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Aging and Growth measurements
Figure 1. Winter – grade lines on shells of
Littorina littorea sampled in middle of June 1997; A – the individual
having survived for three winters, B – the individual having survived
for 8 winters, C – the individual having survived for 12 winters;
+ - growth increment of current season, 1 - 12 – winter – grade
lines numbered according to winters survived.
Figure 2. The diagram of a snail shell measurements; L –
last winter grade line,  -
angle increment, d0.- shell diameter at the moment of
winter – grade line formation (beginning of the current growth season).
Snails were aged using both the number and position
of winter growth interruption lines on the shell’s surface (Figure
1). The mechanism of formation of these lines on the shells of White
Sea intertidal snails is described in Gorbushin (1993). As a natural
mark of the beginning of the snail growth season (late May) the
last winter grade line was used. Two further measurements were taken
from each shell: shell diameter at the level of the last winter
growth interruption line (d0), and shell angle gain ()
from the last winter interruption line to the rim of the shell aperture
(Figure 2). Because the specimens were collected almost at the end
of the snail growth season (autumn frosts are possible already in
early September), the angle gain measured here can be considered
to represent annual growth. All measurements were taken under the
stereomicroscope using a standard ocular provided with a micrometer
and an ocular angle meter. For angle growth measurement, the shell
was vertically oriented so that the collumular axis was perpendicular
to the plane of measurements. The error of the diameter measurement
did not exceed 0.05 mm and the angle gain was measured with pi/32
accuracy.
Data analysis. Since growth rate decreases with snail size (see
results), a comparison of growth rates between infected and uninfected
specimens was carried out by applying a paired two-tailed t-test
on pairs of snails with similar shell diameter at the last winter
line (d0). Pairs of snails were chosen (where possible)
randomly from the database using a computer algorithm. The same
analysis was performed with similar aged pairs. Due to possible
growth rate differences between sexes only individuals of the same
sex were paired.
Evaluation of the method Because angle gain from
an identified last growth interruption line on the shell as a measure
of growth rate rarely has been applied on gastropods, it may be
appropriate to comment on the advantages and disadvantages of this
method in studies of parasite-induced growth changes. In comparison
with the usually applied methods measuring changes in shell-height
(or width) during mark-recapture studies in the field or under controlled
laboratory conditions, the present method has several advantages.
(1) Measurements of angle gain is considerable more accurate than
any measures of growth based on changes in shell height or width,
especially when dealing with slow growing species or specimens.
(2) The method avoids artifacts that may occur due to parasite-induced
shell-deformation unrelated to actual growth rate but nevertheless
affecting shell-height or width (Wesenberg-Lund, 1934; Rothschild,
1936; Sturrock & Sturrock, 1971; Ankel, 19XX; S. R. Martorelli
pers. com. regarding Potamolithus agapetus Pilsbry, 1911). (3) It
avoids problems with spire-erosion that might invalidate conclusions
in mark-recapture studies where only shell-height is measured (see
e.g. Huxham et al., 1993). (4) Measurements of angle gain is easy
and time-saving, and there will be no problems with low rates of
recapture usually experienced in mark-recapture studies. (5) In
comparison with laboratory experiments the method benefits from
its direct in situ approach thereby avoiding artifacts such as unrealistic
availability of food (see e.g. Fenández & Esch, 1991).
Beside the above advantages, also a few disadvantages
should be recognized. (1) The method assumes that measured specimens
all have been infected (or uninfected) throughout the period from
the formation of the last growth interruption line. However, if
this is not true it only increases the variance in the group of
both infected and uninfected snails, and hence, do not necessarily
affects the conclusion if the sample sizes are sufficiently large.
(2) In species of snails where gigantism seems to be present, it
might be argued that a high angle gain (= high growth rate) in a
particular infected individual could be attained because it has
been especially active in feeding and therefore more likely to encounter
infective parasite stages (eggs or miracidia), and consequently
have had a higher probability of becoming infected. However, since
trematode infections appear to increase the growth rate of Hydrobia
spp. (Gorbushin, in press), whereas no or a negative effect is observed
in Littorina littorea (this study) - in both case using the method
of angle gain - that possibility seems only to be minor importance
if present at all. Moreover, in populations of at least longer-lived
snails, the trematode-infections seems to accumulate only very slowly
(Curtis, 1996), which minimize the probability that new infections
arise during a single growth season or period.
Finally, it should be emphasized that the identification
of the last growth interruption line as the critical point in using
the method of angle gain, has shown to be a reasonable easy task
both on Hydrobia and Littorina shells. That these lines actually
represent a period of growth interruption or at least a suitable
starting point for growth estimates, is demonstrated by the observation
of the overall decrease in angle gain with increasing shell size
in both Hydrobia and Littorina (see Gorbushin, in press and Results).
Such negative relationship between growth rate and size is perhaps
the most notorious observation in studies of gastropod growth.
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