MEPS 577:79-91 (2017)  -  DOI: https://doi.org/10.3354/meps12239

ABSTRACT: Oxygen minimum zones (OMZs) are large, persistent regions of the world’s oceans with oxygen concentrations of ≤0.5 ml l^-1. The distribution, abundance, composition, and diversity of benthic fauna shift dramatically where OMZs intersect continental margins, and some species with distributions that extend into the OMZ diverge genetically. These abrupt shifts at the genetic, population, and community levels suggest OMZs might impose a strong selective force on many deep-sea organisms. If larvae are sensitive to hypoxic conditions, the OMZ might preclude dispersal among populations separated by regions of low oxygen and operate essentially as a geographic barrier to gene flow. We investigated this hypothesis by quantifying genetic variation (mitochondrial 16S rRNA) of a wood-boring bivalve, Xylophaga washingtona, along a depth gradient (100 to 1829 m) spanning the OMZ in the Northeast Pacific. Two distinct clades were apparent, one spanning the OMZ and the other restricted to within and below it. These clades likely represent independently evolving lineages, suggestive of cryptic species. The bathymetric divergence is consistent with the OMZ impeding gene flow, although we cannot rule out many other environmental, ecological and evolutionary forces that might have led to the observed divergence. Given the predicted expansion of OMZs as well as the deoxygenation of contemporary oceans, it is critical to develop a better understanding of how hypoxic and suboxic regions might influence the evolution of marine organisms.

KEY WORDS: OMZ · Deep sea · Xylophaga · Genetic divergence