BACKGROUND AND AIMS:Kelps are the primary foundation species in temperate subtidal rocky shores worldwide. However, global change is causing their decline with consequences for the organisms that rely on them. An accurate assessment of these consequences may depend on which attributes of the associated community are considered. This study shows that conventional α-diversity approaches may overlook some of these consequences compared to spatially explicit approaches such as with β-diversity. METHODS:A 1-year seasonal study was conducted to compare the macroalgal understorey between healthy reefs with a Laminaria ochroleuca canopy and degraded reefs where the canopy collapsed years ago due to excessive fish herbivory. At each reef, the understorey seaweed assemblage was recorded in five replicate quadrats to estimate α-diversity (total richness, species density, Shannon index) and β-diversity (intra- and inter-reef scale). KEY RESULTS:The understorey assemblage exhibited a distinct seasonal dynamic in both healthy and degraded reefs. α-Diversity attributes increased in spring and summer; turf-forming algae were particularly dominant in degraded reefs during summer. β-Diversity also showed seasonal variability, but mostly due to the changes in degraded reefs. None of the α-diversity estimates differed significantly between healthy and degraded reefs. In contrast, spatial β-diversity was significantly lower in degraded reefs. CONCLUSIONS:Although the loss of the kelp canopy affected the composition of the macroalgal understorey, none of the conventional indicators of α-diversity detected significant differences between healthy and degraded reefs. In contrast, small-scale spatial β-diversity decreased significantly as a result of deforestation, suggesting that the loss of kelp canopy may not significantly affect the number of species but still have an effect on their spatial arrangement. Our results suggest that small-scale β-diversity may be a good proxy for a more comprehensive assessment of the consequences of kelp forest decline.

Rhodolith seabeds are ‘ecosystem engineers’ composed of free-living calcareous red macroalgae, which create extensive marine habitats. This study addressed how depth influenced the structure (size and morphology) of rhodoliths and the abundance of associated floral and faunal epibionts across the Eastern Atlantic Ocean. Sampling was carried out at two sites within five regions (Brittany, Galicia, Madeira, Gran Canaria, and Principe Island), from temperate to tropical, covering a latitudinal gradient of 47°, in three depth strata (shallow, intermediate and deep), according to the rhodolith bathymetrical range in each region. Depth typically affected the rhodolith size at all regions; the largest nodules were found in the intermediate and deep strata, while rhodolith sphericity was larger at the shallow depth strata. Higher biomasses of attached macroalgae (epiphytes) were observed at depths where rhodoliths were larger. The abundance of epifauna was variable across regions and depth strata. In general, the occurrence, structure, and abundance of the associated biota across rhodolith habitats were affected by depth, with local variability (i.e., sites within regions) often displaying a more significant influence than the regional (large-scale) variation. Overall, this study showed that the rhodolith morphology and associated epibionts (flora and fauna) were mostly affected by depth, irrespective of latitude.

The analysis of biological and ecological traits has a long history in evolutionary and ecological research. However, trait data are often scattered and standardised terminology that transcends taxonomic and biogeographical context are generally missing. As part of the development of a global trait database of marine species, we collated trait information for European seaweeds and structured the data within the standardised framework of the World Register of Marine Species (WoRMS). We collected 45 175 trait records for 21 biologically and ecologically relevant traits of seaweeds. This resulted in a trait database for 1745 European seaweed species of which more than half (56 %) of the records were documented at the species level, while the remaining 44 % were documented at a higher taxonomic level and subsequently inherited at lower levels. The trait database for European seaweeds will serve as a foundation for future research on diversity and evolution of seaweeds and their responses to global changes. The data will contribute to developing detailed trait-based ecosystem models and will be an important tool to inform marine conservation policies. The data are publicly accessible through the AlgaeTraits portal, https://doi.org/10.14284/574 (AlgaeTraits, 2022).

