ECOLOGICAL. STUDIES IN SOUTH BENFLEET CREEK 301
appear to have been washed away from an old cinder tip. There
are three surface-dwelling species present in this area: —Mytilus
edulis, Cardium edule, and Littorina littorea.
Mytilus edulis occurs mainly at the eastern end of area D, their
byssi being attached to other mussel shells, to broken shell deposit
and to the pieces of anthracite coal. Cardium edule and Littorina
littorea are present over the whole area in appreciable numbers.
The shells of all these species are quite heavily encrusted with
the barnacles Balanus balanoides and Elminius modestus as also
are the remaining concrete piles of an old landing stage near-by.
There is also a less extensive shelly deposit a little further up
the Creek. It is of the same character as that described above,
but there only 1 specimen of L. littorea was found, and no other
living species. The explanation for the presence of these species
on one bed and not on the other lies in the fact that the shelly
deposit carrying the mussel bed is uncovered by the tide for about
7 hours, whereas the area carrying no surface forms is uncovered
for not more than 3 hours.
DISCUSSION
The most interesting observations to be made in an ecological
study of Benfleet Creek are on the two species of Corophium
found there. No records have been found of any other area in
the British Isles where these two species occur in close proximity.
C arenarium is also a relatively rare species, or sub-species, and
the only work traced on its ecology is the preliminary work of
Crawford (1937) and a reference to a paper by M. Chevais of
Paris in 'Travaux de la Station biologique de Roscoff'. Therefore
this section is devoted to a discussion of the factors determining
the distribution of these two species in Benfleet Creek. The four
factors studied in most detail with regard to this problem were
the effects of salinity, water content, the composition of the sub-
stratum and slope.
Salinity
A number of writers on the ecology of C. volutator suggest that
salinity is an important factor influencing the location of this
species. Nicol (1953) writing on the Ecology of a Salt Marsh
states that 'the distribution of C. volutator is determined firstly
by salinity' and found adult specimens in pools of 5 %0 salinity.
Crawford (1937) found C. volutator in the Tamar Estuary living
in almost fresh water and in other localities living in water of
nearly full salinity. In a salinity tolerance experiment Hart
(1930) showed that 'this species can endure great ranges of
salinity' and by gradual dilution of the water he kept them alive
in very low salinities all winter. He also showed that they could
live up to 16 days when put straight into fresh water.
No information is available on the survival of C. arenarium in
varying concentrations of sea water. In an experiment to deter-
mine this, 8 petri dishes were filled with 50 mls. of water of