Life on wood - the carnivorous deep-sea mussel Idas argenteus (Bathymodiolinae, Mytilidae, Bivalvia

Deep-sea mussels associated with sunken wood are less well known in terms of anatomy, biology and evolution than their bathymodioline allies from cold seeps and hydrothermal vents. During the Danish 'Ingolf Expedition' (1895-96) to the Northeast Atlantic, two pieces of pinewood were collected from a depth of 1836 m. The wood was inhabited by several hundred individuals of the deep-sea mussel Idas argenteus and the wood-boring pholadid Xyloredo ingolfia. Idas argenteus is the type species of its genus and differs from some of the species until now referred to Idas by having gill filaments like those of suspension-feeding mytilids, with no abfrontal tissue adaptation for symbiotic chemo-autotrophic bacteria. The adaptations in I. argenteus to capture prey as well as the reproductive pattern of the prey, Xyloredo, and its functional dwarf males are described. The population dynamics and adaptation to an ephemeral habitat in the deep sea of both species are described herein. Although larviphagi is known to occur in some filter-feeding bivalves, Idas argenteus is the first mytilid known to be specifically adapted to a carnivorous life. Further, it is argued that the modifications of I. argenteus with regard to its shell development, alimentary system, gill anatomy and life habits provide important clues to the evolution of the Bathymodiolinae.