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ury deeply along the North American coast, compared
to their European conspecifics. The depth/size
relationship, such as determined dining the summer in
San Francisco Bav iVassallo 1971). looks like the winter
curve in Fig. 4. Blundon & Kennedy (1982b) found
individuals of 7 to 10 mm at no less than 5 to 10cm in
the Chesapeake Bay during the summer. Macoma
which reach a size of 30 to 40 mm in the southern part
of their American distribution area (Beukema & Meehan
1985) are also buried at very great depth: 10 to 35
cm (Vassallo 1971. Blundon & Kennedy 1982b).
Hughes (1970a) who measured the winter depth of
Scrobicularia on a mudllat in Wales, found spat of 3 to
4 mm at a depth of 1 to 3 cm. His other measurements
-individuals of 16 mm at 4 to 6 cm and the si/e class >
25 mm at 6 lo 18 cm- correspond with our findings
(Fig. 51.
Body weight, siphon weight and shell size
The body weight/size relationship in Fig. 6 corresponds
with the already published data on Mya
(Munch-Peterson 1973. Warwick & Price 1975.
Miiller & Rosenberg 1983). Cerastoderma (Hibbert
1976, Hancock & Franklin 1972. Chainbeis & Milne
1979. Newell & Bayne 1980, Sutherland 1982a,
Moller & Rosenberg 1983). Macoma (Chambers &
Milne 1975a. Beukema & de Bruin 1977, Bachelet
1980. Cain & Luoma 1986) and Scrobicularia
(Hughes 1970b).
Hodgson 11982a) found the same siphon weights
for Scrobicularia as shown in Fig. 6D. The relationship
between siphon weight and shell size in Macoma
(Fig. 6C) agrees closely with Pekkarinen (1984). but
the siphon weights given by Reading & McGrorty
(1978) are much larger than the 1-2 mg found by
Pekkarinen (184). de Vlas (1985) and ihis study.
Mya invest 40 to 50% of their body weight in
siphon mass (Fig. 12). That is extreme compared to the
other three species, even if it is taken into account that
the inhalant and exhalant siphon of Mya are fused,
whereas Ihey are separate in the other three species
(Fig. 1). The weight of the exhalant siphon oi Scrobicularia
is 70*36 relative to its inhalant siphon (Zwarts
unpubl.). which means lhat the weight of the inhalant
and exhalant siphon equals 8 to 9% of the total body
weight. The siphon weight of Mya is thus 4 to 5 larger
than for Scrobicularia.
SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES
105
In the most shallow-living species. Cerastoderma.
the relative weight of the inhalant siphon is minimal.
viz. 1 to 2%. Macoma invest I to 4'/< of then body
weight and Scrobicularia c. 5%. Comparing the four
species, one sees a clear correlation between deeper
burying depth and the increase of siphon weight in
proportion to total body weighl.
Burying depth and siphon weight
It is clear that burying depth depends to a large degree
on siphon weight, as was already shown by Zwarts
( 1986) who manipulated the burying depth of Scrobicularia
by reduction of the siphon weight. Burying
depth is, however, not proportional to siphon weight
(Figs. 7-9), a fact for which three explanations can be
given. First, depth is not proportional to siphon weighl
because the relation between length and weight is also
not proportional: the siphon is stretched more if its
mass is less i/warts ci al. 1994). Second, there is still
some variation in siphon diameter when animals of a
similar size class are compared (Fig. 10) so that, on the
average, a heavy siphon might be thicker. Third, il is
likely that deposit feeders with a relatively heavy
siphon extend agieatei part of Iheii siphon out of their
burrow to graze the surrounding surface (Zwarts
1986).
Suspension feeders like Mya and Cerastoderma do
not extend Iheir siphon far above the surface, which
means that burying depth depends mainly on siphon
length. The decision to select an optimal depth is more
complex for deposit feeders like Macoma and Scrobicularia.
since a part of their siphon is extended to graze
the surrounding surface. The radius lor deposit feeding
in fact equals total siphon length minus burying depth,
and hence a reduction of burying depth enlarges the
sin face for deposit feeding. For deposit feeders burying
depth is a compromise between feeding and avoidance
of predation. This trade-off is influenced by body
condition in Scrobicularia (Zwarts. 1986): animals
with a small siphon remain at larger depth if they archeavy,
possibly because they are able to survive a period
during which the feeding radius is limited. Individuals
in poor condition have no choice and reduce
their depth to increase their feeding opportunity, thus
accepting an enlarged predation risk (Fig. 1 IB).
Macoma and Scrobicularia do not rely on deposit
feeding only, for they are able to filter food from the