Deep-sea wood-boring bivalves (Xylophagaidae)

Xylophagaid bivalves link terrestrial and deep-sea ecosystems by making energy and nutrients from sunken wood available to other animals. They bore into what can be sulphide-rich wood with their valves and digest it using bacterial enzymes. The evolutionary history of the roughly 60 named xylophagaid species remains largely unknown. We sequenced 18S and 28S rDNA genes of 59 specimens from the northeastern Pacific, southwestern Pacific off New Zealand and the Atlantic Ocean. This dataset contains details about the 59 xylophagaid specimens newly sequenced. We analysed these together with data from GenBank (thus increasing the species represented by sequences from 7 to 22) using maximum likelihood and Bayesian inference to reconstruct the group’s phylogeny. Newly discovered taxa are: Spiniapex gilsonorum n. gen., n. sp.; Feaya n. gen. (for Xylopholas dostwous) and Abditoconus n. gen. (for X. heterosiphon, X. anselli and X. brava that share a two-parted siphon and a periostracal cone). Specimens of Xyloredo from New Zealand, Brazil, the Gulf of Mexico and California USA are a single species. The genus Xylopholas is not unequivocally monophyletic; the presence/absence of a faecal mass in the distal intestine is the most conspicuous difference between the species included. The mesoplax (paired calcified plates over the anterior adductor) evolved convergently in two distinct clades assigned to the genus Xylophaga, which is not monophyletic. All clades represented by at least four taxa occur in every geographic area included. Rather than evolving to exploit sulphide associated with wood falls, xylophagaids may have evolved protection from it. This is indicated by the fact that in four clades, a thick periostracum covers the siphons that extend through the wood, while packed faecal pellets surround the siphons in one subclade, perhaps providing a physical barrier. In only one clade are fleshy siphons exposed to the wood.