THE BOTTOM FAUNA OF THE BLACKWATER ESTUARY 29
theless temperature effects can not be ruled out. Pilumnus is a
warm-temperate Atlantic and Mediterranean species, which
colonises the British Isles from a south-western "headquarters"
and only just penetrates into the North Sea adjacent to the English
Channel. In the Blackwater, it was found only in the immediate
vicinity of the thermal discharge. Likewise, Cereus is a south-
western and western species (Stephenson, 1935) extending
through the Mediterranean Sea and occurring in the warmer
regions of the British Isles. Cereus, however, is otherwise absent
from the North Sea and its presence near the Barrier Wall is
suggestive of local warming of the water as the reason for its
survival. It too was restricted to the area of the thermal dis-
charge, occurring only within 12 m of the outfall culverts.
Phoronis mulleri is known in the adult form from the
western coast of Sweden, from Heligoland and from the
Mediterranean (Silen, 1952). Its larval stage ("Actinotrocha
branchiata") is more widely known, having been frequently re-
corded from the North Sea (Gustafson, 1936; Silen, 1952) and
from the Atlantic coast of Britain (e.g. Crothers, 1966). Gustafson
(1936) was of the opinion that "By further investigations of the
bottoms it will surely be found to be common on all European
coasts" and this record from the Blackwater can probably be
regarded as validation of this prediction. It would certainly not
appear to be a species requiring warm-water conditions for its
survival, being both euryhaline and eurythermal (Gustafson,
1936). In Sweden, it is known from sandy clay substrates at
depths of between nine and fifty metres (Gustafson, 1936; Silen,
1952) and some of the faunal assemblages occurring with P.
mulleri are similar to those present in the Blackwater. The
densities quoted by Gustafson (1936) are considerably lower,
however, than those found near the Barrier Wall. Phoronis is a
filter-feeder and its presence in large numbers near a discharge
carrying fragmented organic material may not be co-incidental.
Therefore, of the species restricted to the area of the outfall,
the nature of the discharge may be responsible for the occurrence
of Pilumnus and Cereus in the Blackwater (in the very small area
immediately adjacent to the discharging culvert) and it may have
a bearing on the numbers of Phoronis, although it would not ap-
pear to be responsible for the initial presence of this species.
Indeed, Gustafson's prediction may well be further verified as
the Sublittoral regions of more British estuaries are investigated.
The authors wish to record their gratitude to Dr Eve South-
ward and Dr J. D. George for kindly identifying material sent to
them; to P. French, Esq., for his help whilst aboard the "Provi-
dence"; to Dr A. I. Bees and the Department of Oceanography,
University of Southampton for the loan of the gravity corer; to
M. Wasson, Esq. for the use of laboratory facilities; and to J. M.
Fleming, Esq. for continual helpful assistance. This paper is