Effective monitoring of non-indigenous seaweeds and combatting their effects relies on a solid confirmation of the non-indigenous status of the respective species. We critically analysed the status of presumed non-indigenous seaweed species reported from the Mediterranean Sea, the Northeast Atlantic Ocean and Macaronesia, resulting in a list of 140 species whose non-indigenous nature is undisputed. For an additional 87 species it is unclear if they are native or non-indigenous (cryptogenic species) or their identity requires confirmation (data deficient species). We discuss the factors underlying both taxonomic and biogeographic uncertainties and outline recommendations to reduce uncertainty about the non-indigenous status of seaweeds. Our dataset consisted of over 19,000 distribution records, half of which can be attributed to only five species (Sargassum muticum, Bonnemaisonia hamifera, Asparagopsis armata, Caulerpa cylindracea and Colpomenia peregrina), while 56 species (40%) are recorded no more than once or twice. In addition, our analyses revealed considerable variation in the diversity of non-indigenous species between the geographic regions. The Eastern Mediterranean Sea is home to the largest fraction of non-indigenous seaweed species, the majority of which have a Red Sea or Indo-Pacific origin and have entered the Mediterranean Sea mostly via the Suez Canal. Non-indigenous seaweeds with native ranges situated in the Northwest Pacific make up a large fraction of the total in the Western Mediterranean Sea, Lusitania and Northern Europe, followed by non-indigenous species with a presumed Australasian origin. Uncertainty remains, however, regarding the native range of a substantial fraction of non-indigenous seaweeds in the study area. In so far as analyses of first detections can serve as a proxy for the introduction rate of non-indigenous seaweeds, these do not reveal a decrease in the introduction rate, indicating that the current measures and policies are insufficient to battle the introduction and spread of non-indigenous species in the study area.

Coralline red algal flora of NW Iberia was re-visited in the framework of a recent doctoral thesis. The present study provides information about 22 species, 9 taxa are new records for Galicia (Harveylithon samoense, Hydrolithon boreale, H. sargassi, Lithophyllum crouaniorum, Pneophyllum confervicola, P. coronatum, P. limitatum, P. myriocarpum and Titanoderma pustulatum var. confine). 8 are new for continental Portugal (Hydrolithon boreale, Lithophyllum bathyporum. L. crouaniorum, Mesophyllum expansum, Pneophyllum myriocarpum, Titanoderma corallinae, T. laminariae and T. pustulatum var. confine) and Phymatolithon lamii for northern Portugal. At province level, 14 taxa are new records and 8 are second records. In addition, Lithophyllum duckerae and Phymatolithon purpureum are excluded from the Atlantic Iberia flora.

Background Syngnathid fishes (Actinopterygii, Syngnathidae) are flagship species strongly associated with seaweed and seagrass habitats. Seahorses and pipefishes are highly vulnerable to anthropogenic and environmental disturbances, but most species are currently Data Deficient according to the IUCN (2019), requiring more biological and ecological research. This study provides the first insights into syngnathid populations in the two marine Spanish National Parks (PNIA—Atlantic- and PNAC—Mediterranean). Fishes were collected periodically, marked, morphologically identified, analysed for size, weight, sex and sexual maturity, and sampled for stable isotope and genetic identification. Due the scarcity of previous information, habitat characteristics were also assessed in PNIA. Results Syngnathid diversity and abundance were low, with two species identified in PNIA ( Hippocampus guttulatus and Syngnathus acus ) and four in PNAC ( S. abaster , S. acus , S. typhle and Nerophis maculatus ). Syngnathids from both National Parks (NP) differed isotopically, with much lower δ 15 N in PNAC than in PNIA. The dominant species were S. abaster in PNAC and S. acus in PNIA. Syngnathids preferred less exposed sites in macroalgal assemblages in PNIA and Cymodocea meadows in PNAC. The occurrence of very large specimens, the absence of small-medium sizes and the isotopic comparison with a nearby population suggest that the population of Syngnathus acus (the dominant syngnathid in PNIA) mainly comprised breeders that migrate seasonally. Mitochondrial cytochrome b sequence variants were detected for H. guttulatus , S. acus, and S. abaster , and a novel 16S rDNA haplotype was obtained in N. maculatus . Our data suggest the presence of a cryptic divergent mitochondrial lineage of Syngnathus abaster species in PNAC. Conclusions This is the first multidisciplinary approach to the study of syngnathids in Spanish marine NPs. Habitat preferences and population characteristics in both NPs differed. Further studies are needed to assess the occurrence of a species complex for S. abaster , discarding potential misidentifications of genus Syngnathus in PNAC, and evaluate migratory events in PNIA. We propose several preferential sites in both NPs for future monitoring of syngnathid populations and some recommendations for their conservation.

