Competition in the deep sea: phylogeny determines destructive impact of wood-boring xylophagaids (Mollusca: Bivalvia)

Abstract

How biotic interactions contribute to structuring deep-sea communities remains poorly known. An example of exploitation competition, in which over time one species dominates a habitat to preclude its use by competitors, is highlighted here. Sunken wood is the obligate habitat of deep-sea wood borers of the Xylophagaidae Purchon, 1941 which eat wood and, with symbiotic bacteria, digest it. Enigmatically, some wood falls remain robust despite xylophagaid boring. Xylophagaids bore other wood falls so heavily that they crumble after just a few months. We perform a meta-analysis of experimental wood deployments and view the results from a phylogenetic perspective. Of 15 deployments recovered after 5 to 25 months from 200 to 3232 m deep, seven were so heavily bored to be crushable by hand. Eight were bored but remained robust. The groups did not significantly differ in wood size, type, temperature, depth, or duration. Members of the Xylophaga dorsalis (Turton, 1819) clade, reported in two studies to recruit and bore faster than confamilials, bored all seven crushable deployments; seven of the eight intact deployments were bored by other xylophagaids. Fecal chimneys line and thus narrow the boreholes of this clade; fluid flow is impeded, a clear liability. The lignin-rich fecal chimneys may, however, cue larval settlement and at resultant high population densities, lower oxygen availability. Members of the X. dorsalis clade, being hypoxia-tolerant, thrive but other xylophagaids appear to suffer, perhaps due to interference competition. The shared derived characters that unite this clade allow them to exploit low-oxygen wood that is intolerable for competitors.