Abstract
A hybrid zone involving the deep‐sea mussels, Bathymodiolus azoricus and B. puteoserpentis , was recently discovered at Broken Spur hydrothermal vent field (29°10′ N, 43°10′ W) along an intermediate segment of the Mid‐Atlantic Ridge axis. Examination of nuclear (allozymes) and cytoplasmic (mitochondrial DNA) gene markers in a new sample from Broken Spur revealed significant cytonuclear disequilibrium caused by an excess of the parental types (coupling phase) and a deficiency of recombinants (repulsion phase). An assignment test of individual multilocus genotypes also revealed an excess of parental genotypes in the admixed population. These results support the hypothesis that the Broken Spur mussel population comprises a nonequilibrium mixture of parental immigrants and hybrid individuals.