22
THE ESSEX NATURALIST
Pilumnus hirtellus was recorded by the May 1970 survey of the
Barrier Wall region, this record having been given in the earlier
paper (Barnes and Coughlan, 1971). Two of the new records
(Cereus and Phoronis) are particularly worthy of comment (see
'Discussion'), — Phoronis was present in abundances of up to
3,000/m2 and was the most abundant species to landwards
of the Barrier Wall, yet it had not been recorded from
the Blackwater hitherto although that estuary has received a
considerable amount of attention since 1958 from the Central
Electricity Research Laboratories and the Ministry of Agricul-
ture, Fisheries and Food.
With these additions to the fauna known from the Barrier
Wall area, a total of 53% of the species recorded from the Black-
water in 1970 and 1971 (Barnes and Coughlan, 1971, and present
paper) have also been recorded from the area adjacent to the
Barrier Wall. When it is considered that many species are known
only from restricted areas, e.g. Thirslet Creek, this represents a
considerable proportion of the known Blackwater fauna occurring
in a single 0.1km2 "sub-region" and it indicates that the Barrier
Wall and its culverts are not causing any general impoverish-
ment of the local fauna. The more detailed distributions of the
various species are considered in a later section.
The 1970 general survey provided evidence that several
species had re-penetrated the Blackwater after absences of
several years (Barnes and Coughlan, 1971). The present surveys
also provided evidence of this recovery. Nucula nucleus and Abra
alba were recorded frequently from the Barrier Wall area in
1971, whereas earlier records were all sited considerably sea-
wards of this point. Conversely, the Barrier Wall area represents
the most seaward point in the distribution of Scoloplos (un-
recorded by Davis, 1967), which is evidently more widespread
than was previously thought (c.f. Barnes and Coughlan, 1971).
Further, a limited amount of dredging carried out near The
Stone in June 1971 captured a Psammechinus and a Macropodia.
This may well be evidence of a continuing recovery and extension
of range on the part of the echinoid (see Orton and Lewis, 1931;
Barnes and Coughlan, 1971) and of the beginning of a recovery
of the spider crab after its decimation and possible local extinc-
tion by the severe winter of 1962/63 (see Davis, 1967: Barnes and
Coughlan, 1971). With the possible return of Macropodia, only
Porcellana longicornis remains on the list of those species which
have failed to recolonise the Blackwater after that severe winter.
THE DISTRIBUTION OF THE FAUNA
The fauna recorded from the grab samples shows an
extremely good correlation with the nature of the substrate (see
Tables 1 and 2). Both of the two major substrate types present
possessed a characteristic and specific fauna, that of the "shell
on mud" being dominated by colonial hydroids, anemones, scale