waders and their estuarine food supplies - Vlaams Instituut voor de ...

5 10 15 20 25 30 35 40 45
size (mm)
Fig. 5. Scrobicularia plana. Frequency distribution of depth per
size class m winter (grey histograms: n = 31)82) and in simmer
(black lii-iogiams: n = 5254). Mean depth indicated. Table 2 gives
statistical analysis,
Scrobicularia which pass the limit of 30 mm remam
at about the same depth. 6 cm in summer and 11
cm in winter (Fig. 5). Depth increases with si/e in the
range of 10 to 30 mm length. Smaller individuals were
not found because there was no new spatlall during the
stiiilv period.
The depth/si/e relationship is significant in all four
species (Table 2). There is a large difference between
winier and summer depm in the deposit feeders. Macoma
and Scrobicularia. but not in the suspension
feeders. Mya and Cerastoderma. In all species there is
remarkable variation in burying depth for a given size
class, especially in the two deposit-feeding species.
SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES
Body weight, siphon weight and shell si/e
The increases of siphon weighl and total body weighl
with si/e in the four species are shown in Fig. 6. There
is a large Variation in body and siphon weight on different
sampling dales within the summer and winter
periods, which explains wh) the exponential increase
of body weight with si/e does not have as perfect a linear
correspondence on a log-log scale as it does for
each sampling date separately.
The increase of siphon weigh! w ilh si/e is also exponential
for all four species, though there is a levelling
off in the siphon weighl for the larger individuals
(Fig. ft).
Burying depth and siphon weight
Siphon weighl was measured instead of siphon length
since length is very difficult to measure. Chapman &
Newell (1956) showed lhat the siphon of Scrobicularia
is elastic in nature: ihe siphon can be stretched more
with a same amount of siphon mass by proportional
thinning of the siphon wall (see aKo Hodgson & Trueman
1981). This elasticity of the siphon probably also
occurs in Macoma. According to Zwarts et al. (1994)
the siphon of this species can be Stretched more if its
weight is low. Average siphon weight and shell length
appeared to be well correlated.
Vassallo I 1971), Reading & McGrorty (1978) and
Ratcliffe et al. (1981) suggested lhat the increase of
depth with size in Macoma can be attributed to the increase
of siphon length and thus siphon weight. The
question, however, remains w heiher ii is the increment
of siphon weight that determines the observed relationship
between burying depth and shell size. Indeed,
burying depth as well as siphon weight increase with
shell si/e i Figs. 2-5 and Fig. 6. respectively) but the in-
Table 2. Results ol 4 two-way analyses of variance lANOVAl to test effect of shell size and season on depth distribution Isame data as in
Figs. 2-5).
Species Seasar Size x Season II
If. lb /' R L % P lf.% /'
Mya arenaria 76.4 0.001 0.6 0.001 0.7 0.001 2528
Cerastoderma edule 21J 0.001 0.7 0.001 l..v 0.002 1288
•na balihica 1.8 0.001 33.3 0.001 U-i 0.001 8320
Scrobicularia plana S.I 0.001 50.2 0.001 0.2 0.001 8336
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