Disjunct Sea of Cortez–Pacific Ocean Gillichthys mirabilis populations and the evolutionary origin of their Sea of Cortez endemic relative, Gillichthys seta

The shortjaw mudsucker, Gillichthys seta, an intertidal goby endemic to the Sea of Cortez, has been proposed to be the paedomorphic derivative of the longjaw mudsucker, Gillichthys mirabilis. G. mirabilis is a disjunct species, with populations found along the Pacific coast of central California to central Baja California, and with isolated populations found in the northern Sea of Cortez. Previous studies have suggested that the endemic paedomorph form speciated in sympatry with the Sea of Cortez population of G. mirabilis. Alternatively, this speciation event could have occurred before the separation of G. mirabilis populations into two disjunct entities. To test these alternative hypotheses, we collected adult individuals from both species throughout their ranges from December 1997 to November 1998. We amplified and sequenced 142 partial 527 base pairs (bp)] mitochondrial cytochrome b regions and 18 nuclear creatine kinase introns (140 bp). We found that Pacific populations of G. mirabilis separated into two distinct clades, possibly reflecting a phylogeographic break found in other fish species along the Baja California coast at Punta Eugenia. These two Pacific populations were well separated from Sea of Cortez populations. Furthermore, our results indicate that the split between Sea of Cortez and Pacific populations of G. mirabilis occurred well after the speciation event that separated G. mirabilis from its paedomorphic counterpart, G. seta. Received: 30 March 2000 / Accepted: 4 September 2000