88
Verena TUNNICLIFFE et al.
medium for molecular diffusion or fluid flow (sediments, sulphide deposits, whale bones) or are located in
the path of fluid discharge; thus, they provide access to
reducing substances or the products of chemosynthesis.
Vesicomyid clams are commonest at sedimented sites,
where they assume a partially-buried position and
take up sulphide through the foot. Certain alvinellid
polychaetes are found only on active hydrothermal
structures (Juniper and Martineu, 1995). Serpulid
polychaetes are limited to rock or shell surfaces, and
at vents occur only away from direct hydrothermal
flow (Fustec et al., 1987). Bathymodiolid mussels, on
the other hand, can form abundant aggregations on
sediment, bare rock and polymetallic sulphide surfaces
wherever there is an appropriate supply of fluids from
vents (Van Dover et al., 1996a) or seeps (Olu et al.,
1996b) for their symbionts. One very large species
(to 36 cm length), Bathymodiolus boomerang, lives
partially buried in mud at low seepages (Von Cosel and
Olu, 1998). Free-living micro-organisms are similarly
flexible with respect to substrata, as is evident in the
growth of filamentous tufts and mats on most animal
or mineral surfaces that provide access to reducing
substances (Jannasch and Wirsen, 1981). Locally, the
availability of appropriate substrata may influence faunal community composition. A few recent studies have
identified flow rate as a key variable influencing the
structure of communities around vents and subductionzone seeps (Henry et al., 1992; Sarrazin et al., 1997).
Fluid flow determines the rate of supply of reducing
substances for microbial chemosynthesis. Vigorous
diffuse flow may even reduce the availability of
dissolved oxygen within faunal aggregations, although
appropriate data remain to be acquired.
In the mid-ocean ridge and back-arc ridge settings of
hydrothermalism, hard substratum is the most common
benthic habitat, occurring as basaltic rock or sulphide
mounds and structures. Low-temperature hydrothermal
fluids flow through fractures in the substratum or diffuse through porous sulphides, supporting aggregations
of vent organisms (Fig. 4.5a). Often vent openings are
multiple, particularly on sulphide edifices, resulting in a
continuous patchwork of vent communities. Organisms
can also exploit high-temperature venting by colonizing
adjacent surfaces which have been diluted and cooled
by ambient seawater (Sarrazin et al., 1997) (Fig. 4.5b).
Geologic settings that favour venting through sediment
are uncommon. The predominance of hard substrata at
vents may limit the presence of some species. Juniper
et al. (1992) suggested that the high species diversity
at one sedimented site may derive partly from the
mixture of soft and hard substrata. Strong affinities
among the faunas of three sedimented hydrothermal
sites in the eastern Pacific provide further evidence for
a strong substratum effect on faunal composition at
vents (Tunnicliffe et al., 1996). The external surfaces of
many vent animals, particularly vestimentiferan tubes,
can represent an important substratum for microbial
growth and colonization by small metazoans (Sarrazin
et al., 1999).
Fig. 4.5. Hydrothermal vents of the mid-ocean ridge. A. Diffuse
venting through basalts on the northern East Pacific Rise.
Vestimentiferans are Riftia pachyptila; bathymodiolid mussels and
galatheid crabs are also seen. Image is about 2 m across. B. High
temperature venting on Juan de Fuca Ridge. Fluid at 370ºC
emerges from chimney spouts atop a sulphide mound 8 m high. The
vestimentiferan Ridgeia piscesae surrounds the high-sulphide fluid.
Subduction-zone and hydrocarbon seeps are predominantly soft-bottom environments, but hard surfaces exist in the form of carbonate concretions formed
by methane oxidation (followed by the reaction of
bicarbonate with calcium in seawater), rocky outcrops,
clam shells and even outcropping methane hydrates
(Juniper and Sibuet, 1987; Olu et al., 1997; Suess
et al., 1999). While there may be considerable variety
Verena TUNNICLIFFE et al.
medium for molecular diffusion or fluid flow (sediments, sulphide deposits, whale bones) or are located in
the path of fluid discharge; thus, they provide access to
reducing substances or the products of chemosynthesis.
