298 THE ESSEX NATURALIST
Corophium arenarium
This species was first collected in 1936 just off the east end
of Leigh Island, about a mile east of the mouth of Benfleet Creek.
As can be seen from Table IV and figure 2. C. arenarium
occurs in considerable numbers in restricted localities in the Creek.
The figure given for Area B in Table IV represents a population of
about 1,700 per square metre1. This species is only to be found
in areas A, B, C, E and F. and their positions and extent coincide
almost exactly with the limits of the 'sand banks' described
earlier. In areas A. E and F, C. arenarium occurs mainly on the
broad flat tops of the sand banks and tends to disappear on the
slopes down to the stream. Area B however is flat on the up-
stream half but the downstream half constitutes quite a steep
bank (see figure la). Here C. arenarium seems to have a definite
distribution pattern with the upper and lower limits varying along
the bank. From the figures for Transect B in Table IV it can be
seen that this zone starts suddenly at station 7. Below station 7
the mud is softer than on the rest of the transect, and covered
with a brown diatom scum. Thick black sulphide mud appears
about 2 cms. below the surface and because it is near low water
mark the water content is high. The C. arenarium zone extends
up to station 26 (which, as far as can be estimated from the
Southend tidal heights, is about M.S.L.) where it suddenly stops
again. This station is at the top of the bank and the nature of
the substratum changes radically on the other side where no
C. arenarium is found.
Although no transect was put down, the position seems to be
essentially the same in area A—near low tide level where the mud
is softer and with a high water content no G. arenarium are to be
found. This also holds for areas E and F.
In all these areas C. arenarium is found in small U-shaped
burrows, extending down to about 5 cms. below the surface, which
because of the nature of the substratum seem to be more per-
manent than the burrows of C. volutator. Areas A, B, E and F
are all below M.S.L., but C. arenarium does not seem to be limited
to the lower tidal levels in its distribution because area C extends
right up to H.W.M.T. level and the corophids are still found in
that part of the bank which is not covered by the Neap Tides
(the surface was sunbaked and cracked when visited).
Corophium volutator
This species is the most common member of the genus
Corophium round the coasts of Britain, but in Benfleet Creek its
distribution is very limited. Table IV shows that C. volutator
is found only in the upper part of the transect in area G. This
species also seems to occur in very large numbers, the figure for
1Square metre sampling seems to have become the accepted custom in
ecological literature and so some attempt has been made to give
estimates for Benfleet Creek as a means of comparison with other
areas.