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

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FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE
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I mg
1 2 3 4 5 6 7
siphon length (cm)
Fig. 7. A. Weighl (mg AFDWi of ihe extended siphon. B. weight
ot the extended siphon ling cm ') and C. weighl of the internal
siphon ling AFDW) in Macoma 15 mm long as a function of the
length of the extended siphon. The three regression lines in each
panel refer to siphons with different total weights: number of cases
given in the upper panel. The weighl of the extended siphon depends
on siphon length (R- = 0.281) and on total siphon weight
(R 2 = 0.295); the interaction term was not significant
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possible if more siphon tissue is protruded, the weight
ol the siphon remaining within the shell must be less
when the siphon is stretched more, and this is exact I v
what was found (Fig. 7C). When extending their
siphon, Macoma kept part of it 120 to 70'.;) within the
shell. The size of this 'internal' part of the siphon decreased
if the siphon was extended further and was
small in Macoma of a low siphon weight.
Discussion
Relation between feeding radius and shell size
As would be expected, there is a linear, rather than exponential,
increase in average feeding radius with shell
size (Figs. 1 and 2). If food is limiting and assuming
lhai ihe proportion of the requirements taken Irom the
surface, and the efficiency wilh which they are collected,
is the same for the different si/e classes, the individual
feeding area should be equivalent to the total
food demands of a deposit-feeding bivalve. This
means that the feeding radius should be a function of
food intake" 5 . Other things being equal, energy requirements
are a function of body weight" 7 \ found
both in comparisons between species and within individual
Scrobicularia of different flesh weights
(Hughes 1970b). The ash-free dry weight of ihe flesh
in Scrobicularia and Macoma is an allometric function
of shell length, with the exponent of 2.9 (Zwarts 1991).
The prediction therefore would be that feeding radius
would be function of shell length with an exponent of
0.5x0.75x2.9, or 1.09. and so close to observed linearity.
Other studies found no. or hardly any. relationship
between feeding radius and shell size (Wikander 1980,
Levinton 1991). This may indicate that larger individuals
lake relatively more food from the overlying water,
and/or are able to graze the surface more efficiently.
On the other hand, the individual variation in
feeding radius of animals of ihe same size is very large
and this is associated with the huge variation in siphon
weight (see below: Figs. 8B and 9C). Hence the relationship
between average feeding radius and shell size
can easily be overlooked if the sample size is small.
Relation between siphon size and shell length
The exponent of the allometric relationship between