ARCTIC-ANTARCTIC DISJUNCTIONS IN THE BENTHIC SEAWEEDS ACROSIPHONIA-ARCTA (CHLOROPHYTA) AND DESMARESTIA-VIRIDIS-WILLII (PHAEOPHYTA) ARE OF RECENT ORIGIN

Two examples of the most extreme biogeographic disjunctions in benthic marine algae are found in Acrosiphonia arcta (Chlorophyta) and Desmarestia viridis/willii (Phaeophyta). Both species are members of the Arctic and Antarctic boreal and subboreal marine floras. Although both genera have temperate species, neither genus has subtropical or tropical representatives. Comparisons of the fast-evolving ribosomal DNA internal transcribed spacer regions among isolates in each of the two species collected from both hemispheres showed an unexpected near sequence identity suggesting that these biogeographic disjunctions are of recent origin, possibly as recent as the last Pleistocene glacial maximum (18000 yr ago). Paleoclimatic explanations that rely on a much earlier transequatorial passage of cold-adapted species through a narrowed and cooler tropical belt during the Oligocene/Miocene (38 to 7 Ma ago) are unlikely. We hypothesize that despite the separated evolutionary histories of the northern and southern hemisphere cold-water marine floras, deep-water dispersal of microthalli has occurred and probably occurs on a regular basis.