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Author Posting. © Oceanography Society, 2007. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 20, 1 (2007): 30-41. The discovery of hydrothermal vents and the unique, often endemic fauna that inhabit them represents one of the most extraordinary scientific discoveries of the latter twentieth century. Not surprisingly, after just 30 years of study of these remarkable—and extremely remote—systems, advances in understanding the animals and microbial communities living around hydrothermal vents seem to occur with every fresh expedition to the seafloor. On average, two new species are described each month—a rate of discovery that has been sustained over the past 25–30 years. Furthermore, the physical, geological, and geochemical features of each part of the ridge system and its associated hydrothermal-vent structures appear to dictate which novel biological species can live where. Only 10 percent of the ridge system has been explored for hydrothermal activity to date (Baker and German, 2004), yet we find different diversity patterns in that small fraction. While it is well known that species composition varies along discrete segments of the global ridge system, this “biogeographic puzzle” has more pieces missing than pieces in place. E. Ramirez-Llodra is supported by the ChEss-Census of Marine Life program (A.P. Sloan Foundation), which is kindly acknowledged. C.R. German also acknowledges support from ChEss- Census of Marine Life and further support from the Natural Environment Research Council (UK) and from the US National Science Foundation (NSF) and National Oceanic and Atmospheric Administration (NOAA). T. Shank acknowledges support from NSF, the US National Aeronautic and Space Administration Astrobiology Program, NOAA-Ocean Exploration, and the Deep-Ocean Exploration Institute at the Woods Hole Oceanographic Institution.
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Oceanography Vol. 20, No. 1
30
Challenger Deep
Unusual life forms may have
evolved under conditions of
extreme pressure in this 11,000-
meter-deep trench, the deepest
part of the world’s oceans.
New Zealand
This region has a full spectrum of
habitats supporting seafl oor life
(hydrothermal vents, cold seeps,
whale carcasses, and wood from
shipwrecks and trees) in close
proximity. How have species
evolved in these diverse settings?
Chile Rise
This region has a variety of
chemosynthetic habitats and
geological features in close prox-
imity. How do seafl oor popula-
tions diverge or converge at this
triple junction on the “highway”
of mid-ocean ridges?
Soon after animal communities were discovered
around seafl oor hydrothermal vents in 1977, sci-
entists found that vents in various regions are
populated by distinct animal species. Scien-
tists have been sorting clues to explain how
seafl oor populations are related and how
they evolved and diverged over Earths
history. Scientists today recognize dis-
tinct assemblages of animal species
in six major seafl oor regions (colored
dots) along the system of volcanic
mountains and deep-sea trenches
that form the borders of Earth’s
tectonic plates. But unexplored
ocean regions remain critical
missing pieces for assembling
the full evolutionary puzzle.
Missing
Pieces
E. Paul Oberlander
On the Seaoor, Dierent Species
Thrive in Dierent Regions
South Atlantic
Powerful currents and huge
seafl oor chasms (fracture zones)
may act as barriers blocking the
dispersal of vent larvae and dis-
connecting vent populations in
the North and South Atlantic.
Caribbean
In this region, methane seeping
from the seafl oor also supports
animal communities. Did
animals migrate between “cold
seeps” and nearby hot vents over
evolutionary history?
Arctic Ocean
The Arctic Ocean has never
had deep connections with
other major oceans. It may
harbor fundamentally
different vent animals that
evolved in isolation over the
past 25 million years.
Southern Ocean
The Drake Passage may act as a
key link or bottleneck for larval
dispersal between the Atlantic
and Pacifi c. Whale carcasses and
shipwrecks (such as Shackletons
Endurance) may offer refuges or
stepping-stones between vents.
Central Indian vent com-
munities are populated by
Western Pacifi c-type fauna, but
also have North Atlantic-type
shrimp species.
Northeast Pacifi c vent
communities are domi-
nated by “bushes” of skinny tube-
worms called Ridgea piscesae.
Western Pacifi c vent com-
munities are dominated by
barnacles and limpets, as well as
hairy gastropods, shown above.
Shallow Atlantic vents
(800-1700-meter depths)
support dense clusters of mussels
on black smoker chimneys.
Eastern Pacifi c vent com-
munities are dominated
by tall, fat tubeworms called
Riftia pachyptila.
Deep Atlantic vent com-
munities (2500-3650-me-
ter depths) are dominated by
swarms of shrimp called Rimica-
ris exoculata.
The discovery of hydrothermal vents and the unique, often endem-
ic fauna that inhabit them represents one of the most extraordinary
scientific discoveries of the latter twentieth century. Not surprising-
ly, after just 30 years of study of these remarkable—and extremely
remote—systems, advances in understanding the animals and mi-
crobial communities living around hydrothermal vents seem to
occur with every fresh expedition to the seafloor. On average, two
new species are described each month—a rate of discovery that has
been sustained over the past 25–30 years (Van Dover et al., 2002;
Fisher et al., this issue). Furthermore, the physical, geological, and
geochemical features of each part of the ridge system and its asso-
ciated hydrothermal-vent structures appear to dictate which nov-
el biological species can live where. Only 10 percent of the ridge
system has been explored for hydrothermal activity to date (Baker
and German, 2004), yet we find different diversity patterns in that
small fraction. While it is well known that species composition var-
ies along discrete segments of the global ridge system, this bio-
geographic puzzle” has more pieces missing than pieces in place
(Figure 1, Table 1).
In this paper, we start with a general picture of the global ridge
system and hydrothermal environments, then continue with a de-
scription of known biodiversity—including physiological adapta-
tions—and species distribution, which leads into a discussion of
the factors that might drive observed biogeography patterns. We
conclude with a look toward the future, describing the main efforts
being made to fill the gaps in our knowledge of vent biogeography.
S P E C I A L I S S U E F E AT U R E
irty Years of Discovery and Investigations
B Y E V A R A M I R E Z L L O D R A ,
T I M O T H Y M . S H A N K , A N D
C H R I S T O P H E R R . G E R M A N
Oceanography Vol. 20, No. 1
30
Biodiversity and Biogeography
of Hydrothermal Vent Species
is article has been published in Oceanography, Volume 20, Number 1, a quarterly journal of e Oceanography Soci-
ety. Copyright 2007 by e Oceanography Society. All rights reserved. Permission is granted to copy this article for use in
teaching and research. Republication, systemmatic reproduction, or collective redistirbution of any portion of this article
by photocopy machine, reposting, or other means is permitted only with the approval of e Oceanography Society.
