REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
95
association with autotrophic bacteria. Symbionts are
extracellular, but appear to be digested intracellularly.
Several very specific extracellular symbioses have
been described from polychaetes, shrimps and gastropods endemic to hydrothermal vents. Polychaetes
of the genus Alvinella which colonize hydrothermal
chimneys on the East Pacific Rise have a characteristic bacterial flora associated with their integument, on which there are well-developed attachment
structures for bacteria (Gaill et al., 1986; Juniper
and Martineu, 1995). Bresiliid shrimps on the MidAtlantic Ridge have specific attachment structures for
bacteria within an enlarged branchial chamber. In the
shrimp the ecto-symbionts likely have a nutritional
function, while oxidation of reduced sulphur on the
polychaete may protect the host’s respiratory process.
Less specific microbial growth on other animal surfaces
at hydrothermal vents are probably best described
as fouling. The question of extracellular growth of
micro-organisms on the tissues, tubes and exoskeletal
surfaces of hydrothermal-vent animals is in need of
systematic comparative study to clarify the specificity
of observed associations and their possible nutritional
and detoxifying roles.
THE FAUNA
Reducing environments span a range of conditions
from uninhabitable to mildly challenging. Despite a
great dependence by some microbial autotrophs on
hydrogen sulphide, it is a metabolic poison for aerobic
metazoans, and is incompatible with an abundant
oxygen supply. Such conditions select for a small pool
of inhabitants compared to the huge diversity available
in the deep sea. The adaptations required to partake of
the enhanced productivity of such environments have
greatly limited the dominant inhabitants to a relatively
few groups. While the oasis analogy is used to describe
these islands of plenty (i.e. Carney, 1994), few taxa
have found it to their liking. Much attention has been
paid to animals from hydrothermal vents because of
their unusual adaptations or systematic positions. While
information from seeps is more limited, data from
recent collections lengthen the list of seep species and
reveal more about their relations with other faunas.
Many discoveries are still so new that systematists
remain busy. A major problem for all work in the deep
sea is the poor support for systematics, which means
that experts in many faunal groups are few.
Common taxonomic characteristics
There are common links across reducing habitats. In
some cases, relationships can be traced among habitats,
although phylogenetic work is limited to a few taxa.
One can imagine that adaptations to sulphide-rich
habitats provide entry to similar habitats throughout
the ocean. However, extreme conditions have invoked
extreme adaptations which often separate taxa at fairly
high levels from their nearest relatives in the deep sea.
Thus, most animals found at hydrothermal vents are
newly-described species, as are about half the genera
(Tunnicliffe et al., 1996); the few species that are also
found in the “normal” deep sea tend to live on the
periphery of the vents or occur as occasional visitors.
Many endemic taxa occur at vents – these animals
are presently not known elsewhere. With the discovery
of seep and whale-bone faunas, the similarity to vent
animals was immediately recognized and the concept
that these habitats might serve as dispersal stepping
stones for vents was broached (Hecker, 1985; Smith
et al., 1989). In fact, while few species are held in
common (Smith and Baco, 1998), many genera and
families are shared.
Tunnicliffe et al. (1996) and Sibuet and Olu (1998)
have discussed the common elements of the faunas
of vents, seeps and deep-sea organic remains. Relationships among these faunas appear very restricted at
the level of species; however, at the level of genera
and families the similarity reflects some evolutionary
connection through common ancestors. Certain taxa reoccur: vesicomyid clams, bathymodiolid mussels and
vestimentiferans – all of which contain symbionts –
are the most famous examples. To date, the species
found in common among vents, seeps and carcasses
number less than five. The polychaete Nereis and
the squat lobster Munidopsis are also present in lowoxygen basins. The former is well-known for its ability
to tolerate low oxygen concentrations in intertidal
sediments, and the latter is wide-ranging throughout the
deep sea.
Vestimentiferans and pogonophorans
Vestimentiferans (= obturate pogonophorans) range
in length from millimetres to metres. They have
been considered to be annelids, pogonophorans or a
separate phylum, depending upon authority. Southward
(1988) has provided extensive descriptions of the
anatomy and functions of these animals. Usually, a
95
association with autotrophic bacteria. Symbionts are
extracellular, but appear to be digested intracellularly.
