98
Verena TUNNICLIFFE et al.
Chorocaris chacei and Rimicaris exoculata encourage growth of filamentous sulphur bacteria on
specialized mouthparts, which they then crop (Gebruk
et al., 1993, 2000; Segonzac et al., 1993). Other species
of shrimp either prey on Rimicaris, or are opportunists
that can utilize organic matter from other shrimps,
mussels and encrusting bacteria (Gebruk et al., 2000).
All the shrimps on the Mid-Atlantic Ridge have
an extended larval life in the water column (Herring
and Dixon, 1998), where they build up reserves of
photosynthetic origin, including essential fatty acids.
These reserves are used up during metamorphosis
to the adult stage, while the bacteriophores develop
(Gebruk et al., 2000). The shrimps are thus not wholly
driven by chemosynthesis.
Faunal relations between seeps and vents
Vent and seep animals occupy geographically separate
habitats, but many similarities in the assemblages
exist. Although few species are shared, the many
similarities among taxonomic groups indicate a strong
historical linkage even if there is not extensive
gene flow today. At most vents and seeps, biomass
is dominated by bathymodiolid mussels, vesicomyid
clams and vestimentiferan worms. Little is known
about their symbiont origins beyond their placement
in the gamma subdivision of the Proteobacteria and
their independent origins among the host groups (Distel
et al., 1988). While symbiont transmission between
generations in the clams likely occurs through the
eggs (Cary and Giovannoni, 1993), the vestimentiferan
juvenile appears to acquire symbionts during a feeding
benthic stage (Southward, 1988). The symbionts of
vent vestimentiferans are very similar – if not the
same – but are distinct from those at seeps (Feldman
et al., 1997).
Genetic studies reveal that four invertebrate groups
show close links between vent and seep species.
Vent mussels are more derived than those at seeps,
suggesting a later invasion of vents perhaps from
seeps (Craddock et al., 1995). Peek et al. (1997)
have described a more complicated situation for clam
species complexes from vents and seeps. While each
clade is limited to a single habitat, there is no clear
overall ancestral habitat. Baco et al. (1999) have
extended the vesicomyid work, to show that specimens
from whale skeletons off California relate to three
different clades found at both vents and seeps. The
vestimentiferan lineage, however, could be a recent
diversification. Lamellibrachia appears to be the basal
taxon, suggesting that the tube worms at vents derive
from seep ancestors (Black et al., 1997). McHugh
(1997) has postulated that both vestimentiferans and
pogonophorans belong within the Annelida; Rouse
and Fauchald (1997) have suggested the family name
Siboglinidae within the Polychaeta.
Among the gastropods, there are many families that
are found at vents and seeps. Shared groups such as
the Neolepetopsidae, Provannidae and Pyropeltidae are
currently found nowhere else, and surely have close
evolutionary ties (Tunnicliffe et al., 1998). However,
some gastropod groups may have been sustained
at vents long after their extinction elsewhere. The
Neomphalina, for instance, are endemic at vents and
show a deep phylogenetic split from their closest
relatives when examined with molecular techniques;
they may represent a relic group which has found a
refuge at vents (McArthur and Koop, 1999).
COMMUNITY CHARACTERISTICS
Biomass and growth
There are few comprehensive estimates of production
and biomass in reducing environments. Quantitative
sampling in the deep sea – particularly on hard
substrata – remains difficult. Karl (1995) has provided
an integrated review of microbial studies at vents. In
diffuse waters, counts of microbes from 10
5 to 10
6 ml
−1
are one to two orders of magnitude above normal
deep-sea water; both water and surface abundances
are patchy and variable. Difficulties with measurement
of microbial production include providing the right
conditions for microbial growth, and interpreting the
results. There are many species of heterotrophic
bacteria; hence, many measurements can often yield
only overall figures for the growth of a mixotrophic
community.
The highly patchy and irregular distributions of
vent assemblages result in a wide range of biomass
estimates: 2 to 15 kg m
−2 (wet weight) for primary
consumers and 0.03 to 0.5 kg m
−2 for detritivores and
carnivores (summarized in Chevaldonn´ e and Jollivet,
1993) for vents on the northern East Pacific Rise.
Local Riftia populations exceed 50 kg m
−2 (Fustec
et al., 1988). At seeps, most of the biomass is
present as bivalves; upper estimates range between
Verena TUNNICLIFFE et al.
