The biogeography of kelps (Laminariales, Phaeophyceae): a global analysis with new insights from recent advances in molecular phylogenetics

Despite their ecological and economic importance, no summary of kelp global biogeography has been produced for almost two decades. The circumscription of the order Laminariales and familial and generic relationships in the group have changed considerably recently, in the light of molecular data. A global summary and geographical analysis of kelp species and their distributions (112 species in 33 genera) is presented. These data are analysed and discussed from the perspective of the new consensus of relationships within the group, and likely evolutionary events. The putative ancestors of the kelps occur and are overwhelmingly most diverse, in the cooler waters of northern Japan. The biogeographical evidence suggests three main lines of subsequent evolution: (a) a diversification producing the four 'derived' families Alariaceae, Costariceae, Laminariaceae, and Lessoniaceae and most extant genera in the temperate northern Pacific, probably during the Miocene. (b) The evolution of an Arctic flora which invaded the North Atlantic following the opening of the Bering Strait ca. 5.5 Ma. (c) At least four separate crossings, by different genera, of tropical regions from Northern to Southern Hemisphere (and one in the opposite direction). The recorded impacts of man on these distributions have thus far been minimal, with the notable exceptions of Undaria pinnatifida and species of Saccharina (grown in aquaculture systems for human food). Most genera are monospecific, with many confined to either the western or eastern temperate North Pacific, whereas the distribution of the most species-rich genera (Alaria, Laminaria, Saccharina) includes the Arctic, and they are widespread in the North Atlantic. This rapid species-level evolution is hypothesised to have been promoted by the relatively recent invasion of the Atlantic by these taxa. The crossing of the tropics has occurred in warm-temperate species some of which occur and are sometimes abundant, in deeper water in today's tropics, refuting the widespread view that kelps are only present in cold-water habitats. Most of these Southern Hemisphere kelps are in the family Lessoniaceae, including the only genus not present in the Northern Hemisphere, Lessonia. The origins of this genus are unclear. Knowledge of the phylogeography of the Ecklonia/Eisenia complex is particularly important in understanding Southern Hemisphere kelp biogeography. A number of groups are undergoing active speciation and, with the lack of a consistent species concept, the species-level taxonomy of the group remains to be clarified. There is a great need for molecular phylogeographical studies to provide evidence for many of the hypotheses presented here based on distributional evidence and the currently accepted taxonomic framework.