ECOLOGICAL STUDIES IN SOUTH BENFLEET CREEK 305
Figure 5. The distribution of C. volutator and C. arenarium in
relation to the water content of the substratum.
A, C. arenarium; B, C. volutator; C, neither species.
C. arenarium areas are even more closely defined by water
content than by substratum composition. This species occurs
only where the water content is between 23.7 % and 28.3 %. It
should be noted that C. arenarium does not occur at station 3 on
the transect in area B where the water content rises to 31*6%. It
may be that this water content is too high for the species to
burrow satisfactorily. On the other hand, C. arenarium was
found in area C, part of which was not covered by the Neap
Tides, having a sun-dried surface, and where the water content
was probably temporarily below 23 %. In this type of substratum
(muddy sand), however, the water content below the immediate
surface tends to remain more constant than if the substratum
were clean sand. It is probable, therefore, that the distribution
shown in figure 5 is typical with regard to water content. C.
volutator occurs where the water content is higher than in the
C. arenarium areas, typically being about 39.2%. I express it like
this because the figure for station 4 in area G is 28.1%. The
reason for this is that at the time of sampling, the substratum
around station 4 was above M.H.W.N. It had not been covered
for a number of days, with the result that the surface layers had
dried out and were suncracked. Although 39% may be nearer
the optimum for C. volutator the position at station 4 suggests
either that (1) C. volutator has a wide range of tolerance of water
conditions—or (2) soil composition is more important than water
content in determining the distribution.
This position at station 4 is also interesting in the light of
the statement by Hart (1930) to the effect that C. volutator is
never found in any numbers where the rate of evaporation of the
surface layers is such that they dry out almost completely. The
surface at station 4 was baked hard by the sun but contained a
population of nearly 2,000 per square metre.