Several very specific extracellular symbioses have
been described from polychaetes, shrimps and gastropods endemic to hydrothermal vents. Polychaetes
of the genus Alvinella which colonize hydrothermal
chimneys on the East Pacific Rise have a characteristic bacterial flora associated with their integument, on which there are well-developed attachment
structures for bacteria (Gaill et al., 1986; Juniper
and Martineu, 1995). Bresiliid shrimps on the MidAtlantic Ridge have specific attachment structures for
bacteria within an enlarged branchial chamber. In the
shrimp the ecto-symbionts likely have a nutritional
function, while oxidation of reduced sulphur on the
polychaete may protect the host’s respiratory process.
Less specific microbial growth on other animal surfaces
at hydrothermal vents are probably best described
as fouling. The question of extracellular growth of
micro-organisms on the tissues, tubes and exoskeletal
surfaces of hydrothermal-vent animals is in need of
systematic comparative study to clarify the specificity
of observed associations and their possible nutritional
and detoxifying roles.
THE FAUNA
Reducing environments span a range of conditions
from uninhabitable to mildly challenging. Despite a
great dependence by some microbial autotrophs on
hydrogen sulphide, it is a metabolic poison for aerobic
metazoans, and is incompatible with an abundant
oxygen supply. Such conditions select for a small pool
of inhabitants compared to the huge diversity available
in the deep sea. The adaptations required to partake of
the enhanced productivity of such environments have
greatly limited the dominant inhabitants to a relatively
few groups. While the oasis analogy is used to describe
these islands of plenty (i.e. Carney, 1994), few taxa
have found it to their liking. Much attention has been
paid to animals from hydrothermal vents because of
their unusual adaptations or systematic positions. While
information from seeps is more limited, data from
recent collections lengthen the list of seep species and
reveal more about their relations with other faunas.
Many discoveries are still so new that systematists
remain busy. A major problem for all work in the deep
sea is the poor support for systematics, which means
that experts in many faunal groups are few.
Common taxonomic characteristics
There are common links across reducing habitats. In
some cases, relationships can be traced among habitats,
although phylogenetic work is limited to a few taxa.
One can imagine that adaptations to sulphide-rich
habitats provide entry to similar habitats throughout
the ocean. However, extreme conditions have invoked
extreme adaptations which often separate taxa at fairly
high levels from their nearest relatives in the deep sea.
Thus, most animals found at hydrothermal vents are
newly-described species, as are about half the genera
(Tunnicliffe et al., 1996); the few species that are also
found in the “normal” deep sea tend to live on the
periphery of the vents or occur as occasional visitors.
Many endemic taxa occur at vents – these animals
are presently not known elsewhere. With the discovery
of seep and whale-bone faunas, the similarity to vent
animals was immediately recognized and the concept
that these habitats might serve as dispersal stepping
stones for vents was broached (Hecker, 1985; Smith
et al., 1989). In fact, while few species are held in
common (Smith and Baco, 1998), many genera and
families are shared.
Tunnicliffe et al. (1996) and Sibuet and Olu (1998)
have discussed the common elements of the faunas
of vents, seeps and deep-sea organic remains. Relationships among these faunas appear very restricted at
the level of species; however, at the level of genera
and families the similarity reflects some evolutionary
connection through common ancestors. Certain taxa reoccur: vesicomyid clams, bathymodiolid mussels and
vestimentiferans – all of which contain symbionts –
are the most famous examples. To date, the species
found in common among vents, seeps and carcasses
number less than five. The polychaete Nereis and
the squat lobster Munidopsis are also present in lowoxygen basins. The former is well-known for its ability
to tolerate low oxygen concentrations in intertidal
sediments, and the latter is wide-ranging throughout the
deep sea.
Vestimentiferans and pogonophorans
Vestimentiferans (= obturate pogonophorans) range
in length from millimetres to metres. They have
been considered to be annelids, pogonophorans or a
separate phylum, depending upon authority. Southward
(1988) has provided extensive descriptions of the
anatomy and functions of these animals. Usually, a