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

sum of iis width and height. This shape index was lowest
in Mytilus 11.51: SE = 0.007, n = 11) and highest io
Scrobicularia plana (1.62; SE = 0.01. n = 10) (see Fig.
7C). There was a very close relationship between shell
length and shell width (Fig. 7A) and between shell
length and shell height (Fig. 7B), so the circumference
could be predieied accurately for each length class
(Fig. 7C).
The circumference of 16 mm long Macoma is 30.6
mm (Fig. 7C). As shown in Fig. 7C. all Cerastoderma
longer than 12 mm could not be ingested, whereas ihe
predicted upper limits for the more slender Mya arenaria.
Scrobicularia plana and Mytilus would be 17.
19 and 21 mm. respectively. Dala to test the predicted
upper size limits are scarce, but agree with these values.
Knot rarely took Cerastoderma longer than 10
mm (Goss-Custard etal. 1977b. Nehls 1992. Piersma
c/ K/. 1993b). but did swallow prey 18 to 19 mm long
among the more oblong Mytilus (Prater 1972. Piersma
pers. comm.). Though lengih itself also might set a
limit to the size of prey Knot could swallow, there are
no dala available for this, so the circumference of 30.6
10 15 20
shell length (mm)
WHY KNOT TAKE MEDIUM-SIZED MACOMA
10 15 20
shell length (mm)
mm measured in Macoma is taken as the limit for all
species. Even the largest Peringia lengih 6 mm. remained
far below this limit.
Accessible prey
All Peringia and Mussels Mytilus were considered accessible,
as ihev live al the surface. The other four bivalve
species occurred at various depths beneath the
surface. Based on work done in the same locality
(Zwarts & Wanink 1989). the proportion of the prey
living in the upper two or three em of the substrate is
shown in Fig. 8.
Which prey would be within reach of a bill 35 nun
long? Assuming that a Knot needs c. 5 mm purchase
on a shell of 10 mm or longer, the distance between ihe
mud surface and the upper edge of the bivalve (burying
depth) would have to be less than 30 mm for a Knot to
take a prey. However, not all of this upper 30 mm
would be fully exploited, as only some probes were
made to the base of the bill. The upper 10 to 20 mm
musl have been searched more thoroughly than ihe
deeper layer. Most probes of the closely related
10 15 20
shell lengih (mm)
Fiji. 7. A. Shell width. B. shell height and C. shell circumference as a funciion of shell length in li\e bivalve species. Length, height and width
were measured in 608 Mm oma (r = 0.9K4. r = 0 993 for width versus length and height versus length, respectively). 400 Cerastoderma
(r = 0.995 and 0.9981. 292 Mya