Freiwald A, Roberts JM (eds), 2005, Cold-water Corals and Ecosystems. Springer-Verlag Berlin Heidelberg, pp 679-690 Genetic circumscription of deep-water coral species in Canada using 18S rRNA Kevin B. Strychar 1,2 , Lorraine C. Hamilton 2 , Ellen L. Kenchington 2 , David B. Scott 1 1 Department of Earth Science, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada (strychar@dal.ca) 2 Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada Abstract. Many deep-water coral species have very broad global distributions and are eurybathic from depths of meters to kilometers. Such ecological breadth may be confounded by the presence of cryptic species. We are currently comparing the genetic distances between Paragorgia sp. and Primnoa sp. across their distribution and depth range in Canada using 18S ribosomal DNA (rDNA) sequences. Initial results show a confusing picture amongst the geographically distant Paragorgia taxa. Specimens of P. arborea from the Canadian Atlantic are very divergent from the specimen from the Canadian Pacific. The placement of Pennatula and Anthomastus relative to these taxa is also unexpected. We expect this topology to alter with the addition of more taxa and further testing. Keywords. Paragorgia, Antipatharian, Anthomastus, Pennatula, molecular genetics, 18S rDNA, Atlantic Canada Introduction Globally, five major taxonomic groups of deep-water corals (DWCs) have been identified (Cimberg et al. 1981), including: Alcyonacea (soft corals), Gorgonacea (sea fans, bamboo corals, and tree corals), Scleractinia (cup corals, stony corals, or true corals), Stylasterina (hydrocorals), and Antipatharia (black corals), many which appear to be concentrated in the northern latitudes. Along the eastern coast of North America, 35 species of DWCs are believed to exist (Breeze et al. 1997) at depths ranging between 100 and 1,300 m but have been observed at depths greater than 4,000 m (Tendal 1992; Cairns and Chapman 2001; Gass 2002), and include seven soft corals (alcyonaceans), ten horny corals (gorgonians), eight sea pens (pennatulaceans), and ~ten stony corals (scleractinians). Approximately ten species of gorgonian corals are known to occur off the coast of Nova Scotia in the Northwest Atlantic (Breeze et al. 1997). Of these, Paragorgia arborea (Linnaeus, 1758) is the largest, attaining heights of greater than 2 m. It is eurybathic, with a global depth range of between 50 and 3000 m (Horridge 1956; Broch and Horridge