Abstract
Molecular studies have reported the coexistence of two species ofAgarophytonin New Zealand: the newly describedA. transtasmanicumwith an apparently restricted distribution to some sites in the North Island, and the more widespreadA.chilense. Here, we compared the distribution, genetic diversity, and structure of bothAgarophytonspecies throughout the archipelago using sequences of the nuclear Internal Transcribed Spacer 2 (ITS2) marker.Agarophyton chilense's distribution was continuous and extensive along the North and South Islands, Stewart Island, and Chatham Island, and the genetic clusters were mostly concordant with boundaries between biogeographic regions. In contrast, specimens ofA. transtasmanicumwere collected in four sites broadly distributed in both the North and South Islands, with no clear spatial structure of the genetic diversity. Populations, where the species co-occurred, tended to display similar levels in genetic diversity for the two species. Demographic inferences supported a postglacial demographic expansion for twoA.chilensegenetic clusters, one present in the South Island and the eastern coast of the North Island, and the other present in northern South Island. A third genetic cluster located on the western coast of the North Island had a signature of long-term demographic stability. ForA. transtasmanicum, the skyline plot also suggested a postglacial demographic expansion. Last, we developed a new molecular tool to quickly and easily distinguish between the twoAgarophytonspecies, which could be used to ease future fine-scale population studies, especially in areas where the two species coexist.