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

4
Q) Si
•a
a. 16-
20-
A
predator: Calidris alba
prey: Exorolana linguitrons
r}r , Excirolana kincaidn
y
I A
0.0 0 5 1.0
SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES
predator: Haematopus oslralegus
prey: Scrobicularia plana
)0 0.5 1.0
relative prey risk
predator: Numenius arquata
prey: Mya arenaria
Fig. II. Predation risk as a function of burying depth in three different studies: A. Sanderling Calidris alba preying on beach crustaceans (Myers
el ul. 1180). B. Ovsicrcaicher Haematopus oslralegus feeding on a benthic bivalve (Wanink .V Zwans 1985) and C Curlew Numenius ariliiatii
feeding on benthic bivalve (/.warts A: Wanink 19X41. Prev risk is expressed in relation to the maximal predalion risk of a single size class
i set to I); •*• indicates a shallow prey taken bv a Curlew, but not occurring in ihe sampling programme, resulting in an infinitely large prey risk.
overlying water while the siphon is just at the surface
(Brafield & Newell 1961. Hughes 1969. de Wilde
1975. Earll 1975. Hummel 1985a). Macoma lake deposit
al a maximum of 4 lo 6 cm from their burrow
(Brafield & Newell 1961. Gilbert 1977) and Scrobicularia
up to 10 cm (Thamdrup 1935. Hughes 1969) or
even 20 cm (Linke 1939). Both species might be able
to double their burying depth by shifting from deposit
feeding to filter feeding. Since filter feeding is limited
to the immersion period and deposit feeding can occur
during immersion as well as during emersion, a vertical
movement within a tidal cycle might be expected.
as suggested by Yonge 11953). No evidence, however,
was found for this in our laboratory (unpublished data)
when we measured continuously the burying depth of
individual Macoma with the aid of thin nylon threads,
which connected the shells with a registration apparatus.
It is likely dial the remarkable difference between
the summer and winter burying depths oi Macoma and
Scrobicularia (Figs. 4 and 5) can be explained by the
absence of deposit feeding in winter (Linke 1939,
Hughes 1969. Hummel 1985a). Individuals with a
106
similar siphon weight live 2-3 times as deep in winter
as in summer (Figs. 7 and 8). The siphon weights in
Scrobicularia are about the same in both seasons (Fig.
6D). whereas for Macoma the siphon is 35% heavier
during winter than in summer (Fig. 6C). so that most
individuals of both species are able to live at greater
depths in the non-feeding season (Figs. 4 and 5). The
relatively low siphon weight of Macoma during the
summer can be attributed lo heavy siphon cropping by
fishes, crabs and shrimps (de Vlas 1985). It would be
of interest to know to what degree die occurrence of
suspension feeding during the summer in Macoma
(Hummel 1985a) is caused by the reduction of siphon
size.
The suspension feeders Mya and Cerastoderma remain
at nearly the same depth during the whole year:
Cerastoderma burrow to a 20'7< greater depth in winter
than in summer (Fig. 3) and in Mya the difference is
only 10 to 15* (Figs. 2 and 9).
Burying depth as a protection against predation
The relationship between burying depth and size can
be described with an S-curve (Figs. 2-5). Burying