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

June 1981 June 1982
is * N.MUD • Q
10 — r*
e \MUD
o
JC 12 -
a. 1>
a
"O
§14 - -
"3
JO
16
\sAND
"
18
" _ • ,t
BURROWING AND FEEDING IN NEREIS
SAND\
,!,,,, - It,
-40 0 +40 +80 -40 0 +40
deviation from mean body weight (%)
+80
Fig. 5. .\, mis diversicolor. Burrow depth of worms of 10(7 lo 141
cm living in mud (•) and sand (O) as function of body condilion in
2 sampling programmes. Mean burrow depth and mean body condition
in mud and sand are indicated by black and white arrows, respectively.
A correction has heen made, .is in Fig 4. to adjust for the
si/c eltect on deplh See Table -I lor slatistical analysis.
Figure 4 shows the relationship between clay content
and burrow depth. Burrow depth appeared to remain
constant if the clay content increased from 6 to
2(l'». However, in sediments with a clay content of
< 5% (defined above as sand) burrow depth increased
when clay content decreased from 5 to 2'7c. Nereis are
not found in sediments where the clay content is well
under 19r (Zwarts 1988b). The relationship between
claj content and burrow depth in the sandy sediments
\\ as more pronounced in September than in June (Fig.
4). The effect of clay content on burrow depth cannoi
be attributed to other measured variables, such as elevation
(or emersion time 1 .
Burrow depth and body condition
The large variation in burrow depth for individuals ul
the same size i Kg, 2) can he explained partly by a difference
in body condition (Fig. 5). The body weight of
deep-living Nereis was on average 2 to 3 limes greater
than thai of individuals of the same si/e class with
shallow burrows. Apparently, worms in good condition
are able to maintain deeper burrows.
It is known that body condition varies throughout
Ihe year and also differs for WOTmS lnund alone the
tidal gradient (Chambers & Milne 1975b, Esselink et
al. 1989). Figure 5 shows that the bodv condition of
worms living in sand was If) to 35% higher than that of
individuals sampled in mud. The effect of clay content
on burrow depth, as described in the previous section,
disappears completely in 3 out of 4 sampling programmes
if body condilion is taken into account (Table
4). The deeper burrows found in sandy sediments
might thus be explained by the average difference in
body weight between individuals inhabiting sandy and
muddy sediments.
There is also a positive relationship between body
condition and burrow depth when worms within a
same season are compared, but this effect appears to be
independeni of ihe seasonal variation in depth (Fig. 6,
Table 5).
Table 4. Kcsulls of four 2-way .111.ily sc- of variance to test the effect of body coiidiliun and clay content on burrow depth in 4 sampling programmes
1 dala lor June and July shown in Rg. 5). Since body condilion is related 10 clay contenl. the total variance explained by the 2 main effect*
can be larger than ihc sum of the variances unique to each of the separale laciors
Nov. 1980 April 1981 June I98J Juls 1982
Sonne of 1: ---5S n = 67 n = 9/ n z
s-arialion R>.% P R 2 .% I' /' R>.% /'
Main effects 36.0 0.001 27.2 0.001 54.2 0.001 11.9 0.001
condition 8.0 0.002 25.0 0.001 Mi 0.001 10.4 0.000
cl.iv contenl 7.5 0.020 4.4 0.055 111! 0.X76 0.7 0.118
Two-way interaction n: 0.734 3.7 0.206 0.4 0.718 7 : 0.190
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