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Table 3. Three-Way analysis Of variance to lest the effect of the
weight ol ihe total siphon (4-6, ...22-24 mg), ihe experimental conditions
(0. 1-9. 10-15 mg removed), and time (Day 0... 5) on the dry
weighl ol the body In = 911: see also Fig. 7.
DEPTH AND SIPHON CROPPING IN SCROBICULARIA
R\ • 1 P
(a) Siphon weighl 26.7 0.001
(b) Exp. condition 6.4 0.026
(c) Time 4.1 0.435
(aix(bi 1 1 (I.S47
(ai xici 4.3 0.977
(b) > 3.2 0.433
Table 3). The animals losing < 9 mg were pooled be­
cause the regression of body weight on siphon weight
was the same. The body weight was relatively low,
however, in the bivalves from which > 10 mg of the
siphon was removed. All animals were alive when they
were dug up 1 to 5 days after the siphon had been removed.
These results agree with the data collected by
Hodgson (1982a) who showed thai the energetic cost
of regeneration was low when 15% ot me siphon was
removed but when 5(W< of more was cut off. regeneration
took place at the expense of the body condition.
Some animals in his experiment died, but no information
is given about survival and siphon loss.
n= 2 5 4 6 1 1
'00
30
ۥ 60
survival
20
0
•
i
5
. ,i
10 15
i
20
amount of siphon cut (mg)
Fig. 8. Survival of Scrobicularia (37 mm) kept 2 weeks in an aquarium,
in relation to the amounl of ihe siphon removed I after Straal.
Wanink & Zwarts unpubl. dala).
91
Direct evidence on the effect of siphon amputation
on subsequent survival is available from an experiment
where Scrobicularia were held in a small aquarium
filled with 20 em mud and sea water for 14 days, without
providing food. All animals from which 13 to 19
mg of the siphon weight has been removed w ere moribund
or dead, whilst all animals remained in perfect
condition when 0 to 6 mg was taken (Fig. 8).
Siphon weight, burying depth and feeding range
The relationship between depth and siphon weight
(Fig. 9A. redrawn from Fig. 5). can be used to calculate
the weight per cm of siphon from the upper edge
of the shell to the surface, assuming that Scrobicularia
have to reach the surface (Fig. 9B). It shows that freeliving
animals with a large siphon live at a depth that
allows them 3 mg per cm. Of more interest is the minimum
value: animals with a short siphon, although
they minimize the distance to the surface, only have
available about I mg per cm. Assuming that the maximum
a feeding siphon can be stretched is 1 or 1.5 mg
per cm. it is possible to estimate the potential reach of
the inhalant siphon during surface feeding (Fig. 9C).
Linke (1939). Green (1967. 1968). and Hughes
(1969) have given extensive descriptions of the feeding
behaviour of Scrobicularia. but more detailed research
is necessary to confirm that the range on the
surface over which the animals can reach really depends
on siphon weight and burying depth, as depicted
in Fig. 9C. The model presented in Fig. 9 can. however,
be used to derive some predictions.
One prediction is that about half the population will
feed within a radius of 5 to 10 cm around the burrow.
This can be measured directly, but can also be derived
from the length of the starlike feeding tracks which a
Scrobicularia makes during the sucking of the substratum.
The predicted mean value of 5 to 10 cm agrees
well with observations of a normal feeding range of 3
lo 10 cm (Thamdrup 1935), 4 to 10 cm (Linke 1939)
and 5 to 8 cm (Hughes 1969). The maximum feeding
range of 20 cm (Linke 1939) falls just within the predicted
range (Fig. 9C I,
It is also to be expected that animals possessing a
heavier siphon and thus a larger feeding space, are in a
better condition than animals with a short siphon. This
was indeed the case in the experimental animals (Fig.
7) as well as in nature: the body weight is 2(1 to 30'-'}