Send all correspondence to: info@tos.org or  e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA.
Oceanography March 2007
31
Challenger Deep
Unusual life forms may have
evolved under conditions of
extreme pressure in this 11,000-
meter-deep trench, the deepest
part of the world’s oceans.
New Zealand
This region has a full spectrum of
habitats supporting seafl oor life
(hydrothermal vents, cold seeps,
whale carcasses, and wood from
shipwrecks and trees) in close
proximity. How have species
evolved in these diverse settings?
Chile Rise
This region has a variety of
chemosynthetic habitats and
geological features in close prox-
imity. How do seafl oor popula-
tions diverge or converge at this
triple junction on the “highway”
of mid-ocean ridges?
Soon after animal communities were discovered
around seafl oor hydrothermal vents in 1977, sci-
entists found that vents in various regions are
populated by distinct animal species. Scien-
tists have been sorting clues to explain how
seafl oor populations are related and how
they evolved and diverged over Earths
history. Scientists today recognize dis-
tinct assemblages of animal species
in six major seafl oor regions (colored
dots) along the system of volcanic
mountains and deep-sea trenches
that form the borders of Earth’s
tectonic plates. But unexplored
ocean regions remain critical
missing pieces for assembling
the full evolutionary puzzle.
Missing
Pieces
E. Paul Oberlander
On the Seaoor, Dierent Species
Thrive in Dierent Regions
South Atlantic
Powerful currents and huge
seafl oor chasms (fracture zones)
may act as barriers blocking the
dispersal of vent larvae and dis-
connecting vent populations in
the North and South Atlantic.
Caribbean
In this region, methane seeping
from the seafl oor also supports
animal communities. Did
animals migrate between “cold
seeps” and nearby hot vents over
evolutionary history?
Arctic Ocean
The Arctic Ocean has never
had deep connections with
other major oceans. It may
harbor fundamentally
different vent animals that
evolved in isolation over the
past 25 million years.
Southern Ocean
The Drake Passage may act as a
key link or bottleneck for larval
dispersal between the Atlantic
and Pacifi c. Whale carcasses and
shipwrecks (such as Shackletons
Endurance) may offer refuges or
stepping-stones between vents.
Central Indian vent com-
munities are populated by
Western Pacifi c-type fauna, but
also have North Atlantic-type
shrimp species.
Northeast Pacifi c vent
communities are domi-
nated by “bushes” of skinny tube-
worms called Ridgea piscesae.
Western Pacifi c vent com-
munities are dominated by
barnacles and limpets, as well as
hairy gastropods, shown above.
Shallow Atlantic vents
(800-1700-meter depths)
support dense clusters of mussels
on black smoker chimneys.
Eastern Pacifi c vent com-
munities are dominated
by tall, fat tubeworms called
Riftia pachyptila.
Deep Atlantic vent com-
munities (2500-3650-me-
ter depths) are dominated by
swarms of shrimp called Rimica-
ris exoculata.
Figure 1. Schematic illustration of the
global mid-ocean ridge system. Scientists
today recognize distinct assemblages of
animal species in six major seafloor regions
(colored dots) along the system of volca-
nic mountains and deep-sea trenches that
form the borders of Earth’s tectonic plates.
However, unexplored ocean regions remain
critical missing pieces for assembling the
full evolutionary puzzle. From Shank (2004)
Oceanography March 2007
31
Oceanography Vol. 20, No. 1
32
ABIOTIC SETUP OF THE
VENT HABITAT: PHYSICAL,
GEOLO GICAL, AND
GEOCHE MICAL PROPERTIE S
Today, satellite technology provides a
clear visualization of the globe-encircling
mid-ocean ridge. It is here, at seafloor-
spreading centers, that lava wells up from
Earths interior to generate fresh oce-
anic crust (see Langmuir and Forsyth,
this issue). While a small proportion
of these volcanic systems are found in
back-arc basins close to subduction
zones (see Martinez et al., this issue), the
vast majority of all Earth’s mid-ocean
ridges form a single, continuous, globe-
encircling volcanic chain that is roughly
60,000 km in length. Mid-ocean ridges
typically lie at around 2000- to 5000-m
depth and therefore represent environ-
ments that experience high pressures,
complete darkness, and ambient tem-
peratures of only ~ 2°C.
Table 1. Dominant fauna of main proposed biogeographical provinces.
Biogeographical Province and Depth Dominating Fauna
Azores (shallow north Atlantic, 800-1700 m) Bathymodiolid mussels, amphipods, and caridean shrimp
Mid-Atlantic Ridge between Azores Triple Junction
and Equator (deep north Atlantic 2500-3650 m)
Caridean shrimp—mainly Rimicaris exoculata—and bathymodiolid mussels
South Mid-Atlantic Ridge Caridean shrimp, bathymodiolid mussels, and clams
East Pacific Rise and Galápagos Rift Vestimentiferan tubeworms—mainly Riftia pachyptila—and bathymodiolid
mussels, vesicomyid clams, alvinellid polychaetes, amphipods, and crabs
Northeast Pacific Vestimentiferan tubeworms excluding Riftiidae, polychaetes, and gastropods
Western Pacific Barnacles, limpets, bathymodiolid mussels, “hairy” gastropod, vesicomyid clams,
and shrimp
Central Indian Ridge Caridean shrimp
Rimicaris kairei, and mussels, “scaly” gastropods, and anemones
Spreading rates for mid-ocean
ridges—the speed at which two tec-
tonic plates are being pulled apart from
each other—vary across the whole sys-
tem. Ridges can be classified accord-
ing to their spreading rates into ultra-
slow spreading (< 20 mm yr
-1
) such
as the Arctic Gakkel Ridge (see Snow
and Edmonds, this issue), slow spread-
ing (20–50 mm yr
-1
) such as the Mid-
Atlantic Ridge, intermediate spreading
(50–90 mm yr
-1
) such as the Central
Indian Ridge, and fast (90–130 mm yr
-1
)
to superfast (130–170 mm yr
-1
) spread-
ing such as the East Pacific Rise. Spread-
ing rates are important in shaping ridge
morphology (MacDonald et al., 1991)
and, as a consequence, the hydrodynam-
ics of the habitat. At slow-spreading
rates, the walls of the ridge axis are
separated by a deep (1–3 km) and wide
(5–15 km) rift valley that constrains
dispersing hydrothermal plumes. Some
of the largest and longest-lived hydro-
thermal vents discovered thus far occur
along the slow-spreading Mid-Atlantic
Ridge. In contrast, fast-spreading cen-
ters exhibit shallow and narrow (order
of tens of meters) summit calderas
where hydrothermal plumes are not
constrained. Along the fastest sections
of the East Pacific Rise, vent sites are so
geographically close that they can form
a continuum along the ridge axis. These
topographic and physical characteristics
have the potential to affect the patterns
of faunal distribution (see below). The
western Pacific Ocean seafloor is char-
acterized by a complex system of back-
arc basins and volcanic arcs with active
hydrothermal venting. These back-arc
basins are isolated spreading centers that
have been active for less than 10 million
years—a short geological time com-
pared to the ages of mid-ocean ridges
(Hessler and Lonsdale, 1991). Their iso-
Oceanography March 2007
33
EVA RAMIREZLLODRA (ezr@icm.csic.es)
is Research Scientist, National Ocean-
ography Centre, Southampton, UK, and
Institut de Ciències del Mar (CMIMA-CSIC),
Barcelona, Spain. TIMOTHY M. SHANK
is Associate Scientist, Biology Department,
Woods Hole Oceanographic Institution,
Woods Hole, MA, USA. CHRISTOPHER
R. GERMAN
is Senior Scientist and Chief
Scientist for Deep Submergence, Geol-
ogy and Geophysics Department, Woods
Hole Oceanographic Institution, Woods
Hole, MA, USA.
