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Tahle 1. Macoma and Scrobicularia. Siphon length (cm) and
siphon weight (mg AFDW per mm) as a function of shell lengUi
(cm). The lengih of tin- siphon was unstretched and measured ili
reclly alter dissection. Macoma(n = 27SO. range: 4 to 21 mm) were
collected in April and May I 9X6 and Scrobicularia < n = 73. range: 6
to 35 mm) in June 1992. The regressions, w Inch are both highly sig-
tiilicaiil. were calculated over the means pei mm class, weighed lor
the number of cases.
Siphon length (IV cm) IIS ,1 turn lion oj shell length (X; mini
Mm imiii balthica
Scrobicularia plana
V= 1.37 ,\ + 0.24
Y = 6.2I X-4.22
FEEDING RADIUS. BURYING DEPTH AND SIPHON SIZE
r=0.90
r = 0.92
Siphon weight t Y: mg . m im ./ function of shell length IX: mm)
Macoma balthica
Scrobicularia plana
Y = 0.145 X- 0.009 r = 0.95
Y = 0.386 X- 0.217 r = 0.97
With Scrobicularia stored in the freezer, the unstretched
siphon length was 2 to 10 cm. about five
times longer than in boiled siphons. Siphon weight
was again highly correlated with siphon length
(r = +0.58) in these 34 Scrobicularia. The correlation
was also significant when siphon weight was plotted
againsi siphon width, measured at the top (r = +0.44),
halfway along the siphon (r = +0.50) and at the base
(r = +0.34). When siphon weight was plotted against
the product of siphon length and the square of siphon
width, the correlation was even higher, r = +0.70.
Siphon weight is thus clearly a function of the combined
effect of siphon length and siphon width. No relationship
was found between siphon weight and shell
si/e (r = +0.10). nor between siphon length and shell
size (r = -0.08). The correlation between shell size and
siphon width was non-significant with the width at the
top (r = +0.15) or halfway up the siphon (r = +0.14),
but highly significant with the width measured al the
base (r =+0.56).
Macoma (15 mm long) that were held in vertical
test tubes with sea water extended their siphons by I to
8 cm. We found for these 26 animals no significant relationship
between the total siphon weight, which varied
between 0.5 and 2.1 mg. and the maximal length of
the extended siphon. The siphons weighed 1.4 mg. on
average. The maximal extension of the siphon was.
120
Scrobicularia plana
Macoma balthica
2.2 n=ll 0.6 n=2148
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siphon weight (mg) siphon weight (mg)
Fig. 6. I engih i cm i ot the inhalant siphon of Scrobicularia (35 mm)
and .Wm iima 115 mm) after disscuion as a lunciion of the siphon
weight: from data given in Tahle I: dala pooled per siphon weight
category. Lines drawn by eye.
also on average, 4.5 cm. so 1 cm siphon length corresponded
with about 0.31 mg siphon tissue.
ln another experiment. Macoma (16 mm long) in
some mm of sea water extended their siphons horizontally.
There was again no relationship between the
length of the extended siphon and its total weight. On
average. Ihe extension was 3.0 cm (SD =1.1 cm) and
the total weight 1.28 mg; this gives for an extension of
1 cm a siphon weight of 0.43 mg. or 0.39 mg cm' 1 for
a standard Macoma of 15 mm. However, the expected
relationship between siphon length and siphon weight
was found in this sample of horizontally stretched
siphons when length was plotted against the weight of
only the extended portion of the siphon (Fig. 7A). As
pari of ihe scatter in die data could be attributed to the
total siphon weight, the relationship is given separately
for total siphon weights of 1. 1.5 and 2 mg. If the
siphon were elastic, the weight per cm extension
would decrease the more the siphon was stretched.
This is clearly not the case, since the weight per unit
length of external siphon was 0.15 mg cm ' for a total
siphon weighing 1 mg and did not increase when the
siphon was stretched by 2 or 7 cm (Fig. 7B). The same
panel shows that with a total siphon weight twice as
greai. the weight per cm levelled off at the higher value
of 0.3 mg cm', also twice as high as in the relatively
lightweight siphons. Since siphon extension is only