rows 1 2
5
(a) overhead view
side wal l of aquarium
snails were placed .in these pots
pots
l
water / / / / / / / / / "' mud / / / / / ( b) end elevation9.1 Diagrammatic representation of the aquaria used in experiment 9.1
centre rows of pots with 1 cm water
(some mud oozed through the holes outer rows of pots to which snails were added
in the bottom)
l
\
to the right or le�t pot in the row was dec ided on the toss o� a c oin,
In Experiment 9.1 the snails were kept in an incubator at 1 6 . 5°C . Relative humidity , recorded by a hair operated hygrograph , was 80-9Q% for the �rst seven d�s , but it must have dropped in the later stages because the soil surface dried and cracked .
U�ortunately no monitoring device was available in the latter period. Randomly selected groups o� snails were removed at various times �om twenty-eight to �ort,y-nine days �ter the experiment began. Their mud filled p ots were placed in glass jars filled with water to a depth o� 1 0 cm. Live and dead snails were c ollected over a three hour period. At the end of the experiment the aquaria were �ooded with tapwater �or twenty-four hours in an attempt to �ind some o� the snails which may have escaped �om their p ot s into the mud outside .
were :
The size ranges of snails at the start of experiment 9 . 1 &· tomentosa - 2 .4 t o 7.0 mm
�.
- 2 . 5 to 1 4.3 mm .The method employed was similar to that for experiment 9 .1 except that there were three aquaria and the depth of mud in the p ots was approximate� 4. 5 c m i . e . about 1 . 5 c m below the level o� the mud outside ( Fig . 9 . 2 ; Plate 9 . 1 ) . Each p ot was c overed with a p iece o� c otton gauze held in place with a rubber band in order to prevent snails escaping �om the pots.
There were unequal numbers of the two species in this
experiment so individual snails were randomly allocated to p ots until there were �our to each pot . The sizes and species in each pot were thus decided randomly , but when there were �our snails in a p ot it was removed �om the possible choices �or the next snail .
vvater
•
.. ..
• • • cotton gauze rubber bandsnails in these pots
. ··
. . .
, . ..
· -- / / / / / / / / / / / / ' / / / / / / mud / / / / / / / /,.
/ / / / / / :/,.
/ /9.2 End elevation of an aquarium from experiment 9.2
(a) aquaria 1 and 4
snails placed on mud surface
(b) aquaria 2 and 3
snails in vvater
• •• • • •
In experiment 9 .2 snails were kept in a thermostatically
c ontrolled room at 22°C . Relative humidity varied between 59 and 97% due to evaporation from other open aquaria in the room, and the
mud surface never dried or cracked as in experiment 9 . 1 • Af'ter thirty-nine days visible snails were removed and the mud in the.pots and aquaria was flooded for twenty-four hours in an attempt to
recover live snails which may have burrowed below the surface.
Experiments 9 .2 and 9 .3 included c onsideration of the
p osition of snails at the end of the experiment i . e . whether they were found on the mud surface , on the side of the pot , had escaped
from it and were recovered from outside , or were lost . at the start of experiment 9 . 2 were :
�. tomentosa - 3 .1 to 7.4 mm
�. - 3 .2 to 1 0 ,1 mm,
Size ranges
In the four aquaria used in this experiment the depth of mud was as in experiment 9 .2 i . e .
6
cm outside the pots and approximately4 . 5 cm inside them. However this time the middle row c ontained mud
and was used in the same way as the rest. This gave a total of sixty p ots , to each of which was randomly allocated one �. tomentosa and one
�.
coluzoolla.In two of the aquaria, replicates one and four, snai ls were placed directly on wet mud. In the other two the pots were three
quarters filled with water which subsided slowly over a few days , p artly by evaporation and partly by diffusion into the mud outside
( Fig . 9 . 3 ) . Selection of aquaria for treatments was as for random
block experiments i . e . having decided that aquaria one and four would c onstitute one block while two and three would be the other, the treatments were decided on the toss of a c oin.
In experiment 9.3 the aquaria were kept in a room whose
temperature varied from 1 5 to 28°C and relative humidity fluctuated
between 34 and 9�. Temperature and relative humidity were recorded
by a hair operated thermo-hygrograph. The range of shell lengths of snails at the start of the experiment was :
�. tomentosa - 3 .4 to 7 . 8 mm
After thirty-�ive �s experiment 9 .3 was terminated by removing visible snails and flooding the pots as in experiments 9.1 and 9 . 2 .
9 . 3 RESULTS
Results are summarised in Appendices 9.1 , 9.2 and 9 . 3 .
Survival
The survival rate of �. tomentosa recovere d at the end o� exp eriment 9 .1 was significantly higher than that o� L. c olumella
(
Table 9.1 ) but there were no significant di�ferences in survival rates between the speci es in the other two experiments(
Tables 9 . 2 ; 9 .3 ) . This was true whether snails remained on mud or climbed ont o the sides o f the pots during experiment 9 . 2(
Table 9 . 9 ; 9 .1 1 ) . There was a surpri sing tendency , not quite significant at the . 05 leve l of probability , for�
. to survive better on the side walls of the p ots during experiment 9 .3 than on the mud surface(
Table 9 . 7) and to be more likely to survive in this location than1· tomentosa
(
Table 9 .1 2 ) . There was a c onverse tendency in experiment 9 . 3 for�.
tomentosa to survive better on mud than on the side walls(
Table 9 . 8 ) and to be more likely to survive on mud than L. c olumella . -(
Table 9 .1 0) . However di�erences did not reach the . 05 level o�significance and there was no evidence o� � of these trends in
experiment 9 . 2
(
Tables 9 . 5 ; 9 . 6 ; 9 . 9 ; 9 .1 1 ). In experiment 9 . 2s nails survived equally well on mu d or on the sides o f the p ots and there were no differenc es in survival rates between the species in either location.
Experiment 9 .3 a�orded an opportunity to test Whether the
snails were more like� to survive when p laced directly on a
mud
sur�ace as i n aquaria one and four , or if subjected to the receding
water level of aquaria two and three . When placed directly onto mud
the survival rates of
�.
tomentosa and 1· were almostidentical
(
Table 9.1 3 ) ; �. tomentosa tended to survive better than�. when subjected to a receding water level although this
result was not statistically signi�cant