lation—both from one another and from
mid-ocean ridges—and their relatively
short active geological ages make back
arcs particularly interesting habitats for
biogeographic and gene-flow analyses
(Desbruyères et al., 2007; see below).
Hydrothermal circulation occurs at
mid-ocean ridges when dense, cold sea-
water percolates downward through frac-
tured oceanic crust near the ridge crest
(Figure 2). The vent fluid is geothermally
heated close to the magma chamber that
feeds the ridge, reaching temperatures
that can exceed 400°C. The fluid is also
chemically modified, losing all dissolved
oxygen and accumulating high con-
centrations of dissolved reduced gases
such as methane and hydrogen sulfide).
They are also strongly acidic (pH 2–3)
compared to the near-neutral character
(~ pH 8) of the deep oceans and are rich
in numerous metals—dominantly iron,
manganese, copper, and zinc, but also
precious metals such as silver, gold, and
platinum, as well as those often thought
of as highly toxic to life (e.g., cadmium,
mercury, arsenic and lead). The buoyant,
superheated fluid is expelled into the wa-
ter column through polymetallic sulfide
chimneys or “black smokers that can
tower tens of meters above the seafloor
(Figure 3). While approximately half of
all hydrothermal fluids are currently be-
lieved to be discharged from the seafloor
in this spectacular black-smoker form,
about an equal amount mixes with cold
seawater and percolates down into the
highly fractured seafloor as more dilute
warm waters. The latter have precipitated
their mineral load but, importantly, still
contain high concentrations of dissolved
methane and hydrogen sulfide, which are
emitted from the seabed in this diffuse-
flow” form, typically at temperatures that
are measured in tens of degrees Celsius
(further detailed information on hydro-
thermal vent chemistry and functioning,
and associated mineral deposits, can be
found in an article by Tivey in this issue).
It is these vent fluids that provide the
necessary energy, in the form of reduced
chemicals, for the development of the
lush faunal communities found at vents.
The hydrothermal trophic web is based
in the production of chemoautotrophic
Figure 2. Schematic illustra-
tion of hydrothermal fluid cir-
culation beneath the seafloor
at mid-ocean ridge crests. From
German and Von Damm (2004)
Oceanography Vol. 20, No. 1
34
Figure 3. Black smoker from the North
Mid-Atlantic Ridge.
Photo credit:
Deutsche-Ridge Schwerpunkt Program
and the MARUM-Quest ROV
microbes (Cavanaugh, 1983; Jannasch,
1984) that use reduced compounds from
the vent fluids to fix carbon dioxide
from seawater. The microbes are found
free-living as well as in highly success-
ful symbiosis with many of the macro-
invertebrates inhabiting the vent habitat
and they are responsible for the very high
production at hydrothermal vents.
BIO GEOGR APHY OF
HYDROTHERMAL VENTS:
A KNOWLED GE PUZZLE
UNDER CONSTRUCTION
Biodiversity and Adaptations
A sound knowledge of an ecosystems
species composition is essential for any
subsequent biogeographic investigations
and to understand the factors driving
distribution patterns. The dawn of bio-
diversity studies in hydrothermal vents
takes us back to nine black-and-white
photographs of large white bivalves on
black basalt, taken by the Deep-Tow
Camera System on May 29, 1976, on the
Galápagos Spreading Center (Lonsdale,
1977). A year later, during Alvin dives re-
visiting the region, geologist Jack Corliss
gave astonishing accounts of the first
observations of giant tubeworms later
described as Riftia pachyptila (Corliss et
al., 1979) and thick beds of mussels as-
sembled around seafloor openings emit-
ting diffuse vent fluids. Only 30 years
have passed since those initial discover-
ies; we now know of more than 550 vent
species and their composition and dis-
tribution at more than 100 vent sites
along the global mid-ocean ridge system
(Desbruyères et al., 2006).
The first impression when looking at
a vent field is the profusion of life, with
high densities and biomass of exotic ani-
mals (Figure 4). This bounty explains
why hydrothermal vents are often re-
ferred to as oases of the deep. While
biomass is extremely high relative to
the surrounding deep sea, biodiversity
at vents is low, especially in relation to
the high biodiversity of non-chemosyn-
thetic, deep-sea benthos (Grassle and
Macioleck, 1992). This relationship is
true for all groups, from meio- and mac-
rofauna found on oceanic basalt and sed-
iments highly modified by hydrothermal
fluid to macro- and megafauna living
on mussel beds or tubeworm fields (Van
Dover, 2000). The low diversity and high
densities of individuals in vent commu-
nities is typical of habitats with high en-
ergy availability and environmental con-
ditions with high or low values within
their range, such as very wide tempera-
ture ranges or high levels of toxic chemi-
cals. The most speciose phyla found at
hydrothermal vents are the arthropods,
followed by the molluscs and the anne-
lids. However, our knowledge of the di-
versity and distribution of vent species is
still limited to an extremely small section
of the global ridge system, and every new
detailed survey and investigation, both
on newly discovered sites and on well-
explored ones, brings new species and
ecologies to be identified and described,
as well as new insight to the phylogeog-
raphy of vent species.
