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

Tahle 1 Weight loss ('•- ± SE) m Macoma (12to21 mm, n = 960
specimens! sUsted III sea waler at 4 "C for I). I. 2 or .' daw after
inlkviion. Average wejghl ol each mm-class on da) u is set to um
Subsequent changes are expressed as relative deviation on day I to
3. The differences are weakly significant according to a one was
analysis of variance (R J *»0.23 p*»0 03, n = 26). The percentage
ash (± SE) differs significantly es
Macoma
AFDW. % nsli. ",
Scrobicularia
\ll>w. c y ash.'
100 21 2 ±0.9 HK) 30.0 ± 1.2
101.8 ±3.3 I2.8±l.() 94.1 ±2.6 17.6 ±0.7
29
of stomach emptying, since about a quarter of the ash
disappeared. Therefore no correction was made for the
duration ol the storage.
i l) Immersion in boiling water mav have dissolved
flesh. This effect also appeared to be negligibly small,
even when the immersion lime was 13 s (Table 2),
Immersion halved the percentage ash. partly because
salt dissolved but mainly because of the loss of sand
particles. The immersion time was kepi as short as
possible and ihe flesh removed as soon as the shells
Stalled to gape. No correction was made for this,
possibly very small, loss of AFDW.
(3) The AFDW of bivalves < 11 mm long could not
be compared directly with that of the larger bivalves
When the shells o( small bivalves are ignited, organic
material in the hard parts (viz. eonchine) disappears
along with water tightly bound in the shell. To
determine how huge an error this introduced, samples
of shells were cleaned by placing them in a solution of
coOagenase at 37 °C for 24 h. so that all the flesh could
be easily removed. The AFDW of the shells, bound
water inclusive, was low (Fig. 2): 2.6% of the DW oi
the shell in Macoma. 1.99 in Scrobicularia (no spat).
2,9% in Mya and 2.2'.. in ("< rastodt rma. These values
are in agreement with data of llibberi (1976),
Beukema (19X0. 1982a), K-leef et al. (1982), and
Goulletquer & Wolowicz (1989). Nonetheless, the
contribution of the shell itself to the total AFDW ol the
shell and flesh together varied between 15% and 40%.
li was greatest in Cerastoderma in winter, when the
DW of the shell was 14 to 15 times that of the flesh, li
was least in Mamma in summer when the DW ol tin.*
shell was still 5 limes that of ihe llesh. In view of this
large contribution made by the AFDW of ihe shell.
Rg. 2 was used lo estimate how much should be subtracted
from the total AFDW on those occasions when
soli and hard parts of bivalves had not been separated.
this being usually the bivalves < 11 mm long.
(4) The DW was determined for all si/e classes, bin
sometimes the ash content was not measured. The
percentage ash appeared independent of size but
varied seasonal!) digs. 3 and 4), The average percentage
ash contenl per month was used to convert
DW into AFDW on those occasions when samples had
not been ignited. A similar seasonal variation in ash
content was found in Macoma by Beukema & de
Bruin (1977). but it occurred at a lower level, probably