ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 307
equal, C. arenarium also is very sensitive to disturbances by tidal
currents. This is borne out by observations of the area round the
sampling stations which had been trampled. The corophid
population had disappeared from these areas completely by the
following day.
CONCLUSIONS
1. The composition of the substratum, i.e. the relative percent-
ages of fine sand, silt and clay, appears to be the chief factor
responsible for the distribution of C. arenarium and C.
volutator, the latter occurring in areas of a slightly muddier
nature than the former.
2. The percentage water content is also an important factor in-
fluencing distribution. From evidence put forward in the
discussion it seems to be a secondary factor to substrate com-
position. Where an area is fairly homogeneous with regard
to substrate composition, e.g. transect in area B, water content
becomes an important factor limiting distribution.
3. Other factors such as disturbance by tidal currents and the
presence of black sulphide mud are of local importance.
4. Salinity is not a factor governing distribution in this area.
5. In Benfleet Creek the habitat of these two species may be
described as follows: —
(i) Corophium arenarium inhabits areas where
(a) the silt and clay content of the substratum is between
14% and 27% (clay content 12% and 17% and silt-
content between 2% and 10%) and the fine sand
content is between 76% and 88% ;
(b) the water content is between 24% and 28.5%, not
usually above 30% and probably not lower than 20%.
(c) This species is found on sandy, mud banks, usually
confined to the flat tops. Where it is found on the
slopes of the banks it avoids the steeper gradients of
the lower slopes.
(d) Black sulphide mud must be below 5 cms. depth,
which is the average length of the burrows.
(ii) Corophium volutator inhabits areas where
(a) the silt and clay content is about 37 % to 38 % (clay
content between 20 % and 25 % and silt content be-
tween 13% and 16%) and the fine sand content is
about 61 % to 65 % ;
(b) the water content of the mud is about 38% to 39%,
probably not much over 40 % but as low as 28 % for
short periods of time.
(c) Like C. arenarium it is usually found on the flat tops
of the banks and is sensitive to tidal currents.
(d) It is never found burrowing in black sulphide mud nor
in association with Paludestrina stagnalis.