One major characteristic of vent bio-
logical communities is the high degree
(~ 85 percent) of species endemism,
with many species showing important
physiological, morphological, and eco-
logical adaptations to particular environ-
Oceanography March 2007
35
mental factors. Arguably, one of the most
remarkable vent animals is the giant
tubeworm Riftia pachyptila from the
Galápagos Rift and East Pacific Rise. This
worm and related species were first de-
scribed as a new phylum, Vestimentifera,
but have now been moved to the family
Siboglinidae in the Phylum Annelida,
Class Polychaeta (Halanych, 2005). Riftia
pachyptila lack a mouth and a diges-
tive system, and depend completely for
growth and reproduction on organic
matter produced by their endosymbiotic
chemosynthetic bacteria. These bacteria
are found densely packed in an organ
that spans the interior of the animal’s
body, called the trophosome. The tube-
worm absorbs hydrogen sulfide, carbon
dioxide, and oxygen through its well-
irrigated plume and transports them to
the trophosome via the vascular system
along with modified hemoglobin, which
transports unbound or available sulfide.
A different fascinating polychaete
from the East Pacific Rise is the Pompeii
worm, Alvinella pompejana. This poly-
chaete lives inside organic tubes that
it builds on the exterior walls of active
chimneys in close proximity to venting
fluid; in contrast to the Riftia habitat,
the chemistry of this fluid is domi-
nated by iron-bound complexes with
hydrogen sulfide (Luther et al., 2001).
The maximum temperature tolerated
by this species is a continuing subject
of debate, with values ranging between
10°C and 80°C (Gaill and Hunt, 1991;
Cary et al., 1998).
In the crustaceans, a remarkable ex-
ample of sensory adaptation is found on
the caridean shrimp Rimicaris exoculata
from the Mid-Atlantic Ridge. These
shrimp live in massive, dense, and dy-
namic aggregations (3000 per liter;
Gebruk et al., 2000) on the chimney
walls, feeding on symbiotic bacteria cul-
tivated within their enlarged gill cham-
bers, on their external carapace, and on
ingested mineral particles. The eyes”
of R. exoculata are considered to have
evolved into a broad ocular plate that
forms part of the shrimps dorsal sur-
face. The plates novel photoreceptors
apparently do not form a distinct image;
instead, these eyes” are highly sensitive
to dim light, perhaps an adaptation for
detecting radiation from the orifices of
hot black smokers and for allowing the
shrimp to orient to the chimney walls
(Van Dover et al., 1989).
Examples of metabolic, morphologi-
cal, sensory, and symbiotic adaptations
are abundant at the vent habitat, and ex-
ploration of new sites still results in the
discovery of striking animals, such as the
“scaly foot gastropod” recently found on
Figure 4. e giant tubeworm Riftia pachyptila, the mussel
Bathymodiolus, and crabs from East Pacific Rise vents.
Image courtesy of Richard A. Lutz
Oceanography Vol. 20, No. 1
36
Figure 5. e “scaly foot
gastropod” from the
Indian Ocean ridge.
Image courtesy
of Charles Fisher
the Central Indian Ridge (Figure 5). This
gastropod, still awaiting description, har-
bors thiotrophic (sulfurous) bacteria in
a greatly enlarged esophageal gland and
has an operculum modified into several
hundred aligned scales covered by thick
layers of iron sulfides, whose purpose is
unknown (Warén et al., 2003).
Known Distribution Patterns
of Vent Species
Although we have only explored a
very limited proportion of the active
hydrothermal vents that potentially exist
worldwide, the description of the fauna
from the vent sites that we do know is
already sufficient to demonstrate that
different faunal communities character-
ize different ocean basins/hydrothermal
regions (Figure 1). Biogeography is the
science that documents and explains
spatial patterns of biodiversity. The pres-
ent distribution of a species represents
(1) the historical events acting on geo-
logical time scales, or vicariance, that
have shaped its geographical range, and
(2) the dispersal potential of the species
determined by its life-history patterns,
topography, and hydrography, and acting
on ecological time scales (Tunnicliffe et
al., 1996). A number of physicochemi-
cal properties of hydrothermal vent
habitats (described above) make these
systems interesting for biogeographical
studies (Tunnicliffe et al., 1991, 1998):
(1) the species are constrained to specific,
linear habitats, (2) vent habitats are glob-
ally distributed, (3) they are discrete and
ephemeral environments, (4) there is a
high degree of endemism, (5) there are
phylogeographic relationships with other
reducing environments, such as cold
seeps on continental margins or whale
falls, and (6) identifying faunal provinces
and their boundaries allows us to rap-
idly characterize a mechanism respon-
sible for creating and/or maintaining
the province. Our present knowledge of
hydrothermal vent biology identifies six
distinct biogeographical domains char-
acterized by specific faunal assemblages:
(1) the Azores shallow Atlantic vent com-
munities (80–1700 m), (2) the fauna of
the deep Mid-Atlantic Ridge vents be-
tween the Azores Triple Junction and the
Equator, (3) the East Pacific Rise com-
munities found from 30°N to the Easter
Micro-Plate, (4) the Northeast Pacific
vent communities of the Explorer, Juan
de Fuca, and Gorda Ridges, (5) the west-
ern Pacific back-arc basin communities,
and (6) the vent fauna of the Central
Indian Ocean (Figure 1, Table 1) (Van
Dover et al., 2002; Shank, 2004).
Speciation Through Vicariance
The processes affecting biogeographic
patterns over geological time scales in-
clude vicariant events, where the move-
ment of oceanic plates plays a defining
role. Vicariance is, for example, used to
explain the formation and boundary of
the Northeast Pacific and East Pacific
Rise biogeographic provinces during the
Mid-Tertiary (~ 28 million years ago
[Ma]). Subduction of the Farallon Plate
under the North American Plate caused
the split of the Farallon-Pacific Ridge,
resulting in the separation of the Gorda-
Juan de Fuca-Explorer Ridge system,
to the north, from the East Pacific Rise,
to the south (Tunnicliffe et al., 1996).
