134
Lisa A. LEVIN and Andrew J. GOODAY
British continental margin
Recent work has concentrated in three main areas of
the Celtic margin of Britain: the Rockall Trough, the
Porcupine Seabight, and the Goban Spur – localities
which are convenient to reach from British ports and
relatively easy to sample.
Rockall Trough (RT): This elongate basin lies
between Scotland and the northern part of Ireland on
the one hand, and the Rockall Bank on the other,
and deepens from the Wyville Thomson Ridge in the
north to the Porcupine Abyssal Plain in the south.
It has been intensively studied for over 20 years by
J.D. Gage and his colleagues at the Scottish Marine
Biological Association/Scottish Association for Marine
Science in Oban. They have compiled a data set
from two deep Permanent Stations, one situated at
2200 m depth, the other at 2900 m – the longest time
series available anywhere in the deep ocean. Mean
standing crop of macrofauna (>420 mm, excluding
Foraminifera), sampled using replicate 0.25 m
2 box
cores, was 1716 individuals m
−2 , with a wet weight
of 14.92 g m
−2 (Gage, 1979). Polychaetes (59.1%
of the individuals) were the dominant taxon followed by tanaids (10.3%), bivalves (10.1%), isopods
(4.4%), scaphopods (3.9%), amphipods (3.3%) and
nemertines (3.3%). The proportions of various taxa
were remarkably similar in box cores taken at 1800 m,
2000 m, 2500 m and 2900 m in other parts of the
Rockall Trough; only isopods tended to be relatively
more abundant at the Permanent Station. The isopod
fauna includes 79 species, dominated by the Eurycopidae, Haploniscidae, Ilyarachnidae, and Ischnomesidae
(Harrison, 1988). The bivalves consist predominantly
(80%) of members of the Nuculanidae. The macrofauna
at the Permanent Station was also sampled using
an epibenthic sledge fitted with a 500 mm-mesh net
(Gage et al., 1980). These catches were dominated by
polychaetes, bivalves and ophiuroids, although there
was considerable variation between individual catches,
caused in particular by summer recruitment of juveniles
of the ophiuroid Ophiura ljungmani.
Gage (1986) reviewed knowledge of the invertebrate megafauna of the Rockall Trough based on
catches from Agassiz trawls or large, single- or twinwarp trawls. The faunas are generally similar to
those found in the Bay of Biscay and Porcupine
Seabight. Echinoderms predominate at all depths, and
species are zoned bathymetrically, the greatest rates
of faunal change occurring between 800 and 1200 m
and (particularly in the case of ophiuroids) around
1800 m. Other invertebrates, particularly cnidarians and
crustaceans, are also important. The cnidarians include
the deep-water coral Lophelia pertusa, a widespread
inhabitant of the upper slope along the European
continental margin, and associated with areas of steady
current around the Rockall Trough (Wilson, 1979). The
influence of currents on the megafauna is particularly
evident on the Feni Ridge on the western flank of the
Rockall Trough, where suspension feeders are more
common than they are at tranquil sites on the eastern
side of the Trough. As well as bathymetry and current
velocity, megafaunal composition seems to reflect
sediment type and bottom water-mass distribution.
Thus, the zone of rapid change in the echinoderm fauna
between 800 and 1200 m occurs within a distinctive
water mass of Mediterranean origin, and also spans
the permanent thermocline. Gage (1986) attributed
differences in the depths of zones of maximum faunal
change between the Rockall Trough and sites in the
Northwest Atlantic and Northeast Pacific to such local
influences superimposed on bathymetric and other
trends which apply globally.
The availability of extensive time-series material
allowed Gage and his colleagues to analyse the
demography and reproductive dynamics among animals
in the Rockall Trough (reviewed by Tyler, 1988; Gage,
1991; Gage and Tyler, 1991). This has led to the
recognition of taxa displaying seasonal reproduction,
leading to the production of juveniles in the spring,
coincident with phytodetrital pulses (Gage and Tyler,
1991). Seasonally breeding taxa include isopods, the
protobranch bivalves Ledella pustulosa and Yoldiella
jeffreysi, and the echinoderms Echinus affinis, Ophiura
ljungmani, Plutonaster bifrons and possibly Dytaster
insignis, Ophiocten gracilis and Ophiomusium lymani.
Other echinoderms have non-seasonal (continuous) reproduction with large or intermediate egg size and low
or intermediate fecundity. Classical growth-rate studies
based on size distributions have been carried out for
E. affinis, L. pustulosa, Ophiomusium lymani, Ophiura
ljungmani and Y. jeffreysi. In the case of Ophiomusium
lymani, it was impossible to disentangle the individual
cohorts present within the single unimodal size peak.
In this species, and in Ophiura ljungmani and the
crinoids Annacrinus wyvillethomsoni and Bathycrinus
carpenteri, growth rates have been determined by
analyzing growth zones in skeletal plates (Duco and
Roux, 1981). These studies suggest that the two crinoid
species have growth and mortality patterns similar
Lisa A. LEVIN and Andrew J. GOODAY
British continental margin
Recent work has concentrated in three main areas of
the Celtic margin of Britain: the Rockall Trough, the
Porcupine Seabight, and the Goban Spur – localities
which are convenient to reach from British ports and
relatively easy to sample.
