At the bottom of the deep blue sea: a new wood-boring bivalve (Mollusca, Pholadidae, Xylophaga) from the Cape Verde Abyssal Plain (subtropical Atlantic)

Bivalves of Xylophaga Turton, 1822 require access to wood or other vegetation on the seafloor, into which they bore. They ingest the wood and, with the aid of bacteria, digest it to survive. Their complete dependence on vegetation for survival suggests that the group would be rare on the abyssal plains, as the availability of terrestrial vegetation declines with distance from land masses. Deployment of a small block of wood on a mooring at 4626 m depth in the Cape Verde Abyssal Plain, over 1600 km west of Africa tested that suggestion. When recovered seven months after deployment, the wood carried an estimated 170 boreholes/cm2 evidence of extremely and surprisingly rapid colonization by a previously unknown species, here described as Xylophaga alexisi n. sp. The species is unique in having an incomplete siphon, a posterior adductor scar made of linear elements and in lacking cirri at both siphonal openings. Atlantic species described by Harvey (1996) are compared to this and other species. The bivalves are estimated to have grown 0.011 mm per day, comparable to growth estimates of X. ricei Harvey, 1996 at 5000 m depth. The high density of this species at this site, the great distance of the site from the continent which is so arid to be Saharan in character and the minimal input the site receives from surface and bottom currents argue strongly that wood-boring species thrive in the largest benthic habitat on Earth, the abyssal plain.