THE PERIPHERAL DEEP SEAS
285
stations along the northern slope of the Gulf of Mexico,
as well as five stations at abyssal depths. At the slope
stations mean density was between 500 and 600 individuals m
−2 , whereas at the abyssal stations in the
eastern Gulf mean density was 19 individuals m
−2 with
a mean biomass of 31 mg m
−2 , and in the western Gulf
the corresponding figures were 112 individuals m
−2
and 82 mg m
−2 , respectively. These values are far less
than those at corresponding depths in the Northwest
Atlantic (Chapter 5; see also Rowe et al., 1974). These
means are proportional to surface productivity, the
offshore rate of decrease in benthic biomass being
proportional to the offshore decrease in surface primary
production.
More recently data have become available for the
deep-water fauna from Mexican waters in the western
Gulf of Mexico (Escobar-Briones and Soto, 1993;
Escobar-Briones et al., 1999). Escobar-Briones and
Soto (1993) have provided a list of the dominant
megafauna and macrofauna below a depth of 200 m.
Many of the species are found in the Atlantic also,
although a number of species have subspecies endemic
to the Gulf of Mexico. In a subsequent analytical paper
(Escobar-Briones et al., 1999), infaunal macrobenthic
density was analysed along a depth gradient from
the shelf break to abyssal depths eastwards along
the 24ºN parallel of latitude. Density increased at
bathyal depths, the highest density (mean 2713 individuals m
−2 ) being found at 1250 m, immediately
below the oxygen minimum. Density was highly
variable on the abyssal plain (means from 794 to
1669 individuals m
−2 ). The main structuring factors
along this portion of the slope of the Gulf of Mexico
were surface production, thermal stratification and the
oxygen minimum zone.
Autochthonous communities
Communities fuelled by either methane or hydrogen
sulphide are found from depths of ~500 to 2200 m
from the Mississippi Fan westwards along the slope to
bathyal depths off Texas (Kennicutt et al., 1985; MacDonald et al., 1990a). Chemosynthetic communities are
also found at the base of the Florida Escarpment, where
sulphide-rich hypersaline waters seep out at a depth of
~3000 m (Paull et al., 1984).
Dominating these chemosynthetically-based communities is an undescribed species of the vestimentiferan
Lamellibrachia (Kennicutt et al., 1988; MacDonald
et al., 1989). This organism is found as dense bushes
at Bush Hill and in Green Canyon, but has not been
observed in the Mississippi Canyon area. Large clumps
of Lamellibrachia sp. are particularly well established
in the Green Canyon area. Where Lamellibrachia
occurs there is often the co-occurrence of a second
vestimentiferan Seepiophilia jonesi (MacDonald et al.,
1989). S. jonesi tends to form irregular small clumps
at the base of the larger Lamellibrachia ‘bushes’.
In contrast to vestimentiferans found at hydrothermal
vents, both these vestimentiferan species are supported
by hydrogen sulphide diffusing up the inside of the
worm tube to the trunk, rather than being absorbed
through the tentacles. Fisher et al. (1997) have shown
that Lamellibrachia grows very slowly (in contast
to vestimentiferans at hydrothermal vents), averaging
0.77 cm y
−1 , and recently Bergquist et al. (2000) have
suggested that this species, with a life span of 170
to 250 years, is the longest-lived metazoan on earth.
Lamellibrachia also supports an epifaunal community.
The bivalve Acesta bullisi (MacDonald et al., 1989),
a species of Echinus (personal observation), and a
recently described sponge Ectyomyxilla methanophila
(Maldonado and Young, 1998) all occur on the tube
of Lamellibrachia. Other associated fauna include the
crabs Bathyplax typhla, Geryon sp. and Rochinia crassa
and the giant isopod Bathynomus giganteus.
The other dominant organisms in this region include the seep mussel Bathymodiolus childressi. The
taxonomy of this genus has recently been clarified.
It has been suggested that, in the Gulf of Mexico,
at least three species exist, one at Bush Hill (600 m)
and Alaminos Canyon (2222 m), and two at the base
of the Florida escarpment (3314 m). There is possibly
a fourth at the oil-drilling station Garden Banks386 (650 m), but this may be more closely related
to the genus Idas (Craddock et al., 1995). All the
Bathymodiolus species are able to use methane as an
energy source (Fisher et al., 1993; Nix et al., 1995),
although there is some evidence that they are also
able to use suspended particulate organic matter (Page
et al., 1990). The fine-scale distribution of mussels
suggests that aggregations of living mussels may form
clusters up to 5 m diameter, separated by sediment or by
clumps of dead and disarticulated mussels (MacDonald
et al., 1990b). A truly remarkable environment on the
Louisiana slope is the Brine Pool (MacDonald et al.,
1990c), which is 22 m long, 11 m wide and some
20 m deep. The salinity is ~121, and there is a strong
pycnocline between the pool and overlying seawater.
