THE MARINE FAUNA OF THE BLACKWATER ESTUARY 7
seem to be the lack of large stones to shelter these animals from
predators and extremes of climate when the tide is out. Some
species such as Carcinus maenas, the Shore Crab, settle on the
shores but migrate sublittorally as they outgrow the limited
amount of shelter available.
The following paragraphs give a general account of the main
habitats that have been distinguished and the common species of
animals found with each.
HABITATS ON THE SHORE
Shell and Shingle Banks: Deposits of shell, sand and stones at
high-water mark form a steep bank which may be dry on top and
colonised by maritime plants. These banks are very mobile and
change shape frequently as a result of wave action. The fauna is
poor with only two species occurring in the sand and under
detritus along the tide line. The species are Ligia oceanica and
Talitrus saltator, both of which are largely nocturnal in habit.
Shell banks occur along the south shore of Mersea Island, Shingle-
head Point, Colne Point, and along the shore from Bradwell to St.
Peters Flats.
Saltings: The areas of saltings now remaining in the Blackwater
have a varied flora dominated at the seaward end by Sueda sp.,
Atriplex sp., Salicornia sp., Limonium sp., Spartina sp., and other
grasses and by Phragmites communis in brackish waters above
Maldon. Only the macrofauna was investigated; the species re-
corded being: Nereis diversicolor (in runnels), Orchestria gam-
marella, Palaemonetes varians (in pools), Carcinus maenas, Liparus
marina, Littorina littorea, Littorina saxatilis, Hydrobia ulvae,
Assiminea grayana, Phytia myosotis and Leucophytia bidentata.
Sea Walls: In some places the sea walls, or dykes, generally
faced with stone, extend below high-water mark. The stable sur-
face offers a good habitat for many types of seaweed and some
animals. Of sedentary forms, the barnacles Balanus balanoides
and Elminius modestus are most important but hydroids such as
Dynamena pumila also occur. Other species present are Ligia
oceanica, Gammarus locusta, Carcinus maenas, Littorina littorea,
Littorina saxatilis, Littorina obtusata, and Mytilus edulis.
Mud: Considerable areas of soft mud occur on the middle and
lower shore, particularly in St. Lawrence Bay and around the
north side of Osea Island. This soft mud is in part residue from
old alluvial deposits and in part recently desposited silt. It is all
anaerobic below the first few inches. There is an abundant
infauna of polychaetes: Nereis sp., and Nephthys sp.; crus-
taceans : Corophium sp.; and molluscs: Scrobicularia plana and
Macoma balthica. The gastropod Hydrobia ulvae is often found in
large numbers on the surface of the mud.