A multigene molecular assessment of cryptic biodiversity in the iconic freshwater blackfishes (Teleostei: Percichthyidae: Gadopsis) of south-eastern Australia MICHAEL P. HAMMER 1,2,3 *, PETER J. UNMACK 4,5 , MARK ADAMS 1,2 , TARMO A. RAADIK 6 and JERALD B. JOHNSON 4 1 Evolutionary Biology Unit, South Australian Museum, North Terrace, SA 5000, Australia 2 Australian Centre for Evolutionary Biology and Biodiversity, School of Earth and Environmental Science, The University of Adelaide, Adelaide, SA 5005, Australia 3 Curator of Fishes, Museum and Art Gallery of the Northern Territory, PO Box 4646, Darwin, NT 0801, Australia 4 WIDB 401, Department of Biology, Brigham Young University, Provo, UT 84602, USA 5 Institute for Applied Ecology and Collaborative Research Network for Murray-Darling Basin Futures, University of Canberra, Canberra, ACT 2601, Australia 6 Aquatic Ecology Section, Arthur Rylah Institute for Environmental Research, Department of Environment and Primary Industries, 123 Brown Street, Heidelberg, VIC 3084, Australia Received 6 September 2013; revised 22 October 2013; accepted for publication 22 October 2013 Freshwater biodiversity is under ever increasing threat from human activities, and its conservation and manage- ment require a sound knowledge of species-level taxonomy. Cryptic biodiversity is a common feature for aquatic systems, particularly in Australia, where recent genetic assessments suggest that the actual number of freshwater fish species may be considerably higher than currently listed. The freshwater blackfishes (genus Gadopsis) are an iconic group in south-eastern Australia and, in combination with their broad, naturally divided distribution and biological attributes that might limit dispersal, as well as ongoing taxonomic uncertainty, they comprise an ideal study group for assessing cryptic biodiversity. We used a multigene molecular assessment including both nuclear (51 allozyme loci; two S7 introns) and matrilineal markers (cytb) to assess species boundaries and broad genetic substructure within freshwater blackfishes. Range-wide examination demonstrates the presence of at least six candidate species across two nominal taxa, Gadopsis marmoratus and Gadopsis bispinosus. Phylogeographical patterns often aligned to purported biogeographical provinces but occasionally reflected more restricted and unexpected relationships. We highlight key issues with taxonomy, conservation, and management for a species group in a highly modified region. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 521–540. ADDITIONAL KEYWORDS: conservation – cryptic species – drainage divides – ESU – phylogeography – sea level changes. INTRODUCTION Australia is regarded as one of the world’s top 17 megadiverse countries (Williams et al., 2001) and this is generally reflected in the species richness and levels of endemism displayed for many organismal groups (Chapman, 2009). Among non-marine verte- brates, however, there is one glaring exception. Aus- tralia’s freshwater fish fauna has long been regarded as depauperate compared to those found in other *Corresponding author. E-mail: michael.hammer@nt.gov.au Biological Journal of the Linnean Society, 2014, 111, 521–540. With 6 figures © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 521–540 521