Chorocaris chacei and Rimicaris exoculata encourage growth of filamentous sulphur bacteria on
specialized mouthparts, which they then crop (Gebruk
et al., 1993, 2000; Segonzac et al., 1993). Other species
of shrimp either prey on Rimicaris, or are opportunists
that can utilize organic matter from other shrimps,
mussels and encrusting bacteria (Gebruk et al., 2000).
All the shrimps on the Mid-Atlantic Ridge have
an extended larval life in the water column (Herring
and Dixon, 1998), where they build up reserves of
photosynthetic origin, including essential fatty acids.
These reserves are used up during metamorphosis
to the adult stage, while the bacteriophores develop
(Gebruk et al., 2000). The shrimps are thus not wholly
driven by chemosynthesis.
Faunal relations between seeps and vents
Vent and seep animals occupy geographically separate
habitats, but many similarities in the assemblages
exist. Although few species are shared, the many
similarities among taxonomic groups indicate a strong
historical linkage even if there is not extensive
gene flow today. At most vents and seeps, biomass
is dominated by bathymodiolid mussels, vesicomyid
clams and vestimentiferan worms. Little is known
about their symbiont origins beyond their placement
in the gamma subdivision of the Proteobacteria and
their independent origins among the host groups (Distel
et al., 1988). While symbiont transmission between
generations in the clams likely occurs through the
eggs (Cary and Giovannoni, 1993), the vestimentiferan
juvenile appears to acquire symbionts during a feeding
benthic stage (Southward, 1988). The symbionts of
vent vestimentiferans are very similar – if not the
same – but are distinct from those at seeps (Feldman
et al., 1997).
Genetic studies reveal that four invertebrate groups
show close links between vent and seep species.
Vent mussels are more derived than those at seeps,
suggesting a later invasion of vents perhaps from
seeps (Craddock et al., 1995). Peek et al. (1997)
have described a more complicated situation for clam
species complexes from vents and seeps. While each
clade is limited to a single habitat, there is no clear
overall ancestral habitat. Baco et al. (1999) have
extended the vesicomyid work, to show that specimens
from whale skeletons off California relate to three
different clades found at both vents and seeps. The
vestimentiferan lineage, however, could be a recent
diversification. Lamellibrachia appears to be the basal
taxon, suggesting that the tube worms at vents derive
from seep ancestors (Black et al., 1997). McHugh
(1997) has postulated that both vestimentiferans and
pogonophorans belong within the Annelida; Rouse
and Fauchald (1997) have suggested the family name
Siboglinidae within the Polychaeta.
Among the gastropods, there are many families that
are found at vents and seeps. Shared groups such as
the Neolepetopsidae, Provannidae and Pyropeltidae are
currently found nowhere else, and surely have close
evolutionary ties (Tunnicliffe et al., 1998). However,
some gastropod groups may have been sustained
at vents long after their extinction elsewhere. The
Neomphalina, for instance, are endemic at vents and
show a deep phylogenetic split from their closest
relatives when examined with molecular techniques;
they may represent a relic group which has found a
refuge at vents (McArthur and Koop, 1999).
COMMUNITY CHARACTERISTICS
Biomass and growth
There are few comprehensive estimates of production
and biomass in reducing environments. Quantitative
sampling in the deep sea – particularly on hard
substrata – remains difficult. Karl (1995) has provided
an integrated review of microbial studies at vents. In
diffuse waters, counts of microbes from 10
5 to 10
6 ml
−1
are one to two orders of magnitude above normal
deep-sea water; both water and surface abundances
are patchy and variable. Difficulties with measurement
of microbial production include providing the right
conditions for microbial growth, and interpreting the
results. There are many species of heterotrophic
bacteria; hence, many measurements can often yield
only overall figures for the growth of a mixotrophic
community.
The highly patchy and irregular distributions of
vent assemblages result in a wide range of biomass
estimates: 2 to 15 kg m
−2 (wet weight) for primary
consumers and 0.03 to 0.5 kg m
−2 for detritivores and
carnivores (summarized in Chevaldonn´ e and Jollivet,
1993) for vents on the northern East Pacific Rise.
Local Riftia populations exceed 50 kg m
−2 (Fustec
et al., 1988). At seeps, most of the biomass is
present as bivalves; upper estimates range between