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

face and so expose themselves to a greater predation
risk.
Ihese observations led Zwarts (1986) lo hypothesise
thai the burying depth of deposit-feeding bivalves
represents a compromise between foraging demands
and predator avoidance. The hypothesis could not be
tested because the feeding radius had not been simultaneously
measured with burying depth and siphon
weight. The aim of this paper is to present some measurements
on the elongation of ihe siphon that are essential
to answer the question about the trade-off between
food and predation. We show that feeding
radius, burying depth, and thus also siphon length, increase
with shell size. We relate individual differences
in feeding radius and burying depth in Scrobicularia
and Macoma of different si/e classes to variations in
siphon weight. We cut off siphons to determine the effect
of siphon cropping on feeding radius and burying
depth. The paper also describes techniques to measure
the length of the inhalant siphon and gives equations lo
predict siphon length from siphon weight. These data
will be used to estimate the feeding radius of a large
sample of Scrobicularia in which burying depth and
siphon weight has been measured.
Materials and methods
Burying depth, feeding radius and siphon weight:
field data
The field data were collected on a tidal mudflal in ihe
eastern pan of the Dutch Wadden Sea along the mainland
coast of the province Friesland (53° 25' N, 6° 04'
E). We determined ihe maximal feeding radius of 75
Scrobicularia in June 1992 using two methods. First,
we observed for 20 min the siphon activity of an individual
bivalve, noted its maximum extension over the
mud. using surface cues such as shell fragments as
markers, and later measured the distance to the siphon
hole. The second method was simply to measure the
distance between the siphon hole and the maximum
extent of the traces in ihe mud: like other deposit-feeding
bivalves. Scrobicularia leave starlikc tracks on the
surface around the burrow caused by ihe siphonal activity
(Hughes 1969. Hulscher 1973. Wikander 1980.
Levinton 1971. 1991). Since both methods gave identical
resulis. we subsequently only used the second
FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE
114
method, selecting stars that were so clear-cut ihal the
maximal feeding radius could be measured precisely.
After that, the burying depth of the selected bivalve
was determined using a circular corer pushed into the
mud. Alter retraction, the core was laid horizontally
and the burying depth of the focal bivalve measured to
the nearest 0.5 cm. Burying depth is defined as ihe distance
between surface of the mud and the upper edge
of the shell; for a detailed description of the methods,
see Zwarts (1986) and Zwarts & Wanink (1989). The
animals were stored in sea water and taken to the laboratory
where shell length was measured and the inhalant
siphon separated from the bcxly, as described
elsewhere (de Vlas 1985. Zwarts & Wanink 1989).
The siphons were dried for 3 days at 70 °C and burned
at 550 "C for 2 h to determine the ash-free dry weight
(AFDW).
To study the relation between burying depth anil
siphon weight alter cropping, we collected 60 Macoma
16 to 17 mm long in May 1986. The animals
were placed in dishes with shallow water, after which
die extended part of the siphon was cut off in 44 individuals.
Using adhesive, a thin nylon thread of known
length was attached to the shell and the animals were
reburied at a depth of 3 cm in the same mudflat from
which they had been collected 8 hours before. The
depth to which the animals subsequently buried themselves
was determined each low water period over five
days, by subtraction, from the length of the thread remaining
above the surface. After that Ihe animals were
collected to determine the ash-free dry weight of the
bcxly weight and the remaining siphon, as well as of
the weight of the part of the siphon that had been removed.
Experiments in the laboratory
Further experiments were conducted on the relationship
between siphon weight, feeding radius and burying
depth in the laboratory. Twenty-six Scrobicularia.
13 lo 37 mm long, and 77 Macoma. II to 20 mm long,
were collected in October 1992 and taken to the laboratory,
where shell length was measured and thin nylon
threads of known length were attached to the shell with
super glue. The bivalves were then allowed to bury
themselves in 20 cm of mud taken from the area where
ihe bivalves had been collected. The mud temperature
was kept constant at 15 °C. The waler table above the