THE MARINE FAUNA OF THE BLACKWATER ESTUARY 45
Severe winters, poor spatfalls and out-dated methods of
management have all contributed to a steady decline of the oyster
stocks in the Blackwater and Colne.
A good account of the biology of oysters is given by Yonge
(1960).
Ostrea virginica Gmelin American Oyster
Stocks were imported to Essex in the 1880's. Shells are rarely
picked up on the shore but the species is now extinct in Essex
waters.
Ostrea angulata (Lamarck) Portuguese Oyster
Mostly imported as juvenile and laid for growing on cultivated
beds in creeks at Brightlingsea, Mersea and Maldon. The species
does not normally breed but some naturally settled specimens
have been obtained from clean bottoms and from the surface of
the Barrier Wall at Bradwell (1965).
Pectinidae
Chlamys varia (Linne)
Shells very common on the bottom and shores of the Black-
water. Only one living specimen taken; in a trawl sample from
the south shore (1.15.1961).
Chlamys opercularis (Linne)
One shell only; taken in a dredge sample off St. Peter's Flats
(19.7.1962).
Sub-class TELEODESMACEA
LUCINIDAE
Phacoides borealis (Linne)
Shells of juveniles collected on St. Peter's Flats (11.6.1961) and
some juvenile specimens collected alive from stations TL.3
(12.8.1960) and C.2 (5.2.1964). The absence of adults in this area
indicates that the planktonic larvae are being brought in from
offshore waters but do not establish a population.
MONTACUTIDAE
Mysella bidentata (Montagu)
Shells and some living specimens occasionally taken in grab
samples from areas of shell and sand. This species is commensal
with the ophiuroid, Acrocnida branchiata.
Cardiidae
Cardium echinatum. Linne Spiny Cockle
Shells occasionally found on the shores of the Blackwater.
Fresh, paired shells have been obtained by dredging in muddy-
sand off Bachelor's Spit (1962).
Cardium exiguum Gmelin
Common in sand and mud in shallow water and on the lower
shore. In the Blackwater; from Thirslet seawards. The greatest