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

Mytllus edults 4m% -
Nephtys hornbergii — ^ -
FOOD SUPPLY HARVESTABLE BY WADERS
•w
Nereis diversicolor H < •w ro
Cerastoderma edule i- i # S
N
C ^ 8
Macoma balthlca y I • 8
IIS
Scrobicularia plana * •
Mya arenana
-m
s mm Crangon crangon s \
mm Crangon crangon s \
mm s \
mm \
Carcinus maenas
+
21
'
22
' , *
23
*
energy content (kJ-g 1 AFDW)
Fig. 2. Average energ) density (kJ g ' AFDW ± SE) Ol ten niver-
tebrates; number of measurements are unhealed. According to a
iine-vvav analysis of variance. Ihe -peek's differ significantly:
Ra=0.05,p=0.027.n=423).
al leasi 20'. to the body weight (e.g. de Wilde &
Berghuis 1978. Zwarts 1991). so a difference between
the energy value of gametes and other flesh would
affect the energy density of the entire animal.
However, de Wilde & Berghuis (1978) found that
gamete production in Macoma would raise the energy
density of females and lower it for males, since the
energy density of eggs was 24.7 kJ g ' and of sperm
IS.9 kJ g '. 3 kJ above and below the average energy
value of Macoma llesh. respectively. At the population
level, it is thus unlikely that gametogenesis would
cause the energy value of an average Macoma to vary
50 : 50. On the other hand, the study of de Wilde &
Berghuis (I97SI shows how food value may vary
between individual prey and that predators may be
able to increase their rate of energy intake by selecting
female pre\ (see e.g. S/aniawska 1984 for Common
Shrimp Crangon crangon: Zwarts & Blomert 1990 for
Fiddler Crab ilea tangeri).
There were also significant differences in the
energy densities of ten tidal invertebrates considered
9
us
f
50
(Fig. 2). Although the worm species had. on average, a
higher energy density than ihe bivalves, the highest
energy density was found in the Common Mussel
Mytilus edulis. Chambers & Milne (1979) found thai,
in ihe Ythan estuary. E. Scotland, the average energy
density differed between Mytilus (22.2 kJ g ' AFDW).
Nereis (21.8 U g '». the Edible Cockle Cerastoderma
edule (20.6 kJ g-i) and Macoma (20.0 kJ g 1 ). The
species ranked in exactly the same order as in fiy 2,
bul the values were in all eases below those found in
our study area.
ln most other studies, estimates of energ) densit)
are similar to those we found. Using conversion factors
for fat. glycogen and protein. Dare & Edwards (1975)
arrived al an average energy density for Mytilus oi 23.3
kJ g '. very close to the value given in Fig. 2. The
energ) densit) measured by Bayne & Worrall (1980)
was slightly higher (24 kJ g '. assuming that ashcontent
was 10%), but Hepplesion (1971) found a
slightly lower value: 22.6 kJ g '. The average value we
found for Macoma is halfway between those given by
Chambers & Milne (1979) (20.0 kJ g ') and Beukema
& de Bruin (1977) (22.9 kJ g •). The published values
for other bivalves are also similar to ours. Thus.
Swennen (1976) found 21.7 kJ g ' for Cerastoderma,
Hughes (1970b) 21.4 kJ g 1 for Scrobicularia, and
Edwards & Huebner (1977) and Winther & Graj
(1985) found 20.8 and 21.7 kJ g*', respectively, for
Mya.
Although a correction of 0.3 kJ g' 1 was made for
the endothennic reaction of CaCO, (see Methods i. the
energ) density oi Carcinus was low. This might be due
to the low energy density of the organic component of
the skeleton (Zwarts & Blomert 1990). This explanation
is strengthened by Klein Breteler (1975)
who found an energy density of 23 kJ g ' in moulting
Can inns with little skeletal material. The presence of
the skeleton probahlv also depresses the energy
density of the amphipod Corophium voluiaior where,
according to Chambers & Milne (1979) and Boates &
Smith (1979). respectively, the energy density is only
19.9 or 20.2 Ug-'.
As fat has a higher energy density than proteins and
carbohydrates. Species differences in biochemical
ci imposition, as well as the amount of skeleton present,
would be expected to cause species differences in
energy density. The energy densitv of Coropfuum is