UNCORRECTED PROOF High levels of genetic variability and panmixia of the tambaqui Colossoma macropomum (Cuvier, 1816) in the main channel of the Amazon River M. C. F. SANTOS*, M. L. RUFFINO† AND I. P. FARIAS*‡ *Universidade Federal do Amazonas (UFAM), Departamento de Cie ˆ ncias Biolo ´gicas, Laborato ´rio de Evoluc xa˜o e Gen etica Animal (LEGAL), Mini Campus, ICB, Av. Gen. Rodrigo Octa´vio Jorda˜o Ramos, 3000 – Coroado, 69077-000, Manaus, AM, Brasil and †ProVa´rzea/Ibama, Rua Ministro Joa˜o Gonc xalves de Souza, s/n° Distrito Industrial 69075-830, Manaus, AM, Brasil (Received 6 December 2005, Accepted 15 March 2007) In this study, the complete mitochondrial control region of 48 tambaqui Colossoma macro- pomum specimens from five localities along a 2200 km transect of the Amazon basin was analysed. High genetic variability was observed in all localities sampled. Analyses of molecular variance indicated that nearly all of the molecular variance was contained within localities, and estimates of gene flow among localities were high. These results suggest that the tambaqui forms a panmictic population along the Solimo˜es-Amazon River channel and are in agreement with species-typical behaviour of semi-migratory movements driven by dispersal for feeding and reproduction during its life cycle. Inspite of the observed high levels of genetic variability and mutation-drift equilibrium of the mtDNA control region, the tambaqui has experienced a 90% demographic decrease and reduction of size at maturity in the past two decades. Population estimates based on molecular markers therefore do not reflect its current demographic status but rather its status in ecological time. # 2007 The Authors Journal compilation # 2007 The Fisheries Society of the British Isles Key words: Colossoma macropomum; control region; genetic structure; genetic variability; mtDNA. INTRODUCTION The Amazonian ichthyofauna is very diverse and is represented by more than 3000 species known to science (Lundberg et al., 2000). Fishery studies carried out by Falabella (1994) in the Amazon State (Brazil) showed that only 36 spe- cies are exploited for consumption, of which the tambaqui Colossoma macropo- mum (Cuvier, 1816) is economically one of the most important species in the region. Numbers of attributes including large size and good flavour make it highly popular. The tambaqui is the largest characid of the Amazon basin, reaching total lengths (L T ) >1 m and weighing >30 kg (Isaac & Ruffino, B J F B 1 5 1 4 Journal Name Manuscript No. Dispatch: 13.4.07 Journal: JFB CE: Jayashree Author Received: No. of pages: 12 ME: Nagalakshmi ‡Author to whom correspondence should be addressed. Tel.: þ55926473244; fax: þ55926474233; email: izeni_farias@ufam.edu.br Journal of Fish Biology (2007) 71 (Supplement X), 1–12 doi:10.1111/j.1095-8649.2007.01514.x, available online at http://www.blackwell-synergy.com 1 # 2007 The Authors Journal compilation # 2007 The Fisheries Society of the British Isles