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FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE
FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE OF
MACOMA BALTHICA AND SCROBICULARIA PLANA
This paper investigates whether siphon weight limits the elongation of the siphon of deposit-feeding
benthic bivalves under natural conditions. Were this to be so. it would imply that foraging and predator
avoidance place conflicting demands on these animals, since an increase in the feeding radius on the surface
would be associated with a decrease in the burying depth, and vice versa. The paper presents methods
with which to measure siphon length and equations to transform siphon weight to siphon length in
two benthic bivalves, Scrobicularia plana and Macoma balihica. Relatively heavy siphons are longer.
but also thicker, than lightweight ones. We conclude that most individual bivalves stretch their siphons
fully while feeding. However, bivalves with heavy siphons keep part of the siphon in reserve within the
shell. Siphon cropping therefore results in an immediate reduction in siphon length, unless the siphon is
heavy and a reserve is available. The feeding radius of Macoma and Scrobicularia is a linear function of
shell size. Scrobicularia use half of their siphon length to feed on the surface and the other half to bury
themselves, but if the siphon weight is below average, the proportion extended over the surface decreases
with siphon size. However, among the animals with a short siphon, those with a good body condition
take no risks and live as deeply as possible. A comparison between species reveals that the weight of the
extended inhalant siphon per cm amounts, on average, to 0.6% of the total body weight.
Introduction
Bivalves that live buried in the substrate arc usually
surrounded by a hostile environment, wiihout oxygen
and food. Suspension-feeding benthic bivalves reach
the surface for respiralion and for filtering fcxxl from
the water by pushing their inhalant siphon through the
overlying mud. Deposit-feeding benthic bivalves have
to protrude their inhalant siphon even further when
ihey feed on the surface of the surrounding substrate,
li is ol crucial importance to benthic bivalves to
elongate their inhalant siphon as much as possible.
since a long siphon enables them to live at a safe depih
beyond the reach of surface predators (Zwarts &
Wanink 1989). On the other hand, il is reasonable to
suppose that deposit-feeding bivalves may increase
their rate of food intake, and thus rate of somatic
growth, by living near the surface so that they can extend
the siphon on the surface over as large a feeding
space as possible. Unless, and improbably, the siphon
is so elastic that bivalves can simply extend it even
more whenever they have to collect distant food and/or
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increase their burying depth to avoid surface predators.
the minimization of prey risk (by burying deeply) and
the maximization of feeding opportunity (by extending
the siphon more) place conflicting demands on these
animals.
Scrobicularia plana can achieve a remarkably long
extension of the siphon (Chapman & Newell 1956.
Green 1967. Hodgson & Trueman 1981). However,
this does not necessarily mean lhat the extent to winch
an individual usually protrudes its siphon is highly
variable. The burying depth of Scrobicularia. and also
oi Macoma balihica and Mya arenaria. increases with
siphon weight when individuals of ihe same size are
compared (Zwarts & Wanink 1989). Moreover, Scrobicularia
reduce iheir burying depth when part of their
inhalant siphon has been removed experimentally
(Zwarts 1986). Furthermore, the degree to which these
cropped Scrobicularia approach the surface also appears
to depend on their body condition, defined as the
individual's body weight relative to the average of its
size class. Only animals vv ith an underweight siphon
that are also in poor condition move closer to the sur-