habitat no. 2 
3.2 An example of the selection of one sample point in 
habitat no. 2 by the random co-ordinate method 
3.3 An example of the selection of sample points in 
habitat no. 3 
3.4 Fixed quadrat sampling method used in habitats 
2 and 3 after July 1 971 
3.5 k? columella populations in pond and marsh habitats 
at Pohangina, mean monthzy temperatures, and 
deviations from a 21 year mean mont? rainfall 
3.6 Population density estimated by 30 mirrute searches 
in two Tokomaru IIRI'sh habitats during 1 973-74 
3.7 Field populations of k? s;?olumella, pond habitat 
3.8 Field populations of !i,. ?olumella. Marsh habitat 
no. 2, Pohangina 
3.9 Field populations of k? s;?olumella. Marsh habitats, 
Tokomaru 
3.10 Field populations of k? tomentosa. Marsh habitat 
no. 3, Tokomaru 
Page 
3 
3 
3 
3 
3 
6 
13 
13 
13 
58 
60 
60 
64 
66 
70 
71 
72 
73 
75 
XV 
Figure 
3.11 
3. 12 
4.1 
4.2 
6.1 
6. 2 
6.3 
6.4 
6.5 
6. 6 
7. 1 
7. 2 
7.3 
Percentage of mature snails in different habitats 
Observed and expected frequencies of ?. columella 
in square metre plots on marsh no. 2 sampled on 
11 /4/70 
Uptake of !? hepatica in tracer sheep in relation 
to temperature, rainfall and snail populations 
The relationship between annual uptake of flukes, 
December/January rainfall, and the percentage 
of .b columella which were sexually mature 
The relationship between shell length and fecundity 
of ?. tomentosa during experiment 6. 1 
The relationship between shell length and fecundity 
of ?. columella during experiment 6. 3 
The relationship between egg production by 
L. coluxoolla and increases in shell length during 
experiment 6. 3 
The relationship between capsule production by 
?? columella and increases in shell length during 
experiment 6.3 
The relationship between eggs per capsule produced 
by &. columella and increases in shell length 
during experiment 6. 3 
The relationship between shell length at the end of 
oviposition and the fecundity of ?. columella 
Oviposition to hatching times of snail eggs 
The relationship between mean oviposition to 
hatching times and temperatures at which egg 
capsules of ?. tomentosa and ?. columella were 
incubated 
Water bath in which the heat stress experiment was 
conducted 
xvi 
Page 
76 
76 
93 
94 
130 
131 
133 
134 
135 
144 
155 
159 
Figure 
8. 1 
8 . 2 
8.3 
8.4 
9.1 
9.2 
9.3 
9.4 
1 0 .1 
10.2 
10.3 
10. 4 
Snails being desiccated on filter paper in an 
incubator 
The relationship between LDSO (resistance to desiccation) and shell length 
The relationship between log LDSO (resistance to desiccation) and log shell length 
A diagrammatic representation or the w? in which 
water may be lost rrom snail tissues 
Diagrammatic representation of the aquaria used 
in experi.Ioont 9.1 
End elevation or an aquarium rrom experiment 9.2 
End elevation or aquaria in experiment 9.3 
showing the two treatments 
Means and 95% conridence limits or shell lengths 
ror groups or ?- tomentosa and ?. columella 
exposed to desiccation on mud 
Parallax errors arise ir both extremities or the 
object are not kept in focus 
Shell shape parameters referred to in the text 
Changes in various shell dimension ratios in groups 
of snails or difrerent shell length 
Changes in PA associated with shell length in 
1!,. columella 
Page 
164 
168 
169 
172 
175 
177 
177 
181 
193 
193 
196 
199 
xvii 
Plate 
2. 1 
2. 2 
2. 3 
2. 4 
3. 1 
9. 1 
LIST OF PLATES 
Pond habitat of ?. columel la 
??sh habitat of ?. tomentosa 
Grids used for sampli? microhabitats 
2 
Two specimens of ?? columel la in a 10 cm quadrat 
A square metre quadrat dug over and tramped to 
sinulate pugging by cattle 
Pots used in experiroont 9.2 with gauze covers 
removed 
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