Subsequent biological isolation permit-
Oceanography March 2007
37
ted divergent evolution of the fauna on
the now-bisected ridges, resulting in an
absence of the tubeworm Riftia pachyp-
tila, the clam Calyptogena magnifica,
and the polychaete Alvinella spp. from
Northeast Pacific vent sites, and the pro-
liferation of vestimentiferan tubeworms
on the East Pacific Rise.
Another example of a major vicari-
ant event is the closure of the Isthmus of
Panama 5 Ma and the resulting isolation
of Pacific marine fauna from the Atlantic
Ocean. Although there was a major deep-
ocean gateway in this region prior to the
closure, there were no ridge connections.
However, comparisons of cold-seep fau-
na from the Gulf of Mexico, the Oregon
margin, and the California margin indi-
cate close taxonomic similarities, suggest-
ing a dispersal pathway for seep species
between the Pacific and Atlantic Oceans
via the (now-closed) Isthmus of Panama
(Tunnicliffe et al., 1996). The only exist-
ing ridge connection between the Pacific
and the Atlantic was through a complex
mid-ocean ridge and subduction system
between South America and Antarctica,
which, in the Tertiary, could have served
as a dispersal pathway between the two
ocean basins (Tunnicliffe et al., 1996).
A similar analysis of vent fauna at the
genus level indicates a relatively close
connection between the western Pacific
back-arc and Central Indian Ridge vent
faunas as described by Desbruyères et al.,
2007. These authors suggest a potential
past dispersal pathway via the Southeast
Indian Ridge, Macquarie Ridge Complex
(south of New Zealand), and Kermadec
Arc (north of New Zealand) that would
explain these similarities. The connec-
tion via the Pacific-Antarctic ridges and
Macquarie Ridge has also been sug-
gested as a potential dispersal route for
vent siboglinid polychaetes between the
Southwest Pacific back-arc basin and the
East Pacific Rise fauna (Tunnicliffe and
Fowler, 1996; Kojima et al., 2003).
Distribution Patterns
and Ecological Processes
While tectonic dynamics affect bioge-
ography at the geological time scale, a
number of ecological and biological fac-
tors play major roles in the distribution
of species, acting within the life span of
the animal. These processes are related
to the life-history patterns of the species
and environmental factors. Marine in-
vertebrates ensure gene flow, dispersal,
and colonization through their larval
phase. The microscopic larvae of in-
vertebrates are transported passively in
the water column, and are affected by
a number of factors, both biotic (e.g.,
metabolic capacity, larval anatomy, be-
havior, swimming capabilities, and mor-
tality rate) and abiotic (e.g., deep-water
currents, geological barriers, distance
between sites, and chemical suitability of
habitats) (Cowen et al., 2000; Mullineaux
et al., 2005). The life-history traits of
marine invertebrates are very diverse,
ranging from species that produce a high
number of small eggs to species that
produce a lower number of larger eggs
(Ramirez-Llodra, 2002). Typically, lar-
vae hatching from smaller eggs develop
during a planktonic phase in which they
feed in the water column, allowing lon-
ger dispersal times but also increasing
mortality risks. In contrast, larvae hatch-
ing from larger eggs exhibit abbreviated,
lecithotrophic development, usually
feeding only on the energetic reserves
provided in the egg. These nonfeeding
larvae have, in general, lower dispersal
potentials but higher survival probabil-
ity. However, recent studies show that
the low temperature of bathyal and abys-
sal waters can result in long metabolic
life spans for larvae of vent species such
as Riftia pachyptila, allowing for long
dispersal distances (Marsh et al., 2001).
Furthermore, studies show that the em-
bryos of the vent polychaete Alvinella
pompejana can achieve long dispersal
times through developmental arrest
when dispersing in cold abyssal waters,
only completing their development upon
encountering warm water (Pradillon et
al., 2001). In hydrothermal vent species,
there are examples of all life-history pat-
terns and larval types, but the relatively
With every new discovery and investigation of
known sites and communities, our knowledge and
understanding of the diversity and functioning
of these remote and exuberant ecosystems increases,
helping us understand the processes driving the
deep sea and the global biosphere.
Oceanography Vol. 20, No. 1
38
limited data on reproduction of vent
species indicates that lecithotrophic lar-
val development is dominant (Tyler and
Young, 1999; Young, 2003). It is the in-
teractions between the life-history traits
of a species and the physical, chemical,
and geological environmental factors of
the larval and juvenile habitat that shape
the dispersal and colonization of a spe-
cies; at the same time, ridge spreading
rates, deep-ocean currents, major frac-
ture zones, and other bathymetric fea-
tures can all also play important roles in
driving biogeographic patterns.
The effect of ridge spreading rate on
larval dispersal is, potentially, directly
related to the distance between active
vent sites, their life span, and the geo-
morphology of the ridge valley (see,
e.g., Baker and German, 2004). On
slow-spreading ridges such as the Mid-
Atlantic Ridge, where the time-averaged
magmatic budget is relatively low, vent
sites may be fewer and longer lived.
Consequently, the distance between po-
tentially suitable recruitment areas may
be greater, and, certainly, the presence
of large fracture zones interspersed with
numerous smaller-scale ( 10 km) off-
sets every few tens of kilometers along
axis creates potential physical barriers to
larval dispersal. Also, the valleys of slow-
spreading ridges have steep walls that
retain the hydrothermal plume within
the valley, potentially limiting across-
axis dispersal. In contrast, magmatically
more robust, fast-spreading ridges such
as the East Pacific Rise harbor vent sites
that may be closer in space and are cer-
tainly less likely to be offset from one
another along axis. The absence of mul-
tiple ridge-crossing fractures, in particu-
lar, creates a virtual continuum of poten-
tially favorable settlement habitats along
the ridge axis that act as stepping stones
for genetic communication among
populations (or gene flow) through
dispersal and successful colonization.
Furthermore, hydrothermal plumes are
not retained within the shallow walls
of the axial East Pacific Rise summit
graben, allowing for across-axis plume
movement and potentially greater larval
dispersal (Van Dover et al., 2002).
