waders and their estuarine food supplies - Vlaams Instituut voor de ...

surface in die containers was varied between 0 and 2
cm. bin we took care to ensure that the mud was always
moist. For practical reasons, the observations
were restricted to the simulated low water situation.
We first recorded the maximal feeding radius of individual
animals that were actively engaged in deposit
feeding during observation periods of 20 min. After
that, we measured the length of the thread remaining
above the substrate lo determine, by subtraction, the
burying depth (Zwarts 1986). Feeding radius and burying
depth were determined 418 times in Macoma and
195 times in Scrobicularia. The bivalves remained in
perfect condition during the ten days of experiment.
If bivalves make a trade-off between the distance
over which they extend their siphon and burying depth,
we would expect them to extend their siphon more,
and to move nearer the surface, as the food supply declined.
We expected a decline in the food supply to occur
over the course of the experiment because of depletion
around the siphon hole (Brey 1989. 1991).
However, the bivalves stopped deposit feeding altogether
after about one day. Therefore we had to turn
over the upper layer of the mud regularly or add new
substrate. This immediately stimulated deposit feeding,
without exception.
The sum of the maximum feeding radius and of the
burying depth for a single individual estimated its
siphon length. Once a consistent estimate had been obtained,
we tried to remove a part of ihe surface-feeding
siphon with a sharp-pointed pair of tweezers to simulate
the effect of siphon-cropping flatfish (de Vlas
1979a). We were able to measure the feeding radius
and burying depth of 11 Macoma and 10 Scrobicularia
alter such cropping action.
Four alternative ways were used to measure siphon
length as a function of siphon weight. These data were
collected in May-June 1986 and October 1992 for Macoma
and in June 1992 and October 1992 for Scrobicularia.
First, we cut the inhalant siphon from the soft
body of 2789 Macoma (4 to 21 mm long) and of 101
Scrobicularia (6 to 35 mm long) that were killed by a
short emersion in boiling water, and measured the
length of the unslretched siphon immediately afterwards.
Second, we separated inhalant siphon and body
in 35 Scrobicularia (18 to 45 mm long) which had
been stored in a freezer, and measured in the unslretched
siphon length, the width at the top. at the base
FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE
116
and halfway along the siphon. Third. 26 Macoma 16
mm long were put in vertically-held lest lubes filled
with sea water. The length of the extended siphon was
measured by reference to a scale at regular intervals,
after which Macoma were sacrificed to separate inhalant
siphon and bcxly. Four. 81 Macoma 15 to 20 mm
long were placed in a dish with 0.5 cm ol sea waler.
When the siphon was extended, its length was measured
and then it was cut oil at the shell edge. Siphon
weight was determined separately for the extended
part and for the part remaining in ihe shell.
Analysis
The feeding radius, burying depth and siphon length
are given as a function of shell size. In order to analyse
ihe data with the effect of si/e ruled out. all weight and
length measurements were transformed to a standard
size class (Scrobicularia 35 mm and Macoma 15 mm
long), using regression equations given in the paper.
SPSS was used for all statistical analyses (NoruSis
1988),
Results
Shell size, feeding radius and burying depth
Feeding radius and burying depth of individual Scrobicularia
simultaneously were measured in the field
(Fig. I A) and in the laboratory (Fig. IB). Burying
depth increased with size, as was also found by
Hughes (1970a) and Zwarts & Wanink (1989. 1993).
In the field. Scrobicularia were buried more deeply in
June 1992 compared with earlier measurements made
in the same area over seven summers (Zwarts &
Wanink 1989. 1993). The bivalves in the laboratory in
October were buried at typical winter depths (Zwarts
& Wanink 1989. 1993). The feeding radius m the field
and in the laboratory varied between 0.5 and 7 cm.
similar to the range mentioned by Thamdrup (1935).
linke (1939) and Hughes (1969). As expected, feeding
radius increased with shell size. but. although significant,
the effect was rather weak. The siphon length
(i.e. sum of bun ing depth and feeding radius) of large
Scrobicularia did not differ between field and laboratory.
However, the smaller specimens extended ihe
siphon more in the laboratory than in the field, due to
their relatively large burying depth. The widening gap