ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 303
These results show that the optimum concentrations for C.
arenarium is about 16‰ and that the animal has a wide range
of tolerance, living for 78.8 hours in concentrations of 2‰. A
sensitivity to slight changes in salinity between 24‰ and 33‰
is suggested by the steep gradient of the graph between these
two points. I was unable to get C. arenarium to live for more
than 0.6 hours when put straight into fresh water (as compared
with Hart's 16 days for C. volutator).
The experiment of Hart with C. volutator and the results of
the above experiment on C. arenarium show that these two
species have a wide range of tolerance with regard to salt con-
centration. It is only in localities where there is an appreciable
range in salinity—e.g. the Tamar Estuary, Spooner and Moore
(1940)—that the distribution is influenced by this factor. Table
1 shows that in Benfleet Creek the salinity variations are almost
negligible, less than 1‰. Despite this, the distribution of C.
volutator and C. arenarium is very restricted. This would suggest
that factors other than salinity are dominant in determining the
distribution pattern of this area.
Soil Composition
It was thought possible, from observations in the field on the
varying firmness of the substratum, that soil composition was an
important factor in determining the distribution of the two species
of Corophium. A number of other writers, Beanland (1939),
Crawford (1937), Hart (1930). Nicol (1953), Percival (1929) and
Watkins (1941) have also noted the effects of substratum com-
position on the distribution of C. volutator, but none have
attempted to define this in specific terms. In the words of Hart
(1930), 'a more accurate description of the substratum is needed'.
A number of soil samples, taken in various parts of the Creek,
have therefore been analysed in the laboratory and the results
presented in Table III. These are graphically represented in
figure 4 and show striking results.
Figure 4. The distribution of C. volutator and C. arenarium in relation
to the silt and clay content of the substratum.
A, C. arenarium; B, C. volutator; C, neither species.