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

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0 4 8 12 16 20 24 0 4 8 12 16 20 24
burrow depth (cm)
Fig. 5. Nereis diversicolor Idem long. Relative Nxl\ condilion i#.
mean ± SE) as a function ol burrow depth during the lour seasons.
The histograms show the depth distributions. The one-way analyses
oi variance, given separate!) for each season, show thai burrow
depth explains a high]) significant pan of the variation in body condition.
and relative body condilion as was found in bivalves,
was also found in this worm (Fig, 5).
The difference between the prey values of deepami
shallow-living animals would be even greater if
deeper-burying animals of a given length also had a
greater energy content per unit of mass. i.e. a greater
'energy density'. This would lie expected if the lean,
shallow-living individuals contained relatively less fat
than the heavy, deep-living ones. No relationship was
found, however, between energy density and burying
depth in Scrobicularia. either in winter or in summer
ACCESSIBLE PREY ARE OFTEN IN POOR CONDITION
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u. I r i i I I I I
4 6 8 10 12
burying depth (cm)
SUMMER
l-'ig. 6. .s. robiculariaplana 35 mm long. Energy content (kJ±Sl
.is a function Ol burying deplh during three summers 112 sampling
dales: upper panel) and three winters (14 sampling dales; lower
panel). There are RO significant relationships between depth jnd en­
ergy content; n = Uh
(Fig. 6). So in terms of energy, relative body mass condition
remains a good predictor of prey value.
Relative body condition and depth for infested and
uninfected animals
The Oystercatcher Haematopus oslralegus. the main
predator of large Macoma. is able to distinguish prey
infested with the parasitic trematode I'arvairema affirm
i Hulscher 19S2). Therefore the relationship between
relative body condition and burying depth was
also examined solely for non-parasitized individuals.
For half a year, each Macoma was checked for infestation.
With infested animals excluded, relative body
condition was even less m ihe shallow-living animals
(Fig. 7A). The reason is that infested individuals
weighed on average W r more, probably due to additional
weight of the parasites, and a larger proportion