10
THE ESSEX NATURALIST
Immediately outside the Blackwater on shelly or sandy bottoms
the polychaete Aphrodite aculeata and echinoderms Psam-
mechinus miliaris and Ophiura texturata, may be found.
Just offshore there are deposits of gravel with stones, the most
important of which occur on the Colne Bar and Eagle. These
may be fluvio-glacial deposits lying almost in situ rather than
material carried down by the existing rivers in more recent times.
The encrusting fauna, of hydroids and polyzoa and the sand tubes
of Sabellaria spinulosa, is very dense. This type of bottom is
locally known as 'ross' and is particularly good for shrimp fishing
(Mistakidis, 1960). On the Colne Bar large pebbles of soft stone
are burrowed by the piddocks, Barnea candida and Barnea parva.
Many other species as already mentioned for the other clean shell
and stone bottom areas also occur here. Where Crepidula
fornicata occurs it is seldom in the form of chains as found on
bottoms of mud and shell but tends to be as individuals or pairs
attached to stones.
CHANGES IN THE FAUNA
Since the 1880's observations have been made on the geography
and biology of the estuaries of the Blackwater and Colne and
records have been published in The Essex Naturalist. The earliest
paper to give lists of marine animals comes as a report of the
Essex Field Club's excursion to Maldon and the River Black-
water in September 1888. Later work deals mainly with specific
topics such as the American Slipper Limpet, Oyster Drills, oysters
and some species whose occurrence was regarded as unusual. Some
information on the River Colne comes from the report of the
Essex Field Club excursion of 1897 and the work of H. C. Sorby
from 1890 to 1903.
Some published work of the staff of the Ministry of Agricul-
ture, Fisheries and Food gives information about the Blackwater
and Colne oyster grounds and more detailed information on the
River Crouch, which has served as a useful comparison. The
present investigation has been the first detailed examination of
the Blackwater fauna.
In the fauna list, some consideration has been given to the
changes in abundance of common species, as seen during the
period 1960-65 and by comparison with the records of previous
workers. For example, an observed disappearance of the
Porcelain Crab (Porcellana longicornis) in 1961-62, and a
withdrawal of the sea urchin (Psammechinus miliaris) to off-
shore waters can be deduced from a comparison of recent records
with those of Orton and Lewis (1931). Similarly it has been
possible to show that the abundance of some species, e.g., the
American Slipper Limpet (Crepidula fornicata) has increased.
The severity of winters has often been quoted as the cause
of major setbacks in animal populations. Great mortalities can
often be observed after hard frosts, but species affected in this
way may often recover within a few years. For example the