BIODIVERSITY RESEARCH Fathom out: biogeographical subdivision across the Western Australian continental margin – a multispecies modelling approach Skipton N. C. Woolley 1 *, Anna W. McCallum 1 , Robin Wilson 1 , Timothy D. O’Hara 1 and Piers K. Dunstan 2 1 Museum Victoria, GPO Box 666, Melbourne, Vic. 3001, Australia, 2 CSIRO Wealth from Oceans Flagship, Hobart, Tas., Australia *Correspondence: Skipton N. C. Woolley, Museum Victoria, GPO Box 666, Melbourne, Vic., 3001, Australia. E-mail: swoolley@museum.vic.gov.au ABSTRACT Aim Biogeographical regions are often used as a basis for management strate- gies, yet a challenge for biodiversity management across broad scales is estab- lishing biogeographical regions that are robust across taxonomic groups. Methods Finite mixture models were developed to predict multiple species assemblages termed archetypes. Modelled species archetypes were developed using Decapoda, Ophiuroidea and Polychaeta species, which were grouped based on their similar responses to oceanographic and geographical gradients. Location Outer-shelf and slope (50–1200 m) of the continental margin of Western Australia (~11° S–36° S). Results Four faunal regions were defined based on cross-taxa surrogates grouped as archetypes. These faunal regions were defined by oxygen, salinity, carbon and temperature gradients across latitude and bathymetry. Two broad latitudinal bands and two bathyal regions were described. Adjacent faunal groups were not defined by abrupt geographical breaks but rather transitions. Main conclusions These results suggest that faunal distributions were less finely resolved than existing marine bioregions on the Western Australian con- tinental margin and that environmental gradients are correlated with distribu- tions of benthic marine invertebrates. Identifying biogeographical regions based on these methods has the potential to inform management across a broad range of environments. Keywords Continental margin, marine biodiversity, finite mixture models, biogeography, bioregions, benthic invertebrates. INTRODUCTION Biogeographical regions, often called ‘bioregions’, are repre- sentative areas used to protect a full spectrum of biodiversity components such as genes, species and higher taxa, along with the communities, evolutionary patterns and ecological processes that sustain this diversity (Last et al., 2010). Biore- gions are ideal conservation units at regional scales because they represent similar communities of flora and fauna (Whit- taker et al., 2005) and have boundaries where key ecological and historical processes most strongly interact (Olson & Dinerstein, 1998; Olson et al., 2001; Spalding et al., 2007). Bioregions are often used as a rationale for spatial management of natural environments and as a starting point for establishing priority areas for conservation (Olson et al., 2001; Spalding et al., 2007). This study uses new data from a number of key faunal groups and a sound statistical frame- work to investigate bioregions across a 3000 km transect on the Western Australian continental margin. Traditional approaches to documenting bioregions have involved qualitative interpretations of endemism and distri- bution patterns (Ekman, 1953; Springer, 1982; Zezina, 1997; Briggs & Bowen, 2013) or are based on environmental, geophysical and biological surrogates (Spalding et al., 2007; the Global Open Oceans and Deep Seabed (GOODS) classification, UNESCO, 2009). Quantitative studies and DOI: 10.1111/ddi.12119 ª 2013 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/ddi 1 Diversity and Distributions, (Diversity Distrib.) (2013) 1–12 A Journal of Conservation Biogeography Diversity and Distributions