It is surrounded by a ‘halo’ of Bathymodiolus and
285
stations along the northern slope of the Gulf of Mexico,
as well as five stations at abyssal depths. At the slope
stations mean density was between 500 and 600 individuals m
−2 , whereas at the abyssal stations in the
eastern Gulf mean density was 19 individuals m
−2 with
a mean biomass of 31 mg m
−2 , and in the western Gulf
the corresponding figures were 112 individuals m
−2
and 82 mg m
−2 , respectively. These values are far less
than those at corresponding depths in the Northwest
Atlantic (Chapter 5; see also Rowe et al., 1974). These
means are proportional to surface productivity, the
offshore rate of decrease in benthic biomass being
proportional to the offshore decrease in surface primary
production.
More recently data have become available for the
deep-water fauna from Mexican waters in the western
Gulf of Mexico (Escobar-Briones and Soto, 1993;
Escobar-Briones et al., 1999). Escobar-Briones and
Soto (1993) have provided a list of the dominant
megafauna and macrofauna below a depth of 200 m.
Many of the species are found in the Atlantic also,
although a number of species have subspecies endemic
to the Gulf of Mexico. In a subsequent analytical paper
(Escobar-Briones et al., 1999), infaunal macrobenthic
density was analysed along a depth gradient from
the shelf break to abyssal depths eastwards along
the 24ºN parallel of latitude. Density increased at
bathyal depths, the highest density (mean 2713 individuals m
−2 ) being found at 1250 m, immediately
below the oxygen minimum. Density was highly
variable on the abyssal plain (means from 794 to
1669 individuals m
−2 ). The main structuring factors
along this portion of the slope of the Gulf of Mexico
were surface production, thermal stratification and the
oxygen minimum zone.
Autochthonous communities
Communities fuelled by either methane or hydrogen
sulphide are found from depths of ~500 to 2200 m
from the Mississippi Fan westwards along the slope to
bathyal depths off Texas (Kennicutt et al., 1985; MacDonald et al., 1990a). Chemosynthetic communities are
also found at the base of the Florida Escarpment, where
sulphide-rich hypersaline waters seep out at a depth of
~3000 m (Paull et al., 1984).
Dominating these chemosynthetically-based communities is an undescribed species of the vestimentiferan
Lamellibrachia (Kennicutt et al., 1988; MacDonald
et al., 1989). This organism is found as dense bushes
at Bush Hill and in Green Canyon, but has not been
observed in the Mississippi Canyon area. Large clumps
of Lamellibrachia sp. are particularly well established
in the Green Canyon area. Where Lamellibrachia
occurs there is often the co-occurrence of a second
vestimentiferan Seepiophilia jonesi (MacDonald et al.,
1989). S. jonesi tends to form irregular small clumps
at the base of the larger Lamellibrachia ‘bushes’.
In contrast to vestimentiferans found at hydrothermal
vents, both these vestimentiferan species are supported
by hydrogen sulphide diffusing up the inside of the
worm tube to the trunk, rather than being absorbed
through the tentacles. Fisher et al. (1997) have shown
that Lamellibrachia grows very slowly (in contast
to vestimentiferans at hydrothermal vents), averaging
0.77 cm y
−1 , and recently Bergquist et al. (2000) have
suggested that this species, with a life span of 170
to 250 years, is the longest-lived metazoan on earth.
Lamellibrachia also supports an epifaunal community.
The bivalve Acesta bullisi (MacDonald et al., 1989),
a species of Echinus (personal observation), and a
recently described sponge Ectyomyxilla methanophila
(Maldonado and Young, 1998) all occur on the tube
of Lamellibrachia. Other associated fauna include the
crabs Bathyplax typhla, Geryon sp. and Rochinia crassa
and the giant isopod Bathynomus giganteus.
The other dominant organisms in this region include the seep mussel Bathymodiolus childressi. The
taxonomy of this genus has recently been clarified.
It has been suggested that, in the Gulf of Mexico,
at least three species exist, one at Bush Hill (600 m)
and Alaminos Canyon (2222 m), and two at the base
of the Florida escarpment (3314 m). There is possibly
a fourth at the oil-drilling station Garden Banks386 (650 m), but this may be more closely related
to the genus Idas (Craddock et al., 1995). All the
Bathymodiolus species are able to use methane as an
energy source (Fisher et al., 1993; Nix et al., 1995),
although there is some evidence that they are also
able to use suspended particulate organic matter (Page
et al., 1990). The fine-scale distribution of mussels
suggests that aggregations of living mussels may form
clusters up to 5 m diameter, separated by sediment or by
clumps of dead and disarticulated mussels (MacDonald
et al., 1990b). A truly remarkable environment on the
Louisiana slope is the Brine Pool (MacDonald et al.,
1990c), which is 22 m long, 11 m wide and some
20 m deep. The salinity is ~121, and there is a strong
pycnocline between the pool and overlying seawater.
It is surrounded by a ‘halo’ of Bathymodiolus and