A New Silverside of the Genus Colpichthys (Atheriniformes: Atherinidae) from the Gulf of California, Mexico

A new silverside (Atheriniformes: Atherinidae) of the previously monotypic genus Colpichthys is described from the Gulf of California, Mexico. Both species occupy near-shore habitats in the northern part of the Gulf of California, with the new species, Colpichthys hubbsi, restricted to the Colorado River delta region. The range of the sister species, C. regis (Jenkins and Evermann), overlaps marginally in the southern reaches of the delta. The new species is most abundant in the mouth of the Colorado River, although several collections have been taken from lagoons to the south. Features that distinguish the new species from C. regis include a more posterior placement of the dorsal fins, large scales, a small head, lack of a posterior extension of the gas bladder beyond the gut cavity, and lack of modified anterior caudal vertebrae characteristic of the other members of this subfamily. The significance of sister species in the same geographical region and the phylogenetic and biogeographic affinities of these Gulf of California species is discussed.