The evolutionary history of seahorses (Syngnathidae: Hippocampus): molecular data suggest a West Pacific origin and two invasions of the Atlantic Ocean Peter R. Teske, * Michael I. Cherry, and Conrad A. Matthee Molecular Laboratory, Zoology Department, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 2 May 2003; revised 19 May 2003 Abstract Sequence data derived from four markers (the nuclear RP1 and Aldolase and the mitochondrial 16S rRNA and cytochrome b genes) were used to determine the phylogenetic relationships among 32 species belonging to the genus Hippocampus. There were marked differences in the rate of evolution among these gene fragments, with Aldolase evolving the slowest and the mtDNA cy- tochrome b gene the fastest. The RP1 gene recovered the highest number of nodes supported by >70% bootstrap values from parsimony analysis and >95% posterior probabilities from Bayesian inference. The combined analysis based on 2317 nucleotides resulted in the most robust phylogeny. A distinct phylogenetic split was identified between the pygmy seahorse, Hippocampus bargibanti, and a clade including all other species. Three species from the western Pacific Ocean included in our study, namely H. bargibanti, H. breviceps, and H. abdominalis occupy basal positions in the phylogeny. This and the high species richness in the region suggests that the genus evolved somewhere in the West Pacific. There is also fairly strong molecular support for the remaining species being subdivided into three main evolutionary lineages: two West Pacific clades and a clade of species present in both the Indo-Pacific and the Atlantic Ocean. The phylogeny obtained herein suggests at least two independent colonization events of the Atlantic Ocean, once before the closure of the Tethyan seaway, and once afterwards. Ó 2003 Elsevier Science (USA). All rights reserved. 1. Introduction Seahorses belong to the Syngnathidae, a teleost family whose oldest fossils date back to the Eocene (Lutetian: 52 mya; Patterson, 1993). The family also includes the pygmy pipehorses (grouped with seahorses in the subfamily Hippocampinae), pipehorses and sea- dragons (Solegnathinae), flag-tail pipefishes (Doryrh- amphinae), and pipefishes (Syngnathinae; Kuiter, 2000). The monophyly of seahorses is supported by a number of synapomorphic morphological characters distin- guishing them from most other Syngnathids. These characters include a prehensile tail, the absence of a caudal fin, the position of the head at a right angle to the trunk, a brood pouch sealed along the midline (except for a small anterior opening), and a raised dorsal fin base (Fritzsche, 1980). Seahorses (genus Hippocampus) and possibly also pygmy pipehorses (genera Amp- helikturus, Acentronura, and Idiotropiscis), are phyloge- netically most closely associated with pipefishes of the genus Syngnathus (Wilson et al., 2001). The worldÕs tropical marine faunas can be divided into those associated with an Atlantic Ocean biome (including the Caribbean and Mediterranean), and those associated with an Indo-Pacific biome (Rosen, 1988). It has been suggested that this pattern arose after the closure of the Tethyan seaway, a tectonic event that resulted from the convergence of the African and Eur- asian plates during the late Oligocene and Miocene (Rosen, 1988). Seahorses are found throughout the tropical and temperate regions of both the Atlantic and Indo-Pacific biomes, but their origin and evolutionary history are not well understood. In a study based on Molecular Phylogenetics and Evolution 30 (2004) 273–286 MOLECULAR PHYLOGENETICS AND EVOLUTION www.elsevier.com/locate/ympev * Corresponding author. E-mail addresses: pt1@sun.ac.za (P.R. Teske). 1055-7903/$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S1055-7903(03)00214-8