The Olympia oyster, Ostrea lurida Carpenter, 1864, is the only native oyster on the west coast of temperate western North America and a conservation target for native species restoration throughout much of its known range, from British Columbia, Canada to Baja California, Mexico. This species was recently demonstrated to be genetically distinct from its southern congener, O. conchaphila Carpenter, 1857, but our new sampling, combined with previously published data, supports a tentative allopatric pattern, with the southern and northern species restricted to either side of Punta Eugenia in Baja California, Mexico, a known biogeographic boundary. We collected O. conchaphila and multiple co-occurring oyster species at 11 sites along the Pacific coast of Baja California Sur or within the Gulf of California, Mexico. Oyster surveys revealed at least six other co-occurring species, including two exotics, of Ostreidae Rafinesque, 1815 that were identified by sequencing 16S ribosomal DNA (16S) and cytochrome oxidase subunit I (COI) mitochondrial markers. In addition to our newly collected material, our phylogenetic analyses included Ostreidae from worldwide localities available in GenBank.

Phylogenetic estimates, using maximum likelihood, supported the sister species relationship between Ostrea lurida Carpenter, 1864 and O. conchaphila Carpenter, 1857. Together, they group as the sister lineage of Myrakeena angelica (Rochebrune, 1895), nested within a grouping of species currently assigned to Ostrea Linnaeus, 1758. Thus, we have revived the original name Ostrea angelica Rochebrune, 1895 and consider the monotypic genus, Myrakeena Harry, 1985 a junior synonym of Ostrea. We also collected O. equestris Say, 1834, native to the Caribbean and not previously reported in the Eastern Pacific. Our results are consistent with the recognition of only four subfamilies within Ostreidae: Ostreinae Rafinesque, 1815, Crassostreinae Scarlato and Starobogatov, 1979, Saccostreinae Salvi et al., 2014, and Striostreinae new subfamily. Another subfamily, Lophinae Vialov, 1936, is best synonymized with Ostreinae because it would otherwise be paraphyletic to that taxon. Sequences of Saccostrea palmula (Carpenter, 1857) revealed a striking lack of genetic variation that contrasted with their substantial phenotypic plasticity. Surprisingly, the morphologically distinctive species, Ostrea tubulifera Dall, 1914, was revealed as an ecotype of S. palmula, and so is herein considered a junior synonym of the latter species.