A broad range of animals in many ecosystems have chemosymbiotic relationships with microbes, but in hydrothermal vent ecosystems only siboglinid
annelids and molluscs (gastropods and bivalves) have intracellular chemoautotrophic symbionts (Childress and Girguis, 2011).
Discrete morphospecies of siboglinid
tubeworms from eastern Pacific vents were found on examination with molecular markers to be developmentally plastic ecophenotypes (Black et al.
polychaete Osedax japonicus and the bathymodiolid mussel Adipicola pacifica (Okutani et ai, 2003), both endemic to the bone surfaces, were also found on the same blocks of bones with P.
Tyler & Young (1999) and Young (2003) reviewed the data, which mainly characterized the larger megafuana including siboglinid
and alvinellid polychaetes, bivalve molluscs, and Crustacea.
(2009) resulted in two hypothetical scenarios in which Osedax split from their siboglinid
relatives either 45 MYA (concurrent with the evolution of large cetaceans) or 135 MYA when large marine reptile carcasses could have provided suitable habitat.
Stable and highly specific invertebrate-hosted symbioses are common (e.g., siboglinid
tubeworms: McMullin et al., 2003; Losekann et al., 2008; bivalves: Hurtado et al., 2003; DeChaine and Cavanaugh, 2005), and the recent application of massively parallel sequencing protocols and in situ hybridization techniques (e.g., Dubilier et al., 2001; Blazejak et al., 2005; Luyten et al., 2006; Ruehland et al., 2008; Stewart and Cavanaugh, 2009) have highlighted the molecular and morphological diversity of bacterial partners engaged in specific associations.
The dimensions of the trophosomes in the different siboglinid
taxa also reflect a high plasticity of this organ.
Most obviously, in many intracellular endosymbioses specific symbiont-housing organs have developed, such as the light organ in bobtail squids, the root nodules in legumes, the bacteriomes in insects, and the trophosome in siboglinid
endosymbionts are typically thought to fall into one of three general metabolic categories: (1) thiotrophic symbionts that oxidize sulfide or other reduced inorganic compounds and fix C[O.sub.2] by the Calvin-Benson cycle, (2) methanotrophic symbionts that use methane as both the source of energy for metabolism and the carbon for assimilation, or (3) heterotrophic symbionts found in the bone-eating worms, Osedax spp.
Osedax, a genus of bone-eating siboglinid
annelids, was first described from a whale carcass at 2893-m depth in Monterey Bay, California (Rouse et al., 2004).
Levels of interspecific COI divergence found between species of deep-sea bivalve molluscs, siboglinid
tubeworms, and decapod crustaceans typically exceed 4%, whereas intraspecific divergence is less than 2% (Peek et al., 1997; Shank et al., 1998; Guinot et al., 2002; Hurtado et al., 2002, 2004; Goffredi et al., 2003; Won et al., 2003; Rouse et al., 2004).
The ingroup comprised four siboglinid
species representing one genus of frenulate and three genera of vestimentiferans, as shown in Table 1.