Experimental deep-sea deployments reveal diverse Northeast Pacific wood-boring bivalves of Xylophagainae (Myoida: Pholadidae)

Abstract

Only a single species of deep-sea wood boring bivalve has been known off the forested northwest coast of North America, although the Xylophagainae are so diverse in other areas that up to five species occur in a given length of wood. To determine whether additional species were present in the Northeast Pacific and if so, how they coexist, lengths of wood were experimentally deployed on heavily sedimented sites on the Cascadia Basin and Escanaba Trough, non-hydrothermally active basalt on Gorda Ridge and Axial Volcano, and an isolated sediment pond within the axis of Juan de Fuca Ridge, at depths of between 1,530 and 3,232 m. All locations were between 418and 488N, at least 240 km off the North American coast. Six previously unknown species, Xylophaga corona n. sp., X. zierenbergi n. sp., X. heterosiphon n. sp., X. oregona n. sp., X. microchira n. sp. and Xylopholas crooki n. sp., were collected and are described here. Dominant species differed in recoveries made after 10 and 24 months at Juan de Fuca Ridge and Cascadia Basin localities. Xylophaga microchira n. sp. dominated four of five deployments recovered within 12 months, but its abundance declined by 24 months, being largely replaced on Juan de Fuca Ridge by X. oregona n. sp., a species known only from this Ridge and from an incidental collection on the Nootka Fracture Zone. At Cascadia Basin sites, the abundances of X. heterosiphon n. sp. and of what are likely to be predatory polyclad flatworms increased as that of X. microchira n. sp. declined. Xylophaga microchira n. sp. is hypothesized to be specialized for early colonization, but to be competitively inferior to X. oregona n. sp., and more vulnerable to predators than X. heterosiphon n. sp. The dominance of competitively superior and more predator-resistant species in older deployments argues that biotic interactions contribute to coexistence of multiple species of wood-borers despite direct competition among them.