Vesicomyid clams are commonest at sedimented sites,
where they assume a partially-buried position and
take up sulphide through the foot. Certain alvinellid
polychaetes are found only on active hydrothermal
structures (Juniper and Martineu, 1995). Serpulid
polychaetes are limited to rock or shell surfaces, and
at vents occur only away from direct hydrothermal
flow (Fustec et al., 1987). Bathymodiolid mussels, on
the other hand, can form abundant aggregations on
sediment, bare rock and polymetallic sulphide surfaces
wherever there is an appropriate supply of fluids from
vents (Van Dover et al., 1996a) or seeps (Olu et al.,
1996b) for their symbionts. One very large species
(to 36 cm length), Bathymodiolus boomerang, lives
partially buried in mud at low seepages (Von Cosel and
Olu, 1998). Free-living micro-organisms are similarly
flexible with respect to substrata, as is evident in the
growth of filamentous tufts and mats on most animal
or mineral surfaces that provide access to reducing
substances (Jannasch and Wirsen, 1981). Locally, the
availability of appropriate substrata may influence faunal community composition. A few recent studies have
identified flow rate as a key variable influencing the
structure of communities around vents and subductionzone seeps (Henry et al., 1992; Sarrazin et al., 1997).
Fluid flow determines the rate of supply of reducing
substances for microbial chemosynthesis. Vigorous
diffuse flow may even reduce the availability of
dissolved oxygen within faunal aggregations, although
appropriate data remain to be acquired.
In the mid-ocean ridge and back-arc ridge settings of
hydrothermalism, hard substratum is the most common
benthic habitat, occurring as basaltic rock or sulphide
mounds and structures. Low-temperature hydrothermal
fluids flow through fractures in the substratum or diffuse through porous sulphides, supporting aggregations
of vent organisms (Fig. 4.5a). Often vent openings are
multiple, particularly on sulphide edifices, resulting in a
continuous patchwork of vent communities. Organisms
can also exploit high-temperature venting by colonizing
adjacent surfaces which have been diluted and cooled
by ambient seawater (Sarrazin et al., 1997) (Fig. 4.5b).
Geologic settings that favour venting through sediment
are uncommon. The predominance of hard substrata at
vents may limit the presence of some species. Juniper
et al. (1992) suggested that the high species diversity
at one sedimented site may derive partly from the
mixture of soft and hard substrata. Strong affinities
among the faunas of three sedimented hydrothermal
sites in the eastern Pacific provide further evidence for
a strong substratum effect on faunal composition at
vents (Tunnicliffe et al., 1996). The external surfaces of
many vent animals, particularly vestimentiferan tubes,
can represent an important substratum for microbial
growth and colonization by small metazoans (Sarrazin
et al., 1999).
Fig. 4.5. Hydrothermal vents of the mid-ocean ridge. A. Diffuse
venting through basalts on the northern East Pacific Rise.
Vestimentiferans are Riftia pachyptila; bathymodiolid mussels and
galatheid crabs are also seen. Image is about 2 m across. B. High
temperature venting on Juan de Fuca Ridge. Fluid at 370ºC
emerges from chimney spouts atop a sulphide mound 8 m high. The
vestimentiferan Ridgeia piscesae surrounds the high-sulphide fluid.
Subduction-zone and hydrocarbon seeps are predominantly soft-bottom environments, but hard surfaces exist in the form of carbonate concretions formed
by methane oxidation (followed by the reaction of
bicarbonate with calcium in seawater), rocky outcrops,
clam shells and even outcropping methane hydrates
(Juniper and Sibuet, 1987; Olu et al., 1997; Suess
et al., 1999). While there may be considerable variety