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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.