Bull. Southern California Acad. Sci. 110(3). 2011. pp. 141-151 © Southern California Academy of Sciences. 2011 Discordant Phylogeographic and Biogeographic Breaks in California Halibut Matthew T. Craig,'* F. Joel Fodrie.- Larry G. Allen,- Laura A. Chartier/ and Robert J. Toonen^ 'University of Puerto Rico, Mayaguez, P.O. Box 9000, Mayaguez, PR 00681, USA -Institute of Marine Sciences and Department of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, NC 28557, USA ^California State University, Northridge, 18111 NordhoffSt., Northridge, CA 91330 USA ^Hawai'i Institute of Marine Biology, School of Ocean & Earth Sciences & Technology. P.O. 1346, Kdneohe, HI 96744, USA Abstract. — The range of the California Halibut. Paralichthys californicus, spans three biogeographic provinces along the coastline of Alto (United States) and Baja (Mexico) California. To assess population genetic structure of the California Halibut, we analyzed mitochondrial cytochrome b sequences from 375 individuals across a large portion of its native range. Nucleotide diversity was consistently low among sampling sites (ti = 0.0026 ± 0.0017). while haplotype diversity was consistently high {h = 0.77 ± 0.024). We found that California Halibut were genetically homogeneous across sampled sites with an overall O^t = 0.0030 (p = 0.22). We saw no evidence of genetic discontinuities at two previously recognized marine phylogeographic breaks in the Los Angeles region or across the California Transition Zone at Point Conception. We conclude that California Halibut are genetically homogeneous and experience substantial gene flow, at least over evolutionary time scales. INTRODUCTION The nearshore marine environment of coastal California (USA) has long been a playground for biogeographers owing to its dynamic composition of marine organisms that has undergone dramatic shifts during the past five decades or so. particularly among marine fishes (Horn, et al., 2006). While southern California's marine ichthyofauna was once thought to share many elements of the cool water "Oregonian" faunal assemblage, a persistent warming trend since the early 1980s precipitated a change in southern California's marine ichthyofauna to a more temperate, sub-tropical fauna with estab- lished communities whose biogeographic affinities lie with faunal assemblages further south along the Pacific Coast (reviewed in Lea and Rosenblatt, 2000). While the biogeographic history of southern California is dynamic, one geographic feature has consistently stood out as a potential dividing point between two distinct faunal provinces at Point Conception, a prominent headland that marks the beginning of the "California Bight" and the Cahfornia Transition Zone (CTZ; Figure 1 [Valentine, 1966]). However, as detailed distributional data on California's marine fishes emerged, this biogeographic "break" appeared to be "leaky", and is now regarded as more of a gradual transition zone (Horn, et a!.. 2006). * Corresponding author: matthe\\craig4''a gmail.com 141