Las algas rojas coralinas conforman un grupo bentónico diverso en el noroeste ibérico, el cual ha sido revisitado en el contexto de una reciente tesis doctoral. En el presente trabajo, se aportan datos para 22 especies, siendo 9 de ellas (Harveylithon samoënse, Hydrolithon boreale, H. cf. sargassi, Lithophyllum cf. crouaniorum, Pneophyllum cf. confervicola, P. coronatum, P. cf. limitatum, P. cf. myriocarpum y Titanoderma pustulatum var. confine) nuevas citas para Galicia. Por otra parte, las 8 especies Hydrolithon boreale, Lithophyllum bathyporum. L. cf. crouaniorum, Mesophyllum expansum, Pneophyllum cf. myriocarpum, Titanoderma corallinae, T. laminariae y T. pustulatum var. confine son novedad para Portugal continental y Phymatolithon lamii para norte de Portugal. Paralelamente, se dan a conocer 14 nuevas citas provinciales y 8 segundas citas provinciales. También se excluye Lithophyllum duckerae y Phymatolithon purpureum de la flora del Atlántico ibérico.

Background: Syngnathid fishes (Actinopterygii, Syngnathidae) are flagship species highly associated to seaweed and seagrass habitats of marine ecosystems biodiversity. Seahorses and pipefish are highly vulnerable to anthropogenic and environmental disturbances, but most species are currently Data Defficient by IUCN (IUCN, 2019), requiring more biological and ecological research. This study provides the first insights on syngnathid populations in two Spanish National Parks (PNIA –Atlantic- and PNAC –Mediterranean-). Fish were collected periodically, marked, morphologically identified, analyzed for size, weight, gender and sexual maturity, and sampled for further stable isotope and genetic identification. Due the scarcity of previous information, habitat characteristics were also assessed in PNIA. Results: Syngnathid diversity and abundances were low, with two species identified in PNIA ( Hippocampus guttulatus and Syngnathus acus ) and four in PNAC ( S. abaster , S. acus , S. typhle and Nerophis maculatus ). Syngnathids from both NPs differed isotopically, with much lower δ15 N in PNAC. The dominant species were S. abaster in PNAC and particularly S. acus in PNIA. Syngnathids preferred less exposed sites in macroalgal assemblages in PNIA and Cymodocea meadows in PNAC. In S. acus from PNIA, the occurrence of very large specimens, the absence of small-medium sizes and the isotopic comparison with a nearby population suggest that the population is mainly founded by breeders that migrate seasonally. Novel 16S rDNA haplotypes and sequence variants were detected for Hippocampus guttulatus , N. maculatus , S. acus, and S. abaster . Our data suggest the presence of a cryptic Syngnathus species in PNAC, Conclusions: This is the first multidisciplinary approach to the study of syngnathids in Spanish marine NPs. Habitat preferences and population characteristics in both NPs differed. Further studies are needed to assess potential misidentifications of Syngnathus genus in PNAC, and migratory events in PNIA. We propose several preferential sites in both NPs for future monitoring of syngnathid populations and some recommendations to undertake.

The loss of canopy‐forming seaweeds from urbanized coasts has intensified in response to warming seas and non‐climatic pressures such as population growth and declining water quality. Surprisingly, there has been little information on the extent of historical losses in the South‐western Atlantic, which limits our ability to place this large marine ecosystem in a global context. Here, we use meta‐analysis to examine long‐term (1969–2017) changes to the cover and biomass of Sargassum spp. and structurally simple algal turfs along more than 1,000 kilometres of Brazil's warm temperate coastline. Analysis revealed major declines in canopy cover that were independent of season (i.e., displaying similar trends for both summer and winter) but varied with coastal environmental setting, whereby sheltered bays experienced greater losses than coastal locations. On average, covers of Sargassum spp. declined by 2.6% per year, to show overall losses of 52% since records began (ranging from 20% to 89%). This contrasted with increases in the cover of filamentous turfs (24% over the last 27 years) which are known to proliferate along human‐impacted coasts. To test the relative influence of climatic versus non‐climatic factors as drivers of this apparent canopy‐to‐turf shift, we examined how well regional warming trends (decadal changes to sea surface temperature) and local proxies of coastal urbanization (population density, thermal pollution, turbidity and nutrient inputs) were able to predict the changes in seaweed communities. Our results revealed that the most pronounced canopy losses over the past 50 years were at sites exhibiting the greatest degree of coastal warming, the highest population growth and those located in semi‐enclosed sheltered bays. These findings contribute knowledge on the drivers of canopy loss in the South‐western Atlantic and join with global efforts to understand and mitigate declines of marine keystone species.

