THE MARINE FAUNA OF THE BLACKWATER ESTUARY 41
There was a slight decline in numbers from 1961 to 1962 but then
a steady increase up to 1965. It seems that no significant mortality
resulted from the severe winter of 1962-63. Breeding takes place
in winter and spring and the juveniles are noticeable in the benthos
from June onwards.
Nassariidae
Nassarius reticulatus (Linne) Netted Dog-Whelk
Common at about low water of spring tides in the larger creeks
and occasionally found in the main body of the Blackwater. The
specimens all have fewer ridges on the body whorl than do those
from the south and west coasts, this being the variety mammilata
(Risso) which also occurs in similar situations on the coast of
Holland (Entrop, 1960).
In the creeks, Nassarius lives on bottoms of mud and shell
often where the polychaete Sabella pavonina is common.
TURRIDAE
Lora turricula (Montagu)
Dead shells are commonly obtained from the bottom sediments
and also from the shore. Living specimens have only occasionally
been observed; station N.4 (1960) and station HB.2 (1964).
Sub-class OPISTHOBRANCHIA
Order Pleurocoela
Retusidae
Retusa alba (Kanmacher)
Shells commonly found in samples of material from the shores
and bottom of the Blackwater. Living specimens have been
obtained from soft mud in St. Lawrence Bay (1964).
Atyidae
Haminoea hydatis (Linne)
Recorded by Sorby (1903) living off Mersea Flat, but recently
only dead shells have been found. These have been collected from
Mersea Flat, Bradwell shore and Bradwell Creek.
Akeratidae
Akera bullata Muller
Shells and shell fragments have been rarely found on the
Mersea Flat, and at Bradwell Waterside. There are no records of
living animals.
Philinidae
Philine aperta quadripartida Ascanius
Some large specimens were collected alive by trawling in Swire
Hole (1962), and other fresh specimens were taken from the
stomachs of fish caught in the Bench Head area (1962). Shells
are frequently encountered in samples of bottom and shore sedi-