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known siphon weight and burying depth. Zwarts
(1986i showed that the burying depth oi Scrobicularia
was not only a function of siphon weight, but also of
body condition. Using the equations from Fig. 8. the
average feeding radius may be estimated for individuals
living at different depths (Fig. 10). All animals ha\ -
ing a body weight > 10% above the average, are able to
take no risks and live in a depth refuge beneath the upper
6 cm of the substrate exploited by the main predator,
the Oystercatcher Haematopus oslralegus (Wanink
& Zwarts 1985). This means that in animals with
a short siphon but reasonable body condition, the
entire siphon length is used for living as deeply as possible
so that there is no extra opportunity for deposit
feeding on the surface. However, animals with a relatively
low body weight have fewer reserves, making it
more important for them lo avoid further starvation,
and so they accept a larger predation risk by living
nearer to the surface. These animals extend about 40%
of the siphon length over the surface to feed, even
when the siphons are quite short. However, as Fig. 10
shows, if the siphon is very short, the bivalves put
mote emphasis on predator protection than on food input,
presumably to avoid the extremely high predation
risk associated with living at a very shallow depth
(Wanink & Zwarts 1985). This supports the suggestion
of Zwarts (1986) that the choice of the burying depth
in deposit-feeding bivalves is the result of a trade-off
between predator avoidance and feeding. The observation
that the feeding radius becomes smaller and burying
depth greater in Macoma whose feeding circumstances
improve (Lin & Hines 1994). indicates that
burying depth is the outcome of conflicting demands
and that, under each set of conditions, an optimum can
be defined.
An interspecific comparison between siphon
liiiLii h and siphon weight
The average siphon weight oi Scrobicularia (35 mm)
amounts to 10 mg. corresponding to an average siphon
length of 10 to 11 cm (Fig. 8A). Scrobicularia live in
winter at an average burying depth of 10 cm (Fig. 5 in
Zwarts & Wanink 1989. 1993). so that they should just
be able to reach the surface. On average. Macoma (15
mm) extend their siphon 5 cm (Fig. 9B). and they live
al an average depth of 5 cm in winter (Fig. 4 in Zw arts
& Wanink 1989). As in Scrobicularia. the estimated
FEEDING RADIUS. BURYING DEPTH AND SIPHON SIZE
126
overage siphon length agrees closely with the measured
average burying depth in winter. This means thai
both bivalve species increase their depth in winter just
to the point at which, on average, they are able to reach
the surface (o acquire oxygen from above the black
mud where they live. This implies that they do not feed
on the surface. Both species live at shallower depths in
summer (Zwarts & Wanink 1989. 1993). At this time
of the year, the predicted feeding radius is on average
4 cm in Scrobicularia (the gap between the two lines in
Fig. 8) and 2 to 3 cm in Macoma. which is similar to
the observed range in both species. These agreements
between predicted and observed burying depths in
winter and surface radius in summer suggest thai the
predictions of siphon length on ihe basis of siphon
weight work out well.
Scrobicularia live in winter at twice the depth of
Macoma and also invest, relative to total body weighl.
Iwice as much material in their inhalant siphon (Figs. 8
and 9). Siphon weight is 5 f f of the total bodv weight in
Scrobicularia (35 mm), but this is only 2 to 3% in Macoma
(15 mm) (Fig. 12 in Zwarts & Wanink 1989).
The weight per cm of extended siphon (1.0 mg cm' 1 in
Scrobicularia and 0.2 mg cm* 1 in Macoma. on average)
is in both species equivalent to 0.5 to 0.6% of the
total body weight (224 mg in Scrobicularia and 33.8
mg in Macoma. on average: Fig. 8 in Zwarts 1991).
The closely-related Tellina tenuis is even smaller than
Macoma (flesh content 15 to 25 mg), yet lives as
deeply as Scrobicularia (10 cm) (Trevellion 1971). Its
siphon weight of I to 2 mg (Trevellion 1971) is relatively
high, representing 7.5% of the total body weight.
However, the weighl per cm extended siphon is similar
to Scrobicularia and Macoma if we assume that the
siphon is protruded I to 3 cm above the surface. Body
and siphon weight and burying depth have also been
determined of a filter-feeding bivalve. Cerastoderma
edule (Zwarts & Wanink 1989). This species lives 1.5
cm below ihe surface and the weight of the inhalant
siphon is 1% of total body weight. Since the siphon is
extended only a short distance above the surface, the
investment per cm siphon lengih is slightly below
0.7% of the total body weight. Comparable data for
more bivalves are needed to know whether the apparent
tendency for siphon investment to equal about
0.6% per cm extension, independent of size, is a general
phenomenon in bivalves.