Sensory structures on the siphons of wood-boring bivalves (Pholadidae: Xylophagainae: Xylophaga)

Deep-sea bivalves of Xylophagainae spend their entire postmetamorphic lives boring into wood that has fallen to the seafloor. Although their boreholes seemingly provide a protected, imperturbable habitat, scanning electron microscopy reveals that the siphons of three species of Xylophaga examined carry elaborate structures that are interpreted as chemoreceptors or mechanoreceptors. Sensory structures occur on the siphonal surface of Xylophaga oregona Voight, 2007, and X. multichela Voight, 2008. The large complex papillae of X. nuiltichela are scattered on the distal incurrent siphon and arrayed in two longitudinal rows along its dorsal surface. The distal incurrent siphon of X. oregona carries minute structures, barely projecting above the surface, that are crowned by tufts of cilia. Both siphonal openings of X. microchira. Voight, 2007, carry cirri. At the excurrent opening, cirri have long cilia emerging from terminal pits. At the incurrent opening, cirri form two rings. The inner cirri appear to be unique in that cilia emerge from between scales that cover their inner surfaces. The structures observed may be useful in species taxonomy and systematics, but we suspect that their elaboration is linked to predation pressure, which might relate to depth distribution.