ISSN 1948‐6596 guest editorial Biogeography and phylogeography of the Atlantic In recent years, the number of biogeographic and phylogeographic studies of marine species in the At‐ lantic Ocean has increased sufficiently that synthesis would be beneficial. In this issue, we present six pa‐ pers promoted by a workshop dedicated to identify‐ ing processes affecting biogeographic patterns of At‐ lantic marine organisms. The talks at the workshop and the articles in this issue help to define new bio‐ geographic hypotheses and provide research direc‐ tions for better understanding biogeographic pat‐ terns in the Atlantic Ocean. Contrary to recent proposals that warm tem‐ perate provinces be subsumed as parts of warm re‐ gions that include the tropics (Briggs & Bowen 2012), Almada et al. (2013) find that the Lusitania Province should be kept separate as evolution of its ichthyo‐ fauna since the Miocene has generated substantial endemism. Divisions also are apparent within the province, in the sharp separation between the ichthy‐ ofauna of the oceanic islands of Azores, Madeira and Canaries versus the mainland shores of western and southern Europe and Northwest Africa. Almada and colleagues also find that the Mediterranean emerges as a center of biological diversity which likely acted both as a refugium during glaciations and as a center of diversification for thermophilic fish. That the north Atlantic biota has been affected dramatically by climate change, particularly during glaciations, also is emphasized by Provan (2013). In his paper, Provan reviews the historical factors that have determined current patterns of genetic variation across the ranges of northern North Atlantic sessile species. Because the loss of genetic diversity is thought to diminish the adaptive potential of species, geographic regions that contain unique genetic varia‐ tion are important from a conservation viewpoint. The North Atlantic ‘rear‐edge’ (southernmost) popu‐ lations of nine species—a bryozoan, a mysid, a fish, a microturbellarian, two gastropods, three seaweeds— apparently were cryptic refugia for temperate marine organisms during the last glacial maximum of the Pleistocene, some 18,000 years ago. These refugia harbour unique genetic variation that is sometimes also associated with high levels of genetic diversity. There are, as always, exceptions and some species have higher than expected genetic diversity at higher latitudes. These and other inconsistencies suggest that there may not be a general pattern fitting all ma‐ rine species distributed on either side of the Atlantic. However, some groups of species show congruent patterns, and greater in‐depth work on individual species will generate knowledge from which can be drawn a general picture of biotic factors that shape biogeographic responses to climate fluctuations. Compilation of individual studies is the purview of comparative phylogeography, which can shed light on historical and contemporary population processes by providing independent and parallel datasets. The paper by Pereira and Almada (2013) presents a study on two migratory lampreys sympatric in western Europe, Petromyzon marinus (which also occurs in the western North Atlantic), and Lampetra fluviatilis (which is restricted to European watersheds). These two lampreys have contrasting levels of genetic diver‐ sity in Europe. P. marinus is much less genetically di‐ verse than L. fluviatilis. Pereira and Almada argue that the probable causes for this difference include the relative recency of P. marinus in Europe, compared to the longer evolutionary history of L. fluviatilis in the region, and higher cold tolerance that enabled L. fluvi‐ atilis to survive in areas with low glacial tempera‐ tures. Comparative phylogeographic analyses in other cases may highlight congruent genetic struc‐ tures among species, which indicates biogeographic boundaries resulting from long‐term barriers to gene flow or secondary contact between previously iso‐ lated lineages. In this issue, von der Heyden et al. (2013) use three phylogenetically related clinid fishes, which are co‐distributed along South Africa's coast but inhabit different tidal heights. This study design provided an opportunity to test for the effects of in‐ tertidal height on genetic population structure. The species inhabiting high intertidal tide pools showed greater levels of population divergence than subtidal and shallow‐water fishes, possibly because move‐ This manuscript is part of the proceedings of the Workshop on the Biogeography and Phylogeography of Atlantic Fish (Lisbon, November 2011). frontiers of biogeography 5.1, 2013 — © 2013 the authors; journal compilation © 2013 The International Biogeography Society