ECOLOGY OF THE COCKLE CARDIUM GLAUCUM 225
described (Mars, 1951 and Grossii, 1961). Even around our
shores C. edule collected from different sites can often be recognised
by their local shell characteristics. Thus Southend cockles can be
separated from those of the estuarine River Crouch, and Wey-
mouth, South Wales and Anglesey cockles are also identifiable
by their shell shape. In sites where the environment is par-
ticularly 'stressed', e.g. sedimentation tanks of power stations, the
C. edule respond to the conditions by exhibiting globular 'odd'
shaped shells. Therefore C. edule is a very variable species as
regards shell shape and is able to produce many 'ecotypic varieties'
which should not be confused with specific differences. However,
on the shore, there is not a range from 'edule' type to 'glaucum'
type, so the occurrence of C. glaucum in pools seems to be in-
dependent of the presence of C. edule on the adjacent shore. This
does not mean to say that C. glaucum cannot occur on the shore
for very occasionally isolated individuals are found between tide
marks. Could the larvae from these animals spread the species
into newly formed pools ? The apparent rarity of these individuals
would probably discount such a hypothesis.
It is interesting at this stage to examine the European distribu-
tion of the two cockles. C. edule extends southwards along the
Atlantic coasts of France and Spain, and northwards to Holland
and Denmark. It is also found in the approaches to the Baltic
where oceanic effects are still evident, e.g. the Kattegat shores
of Sweden and Denmark (G. Hopner-Peterson, personal com-
munication). C. glaucum on the other hand is found in the inner
Baltic (Petersen, 1958) and the Mediterranean (Mars, 1951). Both
seas have a very small tidal rise and fall, thus the cockles remain
permanently submerged in basically still water. The lagoon
'glaucum' sites found throughout southern England are miniature
versions of these non-tidal seas. The habitat of this 'not-so-
common' cockle is not so strange or unusual as first appears.
Petersen (1958) in discussing the occurrence of C. glaucum in
the Baltic Sea, considers this cockle to be a brackish water form.
However, the cockle occurs in the Mediterranean where the
salinity is high. Certainly around our coasts C. glaucum is found
in pools where there is brackish water. The lowered salinity
character of the pools may not be essential, continual submersion
in stagnant saline water being the primary requirement of the
cockle.
In the very recent geological history of England, saline or
brackish water lagoons were probably much more common than
they are at the present day. The region of alluvium shown in the
geological map of S.E. England (see Fig. 1) represents original
areas of marshland where lagoons may well have been abundant.
The area of modern marsh and saltings is much lower and is
mainly retained behind sea defences. Very few modern marshes
contain water of sufficient salinity to harbour C. glaucum. With
the advent of sea walls in about the thirteenth century (Grieve,
1959) the number of potential C. glaucum sites would gradually