Molecular Ecology (2008) 17, 4233–4247 doi: 10.1111/j.1365-294X.2008.03913.x © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Ltd Blackwell Publishing Ltd High connectivity on a global scale in the pelagic wahoo, Acanthocybium solandri (tuna family Scombridae) T. C. THEISEN,* B. W. BOWEN,† W. LANIER‡ and J. D. BALDWIN* *Division of Biological Sciences, Florida Atlantic University, 2912 College Avenue, Davie, Florida 33314, USA, †Hawaii Institute of Marine Biology, University of Hawaii, 46-007 Lilipuna Road, Kaneohe, Hawaii 96744, USA, ‡Genetics Training Program, Department of Biological Sciences, University of Iowa, Iowa City, Iowa 52242, USA Abstract The population genetic structure and phylogeography of wahoo, Acanthocybium solandri, were investigated on a global scale with intron six of lactate dehydrogenase-A (ldhA6, 8 locations, N = 213) and mtDNA cytochrome b (Cytb, 10 locations, N = 322). Results show extensive sharing of haplotypes across the wahoo’s entire global range, and analyses were unable to detect significant structure (nuclear F ST = 0.0125, P = 0.106; mtDNA Φ ST < 0.0001, P = 0.634). Power analyses indicated 95% confidence in detecting nuclear F ST ≥ 0.0389 and mtDNA Φ ST ≥ 0.0148. These findings appear unique, as most other tunas, billfishes, and oceanic sharks exhibit significant population structure on the scale of East–West Atlantic, Atlantic vs. Indian-Pacific, or East–West Pacific. Overall nuclear heterozygosity (H = 0.714) and mtDNA haplotype diversity (h = 0.918) are both high in wahoo, while overall mtDNA nucleotide diversity (π = 0.006) and nuclear nucleotide diversity (π = 0.004) are uniformly low, indicating a recent increase in population size. Coalescence analyses yield an estimate of effective female population size (N eF ) at ~816 000, and a population bottleneck ~690 000 years ago. However, conclusions about population history from our Cytb data set are not concordant with a control region survey, a finding that will require further investi- gation. This is the first example of a vertebrate with a single globally distributed population, a finding we attribute to extensive dispersal at all life stages. The indications of a worldwide stock for wahoo reinforce the mandate for international cooperation on fisheries issues. Keywords: cosmopolitan marine species, mitochondrial DNA, nuclear DNA, pelagic fish dispersal, population genetic structure, stock structure Received 6 February 2008; revision received 3 June 2008; accepted 30 July 2008 Introduction The open tropical and subtropical oceans represent a homogeneous, continuous environment where lack of variation in habitat may limit opportunities for sympatric speciation, and lack of absolute barriers to gene flow, particularly for species with high fecundity and high dispersal capacity, may limit opportunities for allopatric speciation (Palumbi 1994). Despite this generalization, the concept of globally distributed marine fish species has seen a remarkable retreat in the last 50 years. Briggs (1960) published a list of 107 globally distributed shore fishes, but he correctly predicted that advances in taxonomy and genetics would reduce this list, and most of these are now recognized as multiple species. A spectacular example is the bonefishes (Albula spp.), regarded as a single worldwide species (A. vulpes) in 1961, but now divided into 10 species (Bowen et al. 2007). Although the taxonomy of large, highly mobile pelagic predators has proven more stable, genetic analyses have revealed deep evolutionary partitions within some species in this group (Graves 1998; Graves & McDowell 2003), as well as unexpected new species (Bass et al. 2005; Quattro et al. 2006). The wahoo, Acanthocybium solandri (Cuvier 1832), is one of these large, highly mobile, pelagic marine predators. It commonly reaches 20–40 kg (largest recorded 96.4 kg), and has a worldwide distribution within tropical, subtropical, and warm temperate waters (Collette & Nauen 1983). There is no reported variation in morphology or ecology Correspondence: Timothy C. Theisen, Fax: 954-236-1099; E-mail: ttheisen@fau.edu