REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
99
2 and 20 kg m
−2 . The values exceed average deepsea estimates by four orders of magnitude (Hashimoto
et al., 1989; Olu et al., 1996a).
A few time-series studies have noted growth characteristics of some species. Large individuals of
Calyptogena magnifica on the East Pacific Rise are
estimated to reach ages between 20 and 40 years,
and testify to a relative longevity of some vent sites
(Lutz et al., 1988). Rates are highly site-dependent,
the highest values occurring in areas of high fluid flux
and sulphide delivery. Mussels can grow in length by
a centimetre a year (Roux et al., 1989). Tube worms
from vents are most impressive: species colonizing new
vents with optimal conditions grow almost a metre per
year (Lutz et al., 1994; Tunnicliffe et al., 1997). On
the other hand, the longevity of seep vestimentiferans
is equally impressive: Bergquist et al. (2000) have
estimated ages over 200 years for seep Lamellibrachia,
although variability in growth rate is very high.
Benthic fauna of low-oxygen basins has been effectively sampled in many areas using box corers. While
the literature generally reports diminishing biomass
as oxygen content decreases, several studies report
enhanced biological activity in the region of the oxygen
minimum zone on the continental shelf in the eastern
Pacific (Diaz and Rosenberg, 1995). Organisms here
benefit from high flux of relatively undegraded organic
matter, so that abundance and biomass peak on the
edges of the zone of oxygen depletion.
Diversity and succession
Diversity of these communities is extremely low
compared to that in deep-sea sediments (Grassle
et al., 1985); overall, there are only several hundred
species recorded at all vents and seeps (Sibuet and
Olu, 1998; Tunnicliffe et al., 1998), compared to
the millions suspected in the deep sea (Grassle and
Maciolek, 1992). Diversity in mussel clumps from
the Mid-Atlantic Ridge is lower than in intertidal
mussel assemblages from Alaska (Van Dover, 2000).
Species abundances at vents and seeps vary around
the world. Diversification of several vent groups (such
as copepods) has occurred in the equatorial eastern
Pacific region. Many possible factors influencing the
accumulation of species need examination. Seafloor
spreading rate may exert a fundamental control on
abundance of vents on a ridge, the stability of those
vents and their variety – all of which likely are reflected
in diversity (Juniper and Tunnicliffe, 1997). Factors
that influence speciation at hot vents are now under
study. Within regions there is surprisingly good gene
flow for most vent species studied (Jollivet, 1996;
Vrijenhoek, 1997), although range disjunctions clearly
indicate major barriers between regions.
Hydrothermal vent communities live in an unstable
habitat, because of their close links to a fluctuating fluid
supply and their location atop the most tectonically
and volcanically active feature on the Earth. Volcanic
eruptions have been recorded by seafloor acoustic
hydrophones (Fox et al., 1995) three times on Juan
de Fuca Ridge from 1993 to 1998. At one site,
microbial colonization was immediate, and invertebrate
larvae were recruited from a distance of many tens of
kilometres within a year; in two years, tube worms
one metre long dominated a diverse community in
high-sulphide conditions (Tunnicliffe et al., 1997).
In three years, the system was dead. Shank et al.
(1998) described a sequential pattern of colonization
on the northern East Pacific Rise, after pre-existing
communities were overrun with lava. The tube worm
Tevnia was first to appear, then Riftia, and the mussel
Bathymodiolus thermophilus arrived three years later.
This mussel competes for space and vent water with
Riftia, and can displace the vestimentiferan, likely by
its ability to alter local water chemistry and redirect
water flow (Hessler et al., 1985; Johnson et al., 1994;
Desbruy` eres, 1998). On the southern East Pacific Rise
where the ultrafast spreading is fed by high magma
supply, vast areas of apparently new venting were
in early stages of succession when observed; indeed,
different ages of flows could be distinguished by the
associated fauna (Geistdoerfer et al., 1995).
Trophic relations
Tracking trophic connections in these remote communities is not easy. Interpretations from feeding
structures, gut contents, behavioural observations and
comparisons to nearest relatives can slowly describe
the food web. Chemolithoautotrophs supply the basic
organic carbon in the form of symbionts, microbial
mats and suspended cells. The relative importance
of photosynthetic carbon in different sites is not yet
known. For most vents and seeps, the major consumer
biomass lies in the symbiont hosts. On the MidAtlantic Ridge shrimps predominate, thus shifting the
trophic pyramid to the grazers, although a symbiotic
relationship is proposed through epibionts on some
species (Polz et al., 1998). Biofilms and filamentous
99
2 and 20 kg m
−2 . The values exceed average deepsea estimates by four orders of magnitude (Hashimoto
et al., 1989; Olu et al., 1996a).
