Evidence for a role of bathymetry and emergence in speciation in the genus Glycera (Glyceridae, Polychaeta) from the deep Eastern Weddell Sea

Polychaetes in the Southern Ocean are often thought to have wide distribution ranges on a horizontal and vertical scale. Here, this theory is tested for specimens commonly identified as the widely distributed glycerid Glycera kerguelensis using two molecular markers, the mitochondrial cytochrome oxidase c subunit I (COI) and the nuclear 28S rDNA. Identical morphospecies of three “populations” from three different habitats and two depth zones (abyssal plain 5,300 m, continental slope 2,000 m, sea mountain plateau 2,000 m) are compared. High genetic distances suggest the existence of three clades representing distinct species, identifying the investigated specimens as a complex comprising cryptic species with vertically restricted distribution. Two clades were found in sympatry on the Atka Bay slope in 2,000 m depth, one of these also found in similar depth on the plateau of the sea mountain Maud Rise. The third clade was limited to the abyssal plains (5,300 m) indicating the strong role of depth in the distribution of clades, possibly in conjunction with prevailing current systems. Evolution of the different clades is suggested to have resulted from a single emergence event with the origin of clades lying in the abyss.