THE MARINE FAUNA OF THE BLACKWATER ESTUARY 29
Balanus balanoides (Linne)
Very common in the Blackwater and Colne estuaries, on the
upper and middle shores and rarely on the lower shore and below
low water mark. Also occurs as fouling on ships and on the
Barrier Wall.
Balanus improvisum Darwin
Of rare occurrence only. Recorded from fouling on ships
anchored in the Blackwater, 1960 and 1961.
Elminius modestus Darwin
Very common in the Blackwater and Colne estuaries, mostly
on the lower and middle shore but also sublittorally. The species
was introduced to Britain from New Zealand during the period
1943-46, and has since become the dominant species of barnacle
in the Blackwater area. The highest density of population re-
corded from the bottom fauna was 3,750/nr at station N.3
(1.5.1963).
Order Rhizocephala
Sacculinidae
Sacculina carcini Thompson
Parasitic on the swimming crab, Portunus marmoreus, with
often a high level of infection in the population. Never recorded
from Carcinus maenas, which is the usual host of Sacculina.
Peltogaster paguri Rathke
A parasite of the hermit crab, Eupagurus bernhardus. May be
common.
Class MALACOSTRACA
Order Cumacea
Various species of cumaceans are commonly encountered in
the plankton and in fine sediments on the bottom. Only two
specific identifications have been made.
DIASTYLIDAE
Diastylis rathkei (Kr0yer)
Occasionally taken in bottom fauna samples, usually being
associated with muddy ground. Some large specimens were col-
lected from the stations inside the Barrier Wall.
PSEUDOCUMIDAE
Pseudocuma longicornis (Bate)
Occasional records from plankton samples taken in 1961.
Order Isopoda
Anthuridae
Cyathura carinata Kr0yer
Three specimens from the Blackwater on 23.3.1960, one from
the Thirslet area on 12.8.1960 and various other records in the