Material of Jania J.V.Lamour. with a different morphology from the typical one attributed to J. longifurca Zanardini and J. rubens (L.) J.V.Lamour.—the only species in this habitat in Galicia—has been detected in the maerl beds of Galicia. This morphology consists on: thin intergenicula, an irregular pattern of dichotomies and secondary attachment discs that facilitate the refixation to the substrate, all of them similar to those of other southern species of Jania. Nevertheless, the integrative taxonomic study of this material and other collections from rocky habitat confirmed its identification as J. longifurca. Thereby, the atypical morphology with thin intergenicula and secondary attachment disc is dominant in samples of J. longifurca from maerl beds, while the samples from rocky environments showed the typical robust intergenicula. Moreover, molecular analysis resolved some phylogenetic affinities with samples from New Zealand, where this species is not recorded. It is concluded therefore the morphological variation related to the environment and it is evident the need of molecular studies to clarify this genus taxonomy, which is based fundamentally on morphological characters.

Continuando con los estudios para la actualización del inventariado de la flora bentónica marina del Atlántico Ibérico.En este trabajo se incluye nueva información de distribución para 18 especies (3 Cyanobacteria, 9 Rhodophyta, 4 Ochrophyta, 2 Chlorophyta) de algas bentónicas marinas, recolectadas en el intermareal y submareal de 46 localidades del atlántico ibérico norte. Atendiendo a la distribución de las especies recolectadas, cabe destacar una nueva cita para Europa (Plocamium cf. ovicorne), 3 nuevas citas para Galicia (Calothrix consociata, Lyngbya martensiana y Centroceras gasparrinii), 14 nuevas citas provinciales y 8 segundas citas provinciales. Además dos especies (Spermothamnion strictum y Punctaria plantaginea) son excluidas de la flora de Galicia después, de estudiar el único material de herbario testigo de las citas.et al. 2005a-b, 2006, 2008, 2012, 2014, 2015, 2016), se presentan nuevas citas y correcciones florísticas que completan el actual inventario de la biodiversidad marina de Galicia (Bañón 2017). La mayoría de las muestras se conservaron en formalina al 4% y la conservación definitiva se realizó en pliegos de herbario, depositados en el herbario de la Universidad de Santiago de Compostela (SANT), así como en preparaciones semipermanentes en Karo®. También se estudiaron pliegos de herbario del Nationaal Herbarium Nederland, para la verificación o corrección de citas antiguas. Paralelamente, se conservaron fragmentos sin formolar de algunas especies en gel de sílice para estudios moleculares.

Molecular phylogeny of Eudesme specimens collected from various localities in the Northern Hemisphere using mitochondrial cox1 and cox3 and chloroplast atpB, psaA, psbA and rbcL gene sequences revealed an undescribed species in the genus. In the analyses, three distinct clades (clades 1, 2 and 3) with high statistical supports were recognised. Specimens of clade 1 corresponded to the generitype E. virescens because the morphology agreed with the reported descriptions of the species, and the distribution (subboreal Europe) included the type locality. Clade 2 had a broad distribution in the Arctic Sea and coldwater regions of the northern Pacific and northern Atlantic. Specimens of clade 2 were morphologically distinguishable from E. virescens in having broader sporophytes. Specimens of clade 3 collected from western Honshu, Japan, had cox3 sequence identical to one of an old voucher specimen of E. shandongensis (63-519) described from the Yellow Sea. We propose the establishment of a new species, E. borealis, distributed in the Arctic, northern Pacific, and northern Atlantic regions.

The composition, abundance, and distribution of epiphytic macroalgae living in meadows of Zostera marina L. in the northwestern Iberian Peninsula are here analyzed. We identified 63 species: 40 red algae, 16 brown algae, and 7 green algae. Most of them are classified as filamentous or filiform functional forms, while Pneophyllum fragile Kütz. was the only encrusting species. In general, the surface covered by epiphytes on the leaves of Zostera marina was low and a 43% of species were only found in juvenile stages. Regarding their frequency, 10 species were collected in the majority of the areas, while others were rare. Most species were found both epiphytic and in other substrata of the meadows, but 9 were exclusively epiphytic. We detected 9 introduced species.

