THE MARINE FAUNA OF THE BLACKWATER ESTUARY 47
Gastrana fragilis (Linne)
Shells occasionally recorded from mud at the mouth of the
Blackwater (1965).
Macoma balthica (Linne)
Very common in mud and sand on the shore and below low
water mark. Distributed from Maldon to the offshore sand banks,
being tolerant of a wide salinity range.
SCROBICULARIIDAE
Scrobicularia plana (da Costa)
Very common in soft muds along the shore of the Blackwater
and Colne estuaries. This is a truly estuarine species and lives at
some depth in the soft anaerobic mud. In a population that was
studied in Bradwell Creek in 1960-65, a maximum number of
1,304 per nr was recorded (15.6.1961); this including a large
number of juveniles. The populations suffered greatly from the
low temperatures of the 1962-63 winter as in other areas (Crisp
et al., 1964). The population was still at a comparatively low
level in 1965.
Abra alba (W. Wood)
Common on bottoms of mud and sandy-mud, at low water
mark and below, mostly from St. Lawrence Bay and Thirslet,
seawards. In samples of soft mud taken from the shore side of
the Barrier Wall at Bradwell, a maximum number of 65
animals/nr was obtained (31.5.1962).
Abra nitida (Muller)
Shells only, collected from mud at the entrance to Bradwell
Creek (8.6.1961) and at other localities with A. alba.
SOLENIDAE
Cultellus pellucidus (Pennant)
Only one record from the Blackwater; a living specimen taken
in a grab sample from station 2a.4 (15.9.1960).
Ensis ensis (Linne)
Shells only; common on shores at the mouth of the Black-
water, and on the Buxey Sand (1962).
Ensis arcuatus (Jeffreys)
Shells only; occasionally found with E. ensis.
Ensis siliqua (Linne)
Shells only; common on the shores of Mersea Island and on
St. Peter's Flats. Many fresh specimens collected on the Buxey
Sand in the summer of 1962.
Solen marginatus Montagu
Old shells are sometimes found on the shores at the mouth of
the Blackwater.