Deep-sea wood-boring bivalves of the genus Xylopholas Turner, 1972 are unified by a plate-like elaboration lateral to the siphon tips; this genus, known only from specimens under 4 mm in shell length, may provide essential clues to the group's phylogeny. Here three aspects of their biology are treated. A new species, Xylopholas dostwous sp. nov., is distinguished by the absence of a siphonal collar, gills that extend into the incurrent siphon, a curved posterior valve margin and a fleshy cover over the anterior adductors. Second, specimens nearly eight times larger than any in the type series of X. crooki Voight, 2007 show strong positive allometry of the fleshy Anterior Adductor Cover, forcing a reconsideration of the mesoplax (calcified plates over the anterior adductor muscle). Third, apparent dwarf males of Xylopholas may fuse with the female's mantle, allowing sperm to be released inside the mantle near the oviducts. Dramatic differences among hypothesized dwarf xylophagaid males suggest male dwarfism evolved repeatedly. A photomicrograph of the prodissoconch of a suspected dwarf male indicates larvae feed in the plankton. Combined, the unique and potentially plesiomorphic characters discussed here highlight how little we know of wood-borer evolution. Both Xylopholas crooki and X. dostwous sp. nov. are known only from artificial deployments that spent at least 22 months on the seafloor; additional collections are required to determine if the genus is restricted to long-sunken wood.
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