Aim Drivers of extant population genetic structure include past climate‐driven range shifts and vicariant events, as well as gene flow mediated by dispersal and habitat continuity. Their integration as alternative or complementary drivers is often missing or incomplete, potentially overlooking relevant processes and time scales. Here we ask whether it is the imprint of past range shifts or habitat connectivity driven by oceanographic transport that best explain genetic structure in a poorly understood model, a forest‐forming African kelp. Location Southwestern coast of Africa (Benguela current region). Taxon Laminaria pallida. Methods We estimated genetic variability along the species distributional range using 14 microsatellite markers. This genetic variability was compared to estimates of past range shifts derived from species distribution modelling for the Last Glacial Maximum (LGM), the mid‐Holocene (MH) and the present, and estimates of habitat connectivity derived from oceanographic biophysical modelling. Results The species is structured in two clusters, a southern cluster with much richer (allelic richness A: 10.40 ± 0.33) and unique (private alleles PA: 56.69 ± 4.05) genetic diversity, and a northern cluster (A: 4.75 ± 0.17; PA: 6.70 ± 1.45). These clusters matched well‐known biogeographical regions and their transition coincided with a dispersal barrier formed by upwelled offshore transport. No major range shifts or vicariant events were hindcasted along the present range, suggesting population stability from the LGM to the present. Main conclusions Habitat connectivity, rather than past range shifts, explains the extant population structure. Future environmental requirements of the species along the Benguela upwelling system are projected to persist or even intensify, likely preserving the observed genetic patterns for the years to come. Yet, the differentiation and endemicity between clusters, and the isolation structured by the regional oceanography, implies high conservation value for genetic biodiversity, and even more if considering the ecological, social and economic services provided by kelp forests.

Brown algae of the order Laminariales, commonly referred to as kelps, are the largest and most productive primary producers in the coastal inshore environment. The genus Ecklonia (Lessoniaceae, Phaeophyceae) consists of seven species with four species in the Northern Hemisphere and three in the Southern Hemisphere. It was recently transferred to the family Lessoniaceae based on phylogenetic analyses of nuclear and chloroplastic markers, though the type of the genus was not included and its relationship with allied genera Eckloniopsis and Eisenia remained unresolved. The present study is the first to produce a phylogeny focussed on the genus Ecklonia. It included sequences from nuclear, mitochondrial, and chloroplastic DNA, for most of the distribution range of the three current Southern Hemisphere species (Ecklonia radiata, Ecklonia maxima, and a sample of a putative Ecklonia brevipes specimen), sequences for East Asiatic species (Ecklonia cava, Ecklonia kurome, and Ecklonia stolonifera), as well as the closely related genera Eckloniopsis and Eisenia. Results confirmed E. radiata and E. maxima as two distinct species in South Africa, E. radiata as a single species throughout the Southern Hemisphere (in South Africa, Australia, and New Zealand) and East Asiatic species as a distinct lineage from the Southern Hemisphere clade. Results further pointed out a close sister relationship between Eckloniopsis radicosa and two Eisenia species (including the type species: Eisenia arborea) to the genus Ecklonia suggesting that the genera Eckloniopsis and Eisenia are superfluous.

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The genus Laminaria has a wide distribution range compared with other kelp genera because it is found in both the North and the South Atlantic, on both sides of the North Pacific, as well as in the Mediterranean. Hypotheses behind this biogeographical pattern have been discussed by several authors but have not yet been fully evaluated with time-calibrated phylogenies. Based on the analysis of four molecular markers (ITS2, rbcL, atp8 and trnWI), our goal was to reassess the Laminaria species diversity in South Africa, assess its relationship with the other species distributed in the South Atlantic and reconstruct the historical biogeography of the genus. Our results confirm the occurrence of a single species, L. pallida, in southern Africa, and its sister relationship with the North Atlantic L. ochroleuca. Both species belonged to a clade containing the other South Atlantic species: L. abyssalis from Brazil, and the Mediterranean L. rodriguezii. Our time-calibrated phylogenies suggest that Laminaria originated in the northern Pacific around 25 mya, followed by at least two migration events through the Bering Strait after its opening (~5.32 mya). Today, the first is represented by L. solidungula in the Arctic, while the second gave rise to the rest of the Atlantic species. The colonization of the North Atlantic was followed by a gradual colonization southward along the west coast of Europe, into the Mediterranean (~2.07 mya) and two recent, but disconnected, migrations (~1.34 and 0.87 mya) across the equator, giving rise to L. abyssalis in Brazil and L. pallida in southern Africa, respectively.

Long-distance dispersal is one of the main drivers structuring the distribution of marine biodiversity. This study reports the first occurrence of Macrocystis pyrifera and Durvillaea antarctica rafts on the southwestern warm temperate coast of the Atlantic Ocean. Our results indicate that an extreme meteo-oceanographic event, characterized by a northward, displacement of cold sub-Antarctic oceanic waters driven by an extratropical cyclone, could account for these unusual occurrences. A niche model based on known current distribution and maximum entropy principle (MAXENT), revealed the availability of suitable habitats at lower latitudes, outside their actual distribution edges. The distributional boundaries, mainly driven by temperature and irradiance, suggest the existence of environmental suitability in warm temperate areas, as well as in the Northern Hemisphere off Atlantic and Asian coasts. These theoretical edges and respective environmental drivers agree with the physiological affinities of both species, supporting the hypothesis that these variables act as limiting factors for their occurrences in tropical or warmer areas. Emerging regions can function as Diversity 2018, 10, 11; doi:10.3390/d10010011 www.mdpi.com/journal/diversity Diversity 2018, 10, 11 2 of 25 refuges and stepping-stones, providing substrate with adequate habitat conditions for recruitment of propagules, allowing eventual colonization. Long dispersal events reinforce the need for an extensive discussion on selective management of natural dispersion, biological invasions, refuge mapping and conservation initiatives in a transnational perspective.