THE MARINE FAUNA OF THE BLACKWATER ESTUARY
9
phium volutator, and the cumacean Diastylis rathkei, have also
been identified.
Mud and Shell: Most of the oyster grounds which lie between
low-water mark and the 3 fathom line come into this variable
group of sediments. The shell material is mostly composed of the
shells of slipper limpets, mussels, oysters and cockles, with small
stones. It overlies clay or firm mud, and originates from material
deposited by oyster cultivators to improve the ground and from
the natural mortalities of shellfish populations.
Living oysters (Ostrea edulis) are common, but it is the Ameri-
can Slipper Limpet (Crepidula fornicata) which is the most
significant animal in the community, having population densities
of up to 2,000 per nr of bottom. The chain-forming habit of the
slipper limpet provides ample surface for settlement of sedentary
organisms such as hydroids, actinarians, barnacles, polyzoans and
ascidians. There is an infauna of variable quality and quantity
mainly dependent upon the amount of mud present at the locality,
and polychaetes, amphipods, and small bivalves may be common.
Larger, epibenthic forms commonly occurring are: —crabs,
Eupagurus bernhardus, Carcinus maenas and Hyas araneus;
molluscs, Gibbula cineraria and Buccinum undatum; and
echinoderms, Solaster papposus, Asterias rubens and Ophiura sp.
Clean Shell and Small Stones: This is another wide group as it
includes fine shell and sand, coarse shell or 'culch' and gravel.
These may be mixed together or separated into distinct areas and
are distributed from Thirslet, where there is a shingle bank across
the estuary, down to the open sea. The qualifying factor is the
absence of excessive quantities of mud and as a consequence there
is a rich settlement of filter-feeding animals, such as sponges, e.g.,
Halichondria panicea, hydroids and ascidians, particularly the
species Dendrodoa grossularia, of which counts of up to 6,000 per
nr have been made. These cleaner deposits are found mostly
below the 3 fathom line where it seems that the tidal flow may
keep the mud from settling. This type of bottom provides the
greatest variety of animals, nearly seventy species being recorded
from the Thirslet area alone during 1960. Apart from the two
species previously mentioned, the most distinctive species of the
gravel areas are the spider crab, Macropodia rostrata, horse
mussel, Modiolus modiolus and the brittle starfish, Ophiothrix
fragilis.
The shell or 'culch' that occurs mainly in the middle of the
estuary consists of old oyster shells bored by the sponge Cliona
celata, and the polychaete Polydora ciliata and encrusted with the
sand-tubes of another polychaete species, Sabellaria spinulosa,
Crepidula fornicata may be quite common and there are small
areas where Ostrea edulis is present in some numbers. Fine shell
and sandy material commonly contains terebellid polychaetes
and small bivalves such as Nucula nucleus and Abra alba.