Species of the genus Ulva are common in anthropogenically disturbed areas and have been reported as the cause of green tides in many areas of the world. In addition, they rank among the main marine groups used in a wide range of commercial applications. By displaying few distinctive morphological characters, some taxonomical identifications are difficult and the genus is under a conundrum. Our aims were to provide ecophysiological information about three Ulva species in response to abiotic factors and to evaluate the proposal of ecophysiological information and the chlorophyll-a fluorescence technique as auxiliary tool to resolve the long-standing taxonomic confusion. We hypothesize that three cooccurring specimens (U. fasciata Delile, U. lactuca Linnaeus, and U. rigida C. Agardh) have different ecophysiological responses (as measured by the effective quantum yield of photosystem II by pulse amplitude modulated fluorometers) under manipulated conditions of temperature and nutrient concentration. Ulva lactuca and U. rigida showed different photosynthetic efficiencies related to temperature, whereas no difference was recorded for U. fasciata individuals. These results provide a reasonable explanation for the variability in spatial and temporal abundance of these species of Ulva on rocky shores. We proposed the use of ecophysiological information by chlorophyll-a fluorescence as an auxiliary tool to corroborate the taxonomic distinction of Ulva species. We reinforce the statement of U. fasciata and U. lactuca as distinct valid species.

Canopy-forming seaweeds sustain critical ecosystem services in coastal habitats. Around the world, many of these seaweeds are suffering strong declines, mainly attributed to the progressive increase in sea surface temperature, in combination with other stressors due to current global changes. The southernmost part of the NE Atlantic is among those areas most affected by climate change. In this study, we estimated the distributional contractions of seven of the most conspicuous seaweeds from the Atlantic coasts of the Iberian Peninsula using an Extent of Occurrence methodology. Overall, during the last three decades, range shifts have been more pronounced east of the Cantabrian Sea than along the western coast of the Iberian Peninsula. In particular, regions with a semi-permanent summer upwelling seem to be critical to the persistence of brown seaweeds, fucoids and kelps. Range contractions of the cold-temperate fucoids were estimated to be ca. 21% and 45% for Himanthalia elongata and Fucus serratus, respectively; and for the kelps Saccharina latissima and Laminaria hyperborea, 6% and 14%, respectively. Range contractions for warm-temperate kelps were estimated to be ca. 13% and 10% for Saccorhiza polyschides and L. ochroleuca, respectively. Finally, a decline in the warm-temperate red algae Gelidium corneum occurred only in the easternmost area of the Cantabrian Sea (Basque Country), leading to a distributional contraction of 7%. We recommend conservation actions to better manage the remnant populations of these canopy-forming seaweeds, and their inclusion in national and regional catalogues of endangered species and on international Red Lists.

espanolEn los fondos de maerl de Galicia se detecto material de Jania J.V.Lamour. con una morfologia distinta de la tipica atribuida a J. longifurca Zanardini y J. rubens (L.) J.V.Lamour. —unicas especies citadas en este habitat en Galicia—. Esta consiste en: intergeniculos delgados, ramificacion dicotomica irregular y discos secundarios que facilitan la refijacion al substrato, caracteres congruentes con los de otras especies de Jania de distribucion mas meridional. No obstante, el estudio taxonomico integrador de este material y de otras recolecciones de habitat rocoso confirmo su identificacion como J. longifurca. Asi, la morfologia atipica con intergeniculos delgados y discos secundarios de fijacion es dominante en ejemplares de J. longifurca procedentes de fondos de maerl, mientras que los ejemplares de ambientes rocosos presentan los tipicos intergeniculos robustos. Por otra parte, analisis moleculares detectaron cierta afinidad filogenetica de J. longifurca con ejemplares de Nueva Zelanda, donde esta especie no esta citada. Se concluye entonces que la variacion morfologica en J. longifurca esta relacionada con el ambiente y se evidencia la necesidad de estudios moleculares para esclarecer la taxonomia de este genero, que esta basada fundamentalmente en caracteres morfologicos. EnglishMaterial of Jania J.V.Lamour. with a different morphology from the typical one attributed to J. longifurca Zanardini and J. rubens (L.) J.V.Lamour.—the only species in this habitat in Galicia—has been detected in the maerl beds of Galicia. This morphology consists on: thin intergenicula, an irregular pattern of dichotomies and secondary attachment discs that facilitate the refixation to the substrate, all of them similar to those of other southern species of Jania. Nevertheless, the integrative taxonomic study of this material and other collections from rocky habitat confirmed its identification as J. longifurca. Thereby, the atypical morphology with thin intergenicula and secondary attachment disc is dominant in samples of J. longifurca from maerl beds, while the samples from rocky environments showed the typical robust intergenicula. Moreover, molecular analysis resolved some phylogenetic affinities with samples from New Zealand, where this species is not recorded. It is concluded therefore the morphological variation related to the environment and it is evident the need of molecular studies to clarify this genus taxonomy, which is based fundamentally on morphological characters.

