ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 299
station 4 representing about 2,000 per square metre. Figure lb
shows the general configuration of area G and the extent of the
C. volutator zone. Apart from the slope up from the stream the
surface of the bank is roughly level. There is much surface water,
the substratum being quite soft and the thick, black, foul smelling
mud appearing only 5 cms. below the surface. There is a great
change in the substratum on the slopes down to the stream, it is
much firmer, sandier, drier and there is no surface water. It is in
this region that C. volutator rapidly dies out, only two specimens
being found at station 2.
Nereis diversicolor
Except for Scrobicularia plana, the distribution of N. diversi-
color is more general than that of any other species. It is found
in small numbers in most of the Corophium areas as well as
outside them, and from low to high tide levels. The figure for
station 2 on the long transect represents about 250 specimens per
square metre. Here the mud is very thick and wet, with the
black mud very near the surface. The relative absence of N.
diversicolor in the C. arenarium zone of areas A, B and E is of
interest. Whether the presence of Corophium or some other
factor is responsible for this is not quite certain but where the
substratum is softer than in areas A, B or E, N. diversicolor is
more common, becoming very abundant where the mud is very
soft outside these regions. This species occurs down to a depth
of about 15 cms. in the substratum.
Scrobicularia plana
This lamellibranch mollusc is common locally in muddy
estuaries, and is distinguished from Macoma balthica by its larger
size (up to 50 mm. on the long axis of the shell), the greyish
yellow colour of the shell, a large internal ligament and unfrilled
siphons, one of which is very long Table IV and other observa-
tions on the Creek suggest that it has a distribution similar to
that of N. diversicolor. In transect B it is again common near low
tide level, absent in the C. arenarium zone and present above
it as the substratum begins to change. It is more common in
the C. volutator zone of area G and even more common outside
these regions, in the very soft muds. Thus it seems to have a
similar distribution to that of Macoma balthica described by
Fraser (1932) and Bees (1940). S. plana was found in greatest
numbers below about 15 cms. during the period of investigation,
but there is reason to believe that it moves higher up the sub-
stratum in winter.
It is interesting to note that there is a vertical zonation of
these three genera in the Creek. C. arenarium and C. volutator
occur in the top 5 cms., N. diversicolor generally down to about
15 cms., and S. plana predominantly below 15 cms. depth.