Xylotrophic bivalves: Aspects of their biology and the impacts of humans

Abstract

Bivalves of the families Teredinidae and Xylophagaidae bore into and eat wood in shallow water and the deep sea, respectively. After an introduction to these sister taxa, I consider how they bore, focussing on the evolution of the cephalic hood in teredinids, which could contribute to the depth separation between the families. If the hood serves as a counter pressure to the foot, it may be vital in allowing teredinids to bore wood that contains air. The availability of wood has been suggested to determine the distribution of these animals. Mangrove habitats with abundant wood sustain the highest diversity, about one-third of all known teredinids. The open ocean, where wood is all but nonexistent, presents a nearly impassable barrier to most wood borers. Only after wooden sailing ships transported teredinids between ports did introduced species become problematic. Despite the rarity of wood in the open ocean, some xylophagaids live there as, it is hypothesized, do a limited number of teredinids; wood in the open ocean is predicted to be large because large size enhances buoyancy. The life history of open-ocean species may differ fundamentally from that of nearshore species. Physical stressors of temperature, salinity and desiccation appear to affect teredinids minimally; xylophagaids on the seafloor do not experience these variables. How biotic interactions such as competition and predation affect wood borers is yet to be thoroughly studied, but their impact may be greater than has been recognized, especially among teredinids. Although humans regard teredinids as pests that destroy wooden structures, and attempt to eradicate them from some areas, these animals play vital ecological roles. Teredinids make the energy and nutrients locked into wood available to the local community. Xylophagaids are nearly unique in breaking down cellulose and are analogous to primary producers in sustaining the diverse deep-sea wood-fall communities. The survival of borers requires wood to be present. The reduction of trees near water courses, removal of driftwood from navigable rivers and the destruction and fragmentation of the world's mangrove habitats all threaten the predictable supply of wood in the sea and may seriously threaten the survival of these molluscs.