Maerl beds are one of the world’s key coastal ecosystems and are threatened by human activities and global change. In this study, the genetic diversity and structure of one of the major European maerl-forming species, Phymatolithon calcareum, was studied using eight microsatellite markers. Two sampling scales (global: North East Atlantic and regional: Galicia) were investigated and fifteen maerl beds from Atlantic Europe were sampled. At the regional-scale the location of sites outside and within four estuaries allowed to test for the influence of coastal configuration on population connectivity and genetic diversity. Results suggested that clonal reproduction plays an important role in the population dynamics of P. calcareum maerl beds. Clonality was variable among populations, even within the same region. At the European scale, these differences in clonality cannot be explained by the geographic or latitudinal distribution of the populations studied. A significant genetic differentiation was found among almost all population pairs and a positive correlation between geographic and genetic distances showed the limited dispersal capacity of P. calcareum. Moreover, a very clear pattern of genetic structure was revealed at the regional scale between populations located within and at the mouth of the estuaries. Genetic differentiation among estuaries was less marked for the sites located in outer-zones compared to those located in the inner-zones. In addition, variation in level of clonality linked to seascape was also observed: populations situated in the outer-zones of the estuaries were less clonal than those in the inner-zones. Finally, populations from the same estuary generally shared one or several mutilocus genotypes.

Zostera marina meadows represent an important marine ecosystem in the North temperate region. They are common in the northwestern Spain, but their distribution and biological traits remain poorly studied. Improving the knowledge of eelgrasses and identifying potential impact sources is necessary to implement conservation programs. Distribution, cover, density, size and presence of flowers in plants of Z. marina along the northwestern Spain were studied. Furthermore, we analyzed shifts in their distribution, the major human-mediated pressures (shellfishing activities) and their protection status. The study area was divided in 14 sectors that were sampled between 2014 and 2017. The northwestern Spain hosts the majority (61.5%, covering 5.8 km2, 48 meadows) of the total extension of Z. marina meadows in the Iberian Peninsula. Our study revealed 13 meadows that were not recorded before. Conversely, the disappearance of seven previously reported meadows was detected. Cover and density of Z. marina was 51.2% ± 15.1 (n = 180) and 181.1 shoots m−2 ± 79.6, respectively. Length and width of leaves were 41.9 cm ± 13.2 (n = 1800) and 6.3 mm ± 0.7, respectively. Although 98% of Z. marina meadows (5.7 km2) were located within marine protected areas, 80% of the meadows (4.7 km2) are subjected to anthropogenic pressures related to shellfishing activities. Z. marina meadows from the northwestern Spain are particularly relevant for the conservation of the species at its southern distribution limit and specific conservation and monitoring plans should be implemented to ensure its persistence. We propose 15 meadows to be considered in future conservation plans that were selected among the ones not included at present in protected areas, and considering their vulnerability and the presence of other species with interest for conservation.

Glacial vicariance is regarded as one of the most prevalent drivers of phylogeographic structure and speciation among high-latitude organisms, but direct links between ice advances and range fragmentation have been more difficult to establish in marine than in terrestrial systems. Here we investigate the evolution of largely disjunct (and potentially reproductively isolated) phylogeographic lineages within the amphi-boreal kelp Saccharina latissima s. l. Using molecular data (COI, microsatellites) we confirm that S. latissima comprises also the NE Pacific S. cichorioides complex and is composed of divergent lineages with limited range overlap and genetic admixture. Only a few genetic hybrids were detected throughout a Canadian Arctic/NW Greenland contact zone. The degree of genetic differentiation and sympatric isolation of phylogroups suggest that S. latissima s. l. represents a complex of incipient species. Phylogroup distributions compared with paleo-environmental reconstructions of the cryosphere further suggest that diversification within S. latissima results from chronic glacial isolation in disjunct persistence areas intercalated with ephemeral interglacial poleward expansions and admixture at high-latitude (Arctic) contact zones. This study thus supports a role for glaciations not just in redistributing pre-existing marine lineages but also as a speciation pump across multi-glacial cycles for marine organisms otherwise exhibiting cosmopolite amphi-boreal distributions.