Rockall Trough (RT): This elongate basin lies
between Scotland and the northern part of Ireland on
the one hand, and the Rockall Bank on the other,
and deepens from the Wyville Thomson Ridge in the
north to the Porcupine Abyssal Plain in the south.
It has been intensively studied for over 20 years by
J.D. Gage and his colleagues at the Scottish Marine
Biological Association/Scottish Association for Marine
Science in Oban. They have compiled a data set
from two deep Permanent Stations, one situated at
2200 m depth, the other at 2900 m – the longest time
series available anywhere in the deep ocean. Mean
standing crop of macrofauna (>420 mm, excluding
Foraminifera), sampled using replicate 0.25 m
2 box
cores, was 1716 individuals m
−2 , with a wet weight
of 14.92 g m
−2 (Gage, 1979). Polychaetes (59.1%
of the individuals) were the dominant taxon followed by tanaids (10.3%), bivalves (10.1%), isopods
(4.4%), scaphopods (3.9%), amphipods (3.3%) and
nemertines (3.3%). The proportions of various taxa
were remarkably similar in box cores taken at 1800 m,
2000 m, 2500 m and 2900 m in other parts of the
Rockall Trough; only isopods tended to be relatively
more abundant at the Permanent Station. The isopod
fauna includes 79 species, dominated by the Eurycopidae, Haploniscidae, Ilyarachnidae, and Ischnomesidae
(Harrison, 1988). The bivalves consist predominantly
(80%) of members of the Nuculanidae. The macrofauna
at the Permanent Station was also sampled using
an epibenthic sledge fitted with a 500 mm-mesh net
(Gage et al., 1980). These catches were dominated by
polychaetes, bivalves and ophiuroids, although there
was considerable variation between individual catches,
caused in particular by summer recruitment of juveniles
of the ophiuroid Ophiura ljungmani.
Gage (1986) reviewed knowledge of the invertebrate megafauna of the Rockall Trough based on
catches from Agassiz trawls or large, single- or twinwarp trawls. The faunas are generally similar to
those found in the Bay of Biscay and Porcupine
Seabight. Echinoderms predominate at all depths, and
species are zoned bathymetrically, the greatest rates
of faunal change occurring between 800 and 1200 m
and (particularly in the case of ophiuroids) around
1800 m. Other invertebrates, particularly cnidarians and
crustaceans, are also important. The cnidarians include
the deep-water coral Lophelia pertusa, a widespread
inhabitant of the upper slope along the European
continental margin, and associated with areas of steady
current around the Rockall Trough (Wilson, 1979). The
influence of currents on the megafauna is particularly
evident on the Feni Ridge on the western flank of the
Rockall Trough, where suspension feeders are more
common than they are at tranquil sites on the eastern
side of the Trough. As well as bathymetry and current
velocity, megafaunal composition seems to reflect
sediment type and bottom water-mass distribution.
Thus, the zone of rapid change in the echinoderm fauna
between 800 and 1200 m occurs within a distinctive
water mass of Mediterranean origin, and also spans
the permanent thermocline. Gage (1986) attributed
differences in the depths of zones of maximum faunal
change between the Rockall Trough and sites in the
Northwest Atlantic and Northeast Pacific to such local
influences superimposed on bathymetric and other
trends which apply globally.
The availability of extensive time-series material
allowed Gage and his colleagues to analyse the
demography and reproductive dynamics among animals
in the Rockall Trough (reviewed by Tyler, 1988; Gage,
1991; Gage and Tyler, 1991). This has led to the
recognition of taxa displaying seasonal reproduction,
leading to the production of juveniles in the spring,
coincident with phytodetrital pulses (Gage and Tyler,
1991). Seasonally breeding taxa include isopods, the
protobranch bivalves Ledella pustulosa and Yoldiella
jeffreysi, and the echinoderms Echinus affinis, Ophiura
ljungmani, Plutonaster bifrons and possibly Dytaster
insignis, Ophiocten gracilis and Ophiomusium lymani.
Other echinoderms have non-seasonal (continuous) reproduction with large or intermediate egg size and low
or intermediate fecundity. Classical growth-rate studies
based on size distributions have been carried out for
E. affinis, L. pustulosa, Ophiomusium lymani, Ophiura
ljungmani and Y. jeffreysi. In the case of Ophiomusium
lymani, it was impossible to disentangle the individual
cohorts present within the single unimodal size peak.
In this species, and in Ophiura ljungmani and the
crinoids Annacrinus wyvillethomsoni and Bathycrinus
carpenteri, growth rates have been determined by
analyzing growth zones in skeletal plates (Duco and
Roux, 1981). These studies suggest that the two crinoid
species have growth and mortality patterns similar