Cryptic isolation of Gulf of California shovelnose guitarfish evidenced by mitochondrial DNA

The shovelnose guitarfish Rhinobatos productus is an evolutionarily, ecologically, and economically important ray, with a continuous distribution from San Francisco, California (USA), to Mazatlan, Sinaloa, and in the Gulf of California (Mexico). Regional studies have revealed morphometric differences between shovelnose from the Gulf of California and the Pacific coast of Baja California, which may result from phenotypic plasticity in the presence of high levels of gene flow or from a degree of genetic differentiation in the presence of cryptic isolation within a continuous distribution. We used PCR-RFLP of the mitochondrial control region to assess the degree of genetic differentiation between Gulf of California and Pacific shovelnose guitarfish. We found very high levels of molecular diversity (averages: h = 0.77, π = 1.19%), which may be associated with historically large and stable populations, as well as very significant levels of genetic differentiation between gulf and Pacific samples (χ² = 64, P < 0.0001; Φ_ST = 0.63, P < 0.0001, mean nucleotide divergence d = 2.47%). We found a deep phylogeographic break between haplotypes from the gulf and the Pacific, which may suggest the existence of cryptic species but clearly indicates more than one evolutionarily significant unit of R. productus. Our results show a pattern of genetic structure and levels of differentiation consistent with the geological history of the region. Furthermore, these findings have wide-ranging implications for the management and conservation of cartilaginous fish in Mexico, as they reveal the existence of biological diversity that will go unnoticed without the genetic scrutiny of intraspecific variation and that is highly relevant for much needed management and conservation efforts.