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Ecology of the cockle Cardium glaucum Brugiere*
By C. R. Boyden
(Queen Mary College, London)
Received 25 March 1969
The occurrence of the common edible cockle Cardium edule
L. in England is well known. Dense, commercially exploited
beds are found in the Thames estuary and in the Wash. What
is less recognised is the presence of a second related cockle in
the south-east of England.
This rarer cockle, Cardium glaucum. Brugiere is generally
found in land-locked marine pools. The qualities of a
'glaucum' site are simple; a stagnant saline lagoon isolated from
tidal influences. These pools occur around the southern and
eastern coasts of England from Wells in Norfolk to Weymouth in
Dorset. Within this peculiar habitat C. glaucum is very success-
ful and often grows to a quite large size. C. edule, on the other
hand, occurs all around our shores, typically in truly marine, tidal
conditions. Occasionally, the species extends sub-littorally and
into estuaries, but even in this latter case, the overall salinity of
the water must be high.
To the inexperienced eye a cursory glance would not separate
the two species of cockle. However, on more careful examination,
differences can be found. C. glaucum is elongated posteriorily,
has sharp triangular ribs which are visible internally to the umbo,
and bears a short ligament. C. edule, is more or less oval, has
flattened ribs and the ligament is long. Viewed internally, the
ribs fade out a short distance from the shell periphery (for further
reference see Peterson (1958) and Tebble (1966)). If a live speci-
men is available for internal examination the problem of separa-
tion of the two species is made more easy. The digestive gland
or liver of C. glaucum appears as a large, black, globular mass of
tissue beneath the anterior region of the hinge. In C. edule the
gland is much smaller and is coloured a greenish shade of brown;
during the winter, before spawning, the tissue appears dark
green, whereas in summer the tissue is most usually pale brown.
The organ is not as discrete or globular in appearance as it is in
C. glaucum. This internal character has proved most useful in
separating the two kinds of cockles. With additional knowledge
of the shell shape and habitat, positive identification is possible.
*Nomenclature follows Mars (1951), and Russell (Ph.D. thesis. London,
1969).