THE BOTTOM FAUNA OF THE BLACKWATER ESTUARY 23
worms, Sabellaria, Abra and Alcyonidium, with Gibbula and
Nucula also frequent, whilst that of the "mud on clay" was
dominated by large numbers (up to approx. 3,000/m2) of the
small tube-dwelling Phoronis ?mulleri and by the errant Nereis
longissima and Nephtys incisa. If roving predators or scavengers
such as Eupagurus, Carcinus, Buccinum and Asterias are
excluded, only 13% of the fauna present on these two sub-
strates were common to both and only Nereis longissima and
Nephtys incisa were moderately common or widespread on the
two sides of the wall, the former occurring at 60% of the "mud
on clay" stations and 25% of the "shell on mud", and the latter
at 40% and 25% respectively. In total (excluding the roving
predatory and scavenging species), 81% of the "shell on mud"
fauna was absent from the "mud on clay" and 61% of the species
inhabiting the latter were absent from the former. It can also
be seen from Table 2 that the "mud on clay" fauna comprised
fewer species than that associated with the "shell on mud".
This would appear to be the result of the greater number of
niches provided by the more diverse shelly substrate, including
a supply of firm bases for the attachment of sessile animals,
and crevices, etc., for sedentary species. The "mud on clay"
lacked the latter, although it may be noted that if an artificial
hard substrate is added to the general area, e.g. in the form
of concrete ballast (see below), the number of sessile species
rises to levels comparable with those from the "shell on mud"
(Tables 1 and 2). Analysis of Table 2 in this context, in terms
of the numbers of species of (a) epifaunal, (b) scavenging or
predatory, and (c) infaunal organisms, reveals that the "mud
on clay" and "shell on mud" substrates possess similar numbers
of categories (b) and (c), but that the "shell on mud" possesses
four times as many epifaunal species as the "mud on clay".
The fauna of the bare stones in the immediate vicinity of
the intake culvert is reduced, both in terms of numbers of species
and especially in numbers of individuals or colonies. This is
as might be expected, since the velocity of the water current
entering the culvert would not only remove the fine surface
sediment but also rotate the stones one against the other (see
also Barnes, Coughlan & Holmes, 1973), discouraging the suc-
cessful settlement of larvae. Only a few individuals of Sagartia,
Alcyonidium and Flustra were obtained attached to the stones
and to the small amount of concrete rubble which was also
present.
The fauna of the substrate in the immediate vicinity of the
outfall culverts varied with the precise nature of the substrate,
but a total of twenty-two species were taken in the area typified
by ballast and mussel-debris. The fauna was dominated by
mobile predators feeding on the abundant small Mytilus. This
debris, bound into lumps by byssus threads, provided attach-
ment sites for anemones (mainly Tealia) which were abundant