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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
45
Chemosymbiotic animals at methane seeps can be large or
small, form bushes, dense beds, reefs, or live alone, and they
can grow very quickly or exceptionally slowly. Animal commu-
nities at methane seeps include single-celled organisms (pro-
tozoans) and multi-celled animals (metazoans). Most of the
metazoans are invertebrates. Many are sustained, one way or
another, by microbial activity linked to methane. Common ex-
amples include vestimentiferan tubeworms (Fig. 2.5, A), crabs
(Fig. 2.5 B, E), and a diversity of clams (Fig. 2.5, C, F).
All of these taxa are relatively large compared to non-seep,
deep-sea fauna. Many seep-endemic organisms have reduced
or absent digestive systems. Instead, they provide homes to
symbiotic chemoautotrophic bacteria that provide the host
with nutrition through aerobic sulphide and/or methane oxi-
dation (Fig. 2.6).
The seeps and seep organisms support a wealth of grazing,
predatory, and deposit-feeding taxa by providing substrate for
attachment, access to reduced compounds, entrainment of
organic-rich particles, and access to microbial protozoan or
metazoan prey (Carney 1994; Cordes
et al.
2010). Additionally,
the carbonates (limestone is a type of carbonate) precipitated
by microbial AOM consortia form crusts, rocks, boulders,
and even vast landscapes at seeps (Teichert
et al.
2005). These
seeps can support high densities of mussels, tubeworms, and
grazing gastropods (Olu-Le Roy
et al.
1996; Levin
et al.
2010).
Because the chemosynthetic life forms described here re-
quire different chemical balances and concentrations of
methane and sulphide (Sibuet and Olu-Le Roy 1998; 2003;
Levin 2005), distinct habitat patches form in response to the
fluid chemistry and fluid flow rate (flux). Generally, sedi-
ments covered with mats of sulphur-oxidizing bacteria are as-
sociated with the strongest fluid and methane fluxes or near-
surface gas hydrates. Mussel and vesicomyid clam beds are
associated with high to moderate fluxes. Solemyid clam beds,
as well as vestimentiferan frenulate tubeworm fields, are as-
sociated with lower oscillating fluxes or deeper gas hydrates
(Fig. 2.7) (Sahling
et al.
2002; Sibuet and Olu-Le Roy 2003;
Levin 2005; Sommer
et al.
2006). Such connections have
been documented in several methane-seep environments
(e.g. Van Dover
et al.
2003; Olu-Le Roy
et al.
2007; 2009).
The combination of microbial mats, the beds, bushes, and
Vestimentum
Heart like
structure
Dorsal
vessel
Ventral
vessel
Bacteria
Plume
Trophosome
Coelomic
Cavity
O
2
O
2
HS
-
HS
-
CO
2
CO
2
Morphology of a tube worm
hosting sulphide-oxidizing symbionts
Figure 2.6:
Symbiotic relationships for obtaining energy from
sulphide. Morphology of a tube worm (top) and photo of a clam
hosting sulphide-oxidizing symbionts (bottom, photo courtesy of
Greg Rouse, Scripps Institution of Oceanography). Tube worms
host their symbionts in the trophosome, a specialized organ.
Oxygen (O
2
), sulphide (HS
–
), and carbon dioxide (CO
2
) are taken
up from the surrounding water through the animal’s plume and
delivered via the blood stream to the symbionts. Clams harbour
their symbionts in their gills. Oxygen and carbon dioxide are
available from the surrounding water, and sulphide is taken up
from the sediment through the clam’s foot.