Large fracture zones and other topo-
graphic features (e.g., seamount chains,
depressions) can play important roles in
the gene flow of vent species; the research
community is currently addressing their
effects. A recent study on genetic dif-
ferentiation of five vent annelids—Riftia
pachyptila, Tevnia jerichonana, Oasi-
sia alvinae, Alvinella pompejana, and
Branchypolynoe symmytilida—in eastern
Pacific vents revealed barriers to dispersal
affecting one or several of these species
(Hurtado et al., 2004). The 6000-m de-
pression at Hess Deep between the East
Pacific Rise and Galápagos Rift could
limit gene flow of Alvniella pompejana
(Hurtado et al., 2004) and of the am-
phipod Ventiella sulfuris (France et al.,
1992), while the 240-km Rivera Frac-
ture Zone on the northern East Pacific
Rise may be a barrier to dispersal for
O. alvinae, T. jerichonana, and B. sym-
mytilida (Hurtado et al., 2004) as well as
for V. sulfuris (France et al., 1992).
Another controlling factor of bio-
geographic patterns may be ridges in
hydrographic isolation. An example
of a potential topographic barrier to
gene flow is found in the Arctic, where
the ultraslow-spreading ridges of the
Norwegian Sea and Arctic Basin are
isolated from the Atlantic ridge system
by the Iceland hotspot and its off-axis
trace, which generates a shallow-water
sill extending across the full breadth of
the North Atlantic Ocean. Recent geo-
chemical studies indicate the existence
of abundant active vents on the Gakkel
Ridge (Edmonds et al., 2003), but these
habitats and potential associated fauna
have not yet been directly observed. A
biological investigation of the Gakkel
Ridge during the International Polar Year
(2007–2008) will provide crucial data
for understanding the evolution of fauna
that may have thrived in isolation for the
past ~ 28 million years.
In the equatorial region of the Mid-
Atlantic Ridge, the 60-million-year-old
Chain and Romanche Fracture Zones
represent formidable geological features
(4-km high and 935 km of ridge offset)
that greatly affect both the linearity of
the ridge system and large-scale ocean
circulation in this region. It has been sug-
gested that these major facture zones re-
sult in a geological barrier to the disper-
sal of vent species along the Mid-Atlantic
Ridge axis, isolating certain species north
and south of the equator, while others,
such as the shrimp Rimicaris exoculata
whose high pelagic larval and juvenile
dispersal capacity allows it to exploit
phytodetritus in the water column, may
not be affected (Shank et al., 1998).
These hypotheses are being tested by cur-
rent research programs (see below).
Along with their potential as bar-
riers to north-south, along-axis gene
flow, the Mid-Atlantic Ridge fracture
zones may serve as a conduit for lar-
val transport through deep-water cur-
rents from west to east in the Atlantic
Ocean. The North Atlantic Deep Water
(NADW) flows south along the east
Oceanography March 2007
39
coasts of North and South America as
far as the equator before being deflected
east and crossing the Mid-Atlantic Ridge
through the fracture zones (Messias et
al., 1999). These fracture zones could be
pathways for migrants between chemo-
synthetic sites in the Gulf of Mexico to
similar habitats in the Gulf of Guinea.
Moreover, recent physical studies indi-
cate the presence of massive currents or
deep-water jets in these regions (Schmid
et al., 2005) that could increase the ve-
locity of larval transport between the
two sides of the Mid-Atlantic Ridge
and enhance the possibility of success-
ful communication among popula-
tions. It is known, for example, that the
seep-dwelling caridean shrimp Alvino-
caris muricola inhabits both Gulf of
Mexico and Gulf of Guinea cold seeps
(Ramirez-Llodra and Segonzac, 2006);
in addition, the siboglinid tubeworm
Escarpia southwardae has recently been
described at the Gulf of Guinea seeps,
with morphological and genetic charac-
teristics that indicate a close taxonomic
relationship with E. laminata from the
Gulf of Mexico seeps (Andersen et al.,
2004). While these findings may suggest
a certain degree of gene flow across the
Mid-Atlantic Ridge, data are still very
limited. Further molecular and larval
ecology studies are needed in order to
fully understand dispersal pathways of
vent species, and particular attention
should be paid to the effect of larval re-
tention by local hydrodynamics (Marsh
et al., 2001; Mullineaux et al., 2005) and
the decrease in larval concentration by
diffusion and mortality (Cowen et al.,
2000). In the southern oceans, the Cir-
cumpolar Current has been suggested as
a pathway that may link species from the
southern East Pacific Rise, the East Scotia
Rise near Antarctica, the southern Mid-
Atlantic Ridge, and the Southwest Indian
Ridge (Figure 1) (Van Dover et al., 2002;
Hurtado et al., 2004).
Finally, depth may also play a signifi-
cant role in biogeographic patterns, as
suggested by Desbruyères et al. (2001)
for the differences found near the Azores
Triple Junction, where depth decreases
from 2400 m at Rainbow to 850 m at
Menez Gwen. This depth variation
causes changes in fluid toxicity and sus-
pended mineral particles, accompanied
by an impoverishment of vent endemic
species at shallower depths and an in-
crease in non-vent bathyal species. These
authors also suggest that the existence of
several biogeographic islands in this re-
gion is driven by depth.
LO OKING TOWARD
THE FUTURE
Thirty years after the discovery of hy-
drothermal vents, the investigation of
vent habitats and their associated fauna
is still in a critical exploratory and dis-
covery phase. Every new systematic
survey of a ridge section uncovers new
vent sites and their biological commu-
nities, often yielding species new to sci-
ence and sometimes new physiological
or morphological adaptations. The latest
discoveries span from pole to pole and
around the globe, including the detec-
tion of hydrothermal plume signals in
both the Arctic (Edmonds et al., 2003)
and Antarctic regions (German et al.,
2000; Klinkhammer et al., 2001), the dis-
covery of new hydrothermal sites along
the Central Indian Ridge (Van Dover
et al., 2001), and the first investiga-
tions of vent sites on the Mid-Atlantic
Ridge south of the equator (German
et al., 2005; Shank, 2006). The geologi-
cal, geochemical, physical, and biologi-
cal investigations of these regions will
provide essential information toward
completing the biogeographical puzzle
of vent biogeography (Figure 1), increas-
ing our knowledge of species diversity
and distribution, and providing the nec-
essary clues to understand what factors
drive and shape vent communities and
species distribution.
To address the gaps in our knowledge
of biogeography in deep-water chemo-
synthetic ecosystems (including hydro-
thermal vents, cold seeps, whale falls, and
regions of low oxygen and other reduc-
ing habitats), the international scien-
tific community is joining efforts in the
framework of the ChEss program (www.
noc.soton.ac.uk/chess). ChEss is one of
the 14 field projects of the Census of
Marine Life (www.coml.org), a 10-year
global initiative to describe the diver-
sity, abundance, and distribution of life
in the oceans. Within this context, the
main goal of ChEss is to describe the
biogeography of species from deep-water
chemosynthetic ecosystems and under-
stand the forces driving them (Tyler et
al., 2003). To achieve this goal, ChEss
has selected a number of priority study
areas based on our present knowledge of
biogeography (Figure 1). These include:
(1) the Atlantic Equatorial Belt, to ad-
dress scientific questions of gene flow
across and along the Mid-Atlantic Ridge
equatorial fracture zones; (2) the South-
east Pacific region off Chile, a unique
site where the Chile Ridge is subducting
beneath the South American Plate and
where we find all known chemosyn-
thetic habitat types in close proximity;
Oceanography Vol. 20, No. 1
40
(3) the New Zealand region, where vents,
seeps, whale falls, and other reducing
ecosystems are also found in close prox-
imity; and (4) the polar regions, with
development of field programs along the
potentially isolated Gakkel Ridge and in
the Antarctic. Large-scale national and
international collaborations and sharing
of human and infrastructure resources
are making such research programs pos-
sible, greatly facilitated by coordination
of international programs such as ChEss
and InterRidge (www.interridge.org).
The exploration, investigation, and
sampling of remote, dynamic, and
topographically complex ecosystems
such as hydrothermal vents also requires
continuing development of state-of-
the-art technologies for fieldwork and
laboratory analyses. For example, to
continue exploration at ever-higher lati-
tudes, robotic vehicles are increasingly
important—both remotely operated
vehicles and autonomous underwater
vehicles (AUVs). Indeed, with so much
of the global mid-ocean ridge yet to be
investigated, AUVs that can operated
independently of a mother ship are in-
creasingly recognized as one technology,
in particular, that will be vital to push
back the barriers to our knowledge (see,
e.g., Yoerger et al., this issue).
CONCLUSION
Exploration and investigation of new
sites at key locations is essential to fill
in important gaps in the biogeographi-
cal puzzle of hydrothermal vents. The
existence of such ecosystems was com-
pletely unexpected 30 years ago, but
their discovery has changed the way we
understand both Earth and the life upon
it. With every new discovery and inves-
tigation of known sites and communi-
ties, our knowledge and understand-
ing of the diversity and functioning of
these remote and exuberant ecosystems
increases, helping us understand the
processes driving the deep sea and the
global biosphere. Continuing explo-
ration of hydrothermal vents will no
doubt lead to the description of new
species, a better understanding of geo-
logical and geochemical processes affect-
ing their biology (and vice versa), and
the understanding of the phylogenetic
links between vent species and other
faunal communities. As an important
societal benefit, this research will also
undoubtedly continue to provide po-
tentially interesting sources of active
molecules for the biotechnological and
biomedical industries.
ACKNOWLED GEMENTS
E. Ramirez-Llodra is supported by
the ChEss-Census of Marine Life pro-
gram (A.P. Sloan Foundation), which
is kindly acknowledged. C.R. German
also acknowledges support from ChEss-
Census of Marine Life and further sup-
port from the Natural Environment
Research Council (UK) and from the
US National Science Foundation (NSF)
and National Oceanic and Atmospheric
Administration (NOAA). T. Shank
acknowledges support from NSF, the US
National Aeronautic and Space Admin-
istration Astrobiology Program, NOAA-
Ocean Exploration, and the Deep-Ocean
Exploration Institute at the Woods Hole
Oceanographic Institution. The authors
thank Paul Tyler, Chuck Fisher, Kristen
Kusek, and an anonymous reviewer for
their comments and suggestions on an
earlier version of this manuscript.
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... The spreading ridges of the Indian Ocean and their hydrothermal ecosystems (Figure 1) are unique but have less known vent ecosystems than those in the Atlantic and Pacific mid-ocean ridges in terms of their spreading rate (Müller et al., 2008;Beaulieu, 2015), mineral composition (German et al., 2016), and carbon input from the surface (Harms et al., 2021). Indian Ocean spreading ridge vent ecosystems have been hypothesized to serve as a corridor of connectivity between Atlantic and Pacific vent fauna (Ramirez-Llodra et al., 2007;Bachraty et al., 2009;Moalic et al., 2012;Rogers et al., 2012). ...
... Like elsewhere, macrofaunal vent communities in the Indian Ocean also possess high endemicity (Ramirez-Llodra et al., 2007;Rogers et al., 2012; Figure 2). The Indian vent communities are also distinct in composition from other oceans because almost all the dominant taxa above are unique to the Indian vents (Hashimoto et al., 2001;Nakamura et al., 2012;Copley et al., 2016;Zhou et al., 2018;Kim et al., 2020). ...
... One long-held hypothesis is that the Indian ridges act as a corridor of population connectivity between the Atlantic and Western Pacific vents . This idea was brought up by early surveys of global vent biodiversity on the basis of the presence/absence of vent macrofauna, which classified the Indian vents into a distinct biogeographic province with strong connection to both the Atlantic and West Pacific provinces (Ramirez-Llodra et al., 2007;Moalic et al., 2012; Figure 2). Many studies investigating the biogeography of macrofaunal taxa at the species level also highlighted the evolutionary connection between the Atlantic, Pacific, and Indian oceans (Hashimoto et al., 2001;McKiness and Cavanaugh, 2005;Ramirez-Llodra et al., 2007;Miyazaki et al., 2010;Rogers et al., 2012;Borda et al., 2013;Roterman et al., 2013;Breusing et al., 2015Breusing et al., , 2020Chen et al., 2015b;Johnson et al., 2015;Copley et al., 2016;Watanabe et al., 2018;Zhou et al., 2018;Lee et al., 2019;Jang et al., 2020;Han et al., 2021). ...
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To date, 13 biologically active hydrothermal vent (HTV) fields have been described on the West Indian Ocean ridges. Knowledge of benthic communities of these vent ecosystems serves as scientific bases for assessing the resilience of these ecosystems under the global effort to strike an elegant balance between future deep-sea mining and biodiversity conservation. This review aims to summarize our up-to-date knowledge of the benthic community structure and connectivity of these Indian vents and to identify knowledge gaps and key research questions to be prioritized in order to assess the resilience of these communities. The HTVs in the West Indian Ocean are home to many unique invertebrate species such as the remarkable scaly-foot snail. While distinct in composition, the macrofaunal communities of the Indian HTVs share many characteristics with those of other HTVs, including high endemism, strong zonation at the local scale, and a simple food web structure. Furthermore, Indian vent benthic communities are mosaic compositions of Atlantic, Pacific, and Antarctic HTV fauna possibly owning to multiple waves of past colonization. Phylogeographic studies have shed new light into these migratory routes. Current animal connectivity across vent fields appears to be highly influenced by distance and topological barriers. However, contrasting differences in gene flow have been documented across species. Thus, a better understanding of the reproductive biology of the Indian vent animals and the structure of their population at the local scale is crucial for conservation purposes. In addition, increased effort should be given to characterizing the vents’ missing diversity (at both the meio and micro-scale) and elucidating the functional ecology of these vents.
... In addition, dense epibenthic aggregations of Chaetopterus variopedatus (Renier, 1804) (family Chaetopteridae) are reported from the Chilean fjords, both on soft and hard bottoms, within a broad bathymetric range (1-185 m) (Montiel et al., 2005;Försterra et al., 2017;Betti et al., 2021), whereas deep-sea coral-serpulids frameworks dominated by Serpula vermicularis Linnaeus, 1767 are known from the central Mediterranean Sea (Sanfilippo et al., 2013). Finally, siboglinids (e.g., Riftia pachyptila Jones, 1981) aggregations are associated with extreme habitats such as hydrothermal vents (Jones, 1981;Van Dover, 2000;Ramirez-Llodra et al., 2007;Kiel and Tyler, 2010). ...
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This study presents a description of Bispira riccardi sp. nov., a new habitat-forming sabellid polychaete from the mesophotic NW Mediterranean Sea. Individuals, up to 20 cm long, show a peculiar morphology of radioles, thoracic uncini, companion chaetae and ventral shield of the collar. The phylogenetic position of this new taxon in the genus Bispira has been validated using nuclear (18S rRNA) and mitochondrial (COI) markers. Aggregations of B. riccardi sp. nov. were found by ROV on horizontal muddy bottoms between 56 and 85 m, in areas subjected to high trophic inputs. Patches are fragmented and dense (up to 943 individuals m⁻²) probably accounting for various hectares. The 5-days continuous monitoring, carried out using an autonomous lander, revealed that the contraction of the branchial crown was positively affected by temperature and current, rapidly responding to meteorological events. The filtering activity and high density of these fields suggest a considerable impact on the pelagic-benthic coupling and the amount of organic matter in the sediments. Indeed, meiofaunal abundance and diversity within the aggregations resulted significantly higher than in outer stations. These findings highlight the undisclosed potential of the deep Mediterranean Sea for sabellid diversity and their importance as habitat-forming species on mesophotic soft bottoms.
... SMS deposits also provide a variety of benthic habitats that can be broadly divided into three types (Arquit 1990;Kim and Hammerstrom 2012): 1) hydrothermally active areas with venting, which can be through chimneys; 2) hydrothermally inactive areas that can include relict chimneys and may have the potential to become 'reactivated' if hydrothermal flow resumes (see Jamieson and Gartman 2020); and 3) non-hydrothermal hard substrata, such as lava. Active areas are primarily colonised by hydrothermal vent species that cannot survive away from hydrothermal activity (Van Dover 2000;Ramirez-Llodra et al., 2007;Fisher et al., 2013). Hydrothermally inactive and non-hydrothermal hard substrata are colonised by a fauna consisting of 'background' species that occur on hard substrata elsewhere within the deep sea (Galkin 1997;Collins et al., 2012;Boschen et al., 2016;Gerdes et al., 2019). ...
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The deep sea is subject to multiple anthropogenic disturbances, to which may soon be added mining of hydrothermally-formed seafloor massive sulfides (SMS). As a first step towards a full Ecological Risk Assessment (ERA) for SMS mining, ecological sensitivity to mining activities was assessed based on the functional traits of benthic megafaunal taxa. Faunal distribution and abundance data at two different spatial scales (within seamount and within site) were used from two video surveys conducted at un-fished SMS deposit-hosting seamounts on the Kermadec Volcanic Arc, New Zealand. For each of the 157 taxa identified in the seamount and site surveys, sensitivity was scored for six functional traits: adult size, environmental position, living habit, feeding habit, mobility, and structural fragility. Sensitivity (very low, low, intermediate, high, very high) was scored separately for three mining disturbances: passage of mining vehicles along the seafloor, sediment plumes generated by mining activity, and mineral extraction. The effects of different abundance and diversity weightings on the results were explored and transformations chosen based on ecological rationale. Sensitivity to mining impacts was summed within samples and mapped to show the spatial distribution of sensitivity. Samples (assemblages) consisted of 173 individual 200 m transects for seamount survey data and 153 individual 15 m transects for site survey data. For both spatial scales, the sensitivity of taxa and the sensitivity summed within each sample (assemblage) was greatest to mineral extraction, followed by plume impacts, and least sensitive to vehicle impacts. The location of most very highly sensitive assemblages coincided with the occurrence of hydrothermal vent taxa or previously mapped locations for hydrothermally active habitat. Highly sensitive assemblages occurred at hydrothermally inactive sulfide structures, such as chimneys, and other locations where assemblages were dominated by fragile, sessile, suspension-feeding taxa, such as scleractinian branching corals. The approach taken here illustrates spatial patterns in sensitivity within seamounts and sites and provides an important first step towards a more comprehensive ERA. This type of assessment has the potential to inform decisions on spatial management of SMS mining activities, and the suitable placement of area-based management measures, such as protected areas.
... Despite their extremes of temperature and acidity, hydrothermal vents support vast communities of organisms (Ramirez-Llodra, Shank and German, 2007). Chemosynthetic bacteria form the basis of vent ecosystems, and in turn support a large biomass of invertebrates that include molluscs, annelid tube-dwelling worms, and crustaceans (Van Dover et al., 2002). ...
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New report applies the mitigation hierarchy and deep-ocean science to determine risks and impacts of deep-seabed mining - Volume 54 Issue 4 - Pippa Howard, Nicky Jenner, Guy Parker
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