A few time-series studies have noted growth characteristics of some species. Large individuals of
Calyptogena magnifica on the East Pacific Rise are
estimated to reach ages between 20 and 40 years,
and testify to a relative longevity of some vent sites
(Lutz et al., 1988). Rates are highly site-dependent,
the highest values occurring in areas of high fluid flux
and sulphide delivery. Mussels can grow in length by
a centimetre a year (Roux et al., 1989). Tube worms
from vents are most impressive: species colonizing new
vents with optimal conditions grow almost a metre per
year (Lutz et al., 1994; Tunnicliffe et al., 1997). On
the other hand, the longevity of seep vestimentiferans
is equally impressive: Bergquist et al. (2000) have
estimated ages over 200 years for seep Lamellibrachia,
although variability in growth rate is very high.
Benthic fauna of low-oxygen basins has been effectively sampled in many areas using box corers. While
the literature generally reports diminishing biomass
as oxygen content decreases, several studies report
enhanced biological activity in the region of the oxygen
minimum zone on the continental shelf in the eastern
Pacific (Diaz and Rosenberg, 1995). Organisms here
benefit from high flux of relatively undegraded organic
matter, so that abundance and biomass peak on the
edges of the zone of oxygen depletion.
Diversity and succession
Diversity of these communities is extremely low
compared to that in deep-sea sediments (Grassle
et al., 1985); overall, there are only several hundred
species recorded at all vents and seeps (Sibuet and
Olu, 1998; Tunnicliffe et al., 1998), compared to
the millions suspected in the deep sea (Grassle and
Maciolek, 1992). Diversity in mussel clumps from
the Mid-Atlantic Ridge is lower than in intertidal
mussel assemblages from Alaska (Van Dover, 2000).
Species abundances at vents and seeps vary around
the world. Diversification of several vent groups (such
as copepods) has occurred in the equatorial eastern
Pacific region. Many possible factors influencing the
accumulation of species need examination. Seafloor
spreading rate may exert a fundamental control on
abundance of vents on a ridge, the stability of those
vents and their variety – all of which likely are reflected
in diversity (Juniper and Tunnicliffe, 1997). Factors
that influence speciation at hot vents are now under
study. Within regions there is surprisingly good gene
flow for most vent species studied (Jollivet, 1996;
Vrijenhoek, 1997), although range disjunctions clearly
indicate major barriers between regions.
Hydrothermal vent communities live in an unstable
habitat, because of their close links to a fluctuating fluid
supply and their location atop the most tectonically
and volcanically active feature on the Earth. Volcanic
eruptions have been recorded by seafloor acoustic
hydrophones (Fox et al., 1995) three times on Juan
de Fuca Ridge from 1993 to 1998. At one site,
microbial colonization was immediate, and invertebrate
larvae were recruited from a distance of many tens of
kilometres within a year; in two years, tube worms
one metre long dominated a diverse community in
high-sulphide conditions (Tunnicliffe et al., 1997).
In three years, the system was dead. Shank et al.
(1998) described a sequential pattern of colonization
on the northern East Pacific Rise, after pre-existing
communities were overrun with lava. The tube worm
Tevnia was first to appear, then Riftia, and the mussel
Bathymodiolus thermophilus arrived three years later.
This mussel competes for space and vent water with
Riftia, and can displace the vestimentiferan, likely by
its ability to alter local water chemistry and redirect
water flow (Hessler et al., 1985; Johnson et al., 1994;
Desbruy` eres, 1998). On the southern East Pacific Rise
where the ultrafast spreading is fed by high magma
supply, vast areas of apparently new venting were
in early stages of succession when observed; indeed,
different ages of flows could be distinguished by the
associated fauna (Geistdoerfer et al., 1995).
Trophic relations
Tracking trophic connections in these remote communities is not easy. Interpretations from feeding
structures, gut contents, behavioural observations and
comparisons to nearest relatives can slowly describe
the food web. Chemolithoautotrophs supply the basic
organic carbon in the form of symbionts, microbial
mats and suspended cells. The relative importance
of photosynthetic carbon in different sites is not yet
known. For most vents and seeps, the major consumer
biomass lies in the symbiont hosts. On the MidAtlantic Ridge shrimps predominate, thus shifting the
trophic pyramid to the grazers, although a symbiotic
relationship is proposed through epibionts on some
species (Polz et al., 1998). Biofilms and filamentous