304
THE ESSEX NATURALIST
A comparison between percentage silt and clay content of the
substratum for the stations at which C. arenarium and C. volutator
occur shows a marked distinction between the two species. C.
arenarium is found in those areas where the silt and clay content
is between 14.6% and 26.1% . This results in a firmer substratum
of cleaner sand. C. volutator is found in areas where the silt and
clay content is higher than the above, 37.1% and 38.4% for the
two samples taken. The substratum as a result is less firm and
of a decidedly muddier nature. Only two stations were established
in the C. volutator zone, thus not giving evidence of a range of
tolerance in this species. It will be noted, however, that at
stations 1 and 2 on the transect across area G the percentage
silt and clay content falls well below 30%, into the C. arenarium
range, and is coincidental with the disappearance of C. volutator'.
It can be argued therefore that this provides evidence to support
the view that C. volutator occurs in areas in which the silt and
clay content is higher than that for C. arenarium.
The silt and clay content of the areas outside the Corophium
zone is generally higher than the values within this zone, being
between 39 % and 57 %, resulting in an extremely soft substratum.
When these figures are taken in conjunction with those from
station land 2 in area G, it may be argued that these two figures
quoted for C. volutator are representative of its habitat. The
substratum composition at station 3 on the long transect is well
below 39 %, but here another factor, namely the presence of thick
black mud an inch below the surface, is responsible for the absence
of C. volutator.
The percentages of fine sand at all these stations are inversely
proportional to the percentages of silt and clay because the per-
centages of coarse sand in the samples are for all practical
purposes negligible.
These results would suggest a fundamental distinction between
the habitat of these two species, and a firm basis on which to
explain their distribution. In his letter, Mr. G. I. Crawford
mentioned that he thought C. volutator had different burrowing
habits from C. arenarium in that it built less permanent burrows.
I have examined the burrows of both species and they seem to
be identical in form, but since C. volutator constructs its burrows
in a softer muddier substratum perhaps they are more likely
to collapse than those of C. arenarium, built in a firmer sub-
stratum.
Water content
For reasons stated earlier the water content of the
substratum is closely related to its composition. When, therefore,
the distribution of these two species is plotted with respect to
water content the resulting pattern